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Maiden ride didn’t go so well

thejet95

thejet95

CB750 Member
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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?
 
Sounds like fuel is in there but not combusting well. Did you give it a valve job? 2 and 3 are also fed by the same coil.
 
I didn’t do a valve job. I also thought about the coil for 2 and 3. I tested it and it’s definitely powering those spark plugs unless it’s only giving partial spark or something? Not sure how to test for that.
 
thejet95 said:
I didn’t do a valve job. I also thought about the coil for 2 and 3. I tested it and it’s definitely powering those spark plugs unless it’s only giving partial spark or something? Not sure how to test for that.
Click to expand...
Resistance tests are the best way, I think. Very coincidental to have problems on 2 and 3 if it wasn't the coil or spark pulse things.
 
Well I did a thorough test of coils last night.
Coils. These are brand new coils I bought from 4into1.com

1-4. Primary = 4.4

Secondary = 14.94

2-3. Primary = 4.3

Secondary =. 13.13

Wondering if that secondary of 13.3 is too low? That is the coil for 2 and 3.

I also adjusted the cam chain tensioner and valves last night.
 
I think the secondary resistance should be on 1,000s. Wouldn't swear to it but I think so. If they are, I think you're good there.
Have you run it now m
 
Unfortunately no change. Poor throttle response and no power above partial throttle. Still doesn’t sound right either like running on 2 cylinders. Thinking of putting the old points on and trying that?
 
So I got the points back in and surprisingly it ran much better. It’s definitely running on all 4 now! Revved up to 6k but dropped off power quickly. Turned the air fuel screw almost all the way down rich to get it to run best like it’s not getting enough fuel. On a whim I turned the choke on and it actually ran even better. I noticed one carb seems to sucking air and moving up and down with the revs. So I called Murray’s carbs and inquired about re jetting. He walked me through some tests and actually said my carbs were way out of sync. I’m sending them to him so he can re sync them and will make sure all the jets are good etc. I’m hopeful this is at least part of the answer. I ordered new points and condensers and may buy the hondaman ignition. The dyna S one must have been a lemon?
 
thejet95 said:
So I got the points back in and surprisingly it ran much better. It’s definitely running on all 4 now! Revved up to 6k but dropped off power quickly. Turned the air fuel screw almost all the way down rich to get it to run best like it’s not getting enough fuel. On a whim I turned the choke on and it actually ran even better. I noticed one carb seems to sucking air and moving up and down with the revs. So I called Murray’s carbs and inquired about re jetting. He walked me through some tests and actually said my carbs were way out of sync. I’m sending them to him so he can re sync them and will make sure all the jets are good etc. I’m hopeful this is at least part of the answer. I ordered new points and condensers and may buy the hondaman ignition. The dyna S one must have been a lemon?
Click to expand...


Hard to say, you are making too many changes at once to be able to pin down a single cause. The air fuel screw has the most impact at lower rpm and I don't think it would have much effect at higher rpm. As for syncing the carbs, these motors run very well with a proper carb sync, but I would expect that a proper carb sync would have to be performed while the carbs were actually mounted to the motor they are serving, although I don't have any hands on experience with the Mikuni carbs. It could very well be running lean based on the improvement when adding choke, but that could be something to do with the carbs or it could be a vacuum leak where the carb manifold connects to either the carb or the head.
 
Bench syncing a carb could get you to an 80-90% solution. But it needs to be on the bike to finish the job. Madmtnmotors is right about mixture, of course. Only going to influence bike at idle.
 
madmtnmotors said:
Hard to say, you are making too many changes at once to be able to pin down a single cause. The air fuel screw has the most impact at lower rpm and I don't think it would have much effect at higher rpm. As for syncing the carbs, these motors run very well with a proper carb sync, but I would expect that a proper carb sync would have to be performed while the carbs were actually mounted to the motor they are serving, although I don't have any hands on experience with the Mikuni carbs. It could very well be running lean based on the improvement when adding choke, but that could be something to do with the carbs or it could be a vacuum leak where the carb manifold connects to either the carb or the head.
Click to expand...
Yeah I don't know much about carb syncing, but this guy Murray seemed to be saying that it's mostly the throttle tube slides and said he uses an "air flow" test or something. He said he won't charge me to "sync" them, so worth a shot. But I agree I have too many variables that could be problematic, thats why I removed the Electronic ignition. I put an oil catch can from the top of engine and there is fuel in there, so thats probably not good either. I've had trouble with the oil venting and think i was pressurizing the whole engine because developed an oil leak on the shift cover, and was leaking out the oil tank cap. I have an aftermarket oil tank with 3 ports on the bottom labeled Feed, Return, and Vent. I had routed the top of engine vent and bottom of engine through a Y to the "vent" port on the bottom of the oil tank. I opened the tank and see the tube coming to the top of the tank, but unsure how its "vented" since there is not top vent. So I currently have the catch can line coming from the top of engine breather, the lower breather behind the transmission is going to a Y that I have running up the bike to a vented air filter then the other side of the Y to the bottom of the oil tank labeled as vent. None of the forums threads I looked at describe my exact scenario so not sure its right. Oil leak mostly stopped after this configuration. I did change the gasket on the shifter cover and found a tear in the old one. Hopefully I don't end up having to do a top end rebuild....
 
brettp said:
Bench syncing a carb could get you to an 80-90% solution. But it needs to be on the bike to finish the job. Madmtnmotors is right about mixture, of course. Only going to influence bike at idle.
Click to expand...
I hope this is the answer! Yeah I was scolded my Murray Carb guy for messing with the mixure....He said put it back at 1.5 turns out and leave it alone lol.
 

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