Just started playing with Innovate LM-2 on Mikuni 44 2.9L stroker

[Lazeum]

Could your rich AFR at high rpm due to air restriction?

Could it be air starvation at high rpm instead of gas issue?

Could it be an issue with fuel level inside carb too high? Air filtrer / induction? Air jet too small?  

 

you could also display AFR vs. rpm. To do so, you just overlay the area you'd like to plot with the "select tool" and you go to "View" menu to X-Y plot. Much easier to analyze.

Edited March 22, 2013 by Lazeum

[madkaw]

It seems that a hard pull in 5th would be asking a lot from your Mikuni's with a big cam. You are pretty low in the rpm band at 65 in 5th. Adding size to the pumps would make it worse I believe. If anything , maybe a change in the position of the acc. rod to bring things in sooner to get rid of the dead spot, but not sure if it will help you with the 5th gear issue. It seems that your build is to bring in power way above 3000rpm and you are dumping a lot of fuel for the engine to handle below that threshhold. I have read of other builds similiar to yours and 47 pumps seem big for most.

I see the same action with the AFRs when hard tip-in at lower rpm. The AFRs dip quite low waiting for the engine to get in the range to burn off all that fuel. I do have more tuning to do, so I might be talking out my a$$, but this is what I have experienced.

I wouldn't get hung up trying to tune out a hard tip-in issue in 5th-will you really need that?I would be more concerned with wide open AFR's through the power band.

[inline6]

Could your rich AFR at high rpm due to air restriction?

Could it be air starvation at high rpm instead of gas issue?

Could it be an issue with fuel level inside carb too high? Air filtrer / induction? Air jet too small?  

 

you could also display AFR vs. rpm. To do so, you just overlay the area you'd like to plot with the "select tool" and you go to "View" menu to X-Y plot. Much easier to analyze.

 

Regarding air restriction/air starvation: it is possible... but the inlet pipe, at 3", seems to be about as big as I have seen people use.  The air filter on the inlet may be too small.  Certainly can't rule it out as I haven't tested it in any way.

 

Regarding fuel level inside the carb being too high:  I have set the float level as best as I possibly can.  I cut a strip of metal so it was between 12.0 and 12.5 mm in length (specification is per the Mikuni manual) and then used that to set the float height - making sure to check it against each float in the carb (2 per carb).  

 

Regarding air jet too small: I think that is possible too.  See below.

 

It seems that a hard pull in 5th would be asking a lot from your Mikuni's with a big cam. You are pretty low in the rpm band at 65 in 5th.

 

Adding size to the pumps would make it worse I believe. If anything , maybe a change in the position of the acc. rod to bring things in sooner to get rid of the dead spot, but not sure if it will help you with the 5th gear issue. It seems that your build is to bring in power way above 3000rpm and you are dumping a lot of fuel for the engine to handle below that threshhold. I have read of other builds similiar to yours and 47 pumps seem big for most.

 

I see the same action with the AFRs when hard tip-in at lower rpm. The AFRs dip quite low waiting for the engine to get in the range to burn off all that fuel. I do have more tuning to do, so I might be talking out my a$$, but this is what I have experienced.

I wouldn't get hung up trying to tune out a hard tip-in issue in 5th-will you really need that?I would be more concerned with wide open AFR's through the power band.

 

I agree with you about the full throttle in 5th at low RPM.  I'm not sure what is going on, but since I had already tried several jet combinations and seen pretty much identical behavior for each under that scenario, I just wanted to try something radically leaner to see if I could make a noticeable change.  No dice.  So, I'm ditching that test.  I wonder if it has anything to do with the lack of port velocity with that big cam and the low RPMs... not going to dwell on it.  

 

The thing about the pump jet is this... If I understand correctly, the hole settings on the pump rod adjust the volume of fuel.  And the pump jet simply adjusts the hole size that this fixed volume can go through.  A bigger hole translates directly to a shorter duration of time for the stream of fuel.  So, whether I use a 35 pump jet or a 50 pump jet, if I leave the pump rod setting on the middle hole, with either, I get the same amount of fuel when I open the throttle.  The difference is how fast that fuel gets discharged.  So, if I put a bigger pump jet in with the same volume, it has to improve the lean condition - because more fuel is coming out over the same time period in which that lean condition existed previously with the smaller pump jet.  The real question in my mind though is what are the bigger pump jet effects just after the lean spot.  As you said, it may be dumping more fuel at that time as well and cause a richer condition at that point in time.  

