Hey guys new here I have been looking around the forums for ideas but I have had several engine and carb problems over the last year with this project. I have a DOHC 81 750C I am currently dealing with a no start issue after fresh piston rings, top end gaskets and seals. I believe it’s timing related but I don’t know for sure. I followed the Honda manual for reinstallation, aligning the 1,4T mark, #1 cylinder cam lobes pointing to spark plug, and timing marks on cam sprockets parallel to the head. I lapped the valves, and shimmed to .005” After I put all this together and cranked it over it would not start and after about 7-10 cranks it lets out a huge backfire and that’s it. Checked compression and it read 100 on all cylinders. I figured that given all cylinders were even it could be timing related but I just don’t know where it could have gone wrong. Any help would be appreciated as this thing has just left me scratching my head.
‘81 CB750C Low Compression after Rebuild
- Thread starter Lxhggns1
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- compression dohc engine timing
amc49
CB750 Guru
If the cam(s) are off you could have bent valves. Easy to do on these. The backfire could indicate it. At this stage I normally say get backup readings using another gauge as the gauges can give so much trouble with reliability. But risking bending even more parts. You see the problem.
If plugs are all wet maybe you just wetfouled them (7-10 cranks can do it), or maybe coils hooked up backwards yellow and blue. Plugs will have to be dry to try again. Maybe need to recheck cam timing again.
If plugs are all wet maybe you just wetfouled them (7-10 cranks can do it), or maybe coils hooked up backwards yellow and blue. Plugs will have to be dry to try again. Maybe need to recheck cam timing again.
Once I get the valve cover off today I’ll post a pic of the timing marks, I rolled it over slow by hand after install to make sure there was no piston to valve contact etc. And I had all the valves out and lapped them back into the valve seats. All of that went over fine. The only issue I noticed when timing it was that the cams seemed almost like they were a half a tooth or less off the crankshaft timing mark like they wouldn’t sit perfectly parallel to the head but adjusting one tooth on the timing was too far of an adjustment to correct it.
Just for reference which color is for which set of cylinders?
amc49
CB750 Guru
How you get into trouble to begin with.
Yellow is 2,3 and blue is 1,4. Provided the pulsers were hooked back up right too.
If you didn't change chains then the marks are commonly off from chain stretch, you put them in looking at cams and end up with cams slightly retarded, the other way has you off one tooth.
Hope in all the valve work you made sure same valve went back into same hole or you would have mis-lapped as some will cut and form to a seat so much you'd have to lap forever to ever hope of making them seal. Then you have gone through the valve heat treat and valve lasts maybe 6000 miles and then burns.
If off just enough you can roll the engine around and bend them and never feel it too. The valves won't necessarily lock up solid to feel it as they are too small and if the contact is just enough to hang edge. I've run into people who swore they never touched but bent anyway.
Yellow is 2,3 and blue is 1,4. Provided the pulsers were hooked back up right too.
If you didn't change chains then the marks are commonly off from chain stretch, you put them in looking at cams and end up with cams slightly retarded, the other way has you off one tooth.
Hope in all the valve work you made sure same valve went back into same hole or you would have mis-lapped as some will cut and form to a seat so much you'd have to lap forever to ever hope of making them seal. Then you have gone through the valve heat treat and valve lasts maybe 6000 miles and then burns.
If off just enough you can roll the engine around and bend them and never feel it too. The valves won't necessarily lock up solid to feel it as they are too small and if the contact is just enough to hang edge. I've run into people who swore they never touched but bent anyway.
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