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Jumpy carb sync - How to ensure proper vacuum/compression?
Hey guys! I'm finally going to throw in my new carbs this weekend, ('87 Aspencade, (California) carbs on my '86 Interstate). I have had a continual problem of getting a smooth sync on carbs 1 & 3, (right side).
Carbs 2 & 4 have always been smooth on my synchronizers but 1 & 3 ALWAYS jumped around so much I could never get them synched right.
My right foot has always been roasted when riding with my feet on the pegs and I'm not sure if the right cylinders are supposed cook your feet more than the left or not, (on normal 1200s). I was wondering if I have a bad valve that's causing the carbs not to synch or what? My bike was flipped before I bought it but I have never seen any damage to the right cylinders at all.
I would like to know if there is a good way to check positively that cyl. 1 & 3 are right. I do have a compression tester but not a leakdown tester.
I would hate to install the new carbs, (with new O-rings as I always do) and find I still have the jumpy synch issue.
Thanks for your help. Don W.
#1 01-20-2008, 12:08 PM,
If the right side is that radically different than the left side than I would venture to say that the valve timing is incorrect at the timing belt. Pull the timing covers and check the marks.
Ed (Vic) Belanger - 1954-2015
Founder of

#2 01-31-2008, 11:42 AM,
Vic: Is there a way to pull the covers without removing the radiator? I just did the coolant change a few months ago and I hate to have to do it again so soon. So are you telling me it will still run fairly smooth even if it's 1 tooth off?
The engine runs smooth at rpms above idle and the Vacuum Gauges are are equal at 2000 rpms and higher, (mostly). It just bugs me that they jump around so much at idle. I was at least glad I did a compression test and every thing is normal, (thanks for all your help on that).
#3 01-31-2008, 12:05 PM,
It can be 3 teeth off and still run smooth. You can get the covers off without removing the rad but you'll need to crouch down low to see the marks clearly. Have you made a smaller restricter for your vacuum gauges? That would help stabilize the jumping needle.
Ed (Vic) Belanger - 1954-2015
Founder of

#4 01-31-2008, 07:45 PM,
Here is my compression test from the January 2008

Attached Files
.jpg   Dons Compression Test Results small.jpg (Size: 68.08 KB / Downloads: 157)
#5 03-18-2008, 09:00 AM,
The results look very good. Check the valve timing at the belts and move on to inspecting fuel pressure, ignition and carb systems. The manual gives specs on how to inspect each one of these systems. The only way to solve a problem like you're having is to know for certain that all systems are in proper working order.
Ed (Vic) Belanger - 1954-2015
Founder of

#6 03-18-2008, 11:35 AM,
Thanks for some suggestions, Vic.
I'm wondering about the coils and wires since they are original and the bike was flipped before I bought it. I'm going to check everything you mentioned and maybe, just MAYBE I can find the culprit. I really need to get my bike synched and saving gas because with the high price of diesel, it's now cheaper to ride to work than it is to drive my 2002 Jetta Diesel to work, (even at 46 mpg).
#7 03-18-2008, 12:59 PM,
Good luck with it. You'll find that by testing with an ohm meter from one spark plug wire to another on the same coil you can check the whole system on that one side and if the reading is off you then start breaking it down piece by piece to find the defective component. Something has to be wrong or else it would run great.
Ed (Vic) Belanger - 1954-2015
Founder of

#8 03-18-2008, 03:37 PM,
my friends 1100 was getting 2 hard to synchronize. we went for a mechanical synchronization. removed all 4 intake tubes and mechanicaly measured the opening of each butterfly and got all the same

now it runs way better

Smile you should try that Smile
85' GL 1200 Interstate - My Picture Thread
[Image: signaturebg4.jpg]
#9 03-21-2008, 01:30 AM,

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