Took the carbs off and on, now have high vacuum in one carb.

[acadia81]

Hi Guys, hope someone can help.
I took the carbs off my '91 ST1100 to fix a sticking float needle, and I took off the snorkles and the plenum. Everything went back together fine, except now I noticed a thumping noise at idle (someone else here called it a "stomping" sound). I put my hand over each snorkle and found the left front cylinder
pulling a much stronger vacuum than the other three. Any idea why? Any thoughts as to what I screwed up? Thanks.

 

[Dinkie Diesel]

I think you need to do a carb sync. Sounds to me like one butterfly is open farther than the other three therefore that one cylinder is doing more work and making the thumping sound.

 

[Mark]

Yup, Jeff is correct.

You must do a carb sync now...

You might see if someone near you has a flowmeter as they really speed up the sync job...

 

[acadia81]

Thanks guys, I sync'd the carbs this morning using an old Uni-Sync from my MG and Triumph days. Not very precise but it got me pretty close. The bike seems to idle much smoother and the thumping noise is gone.
It stopped raining, the sun is out, it's a holiday, time for a 200-300 mile test ride.

And a heartfelt Thank You to all our veterans and active military members, especially those who have made the ultimate sacrifice, for their noble service to our country.