MIKUNI VM34 HELP!!!!!!!

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boxbodybangin

XS650 Enthusiast
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Hey everybody,

It’s me once again. So my most recent dilemma right now after having my bike running absolutely beautifully for about 2 weeks is that my carbs are crapping out on me again. I have the dual Mikuni VM34s with aluminum velocity stacks (I also am running 1.5” OD straight pipes) Not sure what happened, but I believe it could be because I left my gas on for a few days without riding the bike or draining the float bowls. That wouldn’t make a whole ton of sense, but it’s the only thing I could think of.

Went to ride my bike a few days ago and right as I was getting ready to leave I blew a fuse. When I replaced it I thought everything was good to go, but then the bike wouldn’t start. I cleaned the carbs thoroughly but to no avail. It wasn’t until about a few hours later that I decided to try again and open the throttle while using the kick start and finally it decided to come alive. As soon as the throttle is closed though, it dies. Both with the choke on and off.

With that in mind and after thorough research I decided to buy some jet kits. I used the same size pilot and main jet, needle in the 2nd to bottom position (closest to the tip) as before which is: #170 Main, #25 Pilot. Still having the exact same issue. Brand new jets and thorough cleaning, not sure where to go from here. Bigger Pilot and/or Main Jet? Smaller Pilot and/or Main Jet? Change clip position? Take pictures of me sitting on it to post to instagram but never actually ride it? Help!!!!
 
Another interesting thing I have noticed is that usually the second kick sort of loses compression? It’ll kick fine, kick loose and then kick fine again. That’s the best way I can put it. I noticed a weird tinkering sound when trying to put the RH Carb back in the rubber intake manifolds so maybe there’s a crack that’s causing a vacuum problem? Ordered some aluminum intake manifolds from TC bros to see if that’ll change anything but I am not very hopeful that will fix it.
 
The thing you're describing with the kick start sounds like it might be normal. It's about what position the engine is in.

I always kick start my XS and usually, I push half-way, till it stops up against compression, then bring the lever to the top and give a good shove to start the bike. If I make a kick without that prep, then sometimes it hurts coz I hit comp and the lever stop dead, other times it's like there's not much going on.

Sounds like you might have rich mix bur hard to diagnose remotely.
 
If you can get the bike running, and idling with no throttle twisting, get some water in a squirt bottle and spray around the carb-to-head junction (both where the carb sits in the boot and where the boot sits on the adapter (what most people refer to as the manifold), and where the adapter meets the head). If the bike changes the idle at all then you have a vac leak, possibly a torn gasket or a bad rubber boot.

Some people will use starting fluid or something flammable in the squirt bottle but I use water so that if I do indeed have a vacuum leak then the motor doesn't rev up uncontrollably.

Also, post some pics of your carb setup.
 
your carb is not likely to go bad because the bile sits for a few weeks. If you run point i would start there and make sure the timing is right and the gab is correct. I had same problem with one of my XS, turned out a couple of small adjustment to the points and a re-adjustment of throttle cable and carb sync.
You could have drowned it when trying to start it, turn off the petcock and roll the throttle to full and give it some kicks, that should do the trick if ignition timing is ok.
 
If the bike was running beautifully as you described before it crapped out, then I would not start changing jets. Your problem likely lies in the battery/charging system and or the intake leak. I would also be looking into why your left hand plug is covered in oil. Maybe a compression test and/or leak down test might be in order.
 
Signal said:
Perhaps it’s not your carbs. What sort of ignition are you using, if it needs a battery to operate is the battery charged.
Click to expand...
I believe it’s a Pamco PMA. However, i still have a stator and need a battery to get the bike to accelerate but all electronic controls were deleted. kickstart only. i’m attaching a picture here before I timed it. in the picture you’ll see the timing is fully retarded but that has since been adjusted. I have a fresh 12ah battery in there. Head and tail lights fire up no problem when i flip the ignition switch to on.
IMG_6233.jpeg
IMG_6234.jpeg
 
willis said:
If the bike was running beautifully as you described before it crapped out, then I would not start changing jets. Your problem likely lies in the battery/charging system and or the intake leak. I would also be looking into why your left hand plug is covered in oil. Maybe a compression test and/or leak down test might be in order.
Click to expand...
Compression seems to be good though I need to actually measure the PSI in order to verify fully. Kickstart has great compression. I believe the LH spark plug was covered in oil due to a leak in one of the valve covers. it tends to lightly spray out while the engine is running.
 
Niels B said:
your carb is not likely to go bad because the bile sits for a few weeks. If you run point i would start there and make sure the timing is right and the gab is correct. I had same problem with one of my XS, turned out a couple of small adjustment to the points and a re-adjustment of throttle cable and carb sync.
You could have drowned it when trying to start it, turn off the petcock and roll the throttle to full and give it some kicks, that should do the trick if ignition timing is ok.
Click to expand...
Did you have to advance or retard your timing? PO had the timing set fully retarded, the points were maxed out. not sure if this was by mistake or not, but when it was fully retarded the bike wouldn’t even start at all.
 
grouchie83 said:
If you can get the bike running, and idling with no throttle twisting, get some water in a squirt bottle and spray around the carb-to-head junction (both where the carb sits in the boot and where the boot sits on the adapter (what most people refer to as the manifold), and where the adapter meets the head). If the bike changes the idle at all then you have a vac leak, possibly a torn gasket or a bad rubber boot.

Some people will use starting fluid or something flammable in the squirt bottle but I use water so that if I do indeed have a vacuum leak then the motor doesn't rev up uncontrollably.

Also, post some pics of your carb setup.
Click to expand...
This probably isn’t the best picture but you can kinda see the VM34 with the velocity stacks.
 

Attachments

  • IMG_6233.jpeg
    IMG_6233.jpeg
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willis said:
If the bike was running beautifully as you described before it crapped out, then I would not start changing jets. Your problem likely lies in the battery/charging system and or the intake leak. I would also be looking into why your left hand plug is covered in oil. Maybe a compression test and/or leak down test might be in order.
Click to expand...
Charging system could make sense considering I blew a fuse. I think what could’ve happened is before I tried to fire it up the first time, i reinstalled my headlight that I’d removed when tearing down the front end. No soldering or anything, it’s just one of those little plug connectors. maybe plugging the headlight back in might’ve messed with something since I didn’t disconnect the battery before I plugged it back in. I’m not really sure what/how to test for any problems though. Any suggestions on what to look for/how to diagnose?
 
Raymond said:
The thing you're describing with the kick start sounds like it might be normal. It's about what position the engine is in.

I always kick start my XS and usually, I push half-way, till it stops up against compression, then bring the lever to the top and give a good shove to start the bike. If I make a kick without that prep, then sometimes it hurts coz I hit comp and the lever stop dead, other times it's like there's not much going on.

Sounds like you might have rich mix bur hard to diagnose remotely.
Click to expand...
In the event the mixture is too rich (which i’m almost certain it is) then would you suggest just tinkering with the air screw on my carbs to lean the mixture out?
 

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