Low RPM backfire from one cylinder

jamesp

jamesp

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I have a frustrating problem that I cannot seem to resolve, hope someone can give me an idea where to look. My 74 Commando is mostly rebuilt, new motor, valve adjustments checked several times, new concentric carbs with chokes removed synchronized with gantry, new Tri-Spark ignition double checked for timing, dual Tri-Spark coils wired together with new CNW wiring harness.

Problem is it backfires on left cylinder when warming up, sometimes it never seems to go away. I have double checked settings, changed plugs, etc. I am now thinking I may have a coil with an internal short (but wouldn’t that affect both cylinders?), or a vacuum leak somewhere along that intake tract. Any suggestions? Thanks.
 
jamesp said:
I have a frustrating problem that I cannot seem to resolve, hope someone can give me an idea where to look. My 74 Commando is mostly rebuilt, new motor, valve adjustments checked several times, new concentric carbs with chokes removed synchronized with gantry, new Tri-Spark ignition double checked for timing, dual Tri-Spark coils wired together with new CNW wiring harness.

Problem is it backfires on left cylinder when warming up, sometimes it never seems to go away. I have double checked settings, changed plugs, etc. I am now thinking I may have a coil with an internal short (but wouldn’t that affect both cylinders?), or a vacuum leak somewhere along that intake tract. Any suggestions? Thanks.
Click to expand...
Are the carbs Premiers ?
 
A friend had the same problem on a Triumph. Turned out a helicoil for a sparkplug had been installed in the head, and it was providing a poor ground until the bike warmed up. At which point the bike ran well.
 
Air issue, check valve clearances, spray manifold joints with WD40.

Fuel issue, does the tickover respond to pilot air adjustment screw, check pilot circuit.

Electrical issue, swap plugs, HT leads and see if backfire moves to other side.
 
It may be not sparking reliably on the LH side. A spark in the open does not prove it's sparking under compression.
I use one of these to test...
robs ss

Post in thread 'Condenser question'

marshg246 said:
To see if the condensers have anything to do with your problem, remove them. Their primary job is to prevent sparking at the points. A little running without them won't hurt. If they fail open, the engine will run fine but the points won't last. If they fail closed there will be no spark. If they fail partly closed the spark will be diminished.
Click to expand...
I disagree - they act as filters, removing the AC current through the points, AC current gives a much weaker spark, with that characteristic yellow/orange colour. Will spark in open atmosphere but not under compression.
I am somewhat...
  • Like
 
You did not mention setting the pilot air screws. With a wasted spark system like a Tr-Spark, if a cylinder is running sufficiently rich you can experience popping in the exhaust because the spark also fires on the exhaust stroke and residual fuel mixture will fire.
 
Backfiring or misfiring? Stock cam or something a little racey?

A hot cam can sound like it is misfiring at idle. Little pops are usually air leaks or the carburetion actually isn't 100% synced or one has a different slightly off setting on the fuel/air screw. Worst case would be a burnt valve, or a valve that was not lapped in enough. Leaking head gasket between cylinders, but that is a reach.

If you think it is the coil(s) Matt of cNw uses a big 3 ohm Harley dual coil in his TriSpark kits, and they work very well. Worth buying one if you want to eliminate the what if it is my coils issue. Don't buy a cheap one though. You might have to make up your own plug wires for a dual coil. Matt uses Taylor made, or did.
 
Jamesp,
it seems you are using Don Pender's gantry setup. Please note the gantry can induce a skewness in the flange mounting of carbs and possible warpage. This has been reported before. It will allow one or both cylinders to be fed with a too lean mixture.
I suggest you remove the gantry, check flanges for flatness, refit both carbs, and test. The carbs themselves may need going over too.

Time Warp

Post in thread 'TW wrecks a perfectly good Commando etc.'

First job today is get the inlet manifolds so the carburetors are at the same height and level at the tops.
The manifolds as stock have the holes and studs skewed to each other which is not a problem with twin throttle cables but can present a problem (unless ignored) with a top gantry plate.

man.jpg man5.jpg

Seemed like a good idea at the time.
Test piece insert @ 11 / 10 OD / 8 ID
The real ones will be Loctited in place and the surface re fly cut lightly.

bs1.jpg bs.jpg bs4.jpg

Interference fits on both OD's so drawn in + Loctite 680.

bs9.jpg bs7.jpg...
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- Knut
 
Float level high on the left cylinder and what Knut said. Sorry that float thing just came to mind.
 
kommando said:
Air issue, check valve clearances, spray manifold joints with WD40.

Fuel issue, does the tickover respond to pilot air adjustment screw, check pilot circuit.

Electrical issue, swap plugs, HT leads and see if backfire moves to other side.
Click to expand...
I have checked valve clearances, not really looked at intake connections, but insulators and o rings are new. Again, it was doing same thing with old carbs.
 
mdt-son said:
Jamesp,
it seems you are using Don Pender's gantry setup. Please note the gantry can induce a skewness in the flange mounting of carbs and possible warpage. This has been reported before. It will allow one or both cylinders to be fed with a too lean mixture.
I suggest you remove the gantry, check flanges for flatness, refit both carbs, and test. The carbs themselves may need going over too.

Time Warp

Post in thread 'TW wrecks a perfectly good Commando etc.'

First job today is get the inlet manifolds so the carburetors are at the same height and level at the tops.
The manifolds as stock have the holes and studs skewed to each other which is not a problem with twin throttle cables but can present a problem (unless ignored) with a top gantry plate.

man.jpg man5.jpg

Seemed like a good idea at the time.
Test piece insert @ 11 / 10 OD / 8 ID
The real ones will be Loctited in place and the surface re fly cut lightly.

bs1.jpg bs.jpg bs4.jpg

Interference fits on both OD's so drawn in + Loctite 680.

bs9.jpg bs7.jpg...
  • Like

- Knut
Click to expand...
Thank you, sounds worth checking out. This has been a consistent problem since I rebuilt the bike. Since restarting it, have gone through two sets of carbs, using same gantry. I know the gantry did distort my original carbs, causing one slide to hang up at times, but I thought it was because of their poor metallurgy.
 
Buy Jim Schmidt's 33mm FCR carburetors and kiss your carburetor problems goodbye. You'd be throwing more money at it though, so may not be a solution you'd be comfortable with. Plus it's probably something other than carburetion. Old standard inexpensive advice: try some new gas and new plugs.
 

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