Smoothbore leaking issue

[dardoonk]

Allright you Mikuni VM29 smoothbore gurus.... Here's two I can't figure out.

Question #1 above the pic & question #2 is below the pic

I have rebuilt these carbs & finally have them very, very close to being jetted correct. None of that is really the problem this time. They slowly leak fuel & I can't figure out why they just started now.

I just swapped pilot jets out and made sure of my float heights. I have all four floats set at 25mm - -exactly-- not +/- anything.. they are spot on and no different than they were a week ago when none of them leaked. Problem is - now fuel leaks out of all four - It isn't much, only a few drops per day from each

Some of it looks to be coming out of the small hole at the bottom of the intake side. and the rest is coming from within the venturi itself.

There are no leaks around fittings or bowls etc...

Another interesting thing I noted.....(Bear in mind, I have no vac line to the petcock, so it is closed when "on")
With the petcock in the "on" position, if I start the bike for about 1 minute to run down the fuel in the bowls, then open the four drain plugs on the bottom a small amount of fuel runs out from the bowls, but none comes back to replace it unless I open the petcock... Granted this sounds really obvious, but I have a clear fuel filter located just above the "T" fitting between carbs #2 & #3. It holds about 2 ounces of fuel and it remains full. ..

So, I have four empty bowls, a closed petcock, and fuel in the fuel line collected right above the T fitting.... If I crank the motor, it will not pull the fuel down. I have to put the petcock on prime, then the gas flows.

Question is: Is something wrong, or does the line hold so much back pressure that the fuel won't move without pressure behind it? (sorta like putting your finger on the end of a straw, I guess)

 

[tone]

Hmm this is a bit strange to me :-? i cannot see the reason for swapping the pilot jets unless you are running a massive big bore & big horespower ive never needed to do that yet

this leads me on to my first real thoughts on your problem with the tap in the on position & no vac it will be effectivly off so no fuel should leak but when you put it to prime you get leakage right ? so if thats right it can only be damaged needle jets or float heights not properly stopping the fuel & when running at tickover those damn pilots are too big & its blowing fuel back throught the venturi

with the tap in the on (off) position & the bowls full i wouldnt expect the fuel in the line to go down very quickly when cranking it over untill it fired up (think how long a small pot of fuel lasts when carb balancing)

hope it helps.... tone

 

[doctorgonzo]

On the pilot jet question, I agree. I have a stage 3 kit in my CV carbs and even richening it up for pods and pipe it didn't change the pilot jet.

 

[TheCafeKid]

Are you running a Pingel or the like type petcock? Sounds like your float needles are getting worn and not holding the fuel back. Might check that the floats arent taking on fuel as well.

 

[SqDancerLynn1]

You won't get all of the fuel out of the line. Yes holding your thumb over the end of a straw, same principal. Have you replaced the float needles ??

 

[dardoonk]

You won't get all of the fuel out of the line. Yes holding your thumb over the end of a straw, same principal. Have you replaced the float needles ??

All the parts in the carbs are new. . Pilots, Mains, float needle valve & needle are all Mikuni parts. New floats are from Z-1. All gaskets & O-rings are new as well.

Hmm this is a bit strange to me :-? i cannot see the reason for swapping the pilot jets unless you are running a massive big bore & big horespower ive never needed to do that yet
.... tone

Sudco recommendation for VM29 carbs on this bike are 25 pilots & 115 main jets. That was way too rich and I also learned that 15 pilots were too lean, so I changed them to 17.5. The CB750 & KZ900 both show 17.5 or 20 pilots when using VM29 carbs.

My floats are set at 25mm. Z1 Enterprises reccomended 23mm on this carb, so so my fuel level is lower than reccomended settings now. I suppose I'll have to lower the fuel height more and see how it affects the jetting.

The weird thing about the leaking is that NONE of them leaked fuel a week ago, and swapping out jets only should not make a difference at all. Maybe I could have bumped the float on one carb, but all four???


Cafe, I am using the stock fuel tap. I am not worried that fuel doesn't run out of the fuel line after the petcock is closed, I am just surprised to see that an old 30 year old petcock manages to keep that kind of vaccum on the fuel line... (Update) the bike has sat for four days and not a drop of fuel has run into the carbs. This is good, as I know the petcock will stop the fuel flow.