Backfiring persists after 1 year sitting

[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]

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

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

[baloo]

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

Seafoam to the gas, I assume?
I do use the shutoff valves from the tank whenever it's not running, and I add gas stabilizer to the tank. I
But, I did empty the tank of the old gas and added fresh fuel before this most recent adventure.

[Dukedesmo]

My LM2 backfires a little on the over-run, I can't find anything wrong with it and it runs well otherwise so I just assumed it's 'normal'?


But a couple of years back the crossover pipe between the 2 exhausts cracked where it joins the down pipe, it wasn't a big crack and was barely visible but it created a huge backfire issue. So checking the exhaust for leaks sounds like a plan.

[redrider90]

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.

What kind of ignition do you have. Some 89s came with Moto Plat like my 90 mille. They are terribly unreliable. I'd check plugs first and then coils. Condensers can look ok at low RPM and fail at speed. Stock coils always good bad over the years.   Checking the timing is drop dead easy. Pop off the access cover on the flywheel and check the timing with a timing light. Advanced  is set at 5K RPMs so get out your ear plugs and a fan on the motor. I forgot for the moment what the timing is set at. I have to go out to my shop which I am not near to get the specs.  You can go to https://www.thisoldtractor.com/mg_manuals/owners_manual_california-iii_1989-01_en.pdf The 89 Calli with points is the samehttps://www.thisoldtractor.com/mg_manuals/owners_manual_california-iii_1989-01_en.pdf

[Huzo]

Backfiring…
Is it blowing back out the carb or firing unburnt mixture in the pipe ?
I’ll betting on points or distributor related cause, have you ascertained which side is failing ?

[chuck peterson]

Did you clean the jets while the carb bowls were off? Im not familiar with the round slide carb but jets clog pretty easily when left alone without seafoam and or stabil