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This is a hangover from my rebuild thread.
I overhauled my top end, new rings, valve guide seals, gaskets, etc.
The carbs are completely cleaned, stripped, dipped, etc., new rubber, re-jetted for the pods and pipe. New intake o-rings.
The valves are in spec.
I replaced the points with a Dyna-S ignition.
The bike starts and is rough at idle, with some intermittent backfiring. Plugs are dark and sooty after idling so its a bit rich, but that could be from idling with choke. I haven't run the engine for more than a few minutes at a time, because I can't get it to idle well enough to dial in the air screws, vacuum synch, etc.
Why? Because it's running on only 3 cylinders. Header pipes for 1,2 and 4 are nice and hot. No. 3 pipe is cold, so it is not firing.
Cold compression tests show 120/120/95/120 - so No. 3 is a bit low, but has enough compression to fire. Indeed, initial tests after the build were 90/90/75/90.
No. 2 is firing so that should rule out coils/ignition.
I have tested the No. 3 plug by grounding it at the engine block, and it sparks.
I have swapped the plug connections for 2/3 and the problem did not migrate. Swapped the plugs themselves, still no change.
There is petrol in the float bowl, so it has fuel.
Why the f**k won't this thing fire?
I have one theory, but folks in the rebuild thread were not buying it. To be sure, they know better than I do. Nevertheless, here is the best that I can fathom.
When I rebuilt the top end, I had to fix a botched plug hole insert for No. 3. Some PO had done it before and the insert fused to the plug instead of the head, preventing me from replacing the plug. I used a new kit to install an insert. The instructions said to use loctite to secure the insert. My wacky theory is that I used too much of the stuff and managed to insulate the insert from the head. That would explain the fact that the plug sparks against the engine block for testing, but does not seem to spark in the cylinder.
I have a second cylinder head that I can swap in for this one, and a new head gasket in en route. Before I tear back into that, I wondered if the sparkies in this forum had other ideas on the mystery.
Is my insulation theory completely nuts? If it is, then swapping heads will not fix the problem.
Is there anything else to explain this non-firing cylinder mystery?
TIA
I overhauled my top end, new rings, valve guide seals, gaskets, etc.
The carbs are completely cleaned, stripped, dipped, etc., new rubber, re-jetted for the pods and pipe. New intake o-rings.
The valves are in spec.
I replaced the points with a Dyna-S ignition.
The bike starts and is rough at idle, with some intermittent backfiring. Plugs are dark and sooty after idling so its a bit rich, but that could be from idling with choke. I haven't run the engine for more than a few minutes at a time, because I can't get it to idle well enough to dial in the air screws, vacuum synch, etc.
Why? Because it's running on only 3 cylinders. Header pipes for 1,2 and 4 are nice and hot. No. 3 pipe is cold, so it is not firing.
Cold compression tests show 120/120/95/120 - so No. 3 is a bit low, but has enough compression to fire. Indeed, initial tests after the build were 90/90/75/90.
No. 2 is firing so that should rule out coils/ignition.
I have tested the No. 3 plug by grounding it at the engine block, and it sparks.
I have swapped the plug connections for 2/3 and the problem did not migrate. Swapped the plugs themselves, still no change.
There is petrol in the float bowl, so it has fuel.
Why the f**k won't this thing fire?
I have one theory, but folks in the rebuild thread were not buying it. To be sure, they know better than I do. Nevertheless, here is the best that I can fathom.
When I rebuilt the top end, I had to fix a botched plug hole insert for No. 3. Some PO had done it before and the insert fused to the plug instead of the head, preventing me from replacing the plug. I used a new kit to install an insert. The instructions said to use loctite to secure the insert. My wacky theory is that I used too much of the stuff and managed to insulate the insert from the head. That would explain the fact that the plug sparks against the engine block for testing, but does not seem to spark in the cylinder.
I have a second cylinder head that I can swap in for this one, and a new head gasket in en route. Before I tear back into that, I wondered if the sparkies in this forum had other ideas on the mystery.
Is my insulation theory completely nuts? If it is, then swapping heads will not fix the problem.
Is there anything else to explain this non-firing cylinder mystery?
TIA
Sorry, missed that line. 
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