I'm running a set of 44mm Mikuni's on my Rebello L30 in my 240Z track car. The car runs great at WOT and never stumbles regardless of throttle transition. I'm 95% happy with my current setup but the engineer in me wants to get that last 5%. The problem I am having is that the car runs very rich on 3000rpm, 3rd gear corner exits. It does not stumble but it feels like its drowning in fuel. AFR's are as follows...

4th gear cruise = 13.0

idle = 12.0

WOT 4th gear at 6500rpm = 12.8

3rd gear roll on from 3000rpm = 10ish

The carbs are currently jetted as follows...

37mm venturi

160 fuel

200 air

40 pump

50 pilot

When I roll the throttle on in 3rd it stays rich for quite some time before it slowly leans out towards redline. I've tried playing with both the pump rod length and the pump jet to no avail. It seems that if I lean the things out too much with the accelerator system I develop a transition stumble.

Is this something that can be tuned out of these carbs or do I just have to live with dropping to 2nd gear on corner exit?

Steve -

I believe I would call Dave Rebello since his engine is in your car (hopefully you purchased the engine from him directly) and tell him your story. Dave does a lot with racing and Mikuni's, and engine building - this is right up his alley- he may be able to give you set up that has worked for him in the past - he keeps all his set ups and dyno's so he can back track if needed for future set ups..................just a thought. He and I talked about me getting a larger venturi but I do not want to bore mine out / I want to buy an extra set and then have that set bored out. More air = more power!

Do you have many jets to play with? I have a decent collection if you need to try before you buy. It gets expensive after a while of trying and buying.

Let me know

Steve, Did you live in Palmdale at one time? Work at The Skunk Works?

Are you aware of the flaw in the Mikuni design where fuel will slosh into the air jets and flood? It happens in hard right turns.

What have you tried? Looks like you could come down one size on the main and maybe up one size on the main air.

What happens with AFR's if you don't let the the rpms get below 3500 in third before you go WOT. I have noticed that my AFR's go rich when at WOT starting at a lower rpm. Another words you are combining pilot and main for a while and it drives the AFR's rich.

I would also offer up jets. We should get us Mikuni guys together to have an exchange program for all these jets.

What's crazy is that my jetting is close to yours and some jets are bigger, and I am running the L24. Just goes to show that these engines all run under different parameters!

Are you aware of the flaw in the Mikuni design where fuel will slosh into the air jets and flood? It happens in hard right turns.

Good point!

Thanks for the heads up on the flooding on hard right turns thing. I'm on the track this weekend so I will check out the AFR's in the right handers and report back.

If I go to WOT at 5000RPM in 3rd rather than 3500RPM I don't have nearly as bad a richness issue.

I've been working with Todd at Wolfe Creek with the jetting over the past couple of months. We started by switching from 34mm to 37mm chokes. My starting jets were the following...

34mm venturi

150 fuel

200 air

40 pump

55 pilot

As you can see we moved up two steps on the fuels and down two steps on the pilots. I think the pilots helped a little as the AFR's used to drop into the 9's and now they stay mostly in the 10's on acceleration in 3rd. I also have a set of 230 air jets that I tried. The air jets seemed to help with the mid RPM richness but the AFR at redline in 4th moved into the low 13's. This seemed a little lean so I went back to the 200's.

How lean is too lean at redline? Would a combination of the 230 air's and bigger mains be the next place to look?

I would not think mid 13's as being too lean. Everything I read is always pointing you to mid 12's as the most power happy point to be, but mid 13's is still on the rich side of stoich. I would think you would be doing just great if at the high RPM you are consistently in the 13's.

When I dynoed my L24 with the SU's with SM needles -at WOT I was in the 12's. The owner/racer/engine builder/dyno operator thought that my AFR's were too rich at WOT. For NA, his opinion was low 13's were best for HP. He thought I was loosing around 10 HP because of excessive fuel.

It's obvious that your engine likes it on top, I would move the main airs in that direction. I think when you start getting as close as you are on the tuning, only a dyno will get you that 5% you are looking for.

Then you factor in the weather that day-high humidity=less air= richer AFRs, carbs are a compromise.

Steve, check post #25 here: http://www.classiczcars.com/forums/thread18948.html

Thanks for the link Blue! I will talk to Rebello about the safe AFR for my engine at WOT. If he confirms that low 13's are ok then I'll give that a try at the track.

I spoke to Dave before I headed to the track this weekend. He said that an AFR up to 13.5 is acceptable for my engine combination. He also said that with my cam (mild Rebello race cam) he has never been able to tune out the rich condition at mid RPM's that I describe above without going to fuel injection. He told me to simply gear down, let the engine rev and never load the engine below 4800RPM. He said the engine will run at 7200rpm without issue so simply keep the RPMs up.

I followed his suggestion, dropped to 2nd in the slow corners and ran the car to 7000RPM with each shift. This resulted in more torque coming out of the corners and the ability to rotate the car with throttle when needed. I guess I was driving it like a V8 in the past rather than letting it rev like it was built to do. Problem solved.