Need help diagnosing - Cyls #1 and 2 are getting no spark

StabMasterArson

New Member
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?
 

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teazer

Over 1,000 Posts
DTT BOTM WINNER
Check which coils should be attached to which cylinders first and then check the wiring from points or pickups to the coils. Many 4 cylinder bikes are connected 1-4 and 2-3 or 1-3 2-4 but the FSM will tell you for sure. When you have checked the points and wiring (probably have to take the tank off to see what's up), pop the float bowls off and see how bad it looks in the bottom of the carbs. IIRC, left pair share a fuel feed and so do right pair.


BTW, normal naming convention is from left to right as you sit on the bike, so #1 is far left, and #4 is far right
 

pidjones

Over 1,000 Posts
The airbox is a GOOD thing! Many have chased their tails when going to "pods". Best to get everything else ironed out before pullibg the air box, and make sure you hold on to it in case you need to go back.

Yes, ignition should have plugs 1 & 4 from one coil and 2 & 3 from the other.

Sharp looking bike. There are pdf files of the CB750K factory manual on the web. Check 750 specific forums.
 

CarbsAndCylinders

Careful With That Axe Eugene
From what I just read on the interweb, 1 and 4 are off the same coil and 2 and 3 are off the same coil, so if 1 and 2 are not firing, then I would check to make sure the plug wires did not come off those 2 cylinders and if they are okay I would check the fuel delivery like teazer said.
 

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