Left cylinder problem

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Stuffa

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I’ve had an ongoing issue with the left cylinder backfiring and popping back thru air filter. Had this problem on and off since I’ve had the bike. I haven’t started for about four months due to neck surgery. The last time I rode before the surgery it was hard to start and was running rough on left cylinder but when the motor warmed up it ran fine. Thinking maybe a problem with a valve. Thoughts. Tri-spark ignition
 
What carbs? How old? What settings?

Sounds like a weak pilot jet to me. Could be wrong size (if new) or partially blocked (if old).

You failure mode doesn’t sound like those who have suffered with a Tri Spark failures.
 
Fast Eddie said:
What carbs? How old? What settings?

Sounds like a weak pilot jet to me. Could be wrong size (if new) or partially blocked (if old).

You failure mode doesn’t sound like those who have suffered with a Tri Spark failures.
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Thanks for the reply I’ve cleaned all the carbs out and put a sealer in the tank so I don’t think it is a pitot jet, but won’t rule anything out. It’s not a tri-spark issue just put that in so people don’t say it’s a points problem. Maybe I’ll check out for an air leak. It’s been a bastard I’ve had neck surgery and haven’t been able to ride for so long and with the virus and everything I said f*** it I’m going for a ride and both my bikes said NO
 
How did you clean the carbs out?

I’m not being a dick here, it’s just that lots of folk squirt carb cleaner through and declare it ‘cleaned’.

But all that does is prove it’s not completely blocked. There could EASILY still be a partial blockage, enough to cause significant running issues.
 
Stuffa said:
Thanks for the reply I’ve cleaned all the carbs out and put a sealer in the tank so I don’t think it is a pitot jet, but won’t rule anything out. It’s not a tri-spark issue just put that in so people don’t say it’s a points problem. Maybe I’ll check out for an air leak. It’s been a bastard I’ve had neck surgery and haven’t been able to ride for so long and with the virus and everything I said f*** it I’m going for a ride and both my bikes said NO
Click to expand...
 
Fast Eddie said:
How did you clean the carbs out?

I’m not being a dick here, it’s just that lots of folk squirt carb cleaner through and declare it ‘cleaned’.

But all that does is prove it’s not completely blocked. There could EASILY still be a partial blockage, enough to cause significant running issues.
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Mate cleaned out with fine wire and compressed air. I would never call you a dick but this is baffling. As I said before It was doing this when it was warmed up it ran perfect
 
Good idea to supply us with the answers to Fast Eddies questions; old Amal concentric carbs weren't very good to start with and with age go down hill rapidly. A search of the forum will turn up all their faults, real and imagined.

If your carbs are synced and the air screws are within 1/2 turn of each other, but off the seats by at least one turn and not out beyond 3 turns, then time to adventure elsewhere. I suggest starting with checking head fastener torque, then valve lash, then one form or another of a compression test. Please report your findings as it will help a lot in assisting you and it might keep a few members that like to relate almost any operational malady to tight primary chains at bay...

Best.
 
How do plugs look? Any light or dark smoke out the tail pipe on that side when it is misbehaving? Have you tried swapping the plug ends of the spark cables left for right to see if issue moves to RH side?
 
RoadScholar said:
Good idea to supply us with the answers to Fast Eddies questions; old Amal concentric carbs weren't very good to start with and with age go down hill rapidly. A search of the forum will turn up all their faults, real and imagined.

If your carbs are synced and the air screws are within 1/2 turn of each other, but off the seats by at least one turn and not out beyond 3 turns, then time to adventure elsewhere. I suggest starting with checking head fastener torque, then valve lash, then one form or another of a compression test. Please report your findings as it will help a lot in assisting you and it might keep a few members that like to relate almost any operational malady to tight primary chains at bay...

Best.
Click to expand...
I pulled the carbs of and cleaned again and made sure they were balanced, changed the spark plugs and started the bike without the air filter on. If I put my hand over the the right carbs intake there is a lot of suction there and that cylinder runs better if I put my hand over the left intake there is no where near as much suction and that cylinder starts to cut out
 
Intake valve clearances OK?
Leakdown test OK?

If both are OK then time to look at the cam lobes.
 
maylar said:
Intake valve clearances OK?
Leakdown test OK?

If both are OK then time to look at the cam lobes.
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Mate I have checked valve clearances but have no tools to do a leak down test. What do I need to do that. I can do most things on a bike but have always shied away from actually pulling the motor down, maybe I need to start learning as there is no one close who can help
 
A leakdown tester is a special tool available from any auto parts store. It requires an air compressor to pressurize the cylinder and if a valve is leaking you'd hear the air hissing through the carb or exhaust. Your last test seems to indicate low vacuum on one cylinder which could be a leaky intake valve.

Cam lobes wear too... this was what mine looked like a few years back -

Left cylinder problem
 
Pulling the cam is kind of a big deal as you have to split the cases.
However, the cam can be inspected by pulling the head and barrells.
This can be done in the bike.

I'd at least do a compression check first.
Leak down is better, if available.
 
A simple compression test with a readily available gauge will be diagnostic enough if the results show a difference that exceeds 10%.

Stuffa said:
I pulled the carbs of and cleaned again and made sure they were balanced, changed the spark plugs and started the bike without the air filter on. If I put my hand over the the right carbs intake there is a lot of suction there and that cylinder runs better if I put my hand over the left intake there is no where near as much suction and that cylinder starts to cut out
Click to expand...

If the carbs are out of sync the results you note would be the same. Simple sync: back the slide height (speed screws) adjusters out such that both slides are sitting on the bottom of the carb body bores, insure that each cable has a tad of slack. Operate the twist grip with high frequency and low amplitude, just hitting the end of the free play is fine, adjust the carb cables such that both slides are getting an impulse at the same time. When you arrive at this point, and feel confident, then use a single 1/4" drill bit as a feeler gauge and raise both slides to give the same feel when the drill bit is placed between the apex of the slides cutaway and the center of the carb's bore. Set the air screws at 1. 1/2 turns out from seat and start the engine. When warmed the idle will be high; uniformly lower the slides to achieve an idle RPM between 900 and 1100. If you need to adjust the air screws (likely) you will need to readjust the slide height, the goal is to have a smooth take-off from rest and a smooth transition to the needle. Once this has been established try your home brew vacuum test again. If you end up with the same results read on.

Cam ware can be easily checked by:
1) Seeing if all lash adjusters are, roughly, showing the same number of threads above the jam nuts.
2) Measuring (observing) the maximum distance the valves are opened; this doesn't have to be done to aero-space specs.

If your cam is worn to a performance degrading level the suggestions above will tell the tale.

If no cam ware is discernible, compression test is good, carbs synced, head torque is verified, ignition timing is correct, then I wager a guess that the Amals, if more than 5 years old, are ready to be put out to pasture. You can clean Amals until they are ready to be surgically implanted in living tissue, but their design and metallurgy are so poor that they are way below the term "planned obsolescence". I'm NOT saying that new Amals don't work well, they just don't last, the latest iteration, the Premiers, would seem to have addressed the prior generation of Concentric's shot comings...

Best.
 
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