Low RPM backfire from one cylinder

Fast Eddie said:
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…
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Should be #19 - I haven't sold any with #17 for a long time. I order those for 750s them with #19 installed and if they happen to come with #17, I include a set of #19. Those for 850s come from AMAL with #19. I looked it up - he bought AMAL Part PACK 138 so MK2 850 which came with #19.

I would most definitely take them out, be sure they are open, and that they are the same (3 lines). I don't remember if I was asked to go through them - I do that on request only.
 
jamesp said:
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.
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Focus! Below 1/8 throttle is pilot circuit,. Very unlikely to be anything else assuming the floats are right and the carbs are synced. BTW, the cable has nothing to do with sync when the gantry is involved.

You must consider AMAL carbs to be 4 in 1.

0-1/8 throttle: Pilot Circuit
1/8-1/4 Slide Cutaway
1/4-3/4 Needle and Needle Jet but 106 is the correct needle jet for almost all bikes, so needle. 850 Commandos have a special needle
3/4-WOT Main Jet.
 
I have similar popping out left side exhaust, persists when fully warm. Typically occurs when throttling back slowly from 1/2 or more open and holding at near 1/8th area...pops nearly always around 3k rpm when slowing down to there.

Preimers, go through recently. New plugs, swapped coils for another set. Wassel EI. Tried sealing up exhaust rose joint and silencer joint with some high temp rtv. Checked intake balance tubing.

Will take advice from further up this post and try to lean idle down some more.

Curious these reports usually indicate left side issues.
 
Does that mean air leak between the head and end of the silencer? Or does it include the exhaust valves and chamber in the head?
 
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.
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does the airscrew adjuster have any effect when you turn it in or out? If not there is contaminates in carb, if it does, then it most likely is a manifold leak, check the 4 allen bolts that hold manifolds to cylinder head, I recently discovered one of mine broken, causing a air leak. bolt head was in place, but it spun freely.
 
Thank you to all who gave me good things to think about. Armed with these suggestions I went back after it again today. I pulled the carbs and gantry, checked floats, reset one, and checked surfaces with a flat glass for warpage. Did a little flat filing just to make sure there was no warpage. These are newer carbs bought from Greg Marsh. Reassembled with old throttle cables. Fired it up, still did some popping and rough running, but didn’t seem as bad. Understandable as idle needed to be set. I let it get warm while adjusting carburetors. When it got good and warm, it ran fine. When warm, started and idled with one bump of the starter. Will fire it up tomorrow and see how it does cold. If I need to, I will put choke’s back in, but didn’t really think I needed them in this climate. I can’t say that the gantry had anything to do with it, but can say that throttle operation and synching carbs was much easier with it. Thanks again for all the help, it was more than I expected and I am grateful for it.
 
jamesp said:
Thank you to all who gave me good things to think about. Armed with these suggestions I went back after it again today. I pulled the carbs and gantry, checked floats, reset one, and checked surfaces with a flat glass for warpage. Did a little flat filing just to make sure there was no warpage. These are newer carbs bought from Greg Marsh. Reassembled with old throttle cables. Fired it up, still did some popping and rough running, but didn’t seem as bad. Understandable as idle needed to be set. I let it get warm while adjusting carburetors. When it got good and warm, it ran fine. When warm, started and idled with one bump of the starter. Will fire it up tomorrow and see how it does cold. If I need to, I will put choke’s back in, but didn’t really think I needed them in this climate. I can’t say that the gantry had anything to do with it, but can say that throttle operation and synching carbs was much easier with it. Thanks again for all the help, it was more than I expected and I am grateful for it.
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You did all that and didn’t check the pilot jets …?
 
