Topic: 1972 cb750 timing chain, engine removal and other maintenance

[FondaHonda]

I bought my old 1972 cb750 for $700 and it needs some work. It runs decently. I have cleaned & adjusted the carbs, adjusted the valve clearance. I can't get the timing anywhere close using a timing light. It has electronic type points. Compression tests 90 -100 on all cylinders. I'm thinking I may have a problem with the timing chain being off a link or 2. If this is the case and I remove the engine to fix this, are there some other things you suggest I do while I have it torn down? Odometer says 46,00 but I don't know how accurate that is. The frame serial# indicates it is a 1972, the engine serial# indicates 1973. If I decide to do some paint & minor body work is there a source you suggest for instruction on this? There is a small bit of rust on the frame and a few small creases on the gas tank that I hope I can fix myself. Your advice is appreciated.

                                George (FondaHonda)

[Bodi]

The ignition points are on the crankshaft, the timing chain turns the camshaft. The timing chain does not affect the ignition timing, just the valve (cam) timing.
I would not take the engine out right away, that's a nasty job and should be avoided unless absolutely necessary.
What electronic ignition is fitted? Does it use the stock advance unit, and if so is the advance unit working (or is it jammed)?

[FondaHonda]

I have tried to determine the make of the elec ignition but have been unable to. On the plate the part that would be the points is encased in rubber and that's all that's on the plate but that's probably pretty common. Adjustments either way as far as they will go don't get the timing anywhere close using a timing light. Someone in this group told me that cylinders 1 & 4 receive spark at the same time. I've tried the light on 1 & 4 with the same results. If the timing chain is off would that explain the low compression readings? Any ideas or advice is appreciated.

[TwoTired]

Compression tests 90 -100 on all cylinders.

Conditions for this test are important.
-Engine hot
-Throttle and choke wide open.  All spark plugs removed.
-Either a very fresh battery or kick start for the highest reading.
-The gauge matters, too.  A long large hose adds volume to the cylinder and effectively lowers the compression ratio yeilding lower numbers.
-Also a wet test (squirt of oil in the cylinders) after the dry test can tell you if your rings are seating properly.

Why do you think the timing chain jumped?  Is there a problem with the cam chain tensioner?
I don't know this for sure, but, I thought the pistons would hit and bend the valves with a skipped chain.

Can you get the 2-3 ignition timing adjusted?  Or, does it have the same problem?

Cheers,

[scondon]

I have tried to determine the make of the elec ignition but have been unable to. On the plate the part that would be the points is encased in rubber and that's all that's on the plate but that's probably pretty common. Adjustments either way as far as they will go don't get the timing anywhere close using a timing light. Someone in this group told me that cylinders 1 & 4 receive spark at the same time. I've tried the light on 1 & 4 with the same results. If the timing chain is off would that explain the low compression readings? Any ideas or advice is appreciated.

   In your first post you say the bike runs decently. It would run very poorly, if at all, if your cam chain is off a link or two. If your compression readings are consistent   then I would think the tester/method is suspect. Read TT's post for procedure(damn I type slow).

    Dyna and Boyer-Brandsen? both make ignitions like you have. The timing procedure should be the same for both, I think. With engine at Idle(1000-1300rpm), and using a timing light, you should be seeing the "T" and "F" marks through the view hole. With points/condensor type ignitions timing is usally set to the "F" mark at idle, but not so with your ignition. Bring the rpm's up to 2500 or so and set timing to the advance marks ("1[]4" or "2[]3" depending on which lead you have the timing light hooked to) and let the idle timing fall where it may.

    If the bike is not running well there are many things to consider before tearing the engine apart.You may already know this info, just want to make sure before going further.

   
   

[FondaHonda]

Thanks for your response & info. When I bought the bike they told me it might need a timing chain adjustment or "tune-up". There is some noise from the timing chain area at idle. It goes away when the rpm is increased. I have tried 3 or 4 times to adjust the tension on the timing chain, but the noise is still there. The idle and lower rpm's seem a little rough. I have cleaned & adjusted the carbs 3 times even though they didn't look bad. It does seem to smoke some once it is warmed up. I will try again to test the compression & to test cylinders 2 & 3 with the timing light.