Low RPM backfire from one cylinder

James, are you sure you even have a problem ?

“Problem is it backfires on left cylinder when warming up, sometimes it never seems to go away”.

You have the chokes removed (as do many, I know).

Assuming your carb settings are correct, then with chokes removed the engine is going to be weak when cold, which could cause such symptoms.

Get it fully up to temperature and see what it’s like then, if this really is only happening when warming up, then I would not worry about it.

The other thing is… have you tried NEW spark plugs…?
 
kommando said:
Yes, it's an air screw so turn outwards.
Click to expand...
Turn it outwards and see what happens.

Outwards = more air = weaker.

Kommando’s hypothesis is that richness is causing unburnt fuel to enter the exhaust which then combusts with a bang.

My thinking (would not quite call it a hypothesis) is kinda the opposite in that your cold start issues may be related to having no chokes and therefore being too lean when cold. Too lean means unburnt oxygen enters the exhaust which then combusts with a bang.

So, strange as it may seem, problems that are opposites can actually display the same symptoms !

Playing with your air screws (carefully) will at least help you identify which, if any, of these is your issue.
 
Also…

Jame’s, please tell us under what throttle position does the back firing occur (not what rpm).

It often helps to mark the twist grip with white ‘tipex’ or similar to ascertain this because just guessing your throttle position is often very inaccurate.

Understanding where the slide is will indicate which carb circuits are in play, and which to ignore.
 
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Fast Eddie said:
Turn it outwards and see what happens.

Outwards = more air = weaker.

Kommando’s hypothesis is that richness is causing unburnt fuel to enter the exhaust which then combusts with a bang.

My thinking (would not quite call it a hypothesis) is kinda the opposite in that your cold start issues may be related to having no chokes and therefore being too lean when cold. Too lean means unburnt oxygen enters the exhaust which then combusts with a bang.

So, strange as it may seem, problems that are opposites can actually display the same symptoms !

Playing with your air screws (carefully) will at least help you identify which, if any, of these is your issue.
Click to expand...
I agree this can sound like a lean misfire, but when I bought my 850 Mk 3 last Summer, it was waaaay too rich at idle, so much that it would idle at nearly 2,000 once warmed up. The points were beyond saving (stripped screws would not hold timing), so I installed a Tri-Spark then observed the popping in the exhaust. Once I got the idle mixture dialed in, it went away.
 
TRWham said:
I agree this can sound like a lean misfire, but when I bought my 850 Mk 3 last Summer, it was waaaay too rich at idle, so much that it would idle at nearly 2,000 once warmed up. The points were beyond saving (stripped screws would not hold timing), so I installed a Tri-Spark then observed the popping in the exhaust. Once I got the idle mixture dialed in, it went away.
Click to expand...
Twiddling the screw one way, then t’other, will quickly narrow it down !
 
Trivial: A backfire is back through the intake, a report in the exhaust is an after fire.

My experience with the popping in the exhaust, especially when coasting down with closed throttle has been a bit rich idle mixture. Your mileage may vary...
 
I would carefully recheck:
1) Fuel height in bowls
2) Carb sync

Either can cause too rich or lean in one cylinder and either of those can cause unburnt fuel/air popping in the exhaust.

Also, you said new carbs - if recently new, then they are surely Premier so I would check that the pilot jets are screwed in all the way and that they are both either #17 or #19 (preferred). If Premier, directly opposite the silver air screws are the brass pilot jets that screw in.

As several have said, turning the air screws in and then out a little and noting the change can tell you a lot. If the air screws are out different amounts - something is wrong. If they are out more than 1-1/2 turns, something is wrong. If they are out less than 1-1/4 turns, something it wrong.

Assuming that the slides are perfectly sync'ed If the problem goes away at 1/8 throttle or more, then you almost certainly have a pilot circuit issue. Perfectly sync'ed means that they start lifting at exactly the same time and they reach the top at exactly the same time - in other words, they are moving exactly in sync. Slight variations can cause huge differences in how the cylinder run.
 
On my single mikuni, if I turn the idle air screw for maximum speed, and then lean it 1/8 turn out more, it pops in the exhaust pipe on over run.
That's lean.

So I turn it for maximum speed, then I turn it in (richen) about 1/8th. So no popping.
Love to get your comments on this tuning process
Dennis
 
Fast Eddie said:
James, are you sure you even have a problem ?

