Mikuni VM34 fuel issues

[MrBean]

Hello everyone, i recently purchased some brand new mikuni vm34 carbs from TC Bros for my 78' XS650. I installed them and tried to kick it a couple of times and it wouldn't start. I took the carbs apart, cleaned all the jets and passage ways with some carb cleaner spray. I also made sure the floats were at 24mm from the gasket seat. Petcock from fuel tank is flowing clean and i replaced my fuel lines and fuel filter and made sure the fuel was flowing smoothly before connecting it to the carbs. I reinstalled the carbs and tried kicking it again and still nothing. I sprayed some carb cleaner inside the carbs and kicked it again and it turned on but not for long. I kept repeating this process and the same thing happened everytime. It only turns on with the carb cleaner but does not stay on. I tried playing with the air and fuel mixture but still same problem. What can be the problem???

Carb specs
Needle Jet 159 P6
Needle 6F9
Pilot Jet 18.5
Main Jet 185

 

[retiredgentleman]

Do the carbs have a choke, and are you using it?

 

[MrBean]

carbs do have a choke and yes i am using them

 

[scabber]

Do you have spark ? Fuel in the carbs? Starting fluid might get it going. Fix the Electric starter save the leg.

 

[figure8]

He has to have spark if starts...was it running well before replacing carbs?

 

[grizld1]

Mikuni doesn't offer 18.5 PJ's, so I assume that's a misprint and you have 17.5's. That's way too lean; VM34's usually dial in with 22.5 or 25. Click the Tech section and follow the menus until you find the link for the VM section of the Carb Guide. Bear in mind that the mix screws are air additive type, so that the mix leans as they are turned out.

 

[MrBean]

Do you have spark ? Fuel in the carbs? Starting fluid might get it going. Fix the Electric starter save the leg.

Installed a pamco ignition and it has been timed. i've drain the carbs a couple of times and there is fuel in carbs.

He has to have spark if starts...was it running well before replacing carbs?

it had PWK 32MM carbs and with those it would start and after a minute it would shut off. So i decided to go with VM34 and now im having this issue.

Mikuni doesn't offer 18.5 PJ's, so I assume that's a misprint and you have 17.5's. That's way too lean; VM34's usually dial in with 22.5 or 25. Click the Tech section and follow the menus until you find the link for the VM section of the Carb Guide. Bear in mind that the mix screws are air additive type, so that the mix leans as they are turned out.

Just checked, yes 17.5. i will order 25. I have the air screws at 3/4 turn out

 

[figure8]

It's weird it would just die after a minute with the previous carbs. So you managed to time the Pamco with the PWK carbs?

 

[hooser]

Look in the carb inlet an make sure the cut out is showing.. I had a guy put the sides in the wrong carbs .. you can do this with the left and right carbs set ups.

 

[MrBean]

It's weird it would just die after a minute with the previous carbs. So you managed to time the Pamco with the PWK carbs?

Yeah Pamco was timed with the PWK carbs. I did my research on the PWK carbs and they dont have that many good reviews. I took them apart and cleaned them as well but still no luck. So now im focused on getting these VM34 carbs up and runnin. 25 pilot jets and 200 main jet should be coming in tuesday so i can give those a try.

Look in the carb inlet an make sure the cut out is showing.. I had a guy put the sides in the wrong carbs .. you can do this with the left and right carbs set ups.

cut out is showing

 

[figure8]

Jumping from 17.5 to 25 might be a bit much. 20 and 22.5 might be a better range to try first. Focus on the pilots before you start swapping in main jets.

 

[grizld1]

+1, figure8. MrBean, you can tune or you can thrash around, that's up to you. Those #200 mains are going to be too fat unless you're running a 750 with serious mods. You'd do well to stock a range of jets to work with (22.5 and 25 PJ's, 180 and 185 MJ's). The Sudco Mikuni Tuning Manual has some fine diagnostic tips that would help you understand what your motor is trying to say about the mixture it's getting. Good luck to you.

 

[MrBean]

Jumping from 17.5 to 25 might be a bit much. 20 and 22.5 might be a better range to try first. Focus on the pilots before you start swapping in main jets.

Okay, i will give those a try first.

 

[MrBean]

Okay so i installed 25 pilot jets today and tried to kick it on and still wont turn on

 

[tonyc]

Bean I put my floats in upside down and had the same problem. You may want to check.

