Flames from carb on throttle

[Fingerscrossed]

Hi, so I’ve recently gone through my ‘75 as it was running on one cylinder, replacing the coil fixed that. I also put on a new (old) set of BS38s that I cleaned, just with carb cleaner and an air compressor.

So it starts first kick, but upon throttle there are flames coming out of the left carb, that I can see through the air filter… it is consistent upon throttle.

The mix screws are set to 2 1/4 turns out, they are 77-78 carbs so are to spec.

I went through the point gap, and valve clearances but haven’t touched timing. Could it be I’ve set one sides valve clearances wrong? I did notice the left side (flame side) exhaust was getting hotter quicker than the right side.

I couldn’t find any threads on actual flames coming out from the carb, it’s on every throttle at idle which is a bit concerning. It’s not backfiring, there is no sound, and any help would be appreciated!

 

[40north]

Hi, so I’ve recently gone through my ‘75 as it was running on one cylinder, replacing the coil fixed that. I also put on a new (old) set of BS38s that I cleaned, just with carb cleaner and an air compressor.

So it starts first kick, but upon throttle there are flames coming out of the left carb, that I can see through the air filter… it is consistent upon throttle.

The mix screws are set to 2 1/4 turns out, they are 77-78 carbs so are to spec.

I went through the point gap, and valve clearances but haven’t touched timing. Could it be I’ve set one sides valve clearances wrong? I did notice the left side (flame side) exhaust was getting hotter quicker than the right side.

I couldn’t find any threads on actual flames coming out from the carb, it’s on every throttle at idle which is a bit concerning. It’s not backfiring, there is no sound, and any help would be appreciated!

Others have competence, me not so much specific to these bikes. However...if y'all have flames and sparks and gasoline it's probably a good idea to have a way to put out a fire...

Advice> assume nothing, create truth table. work through every aspect taking notes.

 

[Bruno_c]

Others have competence, me not so much specific to these bikes. However...if y'all have flames and sparks and gasoline it's probably a good idea to have a way to put out a fire...

Advice> assume nothing, create truth table. work through every aspect taking notes.

 

[Bruno_c]

I suggest you check the left inlet valve clearance.

 

[Jan_P]

I suggest you check the left inlet valve clearance.

Yes Bruno You are Right .. also check Ignition timing Both cylinders Strobe light
Fire extinguisher Rags Water or so at hand. Little fuel only in there Preferably outdoors so the garage still stands tonight

 

[Melnic]

FYI, a timing light is invaluable with a points vehicle.
If you don't have a timing light, you can ball park where you are at using a light or a meter.

 

[mike in idaho]

Check the compression in that cylinder, to verify that the intake valve is closing ok.

 

[gggGary]

Way down the list;
but check that the exhaust pipe is clear, the inner wall on a stock pipe has been known to collapse greatly restricting exhaust flow.

 

[Fingerscrossed]

Ok thanks all, I’ll go through the timing, valves and do a compression test today. I have a light just haven’t used it yet. And I have the fire extinguisher ready, and will work on the bike outside.

 

[Melnic]

If the timing looks good at idle, look for it to advance as you slowly move the throttle up.

 

[Fingerscrossed]

The timing was way off valves were spot on although when doing the covers back up one of the bolts stripped, so got an oil leak…

It has happened a few times on this bike, is that pretty common? The oil drain plug needed a pretty big breaker bar to get undone so it could be that the PO was just very extreme…

Also there is still some popping out of one cylinder, mostly on decel. The boots seem good, I have two pairs so could try swapping but I haven’t replaced the butterfly seals, is that recommended? Only thing putting me off is potentially rounding the heads as I’ve heard it’s tricky

 

[bosco659]

The timing was way off valves were spot on although when doing the covers back up one of the bolts stripped, so got an oil leak…

It has happened a few times on this bike, is that pretty common? The oil drain plug needed a pretty big breaker bar to get undone so it could be that the PO was just very extreme…

Also there is still some popping out of one cylinder, mostly on decel. The boots seem good, I have two pairs so could try swapping but I haven’t replaced the butterfly seals, is that recommended? Only thing putting me off is potentially rounding the heads as I’ve heard it’s tricky

For removing butterfly plates, using a Dremel tool, carefully grind the backs of the screws off where they have been peened over. Use a good JIC screwdriver to remove the screws, applying good downward pressure, but not enough to bend the throttle shafts. Some use a piece of wood to support the back of the throttle shaft to support it while removing the screws. This requires slide removal. I’m sure other will chime in on this subject. When I remove throttle shaft screws, I apply slight forward and backward twisting motion of the screwdriver until the screws break free.

 

[Fingerscrossed]

Thank you!! That worked great, now after running it I've found more problems for another thread...