 

We have a pretty good feel for when the pump jet starts squirting fuel (we can see the RPM's increase in response to throttle input) but no real good idea when it stops...  If we could see both of those points on the graph, it would be easier to determine if a larger pump jet is going to add even more fuel on top of that rich spot that is occurring just after the lean spot.

 

 

Here is another graph from tonight.  I put back in what I had before: 180 mains, 220 airs, 57.5 pilots... and switched to the 50 pump jets:

 

 

 

Judging by my graph above, I'd say it seems to have helped the lean spot a bit, but didn't really hurt the rich spot.  The lean spot is still there, as is the rich spot, but the extremes seem to have been reduced.  I'll do some more testing, but look especially at the second measurement that I have on the graph.  I am in 3rd gear at about 4500 RPMs and I floor it.  The lean jump is not that bad, and the rich spot afterwards is not bad either.  

 

But you can see the jump to lean, then rich elsewhere in the chart...  The first measurement is where I double clutch downshift from 4th to 3rd... I blip the throttle to match revs (not high enough though), and there it is... lean then rich in a split second.  You can also see it a bit when I shift out of third into fourth and lightly apply throttle afterwards.

 

Also, from 6500 to 7k the AFR drops from 13.4 to 12.1... Not sure what that means, but as Lazeum says, maybe my air jet is too small. 

 

The results at this point are pretty good.  The car will idle forever without loading up... AFR ranges from high 13's to low 15's depending on road inclination at cruise - with very little throttle.  Going down hill for long periods of time with absolutely no throttle will send the AFR slowly upwards to high 16's, low 17's and it will start popping out of the exhaust a tad if I let it get that high.  And with this jet configuration, the car is developing good power.  It is certainly very driveable at this point.  

 

I'll probably move the rev limiter up to 7700 now and do some more testing.  I'd like to address the quick lean/rich on abrupt throttle openings a bit more, and I am not convinced the mains and air are where they need to be yet, but I am getting there.

Edited March 23, 2013 by inline6

[Tony D]

Sound like air corrector with the repich on top activity, agreed.

[inline6]

"Sound like air corrector with the repich on top activity, agreed."

 

I'll give that a shot when I can get back to testing (blew out the Welsh plug).  So is it just another myth that the Mains and Airs are typically close to each other?  For the runs above, I was using 180 Mains and 220 Air Correction.  

Edited March 25, 2013 by inline6

[Tony D]

You track or datalog MAP? You will find when you come on the cam, that you can see your MAP change... as you run up the RPM range you start going from "'0" manifold vacuum towards maybe 9" on a stock car. This is near redline.... But the fact of the matter is, as you start going from "0" and then actually drawing harder in the main venturi, you go rich. That is what the air-corrector is supposed to 'correct'... 

 

At 9" mercury, you SHOULD be on the IDLE jet, and your MAIN can't draw in through the booster venturi at that point.

 

But once WOT, you are drawing through your booster venturi....and if you start raising vacuum again you go WAY RICH wihtout that air bleed up there to break that suction and siphoning raw fuel into the booster. Actually you have 9" in the main manifold, but in the booster you are back up in the 12" or higher...and sucking that hard starts pulling air in the corrector.

 

I HOPE that makes sense as to how that thing is suppose to work. They are looking at me to LEAVE so I gotta pack up and get out of this office!

[inline6]

I don't track or log manifold air pressure.  

 

I followed you up to here:

At 9" mercury, you SHOULD be on the IDLE jet, and your MAIN can't draw in through the booster venturi at that point.

 

 

Did you mean 'At 0" mercury' in this instance?  This zero manifold vacuum is essentially just after going WOT, before the engine has had any time to speak of to react, right?  The way I envision it is, at this point, there is really not enough draw on the main venturi to bring fuel up through the main fuel jets, into the aux venturis and out into the bore.  

 

If I have this right, then I follow.  And, I'll certainly try some larger air correction jets when I can.

[inline6]

Separately, If I understand what is written in the Mikuni manual correctly, the bypass circuit is changing roles at the point where you go WOT to somewhat address the situation where the mains haven't come online yet.  

