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Backfiring persists after 1 year sitting

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baloo:
1989 Mille GT 1000cc bought in 2021 and still all stock with about 36,000 miles.
Bike ran fine until I again let it sit for over a year. 

So, to re-start, I fogged the cylinders, drained the carb bowls, dumped the old gas and put in new 93 octane gas, changed the battery, and fired it up -- no probs after a few cranks.

BUT, I get bad backfiring above 3000rpm.

I checked the plugs (which look ok) and made sure they were getting spark.

​Any suggestions regarding backfiring?
1) Anything ignition related due to sitting?
2) anything specifically carb-related due to sitting?
3) What could have gone wrong, mechanically, after sitting for a year to cause backfiring at higher rpm?

Thanks.

Tom H:
Could be a number of things. Check to make sure your intake and exhaust bolts are tight. Might pull your carb bowls again and double check for any crud. Change the spark plugs to new or known good ones.

Now for  a weird one. Check you air filter box for rodent nests. as well as the mufflers.

Tom

guzzisteve:
Your points also may have some crud on them. Clean all ports in carbs, mix screws, pilot & main jets, needle & atomizer, and drillings in carbs.
Also the screw in center of distributor w/pad on it. Mahe sure drop oil on pad & loosen screw & just snug it. If it's tight the advance won't work.
Check wires & caps on plugs.

And what Tom said, varments get in airbox

baloo:

--- Quote from: Tom H on September 15, 2025, 10:54:56 AM ---Could be a number of things. Check to make sure your intake and exhaust bolts are tight. Might pull your carb bowls again and double check for any crud. Change the spark plugs to new or known good ones.

Now for  a weird one. Check you air filter box for rodent nests. as well as the mufflers.

Tom

--- End quote ---

Hah!  Rodents!  A possibility, in that it has been sitting under the carport, but covered (cover probably gives the rodents a sense of security, right?)_.
I did look in one of the carb bowls and got out some tiny crud.  Should I just bite the bullet and pull the carbs and clean out all the passageways?

Good thinking about the advance! I'll oil the pad and check the screw.

Likewise, I will check intake and exh bolts -- I assume you're saying that a leak would change the air/fuel ratio enough to backfire....

All good ideas, and thanks.

Moparnut72:
I have a brush chipper that gets used once or twice a year. It has on start up run rough, I have added Seafoam and after a bit will start running normally. This year I put a fuel shut off valve on it so that should solve the issues.
kk

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