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madkaw

Triple Mikuni Help

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The tuning continues;

 

I have the car running pretty good, power feels decent and the wideband shows a nice 14+ at cruise. Still have a slight bobble that only happens at the hardest tip-in. I went ahead and rechecked my adjustment on my pilots(62.5's)and I was out over 2 turns.

So I reinstalled the 65's , and dialed them in-but I am still over 1.5 turns . My useless wideband really didnt give me a good indication of any change. Infact my gauge seemed to get more erratic with the new pilot size. I did get the engine running smoothly and-- at first, it seemed to be a bit torquier at the low end.

 

So I am giving these tid bits of info as background to what I did next. I decided to bolt on my vacuum log again to see if I noticed any difference. The most immediate thing I noticed when driving the car with the vacuum log installed was-- a stabilized AFR on my guage. I couldn't seem to get a stable reading when I first installed the 65's , but with the vacuum log the reading was very stable??

 

Also, the vacuum log also seemed to lean out my AFR. I don't believe I have any vacuum leaks, the car has a rock steady idle at 750rpm.

 

 

One last thing, is there a secret for starting the engine after it's hot? I don't have any issues on initial start up, but after driving it and shutting it down, it is difficult to get started. It usually takes a second try. I've tried different things like -one pump of the gas pedal, or giving no gas at all. Maybe I am trying to run it too lean at idle?

 

Current set-up

65 pilots-1.5+ turns

200-main air

160-main jets

40 acc. pumps-set on the highest setting

 

Vacuum log is 3/8" lines to ALL runners, and 3/8" to the PCV, and 11/32 to the MC booster

post-1083-061406800 1341353526_thumb.jpg

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Sounds like it might be worth it to try 67.5s. FWIW, when my car was hot I never touched the gas, and only gave one pump before starting cold.

 

Timing has a big effect on hot starting. Running without vacuum advance I ran about 18 degrees BTDC and it fired up pretty easily.

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Yes, I think a set of 68's and 70's are in order. A few more main jet sizes and I might be ready for the dyno!

 

As far as starting, I have had some success with just a turn key start when hot-but no always.

From what I have read in these forums and others, it seems these carbs prefer a rich setting, so maybe that is part of the issue.

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"So I am giving these tid bits of info as background to what I did next. I decided to bolt on my vacuum log again to see if I noticed any difference. The most immediate thing I noticed when driving the car with the vacuum log installed was-- a stabilized AFR on my guage. I couldn't seem to get a stable reading when I first installed the 65's , but with the vacuum log the reading was very stable??

Also, the vacuum log also seemed to lean out my AFR. I don't believe I have any vacuum leaks, the car has a rock steady idle at 750rpm.>

I found that my engine smoothed out as well with a similar vacuum log. REMEMBER TO REMOVE IT WHEN YOU SYNCH THE CARBS! With the vacuum log in there, you get a little smoother pulsation in the inlet runner, and not such a "High Spike" so it doesn't 'draw' through the jet as hard for that split fraction of a second that makes it run richer. But as you see, you get more consistent readings.

They way I characterized it on my blowthrough turbo application was that before I had the balance log, it almost felt like I could feel the pulse of each cylinder 'hit' as I accelerated under full boost. With the log on there, it was like a turbine---smooth and just pulled without that pulsation of each cylinder hitting on a power stroke.

I also didn't pump my gas when hot. In fact, I installed the cooling bodies for between the carb body and accel pump which utilize the fuel return that stopped my percolation and allowed easy 'no cracked throttle extended crank' restarts when hot.

usually hard restarts are a problem of too much fuel than not enough. Sloowly crack your throttle about 1/4 way when cranking and see what happens. if it fires easier, you may want to cool the carbs donwn a bit. Chances are good the fuel is perc-ing a little bit and running into the bores causing rich hot restart.

Edited by Tony D

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Thanks for the input Tony.

As far as the starting issue, I think you are correct and I am/was running to rich. I turned in my pilots a 1/4 turn and it seemed to help. I am still dialing things in here and I have been a little paranoid about running things to lean. I have been reading numerous posts that these carbs like to run FAT on the mixture-even at cruise.

 

I just finished a 300 mile cruise yesterday and my AFR's hovered in the 14.5 range most of the day, and I didn't calculate it accuretly, but I believe I got high 20's on the MPG at 75mph with a 3.90 rear. But at idle, it seemed way rich and was hard starting-and pulling the plugs seemed to verify that after put-puting around parking lots and pulling the plug. Is that kind of AFR at cruise concerning? It seems ideal and might lean slightly at an initial slight tip-in, but never past 15.5 or so.

 

Where I am going with this is; I know what the Mikuni manual says about pilot jet selection and the 1 turn out- rule.

It has been posted that the rule is BS, and it is fine to be 2+ turns out. What are the effects of violating the 1 turn rule? Is this where folks start running into the hesistation during transition when they go beyond this 1 turn?

In efforts to FATTEN(close to 2 turns out) up my AFRs with the 65's, my idle got richer and harder to start, and my hesitation got worse in transition. Without naming anyone, they are running a rebello motor and they were told that cruise should be in the low 13's----really? I would think that would be closer to a WOT AFR.