 

The bypass consists of the 4 little holes in the roof of the carburetor bores, just in front of the throttle plates when closed (and somewhat overlapping the edge of the throttle plate).  Before the throttle plate opens to WOT, when the car is idling or has the throttle plate closed, these 4 holes are letting some air into the pilot circuit (there is a tunnel which is part of the pilot circuit, running along the bore in the carb body casting) which already contains a mixture of air and fuel, and after the throttle plate opens to WOT, flow actually reverses direction... so instead of letting air in, these four holes release air fuel mixture out (again, this mixture already metered by the pilot jets).  Oh, and the fuel pump jets are also sending a stream of fuel for a brief period just after WOT too.  

 

A lot of people have talked about the lean spot that they can't get rid of at the transition point from pilots to mains.  I have a theory that they are setting the initial throttle plate position too far open at idle... opening the throttle plates far enough to activate the bypass circuit - to bring some air/fuel mixture out of one or possibly more of the bypass holes (1.5 maybe?).  And this is contributing to their selection of too small a pilot jet as well.  

 

Hard to say for sure at this point, but just before my engine lost the freeze plug the other day, I reset my throttle settings completely making sure not to open the throttle screws any more than 1.5 turns past when they touch the throttle lever.  I don't recall exactly, but they may be closer to 1 turn.  And doing the same with the pilots too: 1.5 turns max.  I warmed up the engine, and then sync'd the carbs by first disconnecting the adjustable throttle arms... and making minute changes to the throttle screws.  Then I set idle at the highest RPM I could achieve while also watching the AFR.  I was adjusting the pilot screws in increments of the screw driver slot width... in the screw, to get the final setting.  With the 57.5 pilots, I ended up at something more than 1.25 and less than 1.5 turns.  

 

I need to look at the data from the runs, but I think my lean spot has been resolved.  I guess after this long post, I should have confirmed already.  It's too late at night for me to look now, but I'll check to confirm when I can.

Edited March 27, 2013 by inline6

[Tony D]

No I meant it as written. As you approach redline, you go towards som inches of hg from the initial zero at WOT.

 

normally, at 9" you are on the idle/transition with a barely cracked throttle. But in this case you are at WOT and come back up to 9" from 0".

 

It is this progressive raise that gives you more fuel.

 

You use accel shot to dump fuel into manifold or 0", no depression exists at idle or transition ports to draw fuel. The accel shot gives you that initial kick from 0" to accelerate the engine to a speed range where a very faint depression (raised by the booster Venturi) starts sucking on the mains.

 

As rpms rise, your depression in the booster Venturi (also "amplifier Venturi") increases pulling harder. On the main jet.

 

In the "swan neck" of jet well, up emulsion tube, past air corrector then back down to booster Venturi...the draw becomes harder and harder, the air corrector lets in more air to lean out he top end. Too small, you go way rich. Drop throttle, loss of amplifier signal, your air corrector acts as a vacuum break so your mains don't siphon themselves into the throats thrown simple siphon action.

 

 

****

 

I would agree that the transition lean is almost always caused by an idle jet too small.

 

 

You just aren't having luck with your ore plugs!!!

 

Keep at it, you sound close!

[inline6]

Ha ha! It's been two more years, and I am finally back to tuning.  I've been through a ridiculous amount of hassle with the engine, but hopefully I am past all bad stuff. Those issues are detailed in other threads however.  So...

 

I've been reading and re-reading Mikuni tuning info in the forums and thinking about how these Mikunis work - at times with spare disassembled carb in hand.  First, a description/re-iteration and some story telling of things using a hypothetical situation:

 

I'm on the chassis dyno in 4th gear with the engine turning say, 2000 RPM.  The throttle plates on the Mikunis aren't open much to do this, so the carbs are running entirely on the pilot circuit.  The pilot circuit is drawing fuel from the same fuel source as the mains... up through the jet block holder - my Mikuni carbs have separate "pipes" for the pilots than those of the mains.  Air for the pilot circuit is drawn from fixed diameter holes in the top of the jet block.  Air and fuel are mixed together within the pilot jet.  Downstream of the pilot jet, the air fuel mixture is traveling, again in a separate pipe.  This pipe travels out of the jet block, along the top of the throat of the carb, crosses over top of 4 bypass holes on the front side of the throttle plate, and one more hole on the backside of the throttle plate.

