Help me troubleshoot a no-spark on cyls 1 and 2

[StabMasterArson]

Tossed a timing light on my 1978 CB750K's wires after noticing that pipes 1 and 2 were cool and 3 and 4 were finger burning hot. Light did not illuminate on the two cylinders on the right side of the bike (as seated on it). I did ride the bike since getting it and noted occasional miss fires when cold but they clear up as the engine gets hot. The bike will easily do 35mph but may be down on power. This is my 1st bike so I have no idea how it should "feel". I feel like there is a bit of knock at idle but I do not know what it should sound like.

Bike has the stock airbox on it. Manifold boots are hard and will get replaced eventually. I have not taken the cover off the points yet to inspect but will tomorrow. What else should I test and where?

Any tips on troubleshooting this?

 

[Sderbyshire]

I think that the points/coils do work in pairs of cylinders, but the pairs are 1&4 and 2&3 so this doesnt help with your symptoms.

on a recent 550 rebuild i found carbs 1&2 much more damaged by corrosion inside than 3&4, so perhaps your symptoms point to carbs.

i recommend a systematic approach to checking, look at the plugs, check for sparks, take the float bowls off the carbs etc to narrow down the cause

good luck!

 

[brad black]

are 1 and 2 the low side on the side stand? that'd help cause carb issues when sitting.

or do you have things hooked up 1 -2 and 3-4 that should be 1 - 4 and 2 - 3?

 

[irk miller]

As stated, coils fire 1/3 and 2/4 on these. Your fuel tank feeds directly into the #2 carb and levels to the other three from there. The sync on #2 isn't adjustable, so you adjust 1, 3 and 4 to #2. I would start there, assuming you have your coils hooked up right. You will certainly get noise if it's firing out of order, if it runs at all. The clutch can make noise at really low idle, but even more so the cam chain tensioner, which is a mechanism with a pair of wheels that puts tension on the chain.