 

[MrBean]

Bean I put my floats in upside down and had the same problem. You may want to check.

Floats were put on correctly.

So after a few more kicks and some carb spray I finally got the bike to start and stay on! Now time to tune it. Thanks everyone for they're help.

 

[MrBean]

So i am still having issues with my carbs (or is can be something else). It takes me anywhere from 10-15 kicks to start it up and at first i thought it was the battery but i connected the charger to it and tried kicking it and still takes me a while. BUT once i've got it on and warmed up it will start on every kick.

2nd issue is I can be riding it around and once i let go of the throttle it will die on me but it will start up right away so maybe my jetting is still not correct? If i leave it idling it will stay on sometimes and other times it will die after a minute or two.

Carbs are synced and air screw is 3/4 turn out.
carb specs right now are...
Needle Jet 159 P6
Needle 6F9 (clip currently 1 notch below top)
Pilot Jet 25
Main Jet 165 (i have tried 185 &175 and still had the same issue)


If its not the carbs then i thought it could be the charging system but previous owner installed a HHB PMA which i took a picture of and although it doesnt say "hughs hand built" on the side, it does look like it. See picture below

I installed the "Ultimate Pamco High Output Electronic Ignition Kit + E-Advancer" recently and it has been timed. One thing i did notice is that the board that is inside the blue box was loose. Everything was connected, just the boards wasn't held down with anything. I don't know if that could be a problem.

Also i am running wise guys drag pipes with no baffles so it has no back pressure but i don't see that would be a problem in why its dying but i can be wrong.

Any help will be greatly appreciated!

 

[MrBean]

Oh and another problem i am having is if i open the throttle to quick it will bogg out a die, so i guess running to rich?

 

[grizld1]

MrBean, you need to stop guessing. What you describe is a lean signal, which is to be expected given that you've dropped the needles to #2 position, which will have a strong effect on the off-idle mixture. Put the needle clips back to #3 (middle slot). The #165 mains are also too lean unless you're operating in very high altitudes. If you bothered to read the VM section of the Carb Guide, you'll recall that I recommend inspecting float level on new carbs. In use, fuel in the bowls buffers movement of the floats, but carbs are of course shipped dry, and floats often get tossed around against the valve needle control arms enough to throw adjustment off.

I'm going to write this one more time and then I'm done. Before you try to tune carburetors you need to do two things. First, you need to read enough to learn some basic fuel system diagnostics. Next, you need to KNOW that everything besides the carburetion is right. Your PMA has timing marks; that's dandy, if and only if the stator marks actually correspond to piston position; that is, if the rotor mark lines up with the TDC stator mark when the piston is actually at TDC. You need a lot more accuracy than you can achieve by poking a stick in a spark plug hole, and you don't know what the PO did. There's been plenty posted on this subject--search and you will find.

So you don't know if a loose circuit board just might cause contact problems? If it hasn't yet, it will.

You're not accomplishing anything with those 1-3/4" straight pipes but making noise and impairing performance (narrowed power band and torque peak occurring way past any rpm your motor can turn). And yes, a system with little restriction is going to require fatter jetting than a system with more restriction. That's why guys who bolt Screaming Eagle pipes on their $30,000 Hardly Ablesons without rejetting get loud popping under engine braking and burn up their valves prematurely.

 

[barncat]

just a few observations late to the game here...

i've had hard to start bikes that ended up being an electrical problem, i.e. weak spark. check for poor grounds, loose wires/connectors/corrosion, failing condenser (swap a known good one), old plug wires with bad connections, loose plugs, failing or fouled plugs.... a potentially tail chasing scenario, especially if you didn't wire your own bike. then there's the whole ignition/timing system.

whenever i reconnect fuel lines to carbs after inspection etc or after draining the bowls, i crack the bowl drains open slightly to insure gas is seeping and therefore filling the bowls. float needles like to hang up occasionally. either they just mechanically stick, or fuel may not overcome atmospheric pressure or vacuum or some such...

+1 on all the expert jetting advice above, and that straight pipes are a power sapping waste of time.

anyway, if the bike was running poorly with your original carbs MrBean, don't discount the electrical system as the culprit, then you can get to the challenge of fine tuning those VM's.