Mikuni's going rich after high lateral G's

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duke
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Mikuni's going rich after high lateral G's

Postby duke » 30 Jan 2015 13:39

I've had this problem for a while, and have finally decided to see if there is anything I can do about it.

After an extended period of high lateral G's (auto-x run, long sweeper, slalom, etc) my car will run overly rich (AFR's aroun 10:1) unless it is "cleared out" by revving it up a few times. If I just let it return to idle (like you would when waiting for a time slip after a auto-x run) the car will die, then be very hard to start.

I'm thinking that it might be fuel sloshing around in the float bowls causing this, but I'm not sure. Has anybody else experienced this? Any help or idea's would be appreciated.

FYI, I'm running dual 44mm Mikuni's on a KA24de. The carbs were rebuild and set up by Todd at Wolf Creek Racing.
Duke Schimmer

'72 2-Door 510
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defdes
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Re: Mikuni's going rich after high lateral G's

Postby defdes » 30 Jan 2015 15:38

I was about to mention the mod that Todd does, sounds like it's working too well. I believe he blocks the vents to eliminate starvation in the turns. Maybe you should unplug them?

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Re: Mikuni's going rich after high lateral G's

Postby bertvorgon » 30 Jan 2015 17:03

Not sure of side drafts but, this is my experience with the same problem, my 32/36 Weber.

Are you meaning it stumbles and dies when you come to a stop? If so, I had the same problem, which I attributed to the float level being a hair too high, as the real world sloshing of the fuel, coupled with the needle and seat maybe opening and closing a bit more than it really should, thus putting more fuel into the bowl, thus spilling out the venturi tubes, making it too rich when it comes down to idle. It also my be a fuel pressure issue. After severe G-force, which we develop at the slalom specially, on and off the throttle so much, I would come into the stop box and my car would stumble, fart, and even die...unless I held the throttle open a hair. This of course gives it the air it needed to clear the rich fuel mixture. I would not "blip" the throttle, as the accelerator pump fires more fuel in there, the LAST thing it needs.

Slaloms really tax a carburetion system, as the transitional side loads are HUGE, even more so the stickier the tires.

Something to think about.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

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Re: Mikuni's going rich after high lateral G's

Postby 510rob » 30 Jan 2015 17:53

...so mucEFIh to thiEFInk abEFIout :P

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Re: Mikuni's going rich after high lateral G's

Postby bertvorgon » 30 Jan 2015 18:35

No.....I have yet to see any of our groups EFI cars run clean through the RPM range.....I adjusted my float...no issue....

Andy's SDS cars are the only ones that run clean and crisp that I have seen......
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

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1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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Re: Mikuni's going rich after high lateral G's

Postby 510rob » 30 Jan 2015 21:45

umm, stock ECCS?

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duke
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Re: Mikuni's going rich after high lateral G's

Postby duke » 31 Jan 2015 11:08

bertvorgon wrote:Not sure of side drafts but, this is my experience with the same problem, my 32/36 Weber.

Are you meaning it stumbles and dies when you come to a stop? If so, I had the same problem, which I attributed to the float level being a hair too high, as the real world sloshing of the fuel, coupled with the needle and seat maybe opening and closing a bit more than it really should, thus putting more fuel into the bowl, thus spilling out the venturi tubes, making it too rich when it comes down to idle. It also my be a fuel pressure issue. After severe G-force, which we develop at the slalom specially, on and off the throttle so much, I would come into the stop box and my car would stumble, fart, and even die...unless I held the throttle open a hair. This of course gives it the air it needed to clear the rich fuel mixture. I would not "blip" the throttle, as the accelerator pump fires more fuel in there, the LAST thing it needs.

Slaloms really tax a carburetion system, as the transitional side loads are HUGE, even more so the stickier the tires.

Something to think about.


Thanks for the reply. Your situation sounds pretty much identical to what I am experiencing. Stumble, fart and die, unless I keep the throttle cracked. Even then it will pop and spit a few times before it clears out. As I said I have confirmed AFR's in the 10:1 region when this happens. Your description of what is going on in the float bowls is exactly what I was picturing.

That being said can anybody tell me how sensitive Mikuni's are to float level changes? In the past I have just set them to factory specs and left them there. I would think that moving the fuel level down on the venturi tubes could cause the car to lean out. I wouldn't really call myself a carb expert though.

As for EFI...maybe in some distant future, but honestly my current setup runs really well 99% of the time that I'm not really seeing the need to make any changes.
Duke Schimmer



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Re: Mikuni's going rich after high lateral G's

Postby Three B's Racing » 13 Feb 2015 05:35

With the lateral G's created by your most excellent handling 510 is it possible with the fuel sloshing to one side of the float bowl the float itself drops thus letting in more fuel over filling the bowl?
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Re: Mikuni's going rich after high lateral G's

Postby Boy Blunder » 16 Aug 2015 07:34

I too have experienced the same issues in my 510 race car. It has left me stumped for quite a while. I will admit I am slightly intimidated by the mikuni's.

