Mikuni's going rich after high lateral G's

Engine, Transmission and related drivetrain.
Chickenman
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Re: Mikuni's going rich after high lateral G's

Post by Chickenman » 18 Sep 2015 08:44

Do a couple of high rpm runs if you can before the Autocross. Watch the high rpm AFR. That will be affected more than the cruise rpm when you lower the fuel level. I would suggest that if you can't make a practice high RPM run before, then immediately jet the Air correctors down one size just to be safe.

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

Post by Chickenman » 18 Sep 2015 09:03

bertvorgon wrote: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......
Sounds like the group needs some help with EFI tuning??? One thing to note. On an EFI conversion car, if you run Autocross with a low fuel level ( for weight issues or balance ), run an external fuel sump supplied with a separate high volume, low pressure pump. -8 AN minimum inlet hose . Then run your high pressure pump off of that.

Carburated fuel tanks do not have the necessary baffling inside to prevent EFI fuel starvation under high lateral G's

The float bowls in a Carb act as a built in surge/sump taank and will handle a short loss of fuel pressure. An EFI system will not. Gas tanks on Carbed cars do not have the extra baffling that EFI tanks do to control fuel starvation.

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

Post by Chickenman » 18 Sep 2015 09:11

^ Integrated Engineering makes some nifty fuel sumps for use with integral EFI fuel pumps:

Single pump:
http://www.performancebyie.com/ie-subme ... surge-tank

Image

Dual pump:
http://www.performancebyie.com/ie-subme ... surge-tank

Image

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

Post by duke » 18 Sep 2015 11:08

Chickenman wrote:Do a couple of high rpm runs if you can before the Autocross. Watch the high rpm AFR. That will be affected more than the cruise rpm when you lower the fuel level. I would suggest that if you can't make a practice high RPM run before, then immediately jet the Air correctors down one size just to be safe.
Done. It's safely in the low 13's up until the shift point.

For reference, the carbs are 44's.
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Boy Blunder
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Re: Mikuni's going rich after high lateral G's

Post by Boy Blunder » 18 Sep 2015 17:31

What are you using to read your AFR on your cars setup? I have thought about installing one but haven't yet. I'd be Interested to see what works for us carb guys.

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

Post by PoorMtnKid » 19 Sep 2015 07:44

[quote="duke"] (as per my WB02 meter)/quote]
BoyBlunder- He is using a Wide Band O2 sensor. I actually have an AEM Uego Widband that works for reading through the gauge however it wont be able to be hooked up to a laptop for tuning purposes if your looking for a cheap one. Its just sitting on a shelf right now in my garage.
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duke
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Re: Mikuni's going rich after high lateral G's

Post by duke » 19 Sep 2015 09:39

Boy Blunder wrote:What are you using to read your AFR on your cars setup? I have thought about installing one but haven't yet. I'd be Interested to see what works for us carb guys.
I run an Innovate wideband. This one in particular: http://www.innovatemotorsports.com/products/MTXL.php

It has been invaluable in setting up and tuning my carbs.
Duke Schimmer

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

Post by bertvorgon » 19 Sep 2015 10:27

I too use the Innovate, has worked flawlessly other than a low voltage issue on my car due to my large alternator pulley. I just hooked a relay in so it would always see battery voltage.

It has been invaluable with my carb/turbo setup. If I had had this 20+ years ago it would have made my life a whole lot easier!!!!!
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Byron510
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Re: Mikuni's going rich after high lateral G's

Post by Byron510 » 19 Sep 2015 17:16

If I had a WB02 sensor 20 years ago, I would have never built the EFI system :-/ and likely could have made those side drafts work. Now that emissions testing is gone here in BC, I'm itching to get those old side drafts back on to something.

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

Post by Chickenman » 24 Sep 2015 10:44

Byron ( and any other local members ) if you ever need any help tuning the side drafts give me a shout via PM. I'm pretty danged good at tuning all carbs including Weber, Mikuni's, Dellorto's, SU's, Holleys and Quadrajets. Have all of my own testing and specialty equipment and some Jet collections. Now that I'm retired I have lots of time on my hands and like helping members out.

I used to tune a lot of local Vintage and Hot Rod cars a few years back,and may get into it again.

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

Post by duke » 24 Sep 2015 12:13

Update from this last weekend. The problem still persists, but it is not as bad as before. In street tires the rich condition is totally gone, but with race tires and the violence of autocross, it is still there. Pulling into the finish box, AFR is still around 10-1 and if I don't keep the RPM's up to clear it out, it will die.

I did however notice a difference in the lower RPM range though. Before lowering the floats, my car would often stumble coming out of really tight corners. That stumble is now gone.
Duke Schimmer

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

Post by Chickenman » 24 Sep 2015 12:31

That may be as good as you can get. As long as she pulls clean out of the corners I'd leave it alone. You might try lowering the fuel level a tad more, but you'll always have " some " issues in the stop box.

I've never seen any carburated Autocross car run absolutely clean after a hard stop in the Stop Box. You can reduce the problem, but on Race tires you can seldom get rid of it entirely.

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

Post by bertvorgon » 24 Sep 2015 12:51

WHAT HE SAID ^^^^^^^^^

I fought that, and just finally gave up as long as it was clean through and out of a corner, I was happy. I even changed my driving style to brake a bit earlier, so I could be on the gas earlier, kinda a "Kentucky windage" as to getting the engine to clean up before I really need to open up the throttle, even slightly accelerating BEFORE the corner entry point, thus the earlier braking point, worked quite well actually.
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1968 Vintage 3HP Mini Bike
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Tedman
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Re: Mikuni's going rich after high lateral G's

Post by Tedman » 04 Apr 2018 21:04

Dredging up a dormant thread here, but maybe this solution can help someone with a similar problem who happens upon this thread while searching or browsing. I too had this problem when autocrossing after I switched from SUs to Mikuni 44s. I consulted one of our local national level 510'ers and he told me to plug the float bowl vents to prevent fuel slosh from messing up the metering. I epoxied the forward vent holes in each carb with JB Weld, and left the rear ones open. Not sure why I only plugged the forward vents, but it totally fixed the problem. Here I have removed the jet cover and you can see where I applied the epoxy.
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Byron510
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Re: Mikuni's going rich after high lateral G's

Post by Byron510 » 04 Apr 2018 22:04

Tedman,

That’s really good to note, and I’ll file that in the grey matter.

Thanks for the tip.

Byron
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