 

Still lots of tuning to do!

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To be honest you can run 16 or up to 18:1 AFR on light cruise. Chrysler lean-burn engines were into the low 20's!

You just have to make sure your MAIN tip-in and accel is correct and you will get GREAT fuel economy (not to mention low emissions) and power when you get on it!

With 40PHH's on a car with an auto box and 4.11 gears on an L24 I would easily get 24-28mpg at a steady-state 65mph going the 357 miles from my house to my bud's in PHX.

They were jetted on the pilots on the verge of coughing and surging at that speed.

Do not get hung on 14.7:1, it's more of an emissions thing to supply enough fuel and O2 for catalytic action than about economical running.

If you aren't concerned about fuel economy, jet for 5%CO or as high as 12%... Idle on these engines seems to like 13:1, but light cruise? 13:1 may show HP Benefits on a Dyno... But you only need 25hp to move the car at. 65mph.... IMO jetting at that cruise point can be leaned to sacrifice power you don't need in lieu of great fuel economy. You go WOT, the mains kick in and you get 12-13:1 anyway.

After your torque peak, you pull fuel -- even on a turbo motor you can pull back to 13.8:1 and keep making power. The inability of carbs to do this under boost combined with their performance in the 3"Hg-3psig turned me to EFI Forevermore. Couldn't get it to do that with carbs...

Be glad you're N/A!

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I don't know about "lots" of tuning, sounds like you're getting it dialed in pretty well. :) At some point that energy should be directed somewhere more useful. They are triple carbs after all, and you're getting very good results. High 20's mpg cruise is great. Definitely want leaner at cruise, 13:1 is WOT afr as far as I'm concerned, I think you're in the right ballpark. As Tony said, you're not needing a lot of power at cruise, so no point in dumping a bunch of extra fuel in there.

 

As to why the pilots need to be 1 or 1.5 turns out, nobody really told me the reasoning why, but I think it has to do with the adjuster screw getting far enough away from the pilot that it's essentially wide open, so that more adjusting isn't really doing anything. Gross exaggeration might be comparing it to adjusting nozzles on SU's in an attempt to change WOT afr's. I'd try the larger pilots with the adjustment in the right range.

 

Fuel percolation is a problem that I never ran into, but it is something you hear about quite a bit, so if you don't have a heat shield, it's not going to hurt to put one on. I did have some vapor lock issues with SU's and I did two things to fix: I got rid of the mechanical pump and used an electric, and I got rid of the metal fuel rail and just used a rubber hose. After making those changes I never had boiling issues again, and I kept those changes with the switch to Mikunis. Still think timing could be a factor if it is too far advanced at idle.

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Maybe more tuning could be translated into- more learning needing to be done:)

I am a bit of a perfectionist - which might be a downfall on a 40 year old machine.

In the end, the dyno will tell all. I will use the same dyno I used for my SU's , so it will be a direct comparison.

 

I have found that I need more heat shielding for my air box. I need to segregate my header with some sheet metal so it doesn't rise up straight on to my airbox.

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"As to why the pilots need to be 1 or 1.5 turns out, nobody really told me the reasoning why, but I think it has to do with the adjuster screw getting far enough away from the pilot that it's essentially wide open, so that more adjusting isn't really doing anything."

It's the linear taper of the adjusting screw compared with the size of the hole it's going into. (Please forgive me if this sounds a bit geekish...but here goes:)

The pilot jet feeds both the idle screw, and the transition ports.

Your fuel supplied by the idle port (throttles near closed and no transition ports exposed to vacuum) is the area supplied by the annular area difference between the volume the screw takes up compared to the size of the hole. You ONLY want enough passing through that annular area enough volume to allow the combined area of all transition ports to be less than the annular area of the pilot jet you install for the vacuum you apply. (Same as the SU Jet Fueling Characteristic when calculating 'station position' at each step in the taper.)

Meaning, if you pull more out than 1-1.5 turns, you have as Jon said, 'basically opened the hole fully" and aren't going to be able to supply transition ports with proper fueling. Back it out two or three turns compare the thread pitch, and you can see where you are on your needle taper compared to only 1 turn out.

BASICALLY what this means is at one turn out, and a BIG JET, you have very little annular space in which to supply fuel, and you therefore allow FAR MORE FUEL to be added during TRANSITION AND OFF IDLE than at idle. This allows you great flexibility in tuning idle mixture, and gives you far less chance of transitional or tip-in bog from enleanment when opening the throttles off idle and before main transition.

As you see, the larger you are going on the pilot jet, and the further closed you are getting the idle screw, the better your tip in transition is getting. This is my feeble attempt to try and explain the mechanics of what is happening and what the engineers at Mikuini had in mind.

I owned countless Toyota 2TG and 18RG engines with factory Toyota OEM Triples, and they ALL acted the EXACT same way: more than 1 to 1.5 turns out on the idle screw and you were flooded pig rich at idle and encountered transitional bogging or popping. You would think the screw being opened more would be a 'zero sum game' but apparently it's not. The transition ports give FAR more annular area for fuel dumping during tip-in than is available from the single annular area around the idle screw.

