Topic: K4 Engine Compression Issues

[Ujeni]

So I finally got my bike (a CB750 K4 with a K1 tank and paint job) finished:



During the restoration, I replaced the piston rings, cylinder gasket, head gasket and valve cover gasket. The pistons and piston rings are standard size.

Now I am finding low compression. Here's what I measured:

Dry Compression Test
Cyl 1: 130
Cyl 2: 90
Cyl 3: 100
Cyl 4: 80

Wet Compression Test (added a bit of oil in the chamber)
Cyl 1: 125
Cyl 2: 115
Cyl 3: 120
Cyl 4: 115

Then I took the engine out, disassembled it and measured the cylinder with a micrometer...

Top of Cylinders
Cyl 1: 60.95mm x 60.93mm
Cyl 2: 60.95mm x 60.93mm
Cyl 3: 60.93mm x 60.94mm
Cyl 4: 60.96mm x 60.97mm

Bottom of Cylinders
Cyl 1: 60.95mm x 60.94mm
Cyl 2: 60.94mm x 60.95mm
Cyl 3: 60.93mm x 60.93mm
Cyl 4: 60.96mm x 60.94mm

According to my manuals, I am within spec....so why the poor compression? Could it be my valves? Thanks for any advice you can give me! And have a happy Xmas!

[MRieck]

Did you have the cylinder honed?

[Bill/BentON Racing]

And how did you do compression test? Throttle wide open or carbs off? Turn it til the needle quits! It'll be going a lot quicker in real life!! Let us know.Bill

[ekpent]

How many miles did you put on since the rebuild,enough to break it in well and was it smoking at all. Big question is how did it run?

[Ujeni]

Some answers....

MRieck,
I did not have the cylinders honed. Would that improve compression with standard pistons and rings?

Bill/BentON Racing,
I connected the compression hose, left the other three plugs in the engine, turned the ignition on, left the kill switch off and used the starter. I waited until the needle wouldn't go any higher, reset the gauge and performed the test again. I went with the highest reading. For the wet tests, I did the same thing with about a table spoon of oil in the combustion chamber. Carbs and airbox were on for all tests. I did not use the accelerator.

ekpent,
I put about 300 miles on it since the rebuild. No smoking (other than a little smoke when it started from cold). The bike ran very smooth of course  It did develop an oil leak at the gasket between the cylinders and head at the first cylinder (the other reason for taking the engine out). Here's a pic of the leak:

[Bill/BentON Racing]

Take all 4 plugs out,open throttle all the way,make sure choke off,compression's going UP,I promise,hone those cylinders also.I recommend CycleX 3pc. headgasket also and cylinder studs so you can really torque it...22lbs vs 16.I hate those seeping headgaskets on CB750's.My '75K was leaking when I got home from buying it new!! Bill

[Ujeni]

I did notice that nuts on the cylinder studs were not very tight when i disassembled the top end despite torquing them to the value in the manual when it pt it together.

Hmmmmm....

So you think my pistons and rings are ok?

What makes the CycleX stuff better than the gaskets I see on ebay and such (like Athena)?

Should I be checking anything with the valves? I didn't touch them.

[Xnavylfr]

Have the cylinders honed with a crosshatch pattern (helps rings to seat better to cylinders)
Change head studs to HEAVY DUTY so MORE torque can be applied to the head( STOPS THE LEAK).
A GOOD QUALITY gasket will help to prevent pulling it apart AGAIN !!!!


Xnavylfr(CHUCK)

[Bill/BentON Racing]

CycleX headgasket multi layer 3pc METAL gasket! Best all around IMO.Bill

[Ujeni]

Ok, sold on the CycleX gasket.

Can someone point me to the heavy duty studs?

Thanks everyone!

[ekpent]

As for the oil leak make sure in case that you didn't, that you replace the 8 round quarter size rubber pucks under the cam towers as these can be a major source of leaks other than the new head gasket.Also make sure a little oil is on the stud threads before you torque,let it sit overnight,loosen them a little and retorque them again after the gasket has settled in before final assembly.

[steam-powered man]

in your description of how you performed the compression test, what does this mean?
"I did not use the accelerator"

was the throttle closed?  if so, i'll bet your compression readings will rise if done with trottle wide the f*ck open, or carbs off. 

nice ride, btw.        

[cbr954]

APE and Cycle X have heavy duty studs

[Ujeni]

in your description of how you performed the compression test, what does this mean?
"I did not use the accelerator"

was the throttle closed?  if so, i'll bet your compression readings will rise if done with trottle wide the f*ck open, or carbs off. 

nice ride, btw.      

I meant that I did not use the throttle; it was closed. Sounds like I did the compression test incorrectly huh?

The rubber pucks we're replaced and sealed well. I don't think they were leaking, but I will inspect again to be sure.

