1973 Commando 850 troubles

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pierodn

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Hi.
The bike of friend of mine, all original (only the bellmouths instead of air filter), has some troubles.
Bike starts only with air choke lever open and engine switchs off if close it.
After a while of starting the left pipe becomes incandescent.
The 932 Amal carbs were overhaulled and well cleaned, all the air and fuel lines are opened, throttle needle central groove, 260 main jet (same problems with 230) and 106 needle jet, float bowls 2 mm under the top line.
Valves were setting like x manual and engine has a good compression.
An old Boyer Brandsen is on with 6 V coils: sparks arrives from both leads.
I can think that the problem comes from I.E. that must have a failure.
To my hear engine works only one cilinder.
What check?.
Thank you.
Piero
 
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Does it kickback when starting?

Sounds like the boyer is advanced due to low voltage. Replace the battery.
 
norton73 said:
Does it kickback when starting?

Sounds like the boyer is advanced due to low voltage. Replace the battery.
Click to expand...
First starts had a not strongly kickback but then was retarded 5 degree so now starts without .
 
Are you absolutely certain the pilot jet bush is clear?
The Boyer should be firing both plugs , so it should be on both cylinders
Go back to basics and check everything again
If it is only running on one cylinder what does the plug look like on the side that won't run?
 
pierodn said:
Hi.

Bike starts only with air choke lever open and engine switches off if close it.
After a while of starting the left pipe becomes incandescent.
Click to expand...

If you mean the choke needs to be down (on) then it's running very lean. That would also make the exhaust hot. A red pipe is not a good sign.
 
baz said:
Are you absolutely certain the pilot jet bush is clear?
The Boyer should be firing both plugs , so it should be on both cylinders
Go back to basics and check everything again
If it is only running on one cylinder what does the plug look like on the side that won't run?
Click to expand...

Pilot jet is clear, the spay carb cleaner from the pilot jet goes out trough all the holes.
The plug of non running side sparks.
 
maylar said:
If you mean the choke needs to be down (on) then it's running very lean. That would also make the exhaust hot. A red pipe is not a good sign.
Click to expand...
Yes, i know, it means engine is running very lean, but all the right stuff is in place.
 
Pipes have balanced tube and so seems hot gas goes out both mufflers but from one side is cold.
 
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Piero, you said that the left exhaust pipe gets very hot ? if so, I would take another close look at the Idle jet.
Did you check the ignition timing of both cylinders with a stroboscope ?
 
Peter R said:
Piero, you said that the left exhaust pipe gets very hot ? if so, I would take another close look at the Idle jet.
Did you check the ignition timing of both cylinders with a stroboscope ?
Click to expand...

idle jets are setted 1,5 turns back from tight for first start.
no stroboscope was already used because the bike doesnt remain on fire
 
A warped float bowl due to overtightening can make a leak in the passage to the idle circuit. Make sure the float bowl is flat and the gasket seals around the idle passage.

It sounds like a lean problem to me also.
 
Manifolds should not be fitted with glue.
Try substituting another pair of carbs ?
Clean out the pilot circuit on the lean side again ...
 
pierodn said:
Pipes have balanced tube and so seems hot gas goes out both mufflers but from one side is cold.
Click to expand...

Here's my guess,... ONE spark plug is bad, so that cylinder is just pumping fuel through the head and it's being ignited in the exhaust pipe by the crossover pipe's hot gas from the side that is firing, and giving the cherry red pipe... The cause could be a bad coil on the "cold" side too.
 
Excessively hot exhaust header pipe means fuel is burning in the exhaust port and header pipe. Cause can be as oOnortonOo states above, exhaust valve failure, or very late timing. In this case, since on one side only, rule out late timing, unless the Boyer Brandson E.I. can somehow be way late on one side only.

A burned exhaust valve is easily ruled out if compression check is good.

Slick
 
switch the left spark plug lead for the right spark plug lead
does the exact too hot side problem remain?
If one side is cool as in not firing then it could be as simple as a fractured plug lead
or insecure contact within the spark plug cap
 
OP has said both plugs are sparking. Though it is possible for a plug to fire at ambient and not under compression.
 
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