 

At 2000 RPM, with the throttle plate is open maybe 5-10%, there is no fuel/air mixture coming out of the auxiliary venturi.  There is a high degree of vacuum in the manifold (9" mercury), as the throttle plates are near shut.  The six pistons going up and down at 2000 RPM are causing some major sucking at the backsides of the throttle plates.  This is drawing the fuel from the pilot system.  The only fuel mixture feeding the engine is coming from the pilot screw outlet hole (backside of throttle plate) and perhaps some of the bypass holes (in front of the throttle plate). The pilot screws are set at exactly 1.5 turns out and the wideband AFR is showing 14.4 (air to fuel).

 

Now, I slap the throttle to the floor as fast as it will go.  The throttle plates jumps to wide-the-frick open.  What previously was major sucking at the backsides of the throttle plates now becomes nearly no sucking at all temporarily (0" mercury).  Without the sucking the pilot circuit goes basically dead... and a massive glob of air leaps towards the pistons - partly drawn by the pistons jumping up and down in their bores at 2000 RPM.  Though it looks like a lean fire in the cylinders is imminent, there is also this stream of fuel that was launched from the pump nozzle right when I started to slap the gas pedal down.  Per the Mikuni manual, once the throttle was open 30%, no more fuel was added to the pump circuit, but the stream (which is suspended in Matrix "bullet time" animation at the moment) will last as long as it takes for the fixed volume of fuel to pass through the hole size in the pump nozzle as dictated by the properties of physics (fluid mechanics specifically).

 

This stream is meant to keep the flames alight when the glob of air gets into the combustion chambers... and until the separate and new supply (a guyser really) of air/fuel mixture can be mustered to come forth from the main circuit.  Now...

 

The main circuit is a bit different in design.  There is basically this big, brass screw (jet block) in the jet block holder.  At the bottom the jet block is another screw with a hole in it.  This main jet is totally immersed in the gas within the carb float chamber.  The hole meters the flow rate of the fuel from the chamber.  At the top of the jet block is another screw with a hole in it. The air corrector jet is totally above the gas in the carb float chamber.  The hole in the air corrector meters the flow rate of the air which comes from outside the carb body, through channeling in the float chamber cover and into the jet block area, above the fuel.

 

At 2000 RPM, the pistons are jumping up and down, the stream of fuel slips into the glob of air headed for the combustion chamber.  As the first bit of fuel from the nozzle gets consumed by the voracious pistons, they speed up a hair.  Additionally, the pipeline of air from the cold air intake to the combustion chamber starts to get a move on.  

 

The part of this air that passes through the auxiliary venturi speeds up relative the to air around the outside of the aux. venturi.  The air passing by a hole in the aux. venturi creates a "depression", which in turns "draws" on the main circuit.  The fuel level in the float chamber is sitting a touch lower than waist high on the jet block.  As the vacuum builds from in the aux. venturi, the "negative pressure" sucks the fuel higher within the jet block hole. Fuel travels through the main jet, then through 4 ovalized holes in the bottom of the jet block and works its way up around the sides of the jet block.  At the same time, air is drawn in through the hole in the air corrector jet... down into the jet block.  It exits via 8 small holes around the periphery of the jet block.  These holes are above the correctly set fuel level in the float chamber.  The main and the air corrector inputs work together to set the mixture of air and fuel that is drawn up high enough in the jet block hole to where it has reached the exit of the jet block holder hole - the pathway to the aux. venturi is downhill from here.  

 

We are nearly to the question of the post now - don't stop me, I'm on a roll!

 

The guyser of fuel now enters the aux. venturi where it is met at 90 degrees by a screaming banshee of air.  It smashes into the fuel constantly... turning it into a spray of bazillions of droplets.  This new source of air/fuel mixture then makes its way to the combustion chamber.  AND... wait for it...

 

Upon first being consumed, the AFR turns sharply downward to around 10.8 (which causes me much stress, as well as washes away the nice protective layer of oil that should be on the cylinder walls).  

 

 

Now, I will admit that for this one run, I saw the 10s and let the throttle off the floor to allow recovery, but by 6.5 seconds I've been back into it, the pumps are outta the picture for sure.

