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'81 CB750 2 and 3 cylinder not firing

nunrleft

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I just bought an '81 CB750. It's in good condition with a sidecar attached. It was one of those "Ran When Parked" deals. 46,xxx miles on the bike. Price was extremely good. Seller said the carbs were rebuilt professionally over the winter by someone in California who also synched them to their engine before sending them back. Seller said the battery needed to be replaced, gas added and should run fine. Seller also stated the #3 cylinder was a little low on compression and it most likely needs a valve job.

I'm not new to bikes and currently own an '87 Honda Super Magna, '82 Kawasaki KZ440 and a '73 Yamaha DT3 which I restored. I put a new battery in the CB, added gas and of course all the carbs leaked out of the over flows. Tapped on them with a pipe and the floats released and they stopped leaking. Went to fire it up and found that the starter didn't want to engaged. Quickly found a bad switch at the clutch lever.

Got her to fire up and the center two cylinder head pipes are cold. Won't rev past 4,000 and obviously only running on two cylinders. I'm pretty good at diagnosing problem and will of course check spark, compression, etc. However, I'm new to the DOHC CB750. Is there anything known issues or things that commonly happen which would cause the two middle cylinders to fail?
 
These use a waste spark type ignition, the coils are twin tower pairs that has each one firing two cylinders that come up and down together, 1-4 and 2-3 are the pairs. Same with the pulsers under left crank cover, each handles two cylinders same way, the entire ignition is basically two halves. Oddly, one plug in each pair fires BACKWARDS, or, outside wire to the tip rather than the norm other way. Because each plug pair forms one half of a circuit, power flows through one plug forward, into head and out the other plug backwards to complete the secondary path.

You can swap coils, spark units, and such to see if the issue swaps sides and that can tell you the errant part.

FYI, under certain circumstances a badly enough fouled single plug can fault the other in pair to not fire correctly if the circuit is faulted enough by the bad plug. Your bad cylinder compression issue...........one wet plug shorts down porcelain bad enough the other doesn't develop enough amperage to fire through gap. Coil spark only develops high enough to find a way out................

Normal compression around 170 psi..................and, syncing carbs to another engine is nowhere near professional work, each engine has different needs once they get older, it shows up in the sync. A sync makes up for slight valve not sealing issues that can mar off idle running. Meaning tuned for each individual engine or a waste of time. You'd be better off with simply bench sync with no engine at all.
 
FYI, under certain circumstances a badly enough fouled single plug can fault the other in pair to not fire correctly if the circuit is faulted enough by the bad plug. Your bad cylinder compression issue...........one wet plug shorts down porcelain bad enough the other doesn't develop enough amperage to fire through gap. Coil spark only develops high enough to find a way out................

This is great information which I would have never figured out on my own. Thank you.
 
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So do you think this air filter would cause it to starve for fuel on the #2 and #3 cylinders?..lol
 
i just had this problem. my solution was the connector for cdi had corrosion on one of the pins cleaned it up and fixed no fire problem
 
FYI, no CDI on these, a common belief. Actually TPI, or Transistorized Pointless Ignition. Yeah, the Haynes and other manuals are wrong. Normal inductive with electronic switching.

Yep, that fix could easily affect a pair of cylinders. So can pulser airgap under the left side cover.
 
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