TOPIC:

I’m near the end of my 76 900 LTD restoration and am waiting on the vacuum gauge kit to try and sync these carbs.

What I have right now for 1-2-3-4 respectively is 7 1/2, 9, 4, 11 inches vacuum. Warm motor, air corrector jets, K&N filters, Kerker exhaust.

As I’ve never done this before, are those numbers way off? My rebuild included cylinder honing and new rings. The bike runs great and idles pretty good. Spark plugs look pretty good. Takes a bit of cranking when cold to start but starts immediately when warm.

Thanks for any advice.
Chris

Here's a pic I took while syncing my VM 28 carbs on my 76 LTD, high miles bike with freshly rebuilt carbs.

Forget the actual vac readings and just get them all pulling the same then set the idle.
These VM slide carbs don't have a register carb ( unlike most CV's ) so just start by adjusting the one that's furthest out from the rest and work from there.
Ideally you should set all the slide adjusters halfway along their threads and bench sync ( level all the slides the same) prior to install so that you are unlikely to run out of adjustment on any individual slide.

Oops, my bad. Yes VM26SS

I did bench sync the slides to all be the same in respect to the slide/needle positions as close as I could visually get. Zed when you say forget the vac readings, how are supposed to know what they are pulling?

 

motomisfit wrote:

Oops, my bad. Yes VM26SS

I did bench sync the slides to all be the same in respect to the slide/needle positions as close as I could visually get. Zed when you say forget the vac readings, how are supposed to know what they are pulling?


 

Just get all the gauge readings the same which means that each cylinder is doing equal work ( pulling the same as it's neighbour) then set the idle.
Actual vac readings are an indication of engine condition and if nothing is amiss your vac readings at idle are what they are and cannot be increased or decreased to conform to a pre determined figure.
Once set the vac will only increase or decrease in line with throttle position and load conditions etc.

 

Before I put the holders and main jets in, I set the slides as best I could by measuring how far the needles stuck out past where the holders screw in. So getting all readings close to the same is done at a higher rpm?

 

Only the frequency of vacuum changes with rpm not the volume. 
Sync them at slightly above idle speed to damp guage needle bounce.
Job done.

Bench set all the slides the same by using a 1mm ish drill bit under the slide on the engine side of the slide. If the drill is trapped by the slide adjust the height of the slide until the bit will just fall out under gravity. Do all four the same and they are bench set. Do this with the idle screw a few turns past touching to allow you to adjust the slides further closed if needs be to get the right idle speed when fitted. 

A bench sync is done using small gauge wire about .07mm but this size isn’t that important. First set the air gap of the idle adjustment screw to about 2mm. Adjusting the slides with the adjustment screw on the pivot arm inside the top chamber. Till the wire has same drag on each slide. Follow the procedure below












 

Thanks for that Mikaw. That’s pretty much what I did but I used how much the needles were protruding out inside the carbs as reference. I only needed to adjust two of them and at that very little.

I took it for a good ride today and it blasted thru the rpm range. Just a little blubbery at idle and off idle and oddly not all the time. Took the plugs out tonight and the #1 plug was very black and sooty. I was having this same issue the last time it ran 13 yrs ago. Cylinders honed, rings new, valves and seats new, valve clearances and ignition timing spot on. Float bowl levels spot on. Scratching my head a little on this one. Will post a pic of the choke plunger for #1 carb. It looks ok to me but…..that could very well be the issue.

This motor hasn’t seen a lot of ethanol fuel. This seal looks ok to me but…..if that indent in the seal is supposed to be flat, that would be a problem.

Zed1015 can rebuilt your choke plungers, it’s good to eliminate that from the mix. When the carb were apart did you check the needle jets for oblong wear? If #1 is oval or the jet needle is worn it could also cause the rich condition. 

EDIT, when the carbs were apart did you verify all of the brass were the same, in 40+ years anything can happen. You need to verify consistency. 
 

Yeah, something is going on in that #1 carb. I’m pretty sure it’s not anything in the ignition because #4 plug is fine. It’s looking like I’ll have to pull the carbs and go thru them again. I never thought to check the needles out of round. Thx