 

Jetting at the time was:

 

Main 180

Air 240 (up from the 220)

Pilot 57.5

Pump 40 

Choke 37 

 

This is really the opposite of what should be happening.  What could cause the AFR to go very rich just after a full throttle snap-open situation such as this?  One obvious answer might be "too rich, go smaller on main jet", but the rest of the run isn't indicating crazy rich at all.  Another potential answer: "go smaller on the pumps".  Well, the chart already is showing that upon initial throttle application, the mixture goes lean for about a second (centered at 1 second).  

 

Full throttle, not much depression to pull on the aux. venturis... a big cam that doesn't pull air through the intake a lower engine speeds...  How does this translate to "hard pull" on the mains... providing an overly rich mixture?  

 

Here is another run a few moments later, also in third.  I start at much higher engine speed to avoid the low speed, throttle dump problem. Pumps are done within the first second or so... the AFR climbs into the 14's from 5700 to 6400... then marches down to ~12 at 7000, where it levels off quite a bit. 

 

 

Sure doesn't look crazy rich in this rpm zone!  

 

Ideas?  Could something be going on with the air supply channel - where the air correctors draw their air from?  I am running a cold air box...  I did remove the air filter for one of my runs on the dyno two weeks ago - it had no noticeable effect.

 

Based on these charts, I've got 250 airs in the car, ready for the next run.  That's where I am for now.

Edited April 9, 2015 by inline6

[madkaw]

It seems contradictory that you are shooting for performance at 2000 rpm with main airs as big as you have them. I think IMHO that you are way too big on the main airs and it's throwing off your tuning.

Running that big of mains and then diluting the mix with that big of main airs seems self defeating.

Since you have had other issues(cam timing), maybe start back on the small side. Remember the smaller the main air jets the richer the main jet mix. How about something completely different like 155 mains and 180 main airs and see what that yeilds?

Once again I'm not sure you are going to get a street tune with that big of cam. I wouldn't even tune for WOT under 3000 rpm. That puts you right in the middle of the transition issue with triples and it also puts you in timing curve issues. I would tune for WOT and cruise and idle. Idle for easy starts, cruise to keep plugs clean, and WOT for WOT.

Staring at the Wide Band during driving could lead to insanity and a bad accident-lol

[gacksen]

Staring at the Wide Band during driving could lead to insanity and a bad accident-lol

 

with a different wideband and a different car almost had been there

 

as for the problem itself i have exactly the same. car is ripped apart for some months now so no tuning is possible but

before that if you go WOT below 2750rpm issue is going in the same direction. with the camed 3.1 and triple dcoe was running

140 mains 195 airs and 40 pumps...

[inline6]

It seems contradictory that you are shooting for performance at 2000 rpm with main airs as big as you have them. I think IMHO that you are way too big on the main airs and it's throwing off your tuning.

 

I don't know if I buy the argument that these carbs can't be tuned to work across the entire engine range.  TonyD has repeatedly chimed in on many threads.  He is saying that it can be done - it is just harder.  From another perspective entirely, when folks go to the dyno, at the end of the day, they can typically take home some kind of chart that has a full RPM sweep with max torque and max hp.  I'd have to concede that with my engine combo, I wouldn't be able to do that.  I'd have to settle for a curve from like 4000 to 7700.  I don't think I'm ready do that yet.

 

Regarding too big on airs: per the Mikuni Service Manual, air correction jets affect "the flow of fuel mainly at medium and high-speed...".  Because my AFR is dropping to low 12s or high 11s right now at the highest RPM range for this engine (6900 - 7400, with redline supposed to be 7700 - I just haven't gone that high yet), and larger air correction jets lean out the mixture, then I should actually be going with bigger air correction jets, not smaller ones to try to raise AFR a bit at this engine speed range.

 

Running that big of mains and then diluting the mix with that big of main airs seems self defeating.

 

Hmmm... I don't know.  As long as I can put more fuel in with "bigger" main fuel jets and with "bigger" air correction jets (speaking relatively here) and still achieve a "power" target AFR, then I should be making more power vs. "smaller" main fuel jets and "smaller" air correction jets, right?.  Even if I do give up on tuning for full throttle at below 3000 RPM, I wouldn't go with smaller mains and airs at this point because my AFRs are not too rich at middle engine speeds.  I mean, I am currently seeing above 14s from 5700 to 6500... and above 13s from 5400 to 6700. That is too high for good power.

 

If I did go smaller, I'd experience lower total power output and probably an increase in fuel economy.  But, that is not really the goal.

