Anyone using Mikuni carb on sprint track?

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Brettm57

Member
My son and I built a couple of SS Animals to run. I tried a 24mm flat slide and he tried a 24mm round slide Mikuni and neither one will work. Apparently the fuel is sloshing away from the jets in the corners. They will sputter and cough in the turns, but once we're back on the straights it will straighten up and run fine.

I don't understand why the stock Animal carb works fine, but a Mikuni won't? Taking them both apart and comparing them, they aren't that much different. I noticed the pilot and main jets are about the same depth in the fuel bowl in the Animal, but the pilot jet in the Mikuni sits quite a bit higher than the main. Thinking that might be the problem, I drilled the hole the pilot jet goes in (I didn't disturb the jet threads, I stopped short of that) and press fit a short piece of tubing into the hole so the pilot jet would be picking up fuel from the bottom of the bowl like the main jet does. That didn't make any difference.

The only other difference I can see is the way they're vented. The Animal carb fuel bowl over flow and vent have very small holes - just a few thousandths of an inch. The Mikuni over flow and vent are more like 3/32" - they're clearly much bigger. I'm wondering if fuel is dumping out of these openings faster than the fuel pump can keep up, and that's what's causing the fuel starvation in the turns? I think I'm going to try restricting these vents somehow - maybe solder them shut - then drill a tiny hole through the solder the same size as the Animal carb has.

I know most people would just run the Animal carb or convert to a Tillotson, but things like this bug the heck out of me and I have to either fix it or figure out exactly what the problem is!

Any ideas?
 
I'm with you . Hard too envision that much fuel sloshing out the vent , worth trying though .
Or put a vent can on the vent line, to monitor how much if any fuel comes out .
The carbs (mikuni) work on motorcycles at funny lean angles and sharp turns , should not be that differnt .
 
This is a major issue on road courses which is why everyone in the shifter world uses a secondary gravity feed tank and return system. Shark shifter and Swedetech make them. Not cheap though.
 
i have ran 28mm mikuni 80cc shifter. run vent hose both sides of carb up to the throttle cable 6-8 inches, tie wrap to cable housing. still vents but you don't loose any fuel. delleto fuel pump (no return line) solves alot of fuel problems. no wrap around or extra fuel can. this fuel pump is lower pressure. you might have to play with needle and seat orfice sizes.
 
Brettm57 said:
All good ideas. Thanks for your advice!
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a return line alone helped me some. I had a 125 with a mikuni super bowl that helped with sloshing and didn’t want to invest in the trick system so I threw a return line off a Y and all the weird corner big stuff went away. But I think that’s because I had a truck fuel bowl to help alleviate the issue.
 

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I think I'm going to try a bypass myself. I'm thinking if I put two tees in the fuel line - one ahead of the fuel pump and another one after - then connect the two tees with another hose and a shut-off valve, I can cut the fuel flow back going to the carb by adjusting the valve. The excess fuel will return back to the inlet side of the pump. I think if I take the spark plug out then spin the engine over with an electric starter I can get an initial setting that I think the engine will need to run, then adjust from there based on how it performs on the track. If it doesn't work I'm only out a few dollars - I think it's worth a try.
 
Just to update this thread: My son tried my fuel bypass idea on his kart (Animal) just before the end of the season. He's using a 26mm Mikuni round slide. It was a race day, so he didn't have a lot of time to work with jetting, but it appears that the fuel bypass solved the flooding problem going through the corners. He also put a 1.5 needle and seat in it beforehand. Between the two, it was going through the corners just fine. We want to rent the track for a day next spring so we can do some thorough testing uninterrupted, but I believe we're on the right path to making a Mikuni carb work.
 
Brettm57 said:
Just to update this thread: My son tried my fuel bypass idea on his kart (Animal) just before the end of the season. He's using a 26mm Mikuni round slide. It was a race day, so he didn't have a lot of time to work with jetting, but it appears that the fuel bypass solved the flooding problem going through the corners. He also put a 1.5 needle and seat in it beforehand. Between the two, it was going through the corners just fine. We want to rent the track for a day next spring so we can do some thorough testing uninterrupted, but I believe we're on the right path to making a Mikuni carb work.
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Good to hear. A lot of the mikuni gods will say the Y return setup is improper... and they’re probably right but it worked well for me and seems to work for you guys. I don’t quite understand the actual schematics of why it works but somehow it does.
 
A bypass system will lower the fuel pressure going to the carb. Being that Mikunis are primarily designed for motorcycles which are gravity fed fuel, it makes sense that high fuel pressure could be a problem.

Right now we're just using a cheapo plastic lawn mower fuel shut off valve like you can buy at any hardware store to control the amount of fuel going through the bypass. A Mikuni main jet fits nicely inside of 1/4" fuel line, so I think I'm going to experiment with using one of those to control fuel return.

We still have a lot more testing to do to sort this out, but I think we're headed in the right direction.
 
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