Has anyone figured out a solution?

Here is a video if it happening to me at a race this summer. The car had to be push started to get it going. It stuttered several times trying to do so. I will admit my driving sucked but this was the first time I've driven it in two years (maybe the third time total?) after completely overhauling the brakes to Wilwoods. Needless to say I was being too gingerly.

http://youtu.be/pAXLEJsO_6Y

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Re: Mikuni's going rich after high lateral G's

Postby Byron510 » 16 Aug 2015 08:06

Keith - I'm sure Dave must have doen somthing on his hill climb car to address this issue - do you know what it was?

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Re: Mikuni's going rich after high lateral G's

Postby Chickenman » 17 Aug 2015 09:52

It's definitely fuel slosh. I ran twin 40mm Mikuni's and even twin 44 Mikuni's on my Datsun 1200 race cars back in the 1970's. Autocrossed as well. The 44PHH Mikuni's had less of a problem than the 40's. What carbs do you have?

Lower your fuel level 1/16" of an inch and give that a try. Watch your AFR reading as as it will lean it out a bit. If it still gives a problem lower it 1/8"... but that is the maximum. At 1/8" you will have to make fueling corrections. Same trick with Weber's. I ran those as well.

Be aware that in Autocross you may not be able to get rid of this problem entirely. Autocross, because of the violent maneuvers, creates a lot of fuel slosh. You may always have a bit of rough running in the stop box, but it shouldn't die and it should run clean in all of the corners, particularly on exit.

One more thing to check. Some of the Mikuni's have a complex shaped float baffle installed. Make sure that your Carb tuner didn't remove this baffle. Some tuners remove it in an attempt to reduce fuel starvation. That's not a good idea in Autocross ( Nor for Road Race cars IMHO ). Edit: Plugging vents is not a good idea either IMHO ( unless different vents are added ) . The float bowls must be vented to Atmo somehow.

Holley carbs also require specific tuning for Autocross. I run Autocross Holley's with the front float bowl 1/16" below the sight glass threads on the front fuel bowl and 1/8" below the sight glass threads on the rear bowls. Then I correct the fuel mixtures accordingly. This is well below the normal Holley " street " or Factory recommendation ( OEM is level with sight glass threads ), but these types of modifications are needed when Autocrossing.

I think Keith can attest to how well my Holley tuned Autocross/Hillclimb Camaro's ran..... Not sure if he ever saw me run my 1200's??

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Re: Mikuni's going rich after high lateral G's

Postby bertvorgon » 17 Aug 2015 10:24

I agree with Richard, today's tires create way more g-force than the day when all these carbs were designed.

I too fought that with my Weber 32/36 and ended up running with a slightly lower float level, which helped in all but the worst violent maneuvers. The stop box issue becomes one of making sure you don't keep "blipping" the engine, as the last thing it needs is the accelerator pumps blasting more fuel in till it cleans itself out.

Richard's Camaro was a treat to watch for sure!
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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Re: Mikuni's going rich after high lateral G's

Postby duke » 18 Aug 2015 20:23

I'm glad to see that this topic is getting some traffic. I've never really tried to solve this problem since my original post, but I'm planning to doing a few auto-x events in the next couple of months, so hopefully I can try playing around with the float levels before then. I will report back with any finding that I have. I was really hoping to pick Todd Walrich's brain about this, but with his passing we lost a wealth of information concerning these carbs.
Duke Schimmer



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Re: Mikuni's going rich after high lateral G's

Postby Three B's Racing » 19 Aug 2015 06:44

duke wrote:I'm glad to see that this topic is getting some traffic. I've never really tried to solve this problem since my original post, but I'm planning to doing a few auto-x events in the next couple of months, so hopefully I can try playing around with the float levels before then. I will report back with any finding that I have. I was really hoping to pick Todd Walrich's brain about this, but with his passing we lost a wealth of information concerning these carbs.



Dave Patten would be my next choice with carb questions.
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Re: Mikuni's going rich after high lateral G's

Postby duke » 18 Sep 2015 08:19

I have an autocross coming up on Sunday. Figured I should start tackling this problem. Last night I lowered the floats 1/16" as per Chickenmans recommendations. Just cruising around town AFR's seem to have leaned out .25 to .5 across the board (as per my WB02 meter) as a result of the change in fuel level. No need to re jet yet, but if I need to lower the floats any more, I might have to bump up a jet size.
Duke Schimmer



'72 2-Door 510

"Simplify and add lightness."


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