Top end undertaking

[nilsthenomad]

Well I've been plagued with problems lately on my '81 750K and after a somewhat extensive carb problems I have lost compression in cylinders 2&3. Consequently, I have managed to deduce from friends and posts on this site that I'm most likely going to have to tackle my valves, rings etc. This would be my first of any such undertaking and while I'm mechanically savvy, I have zero experience with this kind of work. I spoke to my Honda mechanic and he quoted me about $500-$700 just in labor, which is something I can't afford to dish out for right now. My question is: Is valve and ring replacement something that is relatively easy to undertake with limited experience? I have the shop manual and have studied it, but unfortunately it's still a little vague and limited with certain points. Any recommended sources (videos/step-by-step guides) to this kind of work? Looking for any advice or suggestions. Thanks!  

[Lucky 1]

FIRST...

When you performed a compression check was the battery fully charged?
Were all of the spark plugs removed?
Was the throttle wide open? IF not then the air cannot go in to be compressed.

Only after you have done a compression check properly can you then think about rebuilding any part of the engine.

I have seen so many beginners make false assumptions about an engine based on a improper compression check.

" I've lost compression on cylinders 2&3"  This statement alone would make me want to redo your compression check.
ALSO: It is extremely rare for an engine to lose compression on just two cylinders .
And the middle two? Doubtful. Usually unless it is a racing engine all 4 cylinders would be low.

[nilsthenomad]

I stopped by and had my Honda mechanic do the compression test after I had some serious backfiring and loss of power, since I don't have a tester myself. After he quoted me on the massive labor costs just to open up the head and determine the cause, I decided I'd have to do the labor myself with only the initial diagnostic of compression failure from him.

[TOOLS1]

Taking the top end apart is not that hard. Just take lots' of pictures, and notes as you go.
TOOLS

[nilsthenomad]

In all honesty, I would do anything to hear that he might have misdiagnosed the problem and I don't have to open the top end up. I've only used this mechanic for a few things, but he does come well recommended with regard to motorcycles around town.

[TOOLS1]

Go to your local autoparts store, and pick up a good compression tester (do NOT use a Harbor Freight one,) and run another test. Also you could take an old spark plug, and make a leakdown tester, and do that test.
TOOLS

[shinyribs]

I have two compression testers. A Snap-on and a Harbor Freight. <- It was given to me. I used both of them on an engine last night. The HF tester read 10 psi lower than the Snap-on. Who knows which one is actually accurate.

[Hoosier Daddy]

 You said you were fighting carb issues... was it running lean? That is hard on engines.

TOOLS1 wrote

 Also you could take an old spark plug, and make a leakdown tester, and do that test.
TOOLS

^^ Best advise to get an idea of what is going on BEFORE you open up the engine. The Leak down test procedure means to have the cylinder on TDC of the compression stroke where everything should be sealed. Then you put a small amount of regulated air pressure into the cylinder using the adapter TOOLS mentioned in the spark plug hole.  If the air leaks out the carb it's an intake valve, out the muffler an exhaust valve, or out the dipstick / oil fill port it's rings... Helps a lot to know what to look for a head of time.

[Lucky 1]

You still have not given us a compression test result. That is the FIRST STEP!!
And the valves MUST be adjusted and the slack taken out of the cam chain before the compression test.
Any mechanic should know that.


Just because people around town said they knew
the mechanic does not make him the best choice.

I also do not like the fact that he did not give you the results of the compression test?
I do not like the fact that he quoted you a price without even seeing the damage to the engine if any.
He should give you a price for tear down THEN give you an estimate for parts and labor and machine shop services AFTER he inspects for damage,wear and broken parts,if any.

IT can easily cost more that $500-$700. Depends on what parts are needed and machine shop services.
Why pay hiim to run the parts to the machine shop? Do it yourself.

There is engine removal (You could do that yourself and save money)

 

[seestheday]

I did this recently on my bike. I didn't have mechanical experience before owning this bike.

It didn't cost me a lot but did take me a ton of time. I also had valve issues but not ring issues so I only needed to pull the head.

I can point you to lots of great resources once you figure out your compression numbers and do the leak down test (or when I get to a computer).

[nilsthenomad]

Gonna go to the auto parts store tomorrow and get a compression tester and run my own test. I'll post my results shortly after. I re-checked the mechanics notes, he said cylinder 2 & 3 were at 10psi, but we'll see what I get tomorrow. Crossing my fingers and hoping for the best.

[Lucky 1]

10 psi!!??  IT seems like it would have to have a hole in the piston.

[FastCletus]

I have not removed the head off my cb750, but I have on my cb 500T.  I was very intimidated at first of this task, but after doing it, I am no where near nervous about taking it apart.  Hopefully your issue wont coem down to that but if it does.  Do it your self and learn about your bike.  These bikes are old if its not broke now, it will some day, it helps to have some general knowledge on how they work

I found a great vid on you tube that has a guy taking the head off and replacing chain and gasket. will repost if i can find it

good luck!  

[nilsthenomad]

Bought a compression tester today and ran a test. My readings came out cylinders 1-4:

165-140-110-165

Just a few specifics here:

-I dont have the carbs or the valve cover on right now, but charged the battery fully before I tested

-Cylinders 2 & 3 (with the low readings) climbed quickly in psi til they got just below 140 and 110 but then would have kept climbing about 2 psi per crank if I had kept running the starter, but didn't want to strain anything. Dont know if that is an indication of anything, but figure I should note it.

If anyone has recommendations for the next step or interpretations of my readings I could really use them. Thanks!

[seestheday]

That more like it.  Maybe the mechanic meant 110psi when he wrote 10psi.

Carbs off is perfect, valve cover shouldn't matter (just don't get anything pinched in that chain).

Your numbers should be within 10psi of eachother, but you have fantastic compression on your 1 & 4.

Do a wet test next to see if it's rings or valves.  I'm betting it's carbon buildup on your 2 & 3 valves.

Wet test = a few drops of oil through the spark plug hole before you do the compression test.

If the wet readings are normal then you have a ring problem.  If they're still bad then you have a valve problem (likely carbon buildup and they need to be relapped).

[nilsthenomad]

Just did the wet test on cylinders 2 & 3 and they both jumped up to normal

2 went from 140 to 175psi

3 went from 110 to 170psi

I imagine it is safe to assume that it's my piston rings. What am I looking at next? Is this a tough fix?

[seestheday]

Damn, bad rings.  Unless someone more experienced here jumps in I'd say it's time to start planning on pulling the head & cylinder block.

Here is a great guide:
http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=11804&highlight=removing+cylinder+block

You are going to need new gaskets.  It probably makes sense to just buy a whole new set.

While you're in there you should check your valves & combustion chamber for carbon buildup, and check the condition of your cam chains & tensioners.  

[cdaiscool]

How much oil did you put in for the wet test? If more than a capful (aka pouring straight from the bottle, even a couple seconds) can reduce volume of the cylinder and cause the test to be incorrect.

Otherwise, yep, bum rings. That sucks man. :(