“Problem is it backfires on left cylinder when warming up, sometimes it never seems to go away”.

You have the chokes removed (as do many, I know).

Assuming your carb settings are correct, then with chokes removed the engine is going to be weak when cold, which could cause such symptoms.

Get it fully up to temperature and see what it’s like then, if this really is only happening when warming up, then I would not worry about it.

The other thing is… have you tried NEW spark plugs…?
Click to expand...
Yes, tried new plugs.
 
marshg246 said:
I would carefully recheck:
1) Fuel height in bowls
2) Carb sync

Either can cause too rich or lean in one cylinder and either of those can cause unburnt fuel/air popping in the exhaust.

Also, you said new carbs - if recently new, then they are surely Premier so I would check that the pilot jets are screwed in all the way and that they are both either #17 or #19 (preferred). If Premier, directly opposite the silver air screws are the brass pilot jets that screw in.

As several have said, turning the air screws in and then out a little and noting the change can tell you a lot. If the air screws are out different amounts - something is wrong. If they are out more than 1-1/2 turns, something is wrong. If they are out less than 1-1/4 turns, something it wrong.

Assuming that the slides are perfectly sync'ed If the problem goes away at 1/8 throttle or more, then you almost certainly have a pilot circuit issue. Perfectly sync'ed means that they start lifting at exactly the same time and they reach the top at exactly the same time - in other words, they are moving exactly in sync. Slight variations can cause huge differences in how the cylinder run.
Click to expand...
Greg, good to hear from you. I bought the carbs new from you. Best idle is in the range you specified for both screws. Carbs are synched with gantry and minor cable adjustments. However, this problem also existed with my old carbs, which otherwise ran well. Above 1/8 throttle it runs like a scalded dog, a well oiled machine.
 
Fast Eddie said:
Also…

Jame’s, please tell us under what throttle position does the back firing occur (not what rpm).

It often helps to mark the twist grip with white ‘tipex’ or similar to ascertain this because just guessing your throttle position is often very inaccurate.

Understanding where the slide is will indicate which carb circuits are in play, and which to ignore.
Click to expand...
Exhaust popping occurs in lower 1/8 of throttle. Above that, it runs great.
 
Fast Eddie said:
Turn it outwards and see what happens.

Outwards = more air = weaker.

Kommando’s hypothesis is that richness is causing unburnt fuel to enter the exhaust which then combusts with a bang.

My thinking (would not quite call it a hypothesis) is kinda the opposite in that your cold start issues may be related to having no chokes and therefore being too lean when cold. Too lean means unburnt oxygen enters the exhaust which then combusts with a bang.

So, strange as it may seem, problems that are opposites can actually display the same symptoms !

Playing with your air screws (carefully) will at least help you identify which, if any, of these is your issue.
Click to expand...
Yes, I have adjusted the air screws many times. Best adjustment is in 1-1/4 to 1-1/2 turns out.
 
Why not experiment?
Richen the low speed mixture screw of offending cylinder by turning in 1/4 turn to see if the popping goes away?
If that does not work, try a half turn.
Dennis
 
jamesp said:
Yes.
Click to expand...
You mean they ARE Premiers ?

In that case, check the pilot jets.

If they are #17s, change them for #19s.

Some bikes do seem to run ok on #17s, but some do not. I chased poor running when I fitted Premiers to my ‘68 T120 and, long story short, it was the #17s that caused it. I swapped to #19s and goodness was restored !

And you’re sucking in 200cc more than my lil’ T120…
 
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jamesp said:
Yes, I have adjusted the air screws many times. Best adjustment is in 1-1/4 to 1-1/2 turns out.
Click to expand...
That’s where it should be, so that’s good.

But the point is, when adjusting it one way or the other, which way makes the symptoms worse ?

Understanding that will tell you if your issue is leanness or richness or neither.

But… now we know you have Premiers, I wouldn’t do owt else till I’d checked out the pilot jets…
 
You had popping on the old carbs which could have had clogged pilot circuits and or pilot jets. If you now have 17 pilots in new premiers then even though the 1 1/4 to 1 1/2 is in the right ball park the premier pilot circuit runs lean. This may be because of the extra turn the fuel needs to take but the solution is 19 pilot jets or 30 in old money.
 

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