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Mikuni 42 idling through main jet

nascar7613

Active Member
1994 Dyna, Mikuni 42 carb. When running the bike at idle or very low throttle position, the fuel is coming through the main jet in massive amounts. The bike will not idle without cracking the throttle, and with the bike running looking through the carb, fuel is gushing through the needle jet. Carb is clean, new float valve & seat, float adjusted properly. It does not overflow with the motor off. Some reason it is pulling fuel through the main jet when it shouldn't be.
 
I'm by no means an expert on the Mikuni but when i've seen this happen on other carbs the float was either sticking or it wasn't adjusted correctly.
 
There will always be fuel coming through the main jet at any time
the amount of fuel coming through the main jet is controlled by the slide needle which has a taper on it the further out the needle is the thinner the needle is so allowing more fuel to flow
The fuel is drawn from the jet by means of a vacuum created by the flow of air over the top of the jet
There is no device that blocks the main jet to stop fuel from being delivered by it
An incorrectly set float or a leaking float needle valve could cause excess fuel to be delivered through the main jet however there would also be fuel leaking from the float bowl or overflow
the inability to idle may be caused by a leaking inlet gasket allowing air into the system between the carb and the engine causing a weak mixture
A blocked slow valve could also be the cause of this type of problem
A wee bit more investigation may be required to find the problem but a weak mixture is more likely than too much fuel

Brian
 
The fuel flow coming through the main jet by cracking the throttle enough to keep it running is excessive. I have cleaned all the passages in the carb and everything seems to be coming out where it is supposed to. I have worked Mikuni 42 carbs before and have not seen anything like this. I know that some fuel will pass through the main jet at low throttle positions, but this thing is like a fountain. It turns the plugs black. The bike was running fine when it was last ridden, but it sat for a year or so. I would think that at idle or very low throttle position the fuel would be pulled more through the pilot circuit on the engine side of the slide then gushing through the main jet. The carb was cleaned by someone else before it was brought to me, so it is possible that the jet needle was changed, no way for me to know for sure. It does seem possible it has the wrong jet needle.
 
Although the carb has been cleaned, maybe there is an obstruction in the pilot air system and it's causing fuel to be pulled thru the main jet. My Evo with a 42mm Mikuni will idle without a main jet in it. That's about all it will do, but it will idle.
 
I had a kehin carb sitting on the shelf, cleaned it up and put her on there and she fired right up and runs nice. So, we will probably never know whats wrong with the Mikuni.
 
I got thinking about my above post this morning, so I went out and warmed the bike up good. Then I removed the main jet and started it again. It idled just fine, and took throttle blips well. I came in and reread my Mikuni tuning book. About the main and jet needle, it says the needle is straight (no taper) on the bottom, and has no effect below 1/4 throttle. Maybe the needle is shimmed way too high? Just in case you want to persue the Mikuni, I att. the tuning link. Glad to hear you got the bike running well, nothing wrong with a keihn.
http://www.mikuni.com/pdf/hsr_tuningmanual_021003.pdf
 
I understand that you are happy that the bike is now running if it were me I would be pulling the mikuni apart to try and find out what the issue is
But then I personally prefer the mikuni to the stock carb as it works better in my damp environment

Brian
 
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