  

How about something completely different like 155 mains and 180 main airs and see what that yields?

 

Actually, I already did this while at the dyno a couple of weekends ago - with the caveat that the cam timing was off, I know.  But, I changed the main and airs to be much smaller several times, trying to eliminate "rich" completely - trying to get to a "too lean" condition.  Basically, as I went down in size, a "hump of leaness" became more and more prevalent in the mid-range RPMs, and I still was not able to get rid of the 10s at snap open throttle and low RPMs.  You can see more detail about those runs on the dyno in this forum thread if you would like. 

 

Yes the cam was mis-timed, but I don't think that the general behavior vs. correctly timed would be completely different.  In other words, if correctly timed, I believe the overall trends I experienced would be the same... just different RPM points would be in play.

  

Once again I'm not sure you are going to get a street tune with that big of cam. I wouldn't even tune for WOT under 3000 rpm. That puts you right in the middle of the transition issue with triples and it also puts you in timing curve issues. I would tune for WOT and cruise and idle. Idle for easy starts, cruise to keep plugs clean, and WOT for WOT.

 

You may be right, however, I'm not willing to give up just yet.  

Edited April 9, 2015 by inline6

[inline6]

Just came across this thread which discusses reversion.  Kind of seems like what I am experiencing.  Darn it.

[inline6]

I was able to work on the tuning some more yesterday.  I started the day with:

 

Main 180

Air 250

Pilot 57.5

Pump 45 

Choke 37

 

Last change was going from 240 to 250 air correction to try to produce a leaner AFR above 6700 RPM. Here is what actually happened - the dotted line is the previous run with 240 airs - the solid line is the current run with 250 airs, both are in 3rd gear:

 

 

Interesting that the bigger air jet caused the range between 5300 and 6000 to lean out more while causing the range from 6000 up to get richer.  Isn't that the opposite of what you'd expect?

 

Because the AFRs were at or above 13 for all of the graphed RPM range except above 6600, I decided to try upping the mains another step to 185:

 

 

The dotted lines are the 180s from the previous run.  Only the mains were changed - again in 3rd gear.  The AFRs were made richer across the test RPM range.  

 

Here is a comparison of two 4th gear runs.  The runs differ only in that the prior run (dotted lines) had 180 mains.  This run has 185 mains:

 

 

Interestingly, when I went to full throttle on the 185 mains run, AFR didn't dive into the 10's as has typically been the case. I am more conscious of the problem, however, so it may just be that I am squeezing to full throttle over a couple of seconds instead of snapping to full throttle.

 

Since going with the bigger 250 airs richened the top end and leaned what has been consistently the leanest RPM area on many plots, I'm thinking the next step will be to lower the airs to something like 230.  My prediction is that my plot will flatten out - richer in the 5300 - 6000 range and leaner in the 6000 plus range. 

 

But first, I'm going to tighten up the manifold nuts.  I may have some slight leakage at the header, and I want to be sure that is not the case.  I'll do that and make a plot run. If nothing has changed, then I will change to the 230 airs and see what happens.

Edited April 12, 2015 by inline6

[Tony D]

Keep in mind Mikuni PHH were OEM fitments on probably a MILLION or more Toyota DOHC engines from 1967 into the 80's (with catalysts!!!)

 

If you can get Toyota drive ability and reliabilitynout of them in a 1980 Camry GT, you can do it on an L-Series.

 

It's not me just saying it, Toyota proved it! My 76 Celica GT would lug in fifth gear from off-idle to redline.

 

So would my Fairlady Z with the L20A and 40PHH's from a 2TG Toyota.

 

As would my blow-through 44PHH turbocharged 73 240Z....and that was no small feat.

[inline6]

Keep in mind Mikuni PHH were OEM fitments on probably a MILLION or more Toyota DOHC engines from 1967 into the 80's (with catalysts!!!)

 

If you can get Toyota drive ability and reliabilitynout of them in a 1980 Camry GT, you can do it on an L-Series.

 

It's not me just saying it, Toyota proved it! My 76 Celica GT would lug in fifth gear from off-idle to redline.

 

So would my Fairlady Z with the L20A and 40PHH's from a 2TG Toyota.

 

As would my blow-through 44PHH turbocharged 73 240Z....and that was no small feat.

