1982 XS650 Heritage Special - Decel popping on one cylinder

Kevin Werner

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Stock filters, stock exhaust, went up to 145 pilot.

Carbs were completely rebuilt in spring (when bike was bought after sitting for 10 years).
If you verified the pilot jets match, it seems a dirt/varnish problem if they are synced, the timing is the same left and right, the floats are the same left and right, the manifolds don't leak, etc. etc.
I only recently synced my carbs. Guess what an idiot I am. After rides this past 8 or 9 months I often checked 1 plug, alternating left and right assuming both cylinders were working the same but the sync was way off. Foolish me to assume. I should be a textbook on what not to do. I am an armature but learning.
 

jpdevol

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I mean as in checking valve clearance and (further down the road if problem persists) a leak-down test.
 

fenbrancis

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If you verified the pilot jets match, it seems a dirt/varnish problem if they are synced, the timing is the same left and right, the floats are the same left and right, the manifolds don't leak, etc. etc.
I only recently synced my carbs. Guess what an idiot I am. After rides this past 8 or 9 months I often checked 1 plug, alternating left and right assuming both cylinders were working the same but the sync was way off. Foolish me to assume. I am an armature but learning.
I am also a new guy trying to learn these things. This forum has been super helpful.
 

Kevin Werner

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i have checked valves all seem good.

I’ll give mixture screw o-rings a try, along with some sea foam.
SeaFoam will be a tank full or 2 before you get useful data. SeaFoam is a solvent(naptha, Stoddard solvent or some other short chain hydrocarbon), kerosene/diesel for lubrication and an alcohol to absorb moisture. If there is a jet blockage in the pilot circuit it will take a few miles to dissolve.
 

fenbrancis

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Update:

I am checked the fuel mixture screw o-rings and they look fine. I sprayed starter fluid all around the side thats popping and no change, I richened the mixture by going 3.5 turns out on the fuel mixture screw and no real change.

I noticed my axle and axle nut are covered in soot, so now Ive gone so rich trying to mask the decel pop im just blowing unburnt fuel out the pipe.

I went back to the stock setting, 42.5 pilot jet and 3 turns out on the mixture screw. The bike pops so much I don't care to ride it. Also now that I have put the stock pilot screw in, the bike starves for fuel when cold starting, idling, and when accelerating from 1200 rpm-2500 rpm.

The bike runs perfect above 3000 rpm. But the second I let go of the throttle it sounds like my right cyclinder is in a gun fight.
 

jpdevol

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The BS34's are on the lean side with 42.5, but should not be too lean for stock bike with 3ish turns out on the fuel screws. You mentioned stock pilot screw: do you have aftermarket screws? It also should be close with 45 pilots and 2ish turns on fuel screws.

I think, given your symptoms and carb adjustments to date, I'd be curious to verify compression (leaking valve face) and ignition timing (on right side particularly) with timing light to verify your gonzo set-up is good. Both required tools available at local chain auto supply and not expensive.
 

fenbrancis

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The BS34's are on the lean side with 42.5, but should not be too lean for stock bike with 3ish turns out on the fuel screws. You mentioned stock pilot screw: do you have aftermarket screws? It also should be close with 45 pilots and 2ish turns on fuel screws.

I think, given your symptoms and carb adjustments to date, I'd be curious to verify compression (leaking valve face) and ignition timing (on right side particularly) with timing light to verify your gonzo set-up is good.
I think I’ll go back to the 45 with 2-2.5 turns out for now.

Will certainly give the ignition timing a check.

When you say leaking valve face, do you mean the valve cover has a leak or the valve is not seating all the way?
 

jpdevol

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The actual sealing of the valves against their seat inside the combustion chamber. If PO didn't maintain valve adjustment it can cause damage and produce the symptoms you're having. Hope not the case, but pays to verify.
 

jpdevol

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You need a compression gauge with a 14mm adapter (common). Preferably a leakdown tester, but whatever. I'm happy to walk you through procedure
 

5twins

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It sure sounds like your pilot circuit is plugged or partially plugged. There are 4 small holes in the carb's main bore that feed the pilot mix into the carb. Three are clustered together on top about where the butterfly plate closes. The 4th is slightly ahead of the others and comes from the mix screw .....

Outlet Holes.jpg


You must make sure all these holes are clear. Blow cleaner in the pilot jet hole (jet removed) and make sure it exits out all 4 holes. You will need/want to remove the mix screw and it's associated parts (washer, spring, o-ring) during this cleaning process then block off the outside hole where the mix screw went in with a finger. You can alternately direct the air pressure out the 3 holes or the mix screw outlet hole. Block off the 3 holes and the outside mix screw hole, and cleaner will/should come out the mix screw inlet hole. Block off the inside and outside mix screw holes and the cleaner should come out the 3 holes. You can also "back flush" the circuit by blocking off the 4 little holes inside and blowing cleaner in through the mix screw outside hole. It should come out the pilot jet hole on the bottom of the carb.
 

jpdevol

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If you're buying a timing light just get a basic one with clips for battery power. The ones with extra functions or self powered just complicate matters and problematic.
 

fenbrancis

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Hi all,

Bikes timing is correct. The carb has been torn apart and cleaned again and looks fine. I have sprayed started fluid everywhere trying to chase a vacuum leak.

The only thing I can think of it to be is incorrect valve settings. I set them on the popping cylinder awhile back and must have done it incorrectly. Will get them done today and let’s you all know what’s up
 

jpdevol

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Ok, the compression check is a 5 minute process with a $30 tool. Good information to have in your situation👍
 
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