Mikuni carb trouble
 

help!!! i have a 82 rx7 i just got for 250. it has a rb intake and exhaust and a mikuni sidedraft carb. it wouldnt start on itws own until i noticed the distributor was off a tooth. anyhow,the carb,it has a flatr spot where it completely bogs out between 2500 and 2500 rpm,aprox. i have no idea what fuel pump is in it,but the guy said it was around 6psi. also,a peculiar thing is the air fuel adjust screws,do almost nothing,i close them,completely and it doesnt die,im confused,any help or suggestions would be greatly appreciated,id hate to put a stock nikki on b/c i cant fix this set up,cause it sounds mean!!! lol. thanks people. dave

Describe the bog between 2500 and 2500. I bet that's hard to notice. Is it like a little poot between 1 and 8 grand ?

oops!!! sorry i meant between 2500 and 3500 aprox

no takers huh?

mines got the same thing
what type of fuel pump does it have,
do your timing to mazda spec, make sure your carb is clean and your accelorator pump (located undernieth the carb is functioning correctly, try some carb cleaner maybe, mine still bogs, ive tried adjusting it with no luck, if anyone has help for both of us thatd be great, but start from there, it will be better for sure, thats where im sitting now

I have been plaqued with this problem for years ( since 1995). I always attributed my problem to the Mikuni 44 not being enough carb for my ported 13B. I intend to go the RB Holley route with or without supercharger ( from Atkins ) when I have some serious play money again.

If you guys are having bog from the Mikuni with 12A or non-ported 13B, then I guess it may be the nature of the beast. I have been around and around on verifying the jet sizing and mine is supposedly as optimum for the carb in that application as it can be. Hmmm...

Another thing, I have noticed that if I refrain from revving immediately before take off, it will do fine ( no bog and good throttle response ). This is telling me that the shot of gas from idle to WOT needs the bowl full or it will bog. I have played with the fuel pressure lower and higher and that's no help, either.

Please buy a good quality fuel pressure regulator such as Holley, and put a fuel pressure gauge on it. The sidedrafts only want 1.5 - 2.5psi, I set both of mine to 2.25.

If fuel pressure is too high, it'll force fuel into the carb throat and run rich, instead of vacuum drawing it in. Many issues can be attributed to bad fuel pressure, I've learned not to assume anything with pressure.

The idle adjustment screw issue could be a worn or missing O-ring on the screw itself. Or a bent or damaged screw tip(the end in the carb).

The bog is either idle fuel jet or accelerator pump jet wrong size.

Although the 44PPH is not enough carb to get the most out of a ported 13b, it will do okay around town. With a b motor you really want a 48.

If the carb hasn't been rebuit in the last 10 years, it's overdue.

well now that ive taken the carb off and appart i think ive got it. a few things,first and most significantly,the air/fuel mix screw tip is severed off! only one screw. also lots of dirt in the bowls,and...all the gaskets are shot,and there was no gaskets used at all on the intake manifold,either end,so prob good sized leaks there,oh and! the floats seem to be a little out of wack,i have no idea where they are to be set. so once i get these gaskets from whereever,and the floats set,and the screw replaced,and cleaned ill post back and let you know!!! ty for all your help!

you know i have a holley red and a regulator on my mikuni setup and I don't have any of these probs. I get horrible gas mileage but I have mine set at a constant 4 psi. Well thats what the gauge reads. To set the carb take the idle-adjust screw and take it out untill it just rests on the end. you know what I mean. Then take the two mixture adjust screws located on top and screw them all the way in. then back out 2 full turns. Then adjust the idle-set screw so the RPMs are at 1K. Then turn the mixture screws in and out very slowly setting it for the highest rpms it will sit at. Reset the idle-set screw at 1K rpms and repeat then your done.

Now I believe thats how it's done. I worked on my car doing that so much I memorized it. I lost the paper to it as well so good thing right. Try it, don't blame me if anything fails I'm just trying to help get others advice before doing so if they say go for it then try.

Also ask Carl he helped me with that carb a lot too.

The Mikuni manual I have says to start your tuning at 4.5 psi and the idle scews out 1.5 turns. I would start with the idle at 750-800 rpm.

When you turn the idle screws down to seat them, before backing them out, be very gentle. There is more than one person on the forum that has had broken idle screws from a PO screwing them down too tight.

^I seem to recall that set of instructions when I had a 44 PHH on my 13B Sp years ago.

Anyway- sometime ago I debated if I was going to put a 44 on my 12A GSL, but parts are getting tougher to find.. I did however run across this place as a possible good source for parts. They seem to have a nice stock of 44 rebuild kits, jets, rings etc for the 44.

http://www.wolfcreekracing.com/classified.htm

You may want to jingle them and ask their advice as well. Seems they deal with Z cars mostly, but who knows?

I bought 2 rebuild kits from them and a photocopy of the manual was thrown in the deal.

Tim at Wolf Creek is awesome. He has pretty much every part for Mikunis and will help you find what you need. Great service and fast shipping. They also have rebuild kits and manuals on Ebay frequently. I get an audible bog any time I'm holding speed over 3k rpm. It doesn't shake the car or slow down alot, but I do have to push the gas pedal in a little bit more to hold speed. If I could afford it I'd switch to FI. I think I'd do much better that way. I understand computers better than carburetors.

awesome,again thank you for all of your help,im going to order the parts i need tomorrow from wolf creek racing and hopefully get this girl on the road. also,what is a holly red??? and what part number holly reg do you use for a rx7???

oh,anyone know where i can get intake gaskets??? also my mikuni has these plastic spacers,do i need them,and if so,where can i get gaskets for them,didnt see them at wolf creek???

You can make your own gaskets out of sheet gasket material. Use the phenolic spacers, they cut down on heat transfer. As for the o-rings, if Wolf Creek doesn't have them, try the auto parts stores. They usually have an o-ring selection.