 

I'd like to think I can figure it out, but I am wondering if my cam and my intake as well as cylinder head porting haven't made that too tall of task for the limited action of swapping out jets.  I also could be experiencing some reversion...  

 

I'm going to try to get the AFR curve from 3500 RPM up to be "right" and then see where I am with full throttle dumps at 1200 RPM (that is as low as I go - it is my idle speed for this cam) up.  

[Tony D]

You really can't dump from idle.

This is one of the problems with Carburettors: they need velocity or vacuum to work.

Our Bonneville Engine idled at 2,200 rpm son the big predator carb (hey, it's just a big SU, right?) that was as low as it would go.

We swapped to Weber 45 DCOE's and the idle was able to be dropped to 1,700.

This was the exact same engine, just changed induction...

We went to an Electramotive TEC2 and TWM 45 ITB's and we idled at 950! And could turn it down to where the cranking signal was intermittent...it would bang-bang-bang along at 450 RPMS with a triple plate clutch and flywheel that weighed 15# total. I'm convinced even though it made peak power over 8,000 RPM's, we could have put a stock weight flywheel and run that cam on the street with the EMotive/TWM's.

 

The Quoted Procedure of getting it into top gear and making pulls smoothly from 1,000 rpms represents a reasonable driving demand: being in traffic and not wanting to take the car out of top gear.

 

This also assumes the engine idles somewhere around 750 rpms. So 1,000 represents just off-idle by about 33%.

 

The translates roughly into 1,600 in your case with a 1,200 rpm idle. That is a reasonable expectation.

 

On the B-Ville car we could move the car around at 2,500 on the Webers, but more like 3,000 on the Predator.

 

Sometimes I assume too much and think people will make these interpolations. Reason I say it was because the process of writing the Toyota example reminded me of my dunderheaded old USAF Supervisor who had 2TG Toyotas and was convinced he needed to replace his entire brake system because there just wasn't sufficient braking. Long story short was "Wait, is this the car you put the TRD 304/308 Degree Cams into? The one that won't idle below 1,700?" He was convinced the car should work like stock with hella cams in it, and thought idling up to 1,700 was wrong. He had brakes driving down the road, just not at "idle"!

 

Same thing here, I'd say don't expect much below 1,600 at all, you don't put your car into gear and idle it down the road. This is an off-idle test, and off idle is 1,600-1,750 rpms. You just won't work well below that due to carbs needing vacuum to properly atomism the fuel. Up the engine speed and your atomization improves.

 

In Japan they have all sorts of bolt-on TPS and Stand-Off Injector Mounts to strip the venturis out of those PHH's and make them ITB's for better drivability and more power everywhere..."Ctrl-Shift Arrow up up up, or down down down"!

[zbloke]

In Japan they have all sorts of bolt-on TPS and Stand-Off Injector Mounts to strip the venturis out of those PHH's and make them ITB's for better drivability and more power everywhere..

 

You wouldn't have a link to some of the above would you? 

[inline6]

Sometimes I assume too much and think people will make these interpolations. Reason I say it was because the process of writing the Toyota example reminded me of my dunderheaded old USAF Supervisor who had 2TG Toyotas and was convinced he needed to replace his entire brake system because there just wasn't sufficient braking. Long story short was "Wait, is this the car you put the TRD 304/308 Degree Cams into? The one that won't idle below 1,700?" He was convinced the car should work like stock with hella cams in it, and thought idling up to 1,700 was wrong. He had brakes driving down the road, just not at "idle"!

 

Same thing here, I'd say don't expect much below 1,600 at all, you don't put your car into gear and idle it down the road. This is an off-idle test, and off idle is 1,600-1,750 rpms. You just won't work well below that due to carbs needing vacuum to properly atomism the fuel. Up the engine speed and your atomization improves.

 

I started out thinking I could tune these carbs for "acceptable" AFR with full throttle application in 4th gear from 1200-7700 RPM... even with this big cam.  Right now, I can't use full throttle at below 4000 RPM in 4th without a dive of the AFR to mid 10s along with experiencing what I perceive to be engine misfire.  The taller the gear, the more prevalent this issue is.  If I am in first, it isn't a problem.  My understanding is that I am a still a long way from what I can yet achieve with these carbs (and my specific engine) from a tuning standpoint.  But, yeah, I am starting to realize that I won't be able to achieve the original goal until I switch to the Tec GT and the TWM fuel injection.