jamesp said:
Thank you to all who gave me good things to think about. Armed with these suggestions I went back after it again today. I pulled the carbs and gantry, checked floats, reset one, and checked surfaces with a flat glass for warpage. Did a little flat filing just to make sure there was no warpage. These are newer carbs bought from Greg Marsh. Reassembled with old throttle cables. Fired it up, still did some popping and rough running, but didn’t seem as bad. Understandable as idle needed to be set. I let it get warm while adjusting carburetors. When it got good and warm, it ran fine. When warm, started and idled with one bump of the starter. Will fire it up tomorrow and see how it does cold. If I need to, I will put choke’s back in, but didn’t really think I needed them in this climate. I can’t say that the gantry had anything to do with it, but can say that throttle operation and synching carbs was much easier with it. Thanks again for all the help, it was more than I expected and I am grateful for it.
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James,
You don’t need the chokes.
Mike
 
YING said:
James,
You don’t need the chokes.
Mike
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Yes, that’s what I thought “l don’t need the chokes”. I have a 65 GTO that I restomodded. It runs a Holley HP 850 cfm carburetor with no choke. HP carbs have machined throttle bodies that take out the choke tower for better breathing. It also has a fairly radical cam, but it starts right up with two pumps and settles into a nice idle after about a minute, no popping, etc. So, when my Norton is sounding like it’s going to cough up a piston or valve, I got concerned. Also, it never behaved like that before the accident that caused me to completely redo it.
 
Triton Thrasher said:
The silencers on my bike are straight-through and the pipes may be wider than ideal. No air slides.

The exhausts bang on overrun until the engine warms up.
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I would expect that. Until the head and intake track, and piston top warms up, the raw gas is not vaporized and does not burn fully, so it goes out into the exhaust.
 
There is always unburnt fuel in the exhaust gases.

In normal running, there is not enough oxygen in exhaust gases to support combustion in the exhaust.

When the engine is cold, less of the fuel evaporates and combusts in the cylinder, so the mixture is effectively weak. That means less oxygen is used in the cylinder, leaving enough oxygen in the exhaust gases to combust and go bang.
 
To the OP: Have you checked the pick up air gap on your tri-spark? It could be borderline intermittent.
  • Fixed gap for standard kits: Tri-Spark states that the air gap is critical and that a gap that is too great can cause the bike to struggle at higher revolutions. For their standard twin and triple-cylinder kits, the 2mm gap is a required tolerance."""""
 
Steve W. said:
To the OP: Have you checked the pick up air gap on your tri-spark? It could be borderline intermittent.
  • Fixed gap for standard kits: Tri-Spark states that the air gap is critical and that a gap that is too great can cause the bike to struggle at higher revolutions. For their standard twin and triple-cylinder kits, the 2mm gap is a required tolerance."""""
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Thank you for your suggestion. I’m not sure what you mean by “pick up air gap”. I think I have solved the problem by removing the gantry and carbs, then going through the carb settings and checking the flanges. On cold start yesterday, it came up to idle much quicker and quieter. Sure it had some cold start spitting which I think is normal for a bike with no chokes, but none of the shot gun blasts it previously had. If you can let me know more about the air gap, I will check it out. Was that mentioned in the Tri-Spark instructions?
 
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motoalchemist said:
does the airscrew adjuster have any effect when you turn it in or out? If not there is contaminates in carb, if it does, then it most likely is a manifold leak, check the 4 allen bolts that hold manifolds to cylinder head, I recently discovered one of mine broken, causing a air leak. bolt head was in place, but it spun freely.
Click to expand...
Yes, the air adjusters are working. I was able to adjust it to a nice idle when warm.
 
jamesp said:
Thank you for your suggestion. I’m not sure what you mean by “pick up air gap”. I think I have solved the problem by removing the gantry and carbs, then going through the carb settings and checking the flanges. On cold start yesterday, it came up to idle much quicker and quieter. Sure it had some cold start spitting which I think is normal for a bike with no chokes, but none of the shot gun blasts it previously had. If you can let me know more about the air gap, I will check it out. Was that mentioned in the Tri-Spark instructions?
Click to expand...

See step 5 for checking the air gap.
 
Fast Eddie said:
Kinda my point in post #21..
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I understand your point, but there was way too much drama. Not sure what I did to make it better, but it’s much better now. Looking back, the tickler on the left carb would have fuel running out of it when used that was sometimes hard to stop. After adjusting the float a bit lower, that is not happening now that I have seen.
 

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