CarbTune Question

[Condor]

OK, got the CarbTune out and put it on the '91. Figured it was a good time to sync now that the #4 diaphragm was repaired. Surprisingly the carbs weren't that far out. I little minor tweaking was all it took. Next I decided to change out the plugs, and left the carbtune hooked up overnight, rather than undo it, and have to hook it up again in the AM. The next morning I changed the plugs and fired up the motor. As expected the engine at high R's initially pulled lots of vacume, but after warming up, and the R's dropping down to a normal idle, the readings went back to normal. Flat all the way across. Blipped the throttle a little and noticed something which I also noticed at the higher idle RPM's. On the left side the vacume show'd higher than the other. At a steady 2000rpm there was about a 30% difference between the left and right side with the front and back carbs being balanced. 2 and 1 were equal, and 4 and 3 were equal, but the left, 2 and 1, was definitly higher. Now I'm wondering. What would cause that? At idle they are all equal, but the bike doesn't run down the road at idle, and I'd want all the carbs to be pulling equally in the higher R's... Maybe?? I wonder if syncing them at 2000rpm might be better, or is coming off idle equally better??? Whatcha think.

[RandyR]

have you looked at the needles to see if those on the left might be worn a bit?

[Condor]

have you looked at the needles to see if those on the left might be worn a bit?

 

Haven't checked any needles. Which needles are you talking about. Idle or diaphragm slider?? Are you saying that worn needles will cause this??

[RandyR]

one possibilty is the needles in the main jets (slider) are a bit worn and you're getting more fuel from one or two carbs vs the others at some openings. different size needles might cause this too, I would expect.

 

I see where v7Goose says there are different jet sizes in the 2nd gen carbs. I will guess thats to make up for some difference in the air flow to certain carbs. So I'd guess air flow issues might cause an unexpected inbalance.

 

Others who have worked on the 1st gens a lot are likely to have better ideas.

[Venturous Randy]

Keep in mind Condor that the throttle plate opening difference from 800 rpm to 2000 rpm is very little without the bike being under load. Is there any slop in the linkage that is being taken up when you move the throttle? Could you have a vacuum leak that you are compensating for on the idle balanceing sync that is not as much a factor once you open the throttle up a little?

RandyA

Edited September 3, 2009 by Venturous Randy

[Condor]

Keep in mind Condor that the throttle plate opening difference from 800 rpm to 2000 rpm is very little without the bike being under load. It there any slop in the linkage that is being taken up when you move the throttle? Could you have a vacuum leak that you are compensating for on the idle balanceing sync that is not as much a factor once you open the throttle up a little?

RandyA

 

I'm going to grab a can of carb cleaner and start looking for a leak. I noticed also that the idle screw doesn't want to respond like it should. Turn the thing to get the 1000rpm idle and blip the throttle and the R's slowly increase up to around 3K and won't drop back down to idle... staying up around 3K. Even with the throttle back to minimum. And putting finger pressure on the linkage to make sure that the cables aren't stuck. When I back the idle screw way off the R's will drop quickly. I did clean off the outsides of the carbs and then use some Deep Creep on all the moving parts. I think it's time to pull the carbs and take a good look at what's going on....

[MasterGuns]

Jack, If you recall my post early last month concerning my finding a vacuum leak, what I didn't mention in that post was a symptom exactly as you describe. The carbs would sync out perfectly at idle, but when increasing the r's to 2500 or so the left bank would show far less vacuum than the right. Using a can of ether, I found a significant vacuum leak where the rubber manifold of #2 mated with the head. That leak was caused by a stripped bolt. After fixing the stripped bolt and resyning the carbs, my carbtune readings were almost perfect at all r's read. JFYI.

[Condor]

Jack, If you recall my post early last month concerning my finding a vacuum leak, what I didn't mention in that post was a symptom exactly as you describe. The carbs would sync out perfectly at idle, but when increasing the r's to 2500 or so the left bank would show far less vacuum than the right. Using a can of ether, I found a significant vacuum leak where the rubber manifold of #2 mated with the head. That leak was caused by a stripped bolt. After fixing the stripped bolt and resyning the carbs, my carbtune readings were almost perfect at all r's read. JFYI.

Thanks for the heads up Herb. I sorta remember the post, but not to much of the details. I'm up to my ears in getting things ready for Cody. It's getting short time, so I'll have to put things on the back burner, and take a look once I get back. I was plannning on taking the '91, but it's gonna have to sit a while longer while I do the Round-Up on 'old faithfull' the '83. Hey does that mean old faithfull is going to see 'Old Faithfull'?? OR.... one 'OF' is visiting another 'OF'.... Geeze, it's early and the coffee's still perkin'....

[tvking63]

My sync is slightly different at higher RPM than at idle. Since I don't ride at idle, I synced them at 3000rpm and let the idle be a little lumpier.