Jrushtx
CB750 Member
1982 CB750c all stock. Purchased this bike in not running condition, compression 160 lbs on all 4 cylinders. Serviced carbs (I did remove that stupid vacuum controlled petcock) I used factory sized jets I used sizing chart from the shop manual and they were the same size that were installed, new plugs, and the bike started right up, seems to idle ok but could be better. Once the engine is warmed up I get a popping backfire that appears to be coming from the #1, can smell raw fuel on the left exhaust, the #1 exhaust pipe is cooler than the others. Pulled plugs, they all look good. I checked the valve lash, all 16 fell within specs .003 +-.001, checked timing, performed a leakdown test on all 4 cylinders all are less than 15%. Im guessing the #1 is missing intermittently, causing unburned fuel to ignite in the exhaust. The only thing I have not done is new coils and wires, ordered them today, so I can eliminate that, my only reservation is these coils each run 2 cylinders, and I only have one missing cylinder. The bike seems to smooth out at 4k and up, but I dont know if it quits backfiring or I just cannot hear it at that speed, but it does seem worse at lower rpm. The backfire is not super loud, just pops loud enough to make CB feel embarrassed at stop lights, it does not bother me, but it is affecting the self confidence of the bike.
Im hoping one of you DOHC Ninja's reply to this post with, "you forgot to check xyz, dumbass" and I will not take offense as I feel like Im missing something pretty simple. Appreciate any input I can get on this failure.
Im hoping one of you DOHC Ninja's reply to this post with, "you forgot to check xyz, dumbass" and I will not take offense as I feel like Im missing something pretty simple. Appreciate any input I can get on this failure.