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madkaw

Triple Mikuni Help

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L24 -stage 2 cam, headers, electronic ignition.

 

Car ran great before installing Mikuni's

Mikuni's look new inside and out-40phh

180 main air

140 main jet

57.5 pilot

Not sure on the pump nozzle

Float levels checked-seemed low, but after talking to Todd at Wolfcreek I might need to back in and check them

Dialed in for best idle is about 2.5 turns out on the pilots

Electric pump running 3.5 to 4psi

 

Car idles smoothly and engine will rev up pretty freely. But, when I very slowly increase the throttle off idle, it will increase rpm until about 2000 rpm then die, and I mean die. It seems revving up the engine the accelerator pump will bypass this dead spot, but I would need to rev up the motor constantly. As it is, it is undrivable. Wideband showed around 14afr at idle, but plugs were pretty sooty after an attempted drive-probably because i had to constantly rev it up.

 

Todd asked me to make sure the accelerator pumps were actually working-visually, and as best as I can see and hear, the pumps are working. He felt it could be a transition issue.

 

I thought maybe the primary jets might be small, but honestly I am guessing a bit with these carbs.

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What AFR does your wideband show when the engine dies at about 2000 rpm?

 

I'm pretty sure your right about the pilots being too small though, as far as I know when you've got the correct size of pilots in Mikuni's the pilot screw should be 1.5 turns out from fully seated

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I haven't played with Mikunis but they're similar to Webers.

 

Those jets all sound way too big. I have an L24 with stock cam, header, electronic ignition, so a very similar setup. I'm running 40DCOE's and from memory: mains are 125, air correctors are 170, idle jets are 45. I still run too rich at idle and cruise.

 

I'd be surprised if you were running lean. The only functional differences I can expect between the carbs would revolve around the progression hole locations, emulsion tube design and idle/pilot jet design, and I bet they're not worlds different. Again, I don't have direct Mikuni experience.

Edited by Leon

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Leon,

I don't think the numbers correspond . Those jets are exactly what Mikuni installed as a bolt on upgrade. Todd at wolfcreek said the numbers are close.

 

 

The AFRs were 14+ at idle, not sure before it stalls.

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Idle AFR is too high: 13.5-13.6 is normal , may need different accel pump setting, even when moving slowly they dribble fuel out to airstream.

Make sure your distributor is properly recurved to help with this issue!

 

You may need one jet size larger on idle! 57.5-62.5 is normal there!

Edited by Tony D

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I talked to Todd at Wolfcreek.

Todd was emphatic about getting the accelerator pumps working properly. He says that due to the age of these carbs and if they have been sitting, it may take some time to get the pumps fully primed. He said to make sure I get a visual confirmation on this.

Tony, I had a hard time buying the acclerator pump because I was increasing the throttle so slowly that I thought they wouldn't be a factor-but I guesws I might be wrong. Maybe that's old Holley carb mentality!!

 

Secondly, I might have skimped where I shouldn't and installed the carbs on a used intake gasket. The gasket wasn't very old, but it did have SU's mounted to it earlier. So I could have a vacuum leak that is only showing up slightly above idle, which in turn is leaning my primary too much. I shot propane around the gasket at idle-no changes. The car once warmed up idles great.

 

Thirdly, he didn't approve of my float adjustment method. He told me to hang the float on the seat and measure. The manual also shows measuring the fuel level in the bowl, but Todd said forget that.

Todd said once I check all these items, then and only then do I increase jet sizes. From all my searching, the jets in the carbs now should be good enough for my set up.

 

I will also give a SHOUT OUT to Todd for his patience. He comes off gruff-especially if you call him unprepared to answer specific questions, but in the end he was a good guy and realy wanted to help out. I have also ordered from him before and he was fast and accurate with his stuff.

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What idle jets are you running?

You've not mentioned. And it sounds like your issue, as previously mentioned. You should only be adjusting 1.5 turns on your idle screw, +- 1/4 turn. Any more turn, you need to change jet size.

Edited by luvemfast

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You mean the pilot jets? The first post I said 57.5.

It might be the case that they are the wrong size, but I need to get other things squared up first. I am going to replace the intake gasket to assure no vacuum leaks.

I did take the carbs off today and watched the accelerator pumps in action- and there wasn't any on one port. Another nozzle was spraying weak, so this was obviously effecting transition.