I hope that made sense...sad.gif

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Makes good sense!

 

I just experienced it on my 600 mile round trip to the Midwest Heritage Z show.

So I left riding with my 65 pilot jets at just shy of 2.5 turns out. With my gearing set-up, my rpm at highway speed runs between 2700rpm-3100rpm, right in that transitional area with these carbs. Engine sounded smooth at highway speed and not much in the way of a bog during acceleration----but, idle seemed way rich. At the show I pulled a plug and it was sooty around the top of the threads, but the tip was tan.

 

Soooo, I decide to turn in the pilots a full quarter turn to lean out the idle and make it easier to start. Seemed good putzing around the show, but the ride home was different. The engine sounded funky at highway speed/transition area.The bog was worse too. I thought something serious was wrong and pullled over to check things. Then I decided it must be the adjustment, so I turned the pilot back out a good 1/8 turn and it made a big difference. The engine sounded almost normal again at cruise.

 

Your explanation...

 

BASICALLY what this means is at one turn out, and a BIG JET, you have very little annular space in which to supply fuel, and you therefore allow FAR MORE FUEL to be added during TRANSITION AND OFF IDLE than at idle. This allows you great flexibility in tuning idle mixture, and gives you far less chance of transitional or tip-in bog from enleanment when opening the throttles off idle and before main transition.

 

 

As you see, the larger you are going on the pilot jet, and the further closed you are getting the idle screw, the better your tip in transition is getting. This is my feeble attempt to try and explain the mechanics of what is happening and what the engineers at Mikuini had in mind.

 

 

.... does answer my question. I knew Mikuni had a reason for the 1 turn rule.

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As far as cruise mixture, Tony is spot on. Run it as lean as you can without misfire. Whoever told you 13:1 is probably used to 4-barrels or air-cooled engines...

 

The reason behind the carbs "liking" to run rich idea is based on inefficient designs that don't evenly distribute the mixture across all cylinders. Imagine a 4-barrel, or even the stock SUs on an L-series. The intake tracts are not equal length, and are fed by either one or two carburetors. It is impossible to get the same exact AFR to each cylinder. You might be richer on 2 cylinders, but leaner on 4, which would cause issues (surging, rough running, misfire, etc) when trying to lean them out.

 

With triples, you have a dedicated venturi for each cylinder. This allows for much better mixture uniformity, and thus a leaner AFR before experiencing misfire.

 

Set WOT at 12.5:1 to 13:1, set idle as lean as you can without misfire or transition bogs, and set cruise to lean just before misfire.

 

 

I owned countless Toyota 2TG and 18RG engines with factory Toyota OEM Triples...

Tony, where in the world did you find those elusive 2TGs and 18RGs with OEM triples? :P

Edited by Leon

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Been a while since I've checked this thread.

Been an interesting read.

I'm currently running in my 44's- (used the Webber book recommendations for base)

55 pilot (52.5 affected higher rpm's)

160 main

210 airs

50 pump (45 would lean out and hesitate)

 

This has made me want to try larger pilots.

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Well, I tried running 57.5 pilots.

Didn't have good results. Was running too rich at idle and fouled a plug at the track. Thought I had a major issue. But with some advise from some boys at the track and PMC, I went back down to 55's and dialed the idle screws down a 1/4 of a turn. AFR's about 14 and goes great.

Also noticed I was running 230 airs, not 210's.

I'm running a PMC bored L28 2.95L @ 10.5:1 with .59" lift.

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Been a while since I've checked this thread.

Been an interesting read.

I'm currently running in my 44's- (used the Webber book recommendations for base)

55 pilot (52.5 affected higher rpm's)

160 main

210 airs

50 pump (45 would lean out and hesitate)

 

This has made me want to try larger pilots.

 

What made you want to try larger pilots - not sure what you meant?

As was stated to me, the cam has a lot to do with your pilot size and all your jets obviously. But from what I have read the larger the cam, the smaller the pilots- probably because of larger overlap affecting low rpms. This would hold true in my case with a small cam wanting bigger pilots, plus my valve timing being slightly advance . Making my power in the lower rpm band.

So what was your issue?

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So the weather cools off and humidity drops out- so mixtures should lean out a bit in carbed engines?

Had my Mikunis dialed in for the hot and humid and this weather change arrives and my AFR's are suddenly piggy rich- confused!

I'm guessing for some reason a cylinder is not running right or a carby is scewing up causing the richness- unless my understanding of air density is off.

Time to pull some plugs and the CAI and look for fuel.

It's always something:)

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So the weather cools off and humidity drops out- so mixtures should lean out a bit in carbed engines?

Had my Mikunis dialed in for the hot and humid and this weather change arrives and my AFR's are suddenly piggy rich- confused!

I'm guessing for some reason a cylinder is not running right or a carby is scewing up causing the richness- unless my understanding of air density is off.

Time to pull some plugs and the CAI and look for fuel.

It's always something:)

Correct, cooler (denser) air will lean out your mixture. Humidity has a lesser effect.

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