Hello, I was coming home today with no problems then I heard like a air violently popped out of the engine so I stopped and the RPM was like 200 and I had to give it a lot of throttle to get moving and started heavily backfiring from exhaust when decelerating. I rode home and now it barely starts and wont hold even at choke. It runs but then with a loud pop from the engine (not metallic pop like air or electric) dies.
Any ideas ? thanks
Check the bolts between the exhaust header and the head. Your exhaust could be loose. You will likely see residue around the header if this is the case.
If that's OK, check for a massive air/vac leak and start at the carb socks, working your way down the air path and vac connections.
Besides the other comments, Are your valve ajustements up to date?
I haven't checked my valves yet, but I measured my compression which is at 6 bar on both cylinders.
The backfiring on deceleration indicates a very rich mixture. I had a similar problem recently, one of my fuel-screws(adjusters) fell out while riding. You should probably check those and if they are ok, pull the carb bowls and check your jets. Since your problem came on suddenly, you might just find a loose jet lying in the bottom of the bowl.
Hi Chris, we recently had this discussion over on December forum, I've got an old Beatle as well.
There was agreement that popping in the exhaust is very rarely a rich event, I'm thinking that maybe something catastrophic like a jet falling out, as well as an exhaust header leak could possibly lead to something like this. Something catastrophic probably has happened as it was so sudden and the bike isn't even running properly.
I'm almost wondering if possibly an exhaust valve got bent.
Text below was something that came out from that discussion on cars, but I think it's relevant to bikes too. Just my 2c worth😉.
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I've seen those types of posts and explanations. It's not correct. It's almost impossible to run a street car so rich that the mixture does not burn in the cylinder and explodes in the exhaust instead. I've never seen it. If the mixture is that rich, it would consume all oxygen in the burn process, and it would not be able to explode in the exhaust due to lack of oxygen. You will occasionally see this type of rich mixture on heavily modified road race cars, and you'll see actual flames coming out the exhaust as the fuel reaches oxygen at the exhaust outlet. But these cars will not "pop" in the exhaust - they still have a nice exhaust rumble in spite of the rich mixture shooting out the tailpipe.
Rather, what happens is that the mixture is lean enough that it causes an occasional misfire in the cylinder due to the lean condition, which most often happens on deceleration. When the cylinder misfires, it pumps a complete charge of unburned air/fuel mixture into the exhaust, so you actually have fuel and oxygen in the exhaust system. After a couple of times of this, you have a nice charge of both air and fuel in the exhaust system which then lights off and explodes.
Richen it up, and you'll see that the popping will go away (often you can just back out the idle mixture screws a little - if that won't do it, bump the jetting up 2 sizes)
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Cheers
Bmf
it is possible, and happened on my gs.
it would drown on gas, the needle was stuck causing the cylinder to misfire, so not all the gas was burn(or any at all) fushing the cylinder and there was gas on the exhaust.
it was the consequence of a not so great idea of putting foam inside the inner side of a K&N lunchbox, the foam was eaten by the carb blocking the needle-slide so i had gas in the exhaust, it couldn t run more than 10 minutes and will die. i removed the exhaust and was all sludgy with gas.
extreme cirunmstance but possible
if exhaust headers are loose , it can also make very strange things. because flow is changed.
I have changed the valve cover gasket to no avail. The exhaust is not loose anywhere and it keeps dying immediatrly with a pop. This has happened to me obce but changing the valve cover gasket fixed it
Have you checked ignition pickups? Hot and cold, measure the resistance
The valve clearances are OK. It makes this exact sound but dies with the pop. https://www.youtube.com/shorts/ZDJYAawuMQk
you can hear air hissing, with no explosion, for me its compression leak (valves,cylinder head gasket, maybe loose exhaust headers)or misfire, or both.
it happens to my bike and im planning to rebuild the top end anyways.
touch the carburetors while this happen. in my case i can see and feel the carb going backwards, a proper backfire it is for me.
record it on the darkness try to see if there is some sort of fire suputtering to the airbox side.
Quote from: herennow on August 19, 2024, 10:22:36 PMHi Chris, we recently had this discussion over on December forum, I've got an old Beatle as well.
There was agreement that popping in the exhaust is very rarely a rich event, I'm thinking that maybe something catastrophic like a jet falling out, as well as an exhaust header leak could possibly lead to something like this. Something catastrophic probably has happened as it was so sudden and the bike isn't even running properly.
I'm almost wondering if possibly an exhaust valve got bent.
Text below was something that came out from that discussion on cars, but I think it's relevant to bikes too. Just my 2c worth😉.
-----
I've seen those types of posts and explanations. It's not correct. It's almost impossible to run a street car so rich that the mixture does not burn in the cylinder and explodes in the exhaust instead. I've never seen it. If the mixture is that rich, it would consume all oxygen in the burn process, and it would not be able to explode in the exhaust due to lack of oxygen. You will occasionally see this type of rich mixture on heavily modified road race cars, and you'll see actual flames coming out the exhaust as the fuel reaches oxygen at the exhaust outlet. But these cars will not "pop" in the exhaust - they still have a nice exhaust rumble in spite of the rich mixture shooting out the tailpipe.
Rather, what happens is that the mixture is lean enough that it causes an occasional misfire in the cylinder due to the lean condition, which most often happens on deceleration. When the cylinder misfires, it pumps a complete charge of unburned air/fuel mixture into the exhaust, so you actually have fuel and oxygen in the exhaust system. After a couple of times of this, you have a nice charge of both air and fuel in the exhaust system which then lights off and explodes.
Richen it up, and you'll see that the popping will go away (often you can just back out the idle mixture screws a little - if that won't do it, bump the jetting up 2 sizes)
-----
Cheers
Bmf
Popping on decel is caused by some of the mixture not being burned and getting pulled into the hot exhaust. Suzuki makes versions of this engine with PAIR valves to inject air into the exhaust on deceleration, to more completely burn the residual fuel for cleaner emissions. On injected systems, there is a fuel-cut programmed when you cut the throttle, eliminating the issue completely.
Backfiring is a different problem, and is almost always caused by a over-lean condition.
Maybe terminology is the issue. My bike "pops" on decel because I jetted it richer, and changed to a free-flowing exhaust. It sounds good, not like it's misfiring--so maybe the word "pops" isn't that accurate of a description--but everyone uses it.
Ok, backfiring sound almos like a shot through airbox.
Exhaust unburnt fuel igniting=popping.
Not trying to stole the post but i will upload later a similar condition.
Okay I just found out when I disconnect one spark plug it works perfectly on 1 cylinder (doesnt matter which) but when it runs on both it keeps dying. Im completely stumped.
Quote from: Armandorf on August 20, 2024, 03:20:25 PMHave you checked ignition pickups? Hot and cold, measure the resistance
???