My floats were also rich, but not sure if that was hurting me at this point.

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My apologies. Brain fade me thinks......

It's the fact your idle screws are 2.5 turns that looks suspicious. Have you removed them and checked the tapers? Maybe they're damaged?

Defiantly fix the pumps too.

I have 44's. So 57.5 seems too large to me. But the books say 40's have larger pilot jets.

Maybe try a 55, wind out ghe screws 1.5 turns and take it for a drive? 2000rpm is before ghd main jets even kick in, so not a concern at the moment.

Basically, I'd try and get the pumps squirting out (about a meter) and the afr right at 1.5 turns.

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Okay, I'm still having the dead spot. What I have done is:

 

Removed carbs and checked and cleaned accelerator circuit-all carbs are firing a steady pee stream with the slightest of throttle movement.(pumps set on middle hole)

Blew air and carb cleaner thru the pilot circuit and I can feel air coming out of all pilot holes-idle and progressive holes.

Installed 60's pilot air jets-seemed to best around 2.5 turns to 3

Sprayed carb cleaner around while running-no significant change

All O-rings were in good shape between carbs and intake

Removed ALL sources of extra vacuum-brake booster and PCV and removed vacuum log-plugged intake

 

Can get a very smooth idle-even down around 800rpm. Engine will rev-up nicely, but still will die doing a very small, steady opening of the throttle.

I even threw the 57.5's back in, no real change.

 

I pulled the plugs after all this horsing around and some did show rich. It was kind of odd-1.3.6 were cleaner then the other 3??

 

My wideband is a bit sketchy right now, I think I need a new sensor. Most of the times I had the best idle it was showing lean. IT was a challenge to get it below 14 and then it seemed to load up trying to get it any richer. By then I had 3 turns out on the pilot.

 

So if I had a vacuum leak somewhere, would I still be able to get such a smooth idle?

 

I have searched at many set-ups and you rarely see over 57.5 on the pilots, so why would mine be the exception. Would bumping up to a 62 make that much of a difference?

 

Tony, recurving the dizzy would be required to get past this point?

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Was that a L24?

 

Did you exhibit my symptoms with the smaller jets?

 

I have seen sizes all over the place, but usually smaller then 60. I guess every engine is very different!

 

Learning about Mikuni's the hard way I guess.

 

 

I started with pilots that were 52.5's and ended up in the mid/high 60s on 44s. I was told to turn the screws out 1.25 or 1.5 turns, and then if you needed more, go to a bigger pilot.

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L28 sorry. Didn't catch that. My symptom was the hesitation at 2500 rpm. I chased it quite a bit and going bigger on the pilot helped a lot. I think a smaller choke size would have helped too, at the expense of top end.

 

I mostly posted because you were talking about having the adjustments out 2.5 turns, which was pretty much verboten according to the people who taught me how to tune them.

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Yes, pilot may need to be bigger. I've seen 65's in come cars. I have to agree with John, define "2.5 turns out" 900 degrees from seat (360+360+180)? Or is it 450 degrees (180+180+90)...

It's 1AM here, so forgive mathematical errors, you get the idea. As John says, that seems way out (if it's the 900 degrees!.)

 

In ANY case, I will repeat: 57.5's are WAAAAAAAAY too small! You RUN on the pilots at highway speed you know! And I repeat again, lower AFR for idle is better.

 

I would ask if you have 10 or 12mm booster venturis (not necessarily main venturi as most people do), as this will affect when the main system comes into the main bore...but up to that point (around 3,000 rpms) it's ALL your Pilot Jets. If you see a gradual taper on your AFR, that is telling me your pilot jet simply is not flowing enough fuel into the circuit and it just goes dry. If you have 12's you can try 10's to amplify the signal on the main jet and get more fuel in earlier...but this is too early for the main to cause you trouble IMO.

 

The accel pumps 'dribble' under really slow accel but by 2000 you should still be all on the pilot jet.

 

A recurve on the distributor might help, but you can also simply advance your timing to see what happens and give you an idea if you need more advance, earlier.

 

ONE QUICK CHECK: If you can kill the engine (or at least make it run like crap) by a 180 degree turn of the idle mixture screw, your jet is probably the right size and supplying the fuel it needs to---as I recall ideal position was 360 degrees from seat, with an adjustment of 180 in either direction RADICALLY altering your engine operation. This points again to your pilot jet being way too small. Especially with a cam!

