Missfire and difficult running

I recently bought a 73 850 and it had a lot of new parts very inexpertly put together. After remedying all the faults it ran very well. I have sold the bike, to be picked up next week. Suddenly it has started being very rough running and miss firing. It has a new Trispark and new mk1 amal concentrics.
They arnt premiers but have adjustable floats and removable idle plug instead of blanking plugs. It is timed correctly and there is approx 125-135 psi each side. Battery is fully charged. It is missfiring and blowing smoke on right hand cylinder and swapping ht to other side made no difference so I dont think its ignition. I have tried another set of carbs and similar symptoms. It is not impossible that it is the same fault on both carbs, but I am thinking it probably not. I am looking for advice before any major surgery...

Always try the simple things first.

New fuel (if it’s been stood)?

and....

NEW. SPARK. PLUGS.

Sometimes an engine that’s been stood a while simply needs to be run for a while.

Fast Eddie said:

Always try the simple things first.

New fuel (if it’s been stood)?

and....

NEW. SPARK. PLUGS.

Sometimes an engine that’s been stood a while simply needs to be run for a while.

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done both. was running for maybe 80 miles fine

What colour smoke, what does the R/H plug look like?

johnny Lagdon said:

done both. was running for maybe 80 miles fine

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So are you saying it started up fine, ran fine for 80 miles, and THEN generated this symptom?

You’ve not given us a whole lot to work with here sir, but possibilities I can think of are:

1. Some dirt from the tank has blocked a jet.
Would explain the seemingly random nature. But I don’t know if you have good in line filters.

2. You‘ve got a duff plug, I know they’re new, but it happens.
New plugs can be duff, if only 80 miles old its a possibility that’s easy to test.

3. Something has come adrift in a carb, jet loose, needle circlip screwing up the spring, etc.
Again, would explain the random nature. More likely if you‘ve recently been inside ‘em.

4. Coil breaking down.
You swapped leads over but we don’t know what coils you have, if a twin output, this may apply.

5. Intake manifold air leak.
Unlikely unless recently fitted. But easy to test for.

6. Bad earth, wire breaking inside the insulation or eyelet.
Common on these rubber mounted engines that A) won’t earth naturally and B) give earth wires a hard time.

7. Head gasket gone.
Could be blown twixt pushrod gallery or oil drain hole and combustion chamber, but I only way this as you say it’s blowing smoke, but how much, when, under what circumstances, etc ?

You might as well written "It could be anything" and you would be correct.
I'm doing about the same now. One thing at a time. Vexing to say the least.

When presented with a long list of possibilities I work on them in order of cost, start with the cheap fixes.

kommando said:

When presented with a long list of possibilities I work on them in order of cost, start with the cheap fixes.

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Standard procedure north of Hadrian's Wall But an excellent premise, none the less.

Most likely first. Clean the screen on taps and carbs and capture the the petrol from the bowels and check for water there.

johnny Lagdon said:

. It is missfiring and blowing smoke on right hand cylinder and swapping ht to other side made no difference so I don't think its ignition.

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There's your clue right there, so follow the clue. Is it burning oil in the cylinder? look at the spark plug on that side. What color is the smoke? Black smoke is excessive fuel, whitish smoke is burning oil. Once you've determined your exact symptom, eliminate the causes that don't cause that symptom and there will only be a few possibilities left.

In the vein of keeping it simple: The fact that it ran 80 miles without fault leads me to suggest you carry a fresh set of pre-gaped plugs and install them when the symptoms become obvious.
Next. If your motorcycle starts misbehaving (again) open the fuel cap. Do this with great caution and on a smooth road, Before you take this additional risk make sure that both taps are on.

I suggest that you file a flight plan, carry a cell phone, your AAA or AMA card and your medical insurance card, and/or have someone follow you.

When I begin a run-in or trouble shooting routine I travel in ever widening circles, never being too far from my shop. My first set of "laps" is right in the industrial park my shop is based in; it has a small hill that provides enough dead-stick speed for me to get back without pushing.

Lastly. With all adjustments and torques set, new fluids and tire pressures set, get that hunk of British iron up to temp and start using the upper end of the tach with varying loads; not like "ride it like you stole it", but keep adding RPM and load and let the motorcycle speak to you, it does speak English; reminds me of my dog when he was a puppy, he knew his name, but when he needed to make his bladder gladder he spoke perfect English...

Best.