Topic: cb750 engine rebuild??

[daveyd]

so my bike (a 1976 honda cb750k) has been leaking oil, presumably from the puck being shot. i tested compression to see f rebuild is absolutely necessary, and i was 150 151 90 152. crap.  is there anything i can do besides checking those valves? i dont mid pulling out and rebuilding, im just broke and its my daily transportation. if i pulled the engine, i dont mind changing out the pucks only, even though everyone and my dad have said oh just do the rngs and pistons while your there.  any input would be awesome. if i did rebuild at all i would probably for the 850, but all i fan find around me that charge 75 bucks a cylinder and so that would be really expensive. with the head off i suppose i could use a feeler gauge and see if i might only need rings?? i dunno, im kinda in panic mode right now and feel the jab of the dagger in the bellyevery time that 3 cyl drops my idling rpms. any input? sorry for ranting everyone haha.

[phil71]

i've owned bikes with that kind of disparity in compression and they ran fine, for ages. You don't say if it runs well or not, and you don't really have a question here. How exactly are you going to use your feeler gauge to check for worn rings?

[AbbyRider]

My '78 750F has exactly the same issue - I've got 150's across the board except #3 which is at 90. Also have oil leaking out the exhaust valves, which I'm assuming is the pucks. It runs, but has idling issues and when it's running hard you can tell that #3 is not pulling its weight. I've been planning on doing the top end only - have all the gaskets, valve seals, and will lap the valves accordingly. I'm interested in any replies from more experienced members, because I'm comfortable removing the engine and doing the top end stuff, but any talk about "may as well do the pistons and rings while you're in there" reminds me of my mechanical limitations. My thinking is that the difference between doing what I'm planning and getting into the rings etc. is quite substantial.

[Greggo]

The 'while you're in there' mentality takes into consideration how much work is needed to do something like 'doing the rings'.  You have to pull the motor, the top end, the jugs, and replace the gaskets.  SO, if you've got a higher mileage motor that will need a hone/ring job in 5-10k miles, just do it now and save yourself all that extra work.  You may just need new stock rings, and a hone job (which can be done at home by non pros) as compared to using a big bore kit with new pistons that require jug boring and such.  Basically, decide what's right for you right now, taking into consideration all these things, and how long you plan on owning/riding the bike.  You have to make the decision for yourself.

[MCRider]

AbbeyRider. Likely 1 of 2 things or both, cause the low comp in #3. Valves or rings.  When apart, check the #3 valves for leaks with the solvent test, before disassembling. If not substantial, you have no choice but to re-ring it, or all's for naught. IMO.

A valve job will really perk it up, but you may not actually restore the compression without rings. Not that hard on a stocker. Pull the jugs, have them measured and honed. remove the pistons to clean them. Install rings and reassemble. I would say the rings would be a 40% addition of time and effort. Vs everything else you're going to do.

[MCRider]

daveyd: Just to be sure your ealize, the puck leak and the compression are 2 different unrelated items.

As to boring costs, if you're going 850, wouldn't you just send it to CycleX or APE and let them do all the mahining? Haven't checked but they'd do a package deal.

But if its a time/budget problem, I'd strip the top end, clean it (glass bead, etc as needed), lap the valves, replace all gaskets and seals, dingleberry hone the cylinders, re ring it stock and go on your merry way. Cheap and effective.

[Rigid]

Swap in another engine?  $250 and about 6 good hours work. Seriously, if time and money are an issue, a known good engine may be the answer.  Sometimes I have trouble finding them but have been fortunate and have several spares right now.  When someone says, "hey, I was going to build a chopper and I have this extra engine, do you want it?" my answer is always "yes"

[Hondawggie]

Daveyd I have read about a way to test if low compression is due to rings or something else (valves for e.g.).   You drop a little oil into the spark plug hole just before running the compression test on that cylinder.  If your compression improves -- rings are part of the low compression.  If your compression does not improve -- then the rings are not causing the low compression.    Might give that a try if not already.   Another question would be did you run the compression test with a cold or warmed-up motor (sounds like it was a warmed-up situation from the numbers).

If I had tried this 'rings vs. valves' simple test I'd tell you how much difference in the compression I saw before/after the oil dousing but I've only read about it at this point.   Worth a try, it's a simple test.   If it's not the rings maybe all you need is valve work.

[Rigid]

" If it's not the rings maybe all you need is valve work."  that may be true but the point is while the engine is removed from the frame and disassembled it is silly not to hone and rering it.  The only exception to that I see is an engine with known excellent compression and low mileage (below 5K)

[AbbyRider]

That's a lot of good info. Perhaps replacing stock rings is not that big of a job. My engine has about 27,000 km's (17,000 miles) on it so based on what I'm reading it's probably due for more than just a valve lap and seal/gasket replacement. However, I've noticed that rings for a '78 750F are not that easy to find. I'll do a bit of searching here on the forum and see what I can dig up.

[Gordon]

My engine has about 27,000 km's (17,000 miles) on it so based on what I'm reading it's probably due for more than just a valve lap and seal/gasket replacement.

Don't be so sure.  If treated halfway decently, these engines can go way longer than 17K miles before needing any major work.  My K1 is nearing 50K miles, and the only engine parts that have been replaced are the gaskets and seals.  Definitely check the rings and bores while you're in there, but if they're in spec there's no sense in doing work that doesn't need to be done and spending money you don't have to. 

[Rigid]

I believe I got mine off of georgefix out of Florida on eBay.  he has always been very good with prices and fast shipping.

[MCRider]

Abbyrider has that #3 at 90PSI, so somethings amiss. But I'll agree 17K isn't enough by itself to suspect rings. And if it was wearing uniformly the other cylinders should have less than 150PSI which is a pretty good number.

Could be a burned valve in #3.

[AbbyRider]

MCRider - that's making me feel a bit more optimistic. After doing a bit of searching on the forum, I realized that indeed, '78 750F rings are hard to come by. So hopefully it's a valve issue. I'm also thinking that one other explanation is that I didn't set the gap properly on #3 exhaust valve. Once I've got the engine out I'll be able to do that a whole lot easier than using those stupid bent gauges.

[MCRider]

MCRider - that's making me feel a bit more optimistic. After doing a bit of searching on the forum, I realized that indeed, '78 750F rings are hard to come by. So hopefully it's a valve issue. I'm also thinking that one other explanation is that I didn't set the gap properly on #3 exhaust valve. Once I've got the engine out I'll be able to do that a whole lot easier than using those stupid bent gauges.

I didn't realize its a 78F and I think i've read here where those rings are unique and more difficult/expensive to get. OCICBW

You'll be testing the valves once the heads off. Leave the cyls on till then. If #3 leaks like a sieve that may be where you'll stop. If #3 was set too tight, it could burn and then leak compresion. Intake valves don't burn usually, as they are cooled by the fuel and aren't open much to the hot exhaust.

But also the 78s have a reputation for accelerated valve and guide wear. Newer parts are stronger.

[cheapride]

I have a set of rings that I purchased from total seal.  I am going to a 836 kit and don't need them.  PM me if you are interested in them.


Brent

[AbbyRider]

Thanks Brent. I will wait until I tear into things and then will PM you if I think I'm gonna go that route.