So i installed a Mikuni 44 in my stock port 12A with less than 50k on the clock.

Everything is hooked up and it is running, but not running great.

I have it jetted for my area according to the guys in the midwest section.

Venturi 39mm
main fuel #200 jet
main air #240
pilot jet #62.5
accel pump #90 aka idle jet

Like i said it runs and pulls HARD, but the issues i am having are when i am under hard acceleration and then i shift, as the RPMs fall and i shift into the next gear and hit the gas again it hesitates (or bogs maybe).

I have my fuel pressure regulator dialed in to 4.5psi (according to respeed's gauge).

Could variances in fuel pressure reduce said problems, in other words, should i first try adjusting fuel pressure? Would that make any difference? Has anyone else had better luck with more or less fuel pressure?

Furthermore, it doesnt like to cruise at a steady cruising speed on the highway.

Any help/ideas would be great!

 

This is a common problem with the 2 barrel carbs. For now, either keep the rpms up when hitting it hard, or ease back onto the throttle to get past the bog. Then find a wideband for further tuning.

There is a guy selling the Mikuni manual on eBay, it's cheap and on cd. Pick one up, it will give you sometips of street tuning.

 

I'd try 190 Air and 200 fuel and see what happens.

 

Troch, i have a copy of the manual (got it from good ol Todd at Wolf Creek), it definetly helps. What is involved in getting a wideband?

Rx7doc, i will give it a shot!

 

not liking to cruise at speed is unusual.

what rpm is your idle set at?
i like to keep mine around 1200 or so. not only does it kill the bog but it also keeps oil pressure up. trick a local racer taught me.

 

Wideband is simple, drill a hole in the exhaust, weld in an O2 bung, spend 3-400 on a wideband and start checking your afrs. It's easier/cheaper to find someone who has one and knows how to use/read it.

I'm not sure what you are referring to as the accel/pump jet. It's not listed in the manual as such. The only other jets besides the mains are the starter jet, which is #85 in the diagram and sits at the bottom of the fuel well and feeds the AP. Mine are 760 in both the of my Mikunis, one for a 12A, the other for a 13B. The other jet is the pilot jet, which are 52.5/57.5-12A/13B.

The pilot jet can also be considered the idle jet as it feeds the carb from 0-20% throttle.

Doc hit the #s on the main fuel and air jets that I have for the 12A setup.

 

Surging, not hesitation, while crusing means it is running too rich. Lean it out a bit or use smaller idle jet.

 

So then, i should try adjusting the fuel pressure a tad before going further with jets?

I do think wacky is on to something as i may have just said hesitation for lack of the correct term, perhaps it is surging.

The stupid exhaust on my car is so loud its annoying and such a PITA when i am trying to tune this thing, so i think i am going to put further tuning on the back burner till my new RB exhaust gets here...

I know i am close, i just want to get this thing dialed

 

I am bringing this thread back because my new RB muffler will be here tomorrow and i will put that on (my ears are going to thank me) and then i will continue to get this thing dialed.

I took it for a drive a few days ago after i dialed the fuel pressure down a tad, but now it seems that my new fuel pressure gauge from ReSpeed is acting erratically. It was kind of jumping around the other day??? Vibration perhaps? Anyway, i dialed the fuel pressure down a little bit, but then the gauge started acting funny! I get no breaks...

After i dialed the pressure down a bit i cant say that it drove any better, i am using a holley fuel pressure regulator (4.5-9psi) so is 4.5psi as low as it will go? Cause i was getting steady readings of 4psi, but like i said the gauge is acting up now.

Anyway, i will put the muffler on tomorrow, take her for a spin and see if i cant get it right, will keep you all posted.

Again, thanks for all the help.

 

Get the 1-4 psi holley FPR it will work better with the mikuni trust me.

 

Really? That will be my next step if i cant get this just right

 

Yea I bought the holley blue which came with the 4 1/2-9 psi when I swiched to the 1-4 psi its was much better.

 

3.5 PSI seems to be the sweet spot for alot of 12a engines using the Mikuni.

 

+1

 

Yea 3.5 is the sweet spot but during autocross runs I up it to 4psi and switch to a 2nd gen coil noticable difference during runs.

 

what needle valve are you runing ?

 

All the jets i am running are listed in the first post

 

nope its not there. please list all of your mods. what mikuni is yours? I have had a few; 40 - 50 size and a couple of styles. sweet spot fuel psi range for them is between 3.2 to 6 psi. it used to be in the old mikuni tech book. if yours is a 44pph then it should be 4+ lbs and you should tune from there. adjusting the fuel pressure is not the proper way to tune your carb. find the correct presure, set it and leave it alone. your neddle valve should be size 1.x through 2.2 . what is your timming set at? do you have all new gaskets for your carb? is there any rust or dirt in your float bowl? if it is a 44pph it would most likly have a neddle valve of 1.6 or 1.8 . do you have a stock exhuast ? you need to give more info on your setup for better help. did you install the jet in the fuel return line?does it pull hard in 1-2then after the shift to third falls on its face? this would be at or near full throtle. if so then you are most likly in need of a new fuel pump and are experancing starvation. the at cruise problem needs to be better discribed. your problem as stated could be any number of things, it could be one issue or many. better info better help. I may not be able to help you but some one should be able to. there are many here with good knowlage and experance with mikunis the last one I had was a turboed 50pph.

