Topic: Valve Work Honda CB750F2

[NWrider]

I'm new to the group and posted a hello. I have a 1977 CB750F, on which I will be doing a valve job. The engine only has 12K miles, but a leak down test indicated excessive valve leakage. I noticed concentric wear grooves in the seats.

I've read that many 750 owners have put quite a few miles on the factory seats and valves using unleaded fuel. I'm unsure as to whether I should replace seats and valves, assuming they clean up OK and meet valve specifications for runout etc. I can also check the guides for wear as the Clymer manual provides those specs as well.

So here comes the questions. Is the consensus or experience that it's better to replace everything versus machining the seats and valves assuming everything meets the wear specs? Any thoughts on why there would be excessive leakage at 12K miles, when I've read posts which indicate the engine could go well over 50K miles.

Thanks!

[rbmgf7]

try doing a search on the F2 engine for an indept answer. there's one post out there somewhere of why your problem came to be.

short answer: most likely you have excessive valve guide wear and will need to have your exhaust guides replaced...then a valve job. but that won't prevent it from happening again down the road. my PO was lucky and put 30K on my F2 before a cylinder lost all compression.

your best bet is just to swap in a regular K engine or F1 engine.

edit: found it: http://forums.sohc4.net/index.php?topic=10540.0

[NWrider]

Interesting. I wonder why the CB750F would be prone to valve failures? Perhaps better guides are the answer? If its the cam, then a standard 750K cam should do the trick, and leave the 750F head with larger valves on the engine.

Anyone know the full answer?

Thanks

[bryanj]

If you do the search and read all the results you will find out all the secrets of the F2 problem and changing the cam wont alter the wear

[eurban]

Yes read through the entrys in the post that rbmgf7 found for you.  Particularly where OlfFart talks about his experiences as a Honda tech.  Valve angle was the culprit.

[rbmgf7]

i rechecked my head and it's prettyt bad. the valves wiggle a good couple millimeters. i talked to buzz at dynoman a couple years back and they had a remedy to help retard the problem but it's still innevitable. he explained some F2's wouldn't make it to 6000 miles before the valve guides wore out.

[MRieck]

 At least you can get new valves for that head from Buzz.

[NWrider]

Thanks to rbmgf7 for the link I really appreciate the find!!! That really explains quite a lot.

So in summary the key is change oil frequently, use the hardest exhaust guide you can find and increase the exhaust lifter clearance to .004" as noted.

One last question. Do I put a seal on the exhaust valves?

I'm really anxious to get this head back together and want to do it right.

[scondon]

 Not sure about the "seat" question but want to add that you want to check the valve stem tips and tappet adjuster tips for wear. Putting a worn F2 tappet adjuster in with new parts will greatly accelerate wear. Because of the valve angle these parts are usually buggered even in low mile F2 engines.