Mikuni help please

[Noseoil]

The motor is new (60363, the hemi configuration), with the Bullfrog domed high compression piston (11:1), Isky Black Mamba cam, 26# springs, mildly ported intake and exhaust runners, billet connecting rod/flywheel and a free flowing exhaust. It should easily turn somewhere between 7000 and 9000 rpm, but now won't really pull beyond about 4000, that's when it cuts out at 1/4 throttle.

I think I have a problem with the VM22 & I'm hoping you can help me with it.

It runs well at idle and up to 1/4 throttle. As soon as I try to go beyond 1/4 throttle, it shuts down as if it's running out of fuel. When I let it go back to idle, it runs again. The fuel level in the carburetor seems to be correct (it's about 9mm below the mating surface where the body meets the bowl).

Here is what I've tried so far:

1 - Changed pilot jet and it's at 12.5 now (but have tried a 15 & 17.5 as well), it runs well, idles well and pulls well to 1/4 throttle, then it shuts off at anything past a 1/4 opening.

2 - Tried both a 125 and 130 main jet, with different jet needle settings, the same problem happens at 1/4 throttle no matter which jet I use.

3 - Removed the main jet completely based on an article I read, put the jet needle in the center position, installed the carburetor again and ran it. Same thing, it starts, idles and pulls well to 1/4 throttle, then it acts as if it is not getting fuel. Let up on the throttle & it runs like nothing is wrong. It should run up to about 3/4 throttle this way, then start to bog from too much fuel as the needle opens too much.

It's frustrating to have to work like this and ask such a simple question, but any help you could provide would be greatly appreciated. tim

 

[Kentucky Boy]

11:1 is pretty high compression. Are you running gravity feed or pulse pump setup. Sounds like a fuel starvation problem.

 

[Noseoil]

I agree with starvation, it's a gravity feed from the factory tank, but I think there should be enough fuel flowing to at least start to go past 1/4 throttle before it runs out of fuel in the bowl. It happens immediately when the engine is running, no matter for how long. The fuel screw to drain the bowl shows fuel in the carb, if I hold the overflow line up beside the carb to check the level, it's at about 9mm below the bowl-body gasket surface.

I may try a higher fuel tank location, just to rule it out, but I haven't seen anyone with this problem before & I'm still stumped. I guess I can add a pulse pump as well, but am trying to avoid it if possible.

I just can't find anyone who has had this problem before so I'm sort of grabbing at straws now. Wondering if anyone with the Mikuni VM22 has to use a pump or higher tank location on their installation?

 

[KingCobra]

Check the fuel hose because some reports of kinked/pinched while assembled at the factory. I've heard the same story out there before about limited low RPM's and that was the other person's finding.

 

[Noseoil]

Hose is new, new filter, shut-off valve & clamps. It's running clear through the filter still.

 

[KartFab]

When you open the throttle does the slide actually open all the way?

 

[Noseoil]

Yep, everything works like it should, just won't pull past 1/4 throttle. Giving up on this carb & ordered a new one this evening.

 

[2SlickNick]

I would go back to the stock pilot jet. I've had three of these carburetors and I've never had to change the pilot jet. I would however try maybe a 105 Main jet.

Also check your float height. I used to have a nice article on how to set float height, when I come across it I'll send you the link.

---------- Post added at 02:44 PM ---------- Previous post was at 02:44 PM ----------

Too bad you don't live closer I have all the Jets from 95 to 160 for that carburetor LOL

 

[Joe-405]

recheck your intake manifold sounds like its sucking air somewhere to me. or put a air filter on it.

 

[Noseoil]

Checked float height & it was off about 1mm, but that's all so no real fix there.

I did buy a new manifold from gopowersports & it's much better that the one I had. There was a leak with the old one, so that's ruled out as well. The new one they make is real nice. Take a look at it here.
http://www.gopowersports.com/manifold-for-mikuni-vm-22-with-gasket/

They were very good about taking the time to walk me through a few different fixes, but nothing worked, so a new carb is in the mail now. Will post back once the new carb is installed & it's running (again). These are great people do do business with if you need parts. Fast shipping & quality parts.

 

[KingCobra]

Maybe just a few small things contributed to it's lack of wanting to run. Too bad you run 91 octane instead of 87 octane. Heck, you might as well run Nitrous....Here ya go......

Anyway, glad to hear you are finding possible trouble spots.

Where is Go Power Sport's Manufacturing plant or do they just pay someone to put their name or initials on a product like box stores like Walmart/Publix/Target, etc. does? Maybe their items are made in China? Maybe they just pay for monogramming? Can you help me out here? It seems to be a mystery. I'm sure NOT all items are high cost....unless you are paying for a 'name' or initials'...

 

[bob58o]

Can you help me out here?

I just shop around. Some items are the cheapest from there. Can't find a cheaper clone manifold for a 22mm carb. Some other items too. I think they offer just about everyone 10% off.

Since the manifold says "GPS" upside down... I'm guessing it was outsourced and they ordered a bunch.

 

[Joe-405]

You never said what spark plug you have ? I had the same issue with the auto lite 390-x plug and switched to a ngk and it went away.

Can’t remember the actual plug number but I’m close ! Tried them one time and never could get them to work at all.

 

[bob58o]

Autolite 3910x is the one that is commonly recommended for increase on power. Lots seem to have issues with that plug.

 

[KingCobra]

Give this one a try?? Torch LG F6TC

 

[KartFab]

Gotta keep idle at 2k plus with the autolites fyi

 

[Noseoil]

Back from a weekend trip out of town. Swapped out the Autolite X plug for my normal NGK to do testing. New carb is waiting to go on now, so I'll start testing tomorrow when I have a chance. Will post to show what I'm doing.

 

[Noseoil]

VM22-133 is a better choice, it's running now. Need to tune, but it's working well enough finally.

 

[bob58o]

VM22-133 is a better choice, it's running now. Need to tune, but it's working well enough finally.

Better than what?

 

[Noseoil]

The original was a VM22-3487, the replacement is a VM22-133. I drilled the main jet to about a 170 size (#46 drill bit, .080") & it's running pretty well, but I'm still waiting to get real jets delivered to do the fine tuning at higher speeds.

It's a completely different carb than the first one, about twice as heavy, bigger & easier to adjust the main jet, without having to remove the bowl for access.