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VM29 carbs help

  • Thread starter Thread starter razooki
  • Start date Start date
R

razooki

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I was perusing Cliff's web site the other day as I often do and re-visited the "How To" for cleaning VM carbs. I was reading the last section about tuning them and it dawned on me that I may be running lean when cold. I balanced the VM29 carbs not too long ago and now the bike has a hard time idling when cold. The bike runs great through the rev range and holds a steady idle when warm. Does this lean condition apply to VM29 carbs as well? If yes then I guess I'll have to try setting the idle screws but for the life of me I tried using the highest rev method before and I could not hear a difference.
 
When's the last time you've adjusted the valve clearances. Poor running when cold, or having to choke the heck out of it to get it to stay running when cold (hard cold starting as well) are all symptoms of tight valve clearances.
All bikes run a tad lean on cold start. Fuel condenses on the cylinder walls and the burn isnt complete. But, the flip side of that is, if you tune the bike to run richer on cold start, it will be overly rich when warmed up, as the bike will run richer when warm...
 
This cold idling problem only surfaced after I balanced the carbs (about 1000km ago) so I doubt it's the valves - they were done 3000km ago.
 
Well that's plenty good enough then. I'd maybe back off the air screws a 1/8 turn and see what happens. What are your fuel screws set at?
 
Nope. Theres an air screw at the base of the carb infront of the bowl... but i think he means do all of them on all carbs.
 
DannyMotor said:
Nope. Theres an air screw at the base of the carb infront of the bowl... but i think he means do all of them on all carbs.
Click to expand...

VM26s (and 29s?) have two pilot screws. There is a fuel screw underneath the bowl right before the intake port to the head. These are usually 1 turn out to start and are a PITA to change when the carbs are installed on the bike. You can change them, however, without tearing down the carbs, having to resynch, etc. Some people have rigged up tiny screwdrivers to get to them while installed, but I don't like to reach in there next to a hot cylinder head.

The air screw is on the side of each carb pointing to the outside (i.e., left on 1/2 and right on 3/4). These are usually set about 2 turns out and then adjusted to highest idle. They are very easy to access and turn while the bike is running.

As for highest idle, I also have a devil of a time noticing any changes in idle when using that method. Another poster suggested a turn and wait approach, where you turn by 1/4 and wait a few seconds for the new setting to kick in. I'll be trying that out next.

You can also use a colortune to fiddle with the mixture settings.

HTH
 
VM on 1980?

VM on 1980?

I thought the 1980 850 had CV carbs. I have VM 26 on my 79.
 
The VM29s I know never had fuel screws and it looks like somebody must have pinched them from Allens as well:

http://www.allensperformance.co.uk/exp-vm29.html

If the tickover went pants after you balanced your carbs i would have a look to see if you got that right. plus did you check the gauges before you did the balance (depends on the gauges you used). Having said that don't get too beat up about having a poor tickover when cold - VM29s aren't designed to be all nice and sophisticated like that. If you have a nice steady tickover when warm you're on a winner.
 
twotimeGSr said:
I thought the 1980 850 had CV carbs. I have VM 26 on my 79.
Click to expand...

Indeed, however mine came with the VM29s when I got it. Kind of lucked out there because I only found out about these carbs well after I bought the bike. whereas initially I thought I overpaid for the bike, after finding out that they are somewhat rare I did not feel so bad. Have to use KZ100 carb boots and enlarge the hole screws to make them fit.
 
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