 

OR:

 

"Arrow over, arrow down. Ctrl-Shift Arrow-Up" Jets? FAGEDABOUDIT!biggrin.gif

 

Good Luck.

Edited by Tony D

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Yes, 1 turn= 360 degrees, so 2 and 1/2 turns =900 degrees :blink:

 

My car will not run with just 1(360 degrees) out-not even close.

 

900 degrres out gets a nice idle, but that's it. The throttle opens into the progression hole area and it's done(dead)

 

57.5 is a bit arbitrary, but a lot of threads where guys posted their pilot#'s , they weren't to far off or smaller.

 

Todd at Wolfcreek thought I should be "in the ball park"

 

My wideband O2 sensor is a bit erratic, but when it is stable, it is showing lean-like 16+

 

As far a booster venturi's-I might need to school myself on that subject-I'm not sure what you are exactly refering to. Time to search, though it sounds like that is not my issue.

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Schneider 274f I believe-480 lift 270/270

 

Then you throw in dual 50mm exhausts

Larger valves

Mild port

Unschrouded chambers and notched bores

 

I don't know if there is a way to guess, but just try different sizes

 

Sidenote; Just got off the phone with Todd and he made a general statement that the smaller carbs usually wind up taking bigger pilot jets then say a set of 44's. He also believes I should just get out and drive it since a no-load scenerio sitting in the garage might be decieving. Maybe, but I know that an engine shouldn't die under no load just acclerating the throttle in the garage either, and my garage is to steep to be screwing around with a car that stalls. So I have the next 2 sizes coming

 

And Tony, as far as timing I am running 20 initial and 37 full

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Yes, the 40's will take considerably larger pilot jets.

 

I got 28mpg driving at 65-75 to phoenix in an L24 equipped AUTOMATIC Fairlady Z.... Keep out of the main and you run on the pilots, and that's below (generally 3K)

 

If it won't run between 360 and 540 from seat, your pilot jets are too small.

 

I will say if it's dying on free-rev at 2000 now, on the road it's going to COUGH BUCK SNORT! And for that, 99,999,999 times out of 100,000,000 it's pilot jets that are too small.

 

63.5 is my guess, but unless you get 'jet exchange' to try it incrementally up the scale you will buy one or two sets.

 

Going too big on your first try will likely give you what you need when you put them on an L28 or put something with more vacuum in it.

 

Good Luck!

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now just be ginger with those idle screws...next thing that happens is guys twist the tips off in the carb bodies and wonder why turning them has no effect on the barrel constantly popping until you really get into it!

They pop out with a screwdriver, but the bummer is you have to pull the carb to get to them!wink.gif

 

You will be turning them in, that's for sure!

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Buy a set of idle screws as wellsad.gif

 

The top one is trashed and will make it hard as hell to get a proper mixture (linearly) as designed.

 

The bottom one...since there's not a sense of scale I can't readily say if that's the proper "point" -- When the break off in the idle mixture port, they are no longer "Pointy" but have a blunted end where the tip of the screw, the 'bluntness" is equivalent to the diameter of the idle port in the carb body... When you see a new one next to the other you can see one is pointy, the other blunted.

 

I guess you could figure the extruded section of the top needle is about where it breaks off. You can see the end sticking into the air side through the idle port.

 

But for every one that is truncated in your set: replace it. "NFG"--it's on the way to breaking off, and with the larger jets you will want all the linear, fine adjustment you can get!

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I should have stated that I already replaced them. When I pulled these out I thought I had found my issue and got on the phone and ordered new ones. The new pilot screws went in with the 60 pilot jets-but still too lean. I have actually found several issues with these seemingly "new carbs", but I think I do have a very nice set.

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Okay, got some 62.5's and some 65's today. Didn't even bother with the 62 at first, just threw in the 65's.

Engine ran a bit better, but still fell on it's face with 2 turns out. Wouldn't idle with just one turn

Now I'm pulling out my hair thinking I still have other problems and I don't have anything else to try.

 

When I received the carbs it had 2 sets of main air jets, 180 and 200's. I have been running with the 180's so I decided I would try the 200's. Wouldn't you know it raised the threshold of the stumble by atleast 1500 rpm, and I can about save it with a little more throttle.

I ran out of time to do more troubleshooting, but the larger main air did help!

 

Thoughts?

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