Mikuni maxed out

Jthomas

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Have two VM34s on a 81’ XS650:

I’ve read through tons of tech and I also have the manual. I’m still scratching my head.

The bike idles super rich. Plumes of black smoke, fouling plugs, unburnt fuel and misfires.

The idle adjust screws have to be maxed out (screwed all the way in) for the bike to run. Still low rpm. The air mixture adjusting screws have almost no effect on it. Sometimes it will stall out if it’s screwed all the way in.

My first thought was that is has too big of a pilot jet. they both had 17.5

I went down to a 15.0
It started firing on both cylinders more consistently. Air mixture screw still had very little effect. Idle screws still maxed out.

Air jets are 2.0
Main jets are 175

I created a vacuum leak, and the bike immediately started running ‘better’.


There’s no way the pilots are too big. Even when they were 17.5

I haven’t gotten to ride the bike yet, I bought it as a project.

Any thoughts?
 
Forgot to mention:
I set the float height to spec (22-24mm)

It will literally run the carbs dry in less then 10 seconds when I turn the fuel off.
 
There's no way to help you without a complete jet list: pilot jets, slides, needle jets, needles, and mains. Mixture screws on VM carbs control air in the pilot mixture: turn in to richen. This is opposite to the function of the mixture screws in the OE BS series carbs. Screwing the mix screws in and leaning out the PJs will get you nowhere. Click the Tech button and find the link to the VM section of the 650 Garage USA Carb Guide.
 
There's no way to help you without a complete jet list: pilot jets, slides, needle jets, needles, and mains. Mixture screws on VM carbs control air in the pilot mixture: turn in to richen. This is opposite to the function of the mixture screws in the OE BS series carbs. Screwing the mix screws in and leaning out the PJs will get you nowhere. Click the Tech button and find the link to the VM section of the 650 Garage USA Carb Guide.


Needles are on 2nd or 3rd clip.
I’ll check tonight and see what slides and needles are in it.

I kind of assumed that since it’s only idling, that the needle would not have any effect on it. Thats why I didn’t mention.
 
You know what they say: never assume. Secondary effects of components are strong in VM and most older carbs, to the extent that a one step change in needle position will sometimes affect the idle mixture strongly enough to force a PJ change. The main circuit (slide, NJ, JN, MJ) never turns off, it's operating even at lowest idle through the slide cutaway (speaking of which, make sure your slides are installed with the cutaway facing the rear). Need to know which needle you have as well as clip position.
 
Worth a mention? You can fully install the slides and caps with the needle off to the side of the needle jet. Don't ask how I know this. Bike does not idle well in this condition....
 
From the size of the pilot jet you found, what you may have is a set of generic, off-the-shelf VM34s. They are sold set up for two stroke dirt bikes and all the jetting is wrong for 650 use, sometimes the slide too. Also, usually you run these on a 650 with the air jet removed completely (run without one).
 
The generic PJ is a real fatty--#35. The generic 2.5 slide works fine on the XS650, usually with #22.5 or 25 PJ. Sudco installs a richer #2.0 slide in their VM34 jetting for the XS650, with a leaner PJ to compensate--usually #17.5. The generic NJ is 159-Q2. The leaner 159 P-5 or P-6 works well. I'm done here till I see a full jet list.
 
OK, as far as I can tell, if your PJ is series VM22/210 (BS30/96 will thread right in but won't flow right), you have good baseline components. The last time I dealt with a "Too rich no matter what I do" complaint, it turned out that the main switch wasn't passing enough current to enable a clean burn. So let's get some history.
1. Are the carbies new or used?
2. When you set float blade height, where on the blades did you take your measurements?
3. Was the motorcycle running before you mounted the carburetors?
4. What ignition system are you using?
5. What charging system are you using? Have you checked output?
6. Have you checked for voltage drop between battery and ignition coils?
 
Hmmm...sounds like I'm running something very similar to yours...what kind of exhaust do you have? I'm running with Mikes headers and mini-commando's, here's my VM34 set up: 175 Main, 20 Pilot, P6 Needle jet, 6F9 Needle (2nd from bottom), 2.0 Slide, 1 turn out on air (I used the dead cylinder method) and this has worked great for me so far.
 
OK, as far as I can tell, if your PJ is series VM22/210 (BS30/96 will thread right in but won't flow right), you have good baseline components. The last time I dealt with a "Too rich no matter what I do" complaint, it turned out that the main switch wasn't passing enough current to enable a clean burn. So let's get some history.
1. Are the carbies new or used?
2. When you set float blade height, where on the blades did you take your measurements?
3. Was the motorcycle running before you mounted the carburetors?
4. What ignition system are you using?
5. What charging system are you using? Have you checked output?
6. Have you checked for voltage drop between battery and ignition coils?

1. I bought the bike with the carbs on them. They were allegedly new. But, they really do look new.

2. Flipped the carb upside down, measured from the lip on the gasket rail to the top of the arm. (Top of the arm being the side that makes contact with the float)

3. These were the carbs that were on it when I bought it. It was someone else’s unfinished project

4. Just installed Pamco

5. PMA alternator

6. I have not checked anything in the charging/ignition system. When it had the stock charging/ign system, it ran the same as it does now.
 
Hmmm...sounds like I'm running something very similar to yours...what kind of exhaust do you have? I'm running with Mikes headers and mini-commando's, here's my VM34 set up: 175 Main, 20 Pilot, P6 Needle jet, 6F9 Needle (2nd from bottom), 2.0 Slide, 1 turn out on air (I used the dead cylinder method) and this has worked great for me so far.

Short shotgun sidepipes and UNI pods
 
How do I bypass the key switch?

I’ll just run it off the capacitor temporarily to diag.

(I deleted my battery and starter)
 
Wait, re-reading your first post. You're having to screw in the idle adjuster all the way in (essentially raising the slide up as must as possible) to get it to run? Does your bike have vacuum ports on the boots? Are they plugged?
 
Re. float height, did you measure at the point on the arm where the pin on the float makes contact?

You have this in common with the "carburetor problem" that I referred to above: Pamco ignition. The Pamco will produce a spark and advance/retard if it's fed low voltage, but the spark will be too weak to burn clean. The last guy didn't think his "rich" condition could possibly be electrical, so in spite of my advice he struggled for a year until I got hold of his ride at a rally, did a voltage check, and made a jumper to bypass the bad switch. The "jetting problem" cleared up immediately. If you're running a Pamco ignition without a battery you're asking for trouble.

That's all I got. Good luck.
 
You bypass the key switch by hooking one end of a wire to the + terminal of the battery and the other end of the wire to the + terminal of the coil (the side that gets power, not the side that's connected to the signal wire from your igniter box). You'll need some F-crimp terminals and a crimp tool. Before you do anything else make sure your battery has full charge. If you don't have a battery in the system, you might consider installing one.
 
Intake runners are billet and do not have any vacuum ports.

Yes, you read it correctly. Idle screws all the way in to give it as much air as possible.

I have a fresh battery for my truck. I’ll Hotwire the ignition off of that and report back my results tomorrow after work.

I deleted the starter and battery to lighten the bike and clean it up.
 
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