Where Do I Begin?

[RodneyCB750K1979]

I just bought a 1979 CB750K LE with 22k miles. #4 header pipe does not get hot. The #4 carb did leak gasoline a time or two. There is an irregular backfire from #1 or #2 cylinders - bike was modified with 4-into-2 exhausts. Engine will not idle below 1400 RPM. Ignition cables have been taped. I ordered a new set. Clamps on carb inlets and outlets were loose. Compression Test: #1 125 psi dry, 240 psi wet; #2 130 psi dry, 225 wet; #3 145 psi dry, 220 psi wet; #4 0 psi, 55 psi wet. Where do I start? Pull head and jugs or rebuild carbs first?

[rrgunslinger]

Check the valve clearance on #4.  I assume you are using fresh gas,

[cdaiscool]

Rebuilding carbs won't fix a 0 psi issue on cylinder #4.

I would take off the carbs and do a leakdown test. When you did the wet test, how much oil did you put in? Those numbers seem super-high; all it SHOULD do is seal the piston rings to tell you if those or valves are bad, but it sounds like you put too much in and changed the volume of the cylinder.

I would do a leakdown test on all cylinders and go from there.

[Hoosier Daddy]

cdaiscool wrote

I would do a leakdown test on all cylinders and go from there.

Agreed, it will help you troubleshoot to know if the air blows past the intake (valve), exhaust (valve), adjacent cylinder (head gasket) or crankcase (rings).

[750K8]

I'll help out a little: I had this bike in my possession before he came and picked it up (bought it for him). I filled the tank with premium gasoline, and used some seafoam while driving it around.

I know one question that the OP is going to have is where to buy a 4 into 4 exhaust. Obviously, he's not going to find the stock pipes, and if he does, he's going to pay through the roof.

I think the best bet is getting some custom made 4 into 4 exhaust made. Probably will run about $1,000.

I would do the air test that was mentioned. If you wind up having to pull everything out, then I would do a big bore kit on it.

Very nice bike, and it was had for about nine hundred bones. Very complete and unmolested. The only thing it's missing is the side cover emblems.

[robs750]

I agree with checking the valve clearance first before the leakdown test.
And before you buy shims to adjust the tight valves.......buy the shim tool,remove the shims and place quarters in the buckets to make sure the valves are closed.Now do the leakdown test.
No point in spending extra money on shims only to find out the valves are burnt.

[RodneyCB750K1979]

Yes, fresh 92 octane gas with a cleaner added.

[RodneyCB750K1979]

Yes, I added too much oil to the cylinders more than 1 teaspoon.  Leak-down test is next.

[RodneyCB750K1979]

I don't quite understand the DOHC arrangement yet with no rocker arms. Seems like I should be removing shims from under the buckets to ensure the valves are closed completely instead of adding thicker shims (quarters).  

[RodneyCB750K1979]

I definately want to go back to the OEM 4-into-4 exhaust. Doing otherwise will affect air flows and carb settings.  Same with increasing bore size.  Rather stay stock if possible.

[Lucky 1]

Very unlikely result.

I have seen engines with a missing edge on a valve that will develop some pressure.

When you performed the compression test was the throttle all the way open?
Throttle MUST be open.

[Lucky 1]

That increase in bore size is really insignificant.

[cdaiscool]

Staying OEM will cost you QUITE a bit with no real gain. Getting an aftermarket exhaust will change the sound, performance, and any tuning issues will be easy to fix - even with a stock exhaust, you've still gotta have it tuned.

I can understand if you're going for the stock look, but if its just purely an "easier to tune" deal, well, you're going to spend probably $800 or more for a perceived, not-real issue.

Just want to make sure you're going into it with eyes open.

[750K8]

Yep. That's my advice too. There's plenty of exhausts shops out there that will make a 4 into 4 megaphone system that will look close to stock. You just need to be sure if it has the sound you are looking for. I would estimate the cost being up to $1,000 when it's all said and done. Maybe a little less.

Finding OEM exhausts for a 79 would probably be etching on impossible. Even if you find a set, I put money on it being rusted. It's just not worth going that route IMO.

For that bike, I would do 4 into 1, or 4 into 4. 4 into 4 is what I prefer as far as looks are concerned. The Super Sports looked better with the 4 into 1 because they had a narrower shape.

[RodneyCB750K1979]

Thanks for everyone's advise.  I did a pressure test and discovered air passing through #1 exhaust valves.  I also picked up some air noise inside the crankcase which I believe indicates some leakage past the piston rings. I removed the cylinder head today and discovered the top of #1 piston and the head chamber looked like they were peened severely.  This affected the valve seat area. I could think of three likely causes: predetonation, overheating, or a loose piece of metal bouncing around.  The cylinder bore appeared smooth with no nicks or scratches. The #1 piston compression ring does not move inside gland and the piston has some distortion. Can I replace one piston only or do they need to be replaced as a set?  Can the head be repaired?



  

[robs750]

RodneyCB750K1979 wrote

 Can I replace one piston only or do they need to be replaced as a set?  Can the head be repaired?

If the other three pistons are to spec. ,I dont see why you could not just replace the bad one.Of course you should measure the cylinders first.
You can probly re use the head.

[RodneyCB750K1979]

I started out today with a goal of removing the pistons.  OMG. The further I got, the task appears like the entire engine case has to be disassembled including the gearbox.  Any advice on what is necessary to access the piston connecting rod bearing caps?

[RodneyCB750K1979]

With some helpful advice, I don't need to remove the piston connecting rod.  Simply remove the c-clip, extract the wrist pin, and the piston is out. The other pistons appear in good condition except for some cleaning.  Off to Ebay for a used piston.