Air Screw Behavior

I just did a carb sync on my 77 650 with Kerker's, stock airbox, stock filter, and (I think) stock jetting. Two cylinders were close, two weren't. Afterwards, I tried adjusting the air screws and didn't get the behavior I expected. I'm currently at 1-3/4 turns out on all carbs. I arrived at that setting by adjusting for best throttle response off idle. This seems to have resulted in a rich mixture since my plugs are sooty, and removing a vacuum cap (leaning out the mixture) causes the idle to go WAY up. I've read that the proper procedure is to adjust each air screw, one at a time, for highest idle, then re-adjust idle after each air screw adjustment. I tried adjusting one screw out by two whole additional turns, but the idle didn't change at all. What does it mean when the air screw doesn't affect the idle? Is my idle mixture way off? If so, how can I fix it? I thought that the idle mixture was almost totally controlled by the air screw.

Edit to add:
I've installed WG's coil power mod. The points have less than 2500 miles on them, and they were adjusted with a dwell meter. I get a little bit of black smoke when blipping the throttle at idle. Float levels were set with the "wet" method.

No bites, eh? Surely someone's got experience with this.

"What does it mean when the air screw doesn't affect the idle?"

I've found that that means there is a jet or a passage that's plugged and preventing the air screw adjustment from being effective.

KZCSI

Are all of your plugs equally sooty? Are the insulators black and dry or black and wet?

Are those VM26 carbs? If it wasn't rich before and is now after you adjusted only the air screws and the sync on the slides, I'd have to go with you put something out of spec when you sync'd them.

Maybe if you detailed what you did in the sync process and any other tweaks at that time it would help pinpoint the problem. I'm a firm believer in the last thing touched law of causes for problems but that get complicated when you do several things at once which is a fault of mine for sure! :silly:

KZ_Rage wrote:

Are all of your plugs equally sooty? Are the insulators black and dry or black and wet?

The plugs were dry and sooty before sync'ing the carbs.

KZ_Rage wrote:

Are those VM26 carbs? If it wasn't rich before and is now after you adjusted only the air screws and the sync on the slides, I'd have to go with you put something out of spec when you sync'd them.

Not sure which carbs I have. They're the stock units for a 77 650C1. The rich/lean condition hasn't changed as a result of the sync. I adjusted the air mixture screws trying to lean it out as much as I could and still retain good throttle pickup off idle. I wound up adjusting the screws out 1/4 turn, and I may have to go back to the original setting, as it sometimes stumbles of idle now. So it appears that the only effect of the carb sync is a much smoother idle now. I'm hoping for better fuel mileage, too.

KZ_Rage wrote:

Maybe if you detailed what you did in the sync process and any other tweaks at that time it would help pinpoint the problem. I'm a firm believer in the last thing touched law of causes for problems but that get complicated when you do several things at once which is a fault of mine for sure! :silly:

For my sync tool, I used a cheap 20ft clear plastic tube with water in it as a differential manometer. I decided to use carb 3 as the reference since it appeared to be the hardest to get to. So I removed the caps from carbs 1, 2 and 4. I cranked the engine and let it warm up. I shut it off and I hooked the sync tool to the vacuum ports on 3 & 4. I cranked it and sync'ed 3 & 4 by adjusting carb 4. I repeated this for carbs 1 & 3, and 2 & 3. Then I tried to set the air mixture screws for max idle speed, but they didn't seem to have any effect. So I put the bike back together and hoped that I could set them by test riding and adjusting for best throttle response, hoping that I'd be able to lean out the mixture due to having the carbs in sync. I started off at 1-3/4 turns out on all the screws, and finished at 2 turns out, and it may be too lean now as I'm getting a stumble off idle when the bike gets up to temp.

Long shot but....

I've read that it is important to sync the manometer too. This is done by running all four lines off of one vacume source through a one to four splitter. Apparently there can be differences in the readings from tube to tube. If there are differences in the levels, then this can be noted, marked or amended prior to testing on the carbs.

If this was the case here, then it is possible that you synced your carbs to be equal, but if there were differences in the readings from line to line, there will be inequalities transferred to the carbs.

As far as the air-screws, I experience the same thing with my bike as well.

I recently ran a can of sea-foam gas treatment through the system, and it has greatly affected the performance of the bike. I will try the screws again to see if any deposits have been dislodged that might cause the air screws to act correctly again.

Sea foam did help my bike to run more smoothly.

I'll give SeaFoam another try. I've run one treatment through. I'll try it again.

Thanks.

Its my understanding you synch the cards by adjusting the slide heigth,thus the needle position within the needle jet not the air screws. Set the air screws at the default setting, pull the caps off the carbs and do it that way. Use the corrct mercury manometer set up. Mercury weighs the same no matter what, so calibration is not an issue. Its either pulling 22" hg across the board or not. Its all in the manual.