Mikuni Problems
 

Howdy. I have a stock port 12A with a Mikuni 44PHH, Holley FPR, and stock fuel pump. I have the FPR at around 3-3.5 psi and a new fuel filter installed. The vacuum advance is plugged off for now. I was told the carb was set up for a 12A stockport by the seller. The car was running fine before I uninstalled the Nikki.

I started it up today for the first time with the new carb. I ran it for about 20-30 minutes in total. It idled pretty good, albeit a little high, with the choke on. Once it warmed up however it would still idle but not without the choke on. If you try to unchoke it the rpms just drops and everything dies. I also had to unflood the car a couple times. Also is it normal for the intake manifold to be hot enough that it's uncomfortable to touch? Any help would be greatly appreciated, thanks.


-Ken

Have you adjusted the air/fuel mix for your car? It took a bit of fiddling around with mine before it would idle right (idle at 700-800, choke at 1,900-2,100) even though it came straight off another stockport 12a. If you haven't, start by warming up the car and backing the pilot screws out *almost* all the way, then slowly screw them in until you get to the idle you want. I also had to mess around with the throttle settings a little.

The stock pump usually won't flow enough for the Mikuni. Your fuel pressure needs to be 4-5 psi and the intake is hot because it is running too lean. I've had them nearly ice cold on 85* days at idle.

Alrighty. So back them out=higher rpm? Screw them in=lower?



I heard of people running fine at or near idle with the stock pump but I could be wrong. Would this explain why it's shutting off when it's offchoke because it's too lean? I might have to install the 72gph carter then and see what happens.

Yes. Whether or not this is obvious, be carefull not to screw them too tight. The previous owner of mine screwed one down too tight and when I went to back it out it broke off the tapered part of the screw. It wasn't very fun to get it out.

Mine runs fine at idle and normal driving with the stock pump, but it bogs if you go WOT from idle. It also bogs at WOT in 3rd and 4th. As soon as I have the cash I'm getting a Mallory.

can you tell us how it is jetted right now?

Screw your idle circuit screws all the way in, gently, then back them out 1-1/2 turns, start the engine. (The 2 screws on the top of the carb.)

Turn your fuel pressure up to 4.5 psi.

Set your rpm idle with the screw on the throttle cable bracket. You may need to raise the rpms, while the choke is still on. Once the engine warms up, pull off the choke and reset the rpm idle.

Rev up the engine and watch your fuel pressure. If it won't hold steady at the preset psi, you need a better fuel pump.

Once the engine has warmed up, turn in the idle screws, 1/8 th turn at a time, alternating back and forth. When it starts to stumble, back them out until you have a smooth idle. If you can't maintain your fuel pressure, don't take it out and beat on it, it will lean out on you.

Have you opened up the carb? How long has it sat? It may need gone though. Wolf Creek Racing has rebuild kits, jets, parts, and a tuning manual.

Ok, I installed the carter and tried running it but it flooded out on me. I'm going to get a better fuel pressure gauge and see if I can get things to work. Thanks Trochoid and RustyRotary for your advice, it helped me out a lot. Oh, and for the jetting



http://i3.photobucket.com/albums/y54...e/untitled.bmp


Sorry if it's a bit crude. Hopefully someone can decipher it. :D

Nice drawing. I'll check later tonite and see if I can find my list for the jetting I have in the Mikuni.

I highly recommend the book. It helped me understand how these carbs work and how to tune and trouble shoot them. It is not any where near the depth of the the Holley or Weber books, but it will give you the basics.

I have 12a and 13b set ups I will list the jets I have for each. These are the way I bought them

From left to right, per your diagram.

Main air jet 12a -190, 13b-240 Meters air, higher # is leaner

Main jet 12a-200, 13b-175 Meters gas, higher # is richer.

Pilot jet 12a-52.5, 13b-57.5 Meters idle system Higher # is richer.

There is also a starter jet, both of them are 760's, or 76.0 can't tell for sure.

Discalimer. Both carbs are how I bought them, I haven't tried out the 13b one yet.

The 12a ran pretty decent, stock port with headers, presilencer and a small cat just before the axle with an oem muffler. Both carb and exhaust systen came off of a car in the junkyard. :D Looked like it had been raced. There was a stumble when flat shifting at 8k, other than that it seemed to run well. I could keep up with a GSL-SE that had headers, a glass pack and oem muffler. I also had a 250# passenger. The SE got me at the line due to bigger tires and the passenger, but I could stay even with him from 10-100mph.

The pilot systems regulates idle mix and low throttle cruising, the main system regulates acceleration and driving under load (going up hill).

It looks like your pilot jets may be a bit large, accounting for the flooding. Your best bet is to get a decent fuel pressure gauge, pump and fpr, before any jet changes. Once you get those set up, find a wideband to tune with. That will be the easiest and most accurate. You won't have much success tuning until you have a good understanding of how the carb functions, (get the tuning manual) and real data on how it is performing (wideband).

Happy tuning.

The stock pump isn't what I ran on my car I ran a holley red withmy mikuni. I also had to mes with that carb a lot. Adjusting that carb is pretty simple. I'll see if I can get my instructions put up on this site sometime. I have a lot of info on this carb. But it's in my car. and we all know where my car is......I'll try and get it out if theyan find my keys.