Unless someone thinks this is the wrong thing to do, my new plan is to get the cyclex gaskets, heavy duty studs and hone the cylinders. Should I do anything else to improve compression and reduce the chance for leaks? I've been told to have the cylinder block machined to get a perfectly flat surface...think it's a good idea?

[swan]

Ujeni,

Nice bike and you are on the right path. But, before you throw money at it to solve your problems, ride your bike to warm it up, retest you compression as suggested with throttle wide open, choke open, plugs out and crank it over until the needle stops. You should see a rise in PSI. Your goal is even compression across all the cylinders. Also, what compression gauge are you using ? A cheap gauge may give you lower numbers.

Next, I recommend a leakdown test. Search on this forum and Goggle and you will find simple instructions and tools are parts needed to do it. Basically a leakdown test fills each cylinder with pressured air allowing you to identify sources of compression losses. A hissing sound in the carb inlet indicates a badly seated valve or worn valve guide. Same so in the exhaust and/or around the head gasket.

Currently I am rebuilding a CB750 K4 motor and my cylinder bores and rings are within factory spec (like yours) but my exhaust valve guides were out of spec. If you are going to breakdown your motor again to hone the cylinders for your new rings (standard practice with new rings) be sure your valves are seated well. In a dark room, shine a small flashlight in to each of the intake and exhaust ports and look for any slivers of light around the face of your valves. If you find one, they are not seated correctly. Remove the valves and look at the valve seats for pitting. Did you properly clean and seat your valves when you built this?

Next, take your cylinder head to a machine shop so the can measure the inside diameter of your valve guides with their fancy and expensive small bore internal micrometer and measure the diameter of the valve stems. If your guides are too big or stem diameter too worn, replace them with new and have the valve seats touched up. You can also have the shop measure and/or machine the surfaces cylinder and cylinder head to be perfectly flat. Do not remove too much material and consult the manual for specs. A Cycle-x gasket and heavy duty studs will help you leaky head gasket as noted.

Be sure your cylinders get a proper 45 degree cross hatch hone with the right grit for your rings. CLEAN,CLEAN and CLEAN the bores again until you can wipe a white cloth inside and it comes out clean. your bike will need to be broken in gently for 500 miles or so to ensure the rings are bores seat correctly.

If this was easy, everyone be would be riding a classic Honda. Good luck, happy holidays and keep us updated.

[Ujeni]

Merry Christmas everyone!

I guess the christmas break has me moving too quick. I'd love to do a new compression and leakdown test, but the engine is already out of the frame and the top end disassembled:



I'll check the valve seating later today and provide an update.

Do you think I should put new rings on? These rings only have ~300 miles on them, but they were with cylinders that weren't honed. I'm guessing I can keep these rings, but I thought I would ask.

Ordering the gasket kit and heavy duty studs later today.

I'm grateful for all the advice you guys are giving me!

[swan]

Your rings should be fine but you can measure their gap by removing each one individually and placing them 3/4" to 1 1/2" even in their respective cylinder, measure the gap and compare to your manual's specifications.

[MRieck]

The leakdown test is a good idea but the top end is apart. If you take it to a machine shop with a Serdi valve machine they can vacuum test the head and tell you if the intake/exhaust valve seal is good.

[MRieck]

Your rings should be fine but you can measure their gap by removing each one individually and placing them 3/4" to 1 1/2" even in their respective cylinder, measure the gap and compare to your manual's specifications.

Good idea too. Put 2 wrist pins in a piston, insert ring and push the ring down with the piston crown side facing down. This will center the ring.

[Ujeni]

I'm not able to see any light shining from around the valves. My valves are pretty dirty though...is it worth buying a valve spring compression tool, removing the valves and cleaning everything in there? Their current state:



I also checked all of the piston rings. They are easily within spec, so they are good to go!

Placed an order for the CycleX head gasket (not cheap!) and found a place in San Francisco thats appears to have heavy duty studs...so I will try to buy them later this week in person.

[70CB750]

Leak down test on valves. As far as spring compressor they could be rented from stores like Autozone.

[MRieck]

Leak down test on valves. As far as spring compressor they could be rented from stores like Autozone.

Are you saying vacuum test on a valve machine? I would agree with that.

[70CB750]

No, flip the head upside down and test closed valves for leaking. Valve sealing well will not leak.

[seanbarney41]

Here is another test for your valves...turn your head so the intake ports are facing up.  Dump some solvent or very light oil(like wd40) into the port and note if a leak quickly develops in the combustion chamber, do the same with the exhausts. 

[Ujeni]

Ah, lots of good ideas to test the valves. I'll perform them in the morning.

If I detect a leak, then ill get the tool to take them out. If no leak, then I plan to leave them be.

More to follow!