thejet95
CB750 Member
Hoping to get some ideas of what could be wrong with my freshly restored 1976 cb750. The bike on its first ride 2days ago had very little power at partial throttle and even less with full throttle. I’ve been trying to troubleshoot since then but decided I need help from others.
Let me back up a bit. I purchased a mess of a project 2 years ago that had bad wiring, badly cut up frame and barely ran. It did come with brand new Dual VM34 Mikunis from Murrays Carbs which from what I’ve read add more torque? I’ve been doing a very slow bolt by bolt restoration.
I did a compression test initially which showed on 1,2,3,4 = 143, 132, 136, 141. So I thought it was probably okay. Along the way I bought mostly all new parts for it. I installed new plug wires and coils (4.7ohm), a Dyna S ignition, and an M unit blue. It fired up immediately this week and idles well. Was stumbling on revving so I adjusted the air/fuel mixture on the carbs until I got it revving nicely. I tested and have spark on every cylinder. I used a timing light on the dyna ignition at 2500 rpm and got it dialed in to the marks on 1/4. After a few mile test ride I used a thermometer on each exhaust and got 250 degrees approx on 1 and 4 cylinder, and only 125 or so on 2 and 3. So I’m thinking those cylinders might not be firing despite spark? I pulled the plugs and 2 and 3 are pretty black while 1 and 4 look about normal. I’ve never heard or ridden another CB 750 but it sounds very loud and not like a sport bike but more like a Harley which may be because it’s only running on 2 cylinders? Any ideas of what the problem might be?
Let me back up a bit. I purchased a mess of a project 2 years ago that had bad wiring, badly cut up frame and barely ran. It did come with brand new Dual VM34 Mikunis from Murrays Carbs which from what I’ve read add more torque? I’ve been doing a very slow bolt by bolt restoration.
I did a compression test initially which showed on 1,2,3,4 = 143, 132, 136, 141. So I thought it was probably okay. Along the way I bought mostly all new parts for it. I installed new plug wires and coils (4.7ohm), a Dyna S ignition, and an M unit blue. It fired up immediately this week and idles well. Was stumbling on revving so I adjusted the air/fuel mixture on the carbs until I got it revving nicely. I tested and have spark on every cylinder. I used a timing light on the dyna ignition at 2500 rpm and got it dialed in to the marks on 1/4. After a few mile test ride I used a thermometer on each exhaust and got 250 degrees approx on 1 and 4 cylinder, and only 125 or so on 2 and 3. So I’m thinking those cylinders might not be firing despite spark? I pulled the plugs and 2 and 3 are pretty black while 1 and 4 look about normal. I’ve never heard or ridden another CB 750 but it sounds very loud and not like a sport bike but more like a Harley which may be because it’s only running on 2 cylinders? Any ideas of what the problem might be?