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Bench sync a VM carb?

  • Thread starter Thread starter tlepac
  • Start date Start date
T

tlepac

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I have read a lot of posts on Bench syncing your carbs and from what I can glean this ONLY is for CV carbs and not VM carbs? If not what is the specific procedure? I am having problems with a very poor idle and way out of whack on the mixture screws Colortune and right angled pilot screw adjusting screwdriver on the way. I am tired of getting roasted every time I try to set pilot screw on 2 and 3 carb. Why did they have to make them so hard to get to?

I have already dipped the carbs for 24 hrs, changed the O rings between cylinder heads and carbs, adjusted the valve clearances, have good spark on all four plugs and very good output from the battery.

Also I am in the process of rechecking points and timing but I have a bit of a problem in that I cannot see the marks! Cataracts make me not able to adjust to the flashing light and all I get is dazzled by the light. I tried marking with colored pencil to help but I couldn't see a d@#$%d thing! Anyone out there have any suggestions? I am currently waiting for my youngest son to make some time to help me.

Bike is a 79 GS750 L with VM 26 carbs --- BTW what are the settings for the pilot screw and air mixture screw? I have read so many different things and different numbers on the site I am more confused than ever. And the Mikuni Tuning guide doesn't guide on this at all that I can see

Frustrating:confused:

Any help appreciated but please be specific to VM carbs ONLY--Thanks
 
Bench syncing carbs is nothing more than making sure all the throttle plates open same amount.

I take the carb rack, hold it up to a strong light, adjust the base carb until it is open a small amount and make the others look like the are open by the same amount. vm's are no different than cv's, except there is no base line carb.

"static timing" can be set with a 12 volt light connected up between where the condenser-wire to coil is nutted on the points and ground. align your #1-4 firing marks up, then turn the main breaker plate opposite the normal rotation of the point cam, until the 12 volt light bulb illuminates. do the same with the 2-3 firing marks, but you adjust the smaller plate that the #2-3 points are on.
 
Yes, a bench sync will be helpful on VMs

The standard settings are fuel screw - 3/4 to 1 turn out, air screw - 1 1/2 out

Check the carb spec sticky at the top of the page

There are many variables for bad idle

1. Valve clearances done?
2. New O rings in carbs?
3. Carb sync?

Ignition timing is essential also

Where are you in the Land of 10,000 Lakes? Maybe some nearby member can give you the benefit of their experience
 
Bench sync is mostly to get the bike running well enough to vacuum sync. One other thing with the VM carbs is to make sure the slides open all the way, but not more than 1/8" over the top of the carb throat.

Please don't skip the critical vacuum sync stage.
 
Thanks all for the input
Rustybronco
"static timing" can be set with a 12 volt light connected up between where the condenser-wire to coil is nutted on the points and ground. align your #1-4 firing marks up, then turn the main breaker plate opposite the normal rotation of the point cam, until the 12 volt light bulb illuminates. do the same with the 2-3 firing marks, but you adjust the smaller plate that the #2-3 points are on.
I followed the Clymers manual procedure which was similar but not quite the same. I am going to review it tonight to see what the differences mean

BigT
There are many variables for bad idle

1. Valve clearances done?
2. New O rings in carbs?
3. Carb sync?

All three above done with RYonker. Seemed okay but the performance over the next few days deteriorated .

Nessism
Please don't skip the critical vacuum sync stage.

Did the Carbtune Vac sync procedure Seemed to go well but idle still irregular and got worse over the next 10- 20 miles to the point that the bike barely starts on full choke and only stays running by keeping the throttle over 3-4000

I don't know how old the points are but am thinking of changing them just on principle. Any other suggestions?

Thanks in advance
 
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