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I got off the phone today with RodneyCB750K1979, and he noted that his compression is around 130PSI on all cylinders.
I had always thought that fresh CB750s should be pushing 180-190PSI, +/- 15.
I thought I would start a thread here to get some insight on this. Is this expected until the engine is "broke in"? Could it be that the valves need further adjustment? Should the engine be reading full compression right off the bat?
I also ask because he will be handling my CB450 engine, so I wanted to be enlightened on expectations.
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My 81 cb750K had about 130 psi on all cylinders before my rebuild. The bike will run reasonably well with those numbers, but the valves should be lapped when they're that low. Now that I've lapped my valves I notice a massive difference in my bike's performance. The front wheel wants to come up all the time now...
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Administrator
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If they are even across, your gauge is probably off.
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1976 CB 750-A X 2
1977 CB 750-A X 4
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1976 CB 750 F
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I went back and looked up what mine were pre-rebuild:
135
140
140
150
This was with the carbs off (equivalent of WOT).
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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I'll let him weigh in, but I thought he had is valves lapped.....
Basically, when should compression be the highest?
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Not sure what you mean by when. You do a compression test with the starter cycling 4-5 times with WOT.
If your valves need lapping, you have worn out guides, or bad rings you'll have compression issues. Other more serious problems also cause compressions issues (e.g. bent valves).
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Well, I mean when does everything seal enough to do a compression test? Right away? 100 miles? 500 miles? 1,000 miles?
It just seemed to me, that 135ish PSI was a bit too low. Hell, I think my 1978 SOHC w/ 12,000 miles has that.
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12k miles isn't that much, so don't let that be your guide to a 'worn' motor.
After a refresh, your rings are still sealing to the bore. As you ride, it'll go up. When will this be done? Depends on how your motor's feeling. Generally between 500-1000 miles, at least for an automotive motor. The cross-hatching marks are gone within 20 miles typically, but they do keep sealing up for a time after that.
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Lol, I consider 12K to be basically a new engine.
The numbers I posted above were from my engine with 90K km's on it, (56K miles for you Americans  ).
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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After a valve adjustment, my '72 CB450 was spitting out about 125ish PSI. Decided to pull the motor. 24,000 miles.
I think we're derailing. Stock is 190 PSI. His compression shows 130 PSI. I compared the gauge he used with another gauge, and it was consistant.
However, I keep reading stories that you should use a small engine compression gauge.
Have any of you rebuilt motors, and done a compression check? Was it always back up to stock? What gauge did you use?
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