1973 Commando 850 troubles

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I would put the piston at tdc, blow pressurized air into the spark plug hole and listen at the exhaust pipe for leaking. A leaky (burnt) exhaust valve will cause the cherry-red pipe glow.
 
Is this a new problem that just arose or are you trying to get a bike that has been sitting to run ? Check for vacuum leaks (air leaks) around the carbs. you mentioned the needle clips were at the middle position, my 73 850 I have the clips on the top position, it's possible that you may be getting too much fuel and excess is burning in the exhaust pipes.
 
I really don't think the Ignition is the issue as both plugs fire from the same pulse (coils wired in series from a single input), and the other cylinder runs fine?
One coil could be defective, so swap over and see?
Agree with the lean running but I noticed that you say both pilot screws are set 1.5 turns out?
They should be set to whatever gives the best low-rev performance. My 850s seemed best at one turn out only, but I ran filters.
How sure are you that the pilot circuits are clear, and how did you check?
Is one of the bellmouths blanking the pilot air port?

I still can't get past a plugged pilot circuit.... they block up for fun ;)
However, a poorly seating exhaust valve could be well worth investigating.
 
maylar said:
OP has said both plugs are sparking. Though it is possible for a plug to fire at ambient and not under compression.
Click to expand...

I had that situation happen once and it tortured the life out of me. I used a spring clamp to pinch the spark plug to the fins of the head so I could test them. Since they both fired every time, I assumed the plugs were both good. One of them must of had a cracked insulator up inside the electrode so once under pressure it didn't spark at the tip. Since it tested good when held against the head, I was adamant that the plugs were good since I tested them.

IF my friend hadn't seen me pulling my hair out and offered to pick me up lunch and a new set of plugs, I don't know how long it would have taken me to find out that one of the plugs that tested good, was actually not working in the head under compression... When I got the new plugs (and lunch) I think the bike started on the second or third kick... and ran well enough to know that one of the plugs was the problem all along...
 
Sounds like a carb problem. When running, press down the tickler on the bad cylinder. See if it fires.
 
If your problem is ignition related, the ignition pick-up sensor and electronic unit can be ruled out since one cylinder seem to run normal.
It could be a bad ignition coil, did you already swap the ignition coils and leads from left to right and vice versa ?
And as texas slick suggested, a compression check would help here also.

However, I personally still have the impression that the fault is carburator related.
 
Peter R said:
If your problem is ignition related, the ignition pick-up sensor and electronic unit can be ruled out since one cylinder seem to run normal.
It could be a bad ignition coil, did you already swap the ignition coils and leads from left to right and vice versa ?
And as texas slick suggested, a compression check would help here also.

However, I personally still have the impression that the fault is carburator related.
Click to expand...
Hi.
Sometime the IE rotor fault because one magnet looses power.
In any case is right to change the carbs of the cylinder not starting.
My friend will do, rotor and carbs.
I tcan believe it could be the rotor fault.
Piero
 
If the ignition timing is significantly retarded the exhaust pipes will glow red but I would expect both pipes to be the same. I would check the timing with a strobe light on the bad cylinder for peace of mind.

Ian
 
Changed carbs with other from well running bike.
The right cylinder doesnt work the same.
Changed coils, spark plugs, I. E.
It doesnt work.
Ciao
Piero
 
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Time for a compression or leak down test.
Would also have a look through exhaust rocker openings while turning engine over slowly to confirm pushrods are correctly seats to rockers and that they are moving fully (indicating cam followers arestill ok).
 
Nortoniggy said:
If the ignition timing is significantly retarded the exhaust pipes will glow red but I would expect both pipes to be the same. I would check the timing with a strobe light on the bad cylinder for peace of mind.

Ian
Click to expand...
Hi.
What means when the pipes become blue?.
Thanks.
Piero
 
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Certainly a glowing exhaust pipe indicates something is wrong with the fuel/air mixture. However, I disagree with most of that article. Several points:

1. Polishing blue off the pipes just makes blue appear more quickly the next time.
2. Blueing (or not) is totally dependent on the pipe itself and saying that it's a sign of a mixture problem is not accurate. YES, it MIGHT be the sign of an improper mixture but more commonly, it's completely normal. The design of the pipe and the thickness of the chrome plating affects whether it will blue or not under normal conditions.
3. To prevent the discoloration, many motorcycle manufacturers use double-wall pipes so the outer (chromed) surface is not subjected to as much heat. Honda has been doing that on (some/all?) street bikes at least since the late '60's.

Though not related, I like the blue on the pipes; to me it looks "right," especially on old BrittBIkes. Heck, I wish the pipes would 'blue' on my '19 Kawasaki Z900RS Cafe but, since the pipes are double-wall, they won't.
 
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