 

Here is the whole setup:

1984 GSL

Stockport 12A
Racing Beat two piece sidedraft manifold
Mikuni 44 (got it from the RX7Doctor, he said it checked out great)
K&N filter setup on the Mikuni
Racing Beat header to a RB exhaust
MSD Direct fire ingnition mod
Blocked OMP (or MOP, lol) running 87 octance
New NGK plugs and wires along with new distributor cap and rotor
Brand new 3 row radiator
FMOC with stainless lines
Carter fuel pump
Holley 4.5-9psi fuel pressure regulator
Dual fuel filters (one pre pump, one post fpr)
Respeed fuel pressure gauge
Fuel line deadheaded at the other end of the T-Fitting on the Mikuni
All new fuel lines
Stock timing

I do not know the needle valve size (how do i check this, yes i have the manual)

I beleive all of the gaskets to be okay (RX7Doctor said it checked out)

I have not looked at the float bowl again because RX7Doctor said he checked it out

I do not know what the jet in the return line is???

Yes, it does fall on its face when i shift gears at WOT

I will try to better describe my steady cruise problem; Accelerating at a moderate level and shifting at 3k rpms all the way into 3rd gear it feels great, just like it should, then as you taper off your throttle to hold a steady 25mph speed in 3rd gear around 3k rpm
it starts to kind of sound like its got to clear its throat, like a girgling coming from the exhaust. If i accelerate a little bit it sounds/drives normal. Again it happens only when i try to hold a steady speed.

Repeated from above it does fall on its face between shifting of gears at or around WOT.

This is the extent of what i know to be in the carb as i put all new ones in:

The venturi's are 39mm
Fuel jets are 200
Air is 240
Pilot 62.5
accel pump #90 aka idle jet

Sorry for the long post, but it was requested!

And to make this thread a bit more fun a pic under the bonnet:

 

The last fuel filter needs to be before the fpr, not after.

 

your responce and post length with pic will help you. now for a couple of questions
1) why did you chose to dead head the fuel, instead of running a return line from the carb?
2)does it fall on its face between all shifts at or near wot? or is it only when you get to higher gears from 1 st or 2nd? example standing start floor it 1st gear good hard pull shift 2nd gear good hard pull shift third good start then it just fall on its face.
3) the cruise problem is it just sound or is it a driveablity problem?
4)if not just sound does it happen at differant speed levels?
5)is it just after accelerating or constant on cruise?
6)what other jets have you tried and what was the change?
7) the rb exhuast is it the dual back or single?
8)how do you have your vac advance set up on your distributor?
nice pic

sugestions: get the heat shield, try the jets listed above. IMHO others may not agree, but I would set up the fuel supply with a return set up instead of the dead head setup. It might help you if the fpr was after the carb. you should have a fpr that puts you in the above mentioned range, I would try 4psi fuel, timing just a little advanced . I sold my mikuni tech manual a couple of years ago so I will try to find a pdf to help

 

That's pretty.

 

Good to know, i will try that (should that make any difference).


1) Everywhere i looked on this forum about the Mikuni setup, they all were deadheaded as mine is.

2) Dont know, will find out tomorrow

3) Doesnt "feel" like its driving too well at that point, but might just be sound influencing me.

4) Yes, it does this at any speed that i try to keep at a steady cruise

5) It happens only while trying to hold a steady speed

6) Havent tried/ordered any other jets yet...

7) RB header (single exit) to a Magnaflow silencer to the RB rear muffler

8) Vac advance is open to the air

Now you are saying 4 psi????


Thank you! I have put quite a few hours into it thus far and those little comments really really make me feel good.

 

I would set up vac lines to the vac advance. It may or may not make a differance but I lean more toward they were set up to have them so it may help. the exhuast set up you have will sound that way. yeah I would try 4psi, what you got to lose? whats the manual you have say for psi range ? try it and let us know.

 

The vacuum advance is fine left undone. I have never run vacuum advance on a Mikuni set up ever.

What trochoid is trying to point out in the FPR is that you are regulating pressure then you have a filter inline afterwards which may drop pressure again. You want it to drop right before it enters the carb.
Fuel pressure was around 4.5 PSI if I remember correctly. But the gauge that was on their was a cheap one so I can't confirm if that was the actual pressure.

If the only issue is surging when trying to stay at a steady cruise speed. Then the issue is jetting. As I have stated in the past. It takes experimentation to dial in an aftermarket carb for your specific driving habits.
If you are having an issue when you go WOT and leave it there and it falls on it's face between shifts. That is a fuel supply issue not a jetting issue. The most common things there are. 1. Kinked supply hose from the pump.
2. Low voltage supply at the pump or poor grounding there.

The carb was in perfect working order. Float level set correctly. Same set up as you have now. Even same type fuel pump.
The engine made excellent power and no driveability issues with the jetting that is in there now. This was at near sea level though.