1978 CB750K Top End Rebuild

[fiedler.casey]

Hey all,

Just got ahold of a 78 CB750k for $175 (with title). Needless to say, I'm pretty stoked about the price. Cleaned it up, did a shit load of odds and ends work, and then fired her right up, third kick. Rear brake was seized up, got it free and cleaned, lubed. Front brake is shot, master cylinder needs a rebuild and the caliper probably does too (as soon as I get it off).

She's burning oil pretty hard and cylinder 4 isn't firing (or is only firing 1/10 rotations or so). All cylinders have spark. Carbs got a quick cleaning and all float bowls are full of new clean fuel. Compression in #4 is about 95psi.

How else might I determine the cause of cylinder four not firing?

I also am under the sneaking suspicion that it desperately needs a top end rebuild. What parts do I need to order to do the top end on this girl? (gaskets, piston rings, valve guides, anything specific to this model?) I know that some of what I will need will depend on the shape of things when I get it apart.

I've been seeing some stuff about keeping the cam chain tight so it doesn't come off the sprocket in the bottom end... is this necessary with this model of bike when doing a top end?

Any wisdom about this model would be wonderful! I'm in the process of soaking up information about this specific bike.

Also, frame says CB750K, engine says CB750E... are these mismatched?

Thanks all!

 

[gben]

You already said the engine has spark and fuel to all cylinders right? Pretty hard not to have a fire when you mix gasoline and sparks...

Do a compression test. All the cylinders should have the same compression. If one is low squirt some oil in the cylinder and if it comes up it is worn rings, if it does not come up it is a bad valve, maybe burned or bent.

Bad rings should not keep a cylinder from firing, but a bent or burned valve can let the fuel/air escape and it will backfire or pop through one of the carbs.

If the compression is good, then you have simply made a mistake and one of the cylinders is not getting fuel or air. A "quick" carb cleaning might be your problem, I don't know what that means but the only way to clean a carburetor is 100%, and you have to have a 100% clean tank and lines or a filter to keep debris out of your 100% clean carbs or they will quickly become not 100% clean.

 

[fiedler.casey]

I did a meticulous compression test and this is what I found:

Dry Oiled
Cyl 1) 145psi / 153psi
Cyl 2) 135psi / 150psi
Cyl 3) 137psi / 143psi
Cyl 4) 133psi / 145psi

I'm trying to get the bike running for a friend who is on a tight budget and I don't want to sink any more of my time into that I need to in order to get it moving down the road in whatever marginally functional crappy shape that might be.

If it were my baby, I'd be doing a 100% rebuild of everything, but it's not.

Do these compression numbers seem within reasonable limits to get Cyl 4 running?

If so, I'll focus on doing a more thorough cleaning of the carburetor and intake. A few of the carb boots are less than perfect, I'm sure that's messing with the mixture a tad. The airbox is there, but the filter is not so I'm sure that's probably also messing around with the mixture a bit.

There's a Dyna S ignition on the bike, should I try timing the ignition? Could that be all or part of the non-firing #4? My suspicion is that if timing were off then #1 and #4 both wouldn't be firing since they're both firing at the same time, right?

I'll wait to hear some advice from those of you more experienced than I, but my suspicion (now) is that it's a carburetor problem on barrel 4.

 

[JWExperience]

I'm 99% sure it's carbs. Those compression numbers look great for the age. Make sure you spray out all the little passages and the floats are set correctly. You should prob also put clear tubes onto all the bowls and see where the fuel level is.
A timing and valve check wouldn't hurt for piece of mind then when it's running better do a synch.

 

[fiedler.casey]

After absurdly obsessive testing and three full days of troubleshooting I found out that the 1/4 coil is broken. It sparks both plugs when grounded against the engine testing spark but when the plug lead for cylinder four is inserted into the head, it faults. I hooked up the coil for 2/3 and ran the bike flawlessly off of only cylinder four. New coil it is, then!

Remember kids, just because you have spark doesn't mean you have spark!

 

[fiedler.casey]

I'm 99% sure it's carbs. Those compression numbers look great for the age. Make sure you spray out all the little passages and the floats are set correctly. You should prob also put clear tubes onto all the bowls and see where the fuel level is.
A timing and valve check wouldn't hurt for piece of mind then when it's running better do a synch.

Thanks for the response by the way!

 

[JWExperience]

Well done! Enjoy riding your healthier bike