Topic: once bike is hot one cylinder stops firing

[trevordonovan]

i have a 1978 cb750f  runs perfect when first started after you let it idle or ride for 5-10 min one of the cylinders stops firing. or possibly 2 of them stop  leaving the bike only running on2/3 cylinders. once its hot it will cut from from running perfect to only a couple cylinders firing and then back to running perfect again for a few seconds and so on .   i just put new ignition coils, spark plug cables, and plugs, also did a compression check when motor was cold and hot, all cylinders had 125 psi of pressure cold and hot . i also have a aftermarket electronic ignition (dyna) on the bike.  my thoughts are maybe its something to do with fuel or possible electronic  . any ideas? thanks in advance trevor

[dazemc]

Once bike is up to operating temp, check rubber boots for airleaks. Check float bowls levels. Make sure there is proper ventilation for your gas tank. In-line fuel filters? 5.5mm fuel hose routed correctly? Otherwise, it sounds like an electrical component is overheating. I know that dynas are sensitive to over/under voltage so check what your output is at different revs. Check grounds, especially the main one from negative battery terminal. Don't assume the coils are good just because they are new either. Also check spark plugs to see if they are fouling up.

[Bankerdanny]

If it is Dyna related (and I had a different EI brands faul like this) Try taking off the points cover to expose the Dyna and riding it that way. With the unit that failed on mine, exposing it to air kept it cool enough so that it didn't overheat.

[BenelliSEI]

If you are running NGK plugs, try a new set. I have had new ones show exactly the symptoms you describe! Their quality is not what it used to be.....

[scottly]

Older Dyna S ignitions are notorious for failing on two cylinders, especially when hot. Trevor, the Dyna is the #1 suspect.

[trevordonovan]

thanks everyone that reached out with ideas ! so i ended up going over every wire and connection on the bike. sure enough it was the electronic ignition! the ignition didn't go bad, but the wires that run out along the bottom of the frame from the ignition cover held on by metal wire tie had wore threw the outer plastic and 2 of the wires inside grounding or arcing together and or my frame (as shown in photo attached) . i just put a new ignition in today, dyna s , static timed her and shes back making noise in the neighborhood. cheers trevor