New kid on the block

Posted: Mon Sep 28, 2015 9:44 am

Hey everybody, this weekend i just picked up a CB750K as a project bike. This is my first motorcycle, and first time working on one. I've done a bunch of car mechanics, so i'm no noob to turning the wrench. I will admit that the only internal engine work I have done is a valve adjustment to a J30A4 and a d16z6. This will be a learning experience for my brother (who just began schooling for aircraft mechanics) and I (a mechanically inclined IT guy).

But enough about me, let's talk about the bike. I picked her up with ~20K miles on her, with a few issues, and a donor engine with a good top end. The main issue with the original is a hole in the cylinder jugs made by a loose timing chain. It looks like the adjustment bolt has been snapped by a previous owner, and the hole has been patched with quick steel. She's tricky to start (need to mess with the choke in very small increments) and idles a bit rough (around 1k). The guy i bought it from says the carbs probably need to be cleaned at this point as well.

So far I have replaced the engine oil and gas because I had no idea how long it was sitting in there, and the bike seems to idle and start a bit better.
I did notice some metallic fluid in the oil when draining, which is expected given the chain rub.

Just wanted to say hi and give people the run down on what I have on my hands. If anybody has some sort of direction or advice to give me, it is more than welcome and will be appreciated.

Posted: Mon Sep 28, 2015 10:01 am

Welcome to the site 1337pi107 ... Smile

Post a of the patch job and the rest of your bike .

If the chain is that loose , I wouldn't run the motor .

Posted: Mon Sep 28, 2015 10:43 am

hello

Posted: Mon Sep 28, 2015 1:08 pm

I'll see if I can get the girlfriend to go take some pictures of the patch, if not ill post them up once i get off work.

Here is the unpatched crack- a picture the previous owner sent me.

Posted: Mon Sep 28, 2015 1:19 pm

here and enjoy the site

Posted: Mon Sep 28, 2015 2:29 pm

Welcome here wave

I don't get it, just what are we supposed to be seeing there?

Quote:

Here is the unpatched crack- a picture the previous owner sent me.

Posted: Mon Sep 28, 2015 3:16 pm

F model tail on a K model bike. Very interesting.

O RLY?

Posted: Mon Sep 28, 2015 4:22 pm

genesound wrote:

Welcome here wave

I don't get it, just what are we supposed to be seeing there?

Quote:

Here is the unpatched crack- a picture the previous owner sent me.

That's where the A chain wore through the cylinder case and supposedly the hole repaired with liquid steel or the like .

Posted: Mon Sep 28, 2015 4:27 pm

genesound- The picture is of the non-patched crack in the cylinder jugs made from the cam chain. PD750 requested a few..

Whozaa- Yeah it made it a bit weird finding out the model. had to ask the previous owner.

PD750- I got the GF to take some but they are a bit blurry. if not good enough, ill get better ones at home.

This is the crack

The rest are of the patch job

Posted: Mon Sep 28, 2015 6:10 pm

That patch job is pretty ugly . Does it leak at all ?

And , what did the PO do to correct the problem , other than slobber some whatever it is over the hole ?

With that much damage , I'd have to say the A chain was totaled as well as the guide and most likely the tensioner .

Posted: Tue Sep 29, 2015 9:33 am

Yeah it is. He also did me the favor of tightening the tensioner and snapping the adjustment bolt. It does leak a small amount above the fin on the right side. I'm currently making a list of expenses for my brother and I to plan out, so ill mark that down as a probably. Any good brands you recommend?

Posted: Tue Sep 29, 2015 9:48 am

You might consider replacing the cylinders altogether. In any case, I'd tear the motor down to see what else is broken. You'll want to replace the chain, tensioner, and guide (obviously).

Posted: Tue Sep 29, 2015 1:29 pm

What is the matter with the donor engine. You pretty much have to pull the engine. Swap them out...easy-peasy Wink

Posted: Tue Sep 29, 2015 3:42 pm

I'd make every attempt to get rid of that cylinder block and the associated cam chain parts pending close inspection. Check the foot in the case too, where the A guide rests.

Is the donor engine usable? It might be in better shape overall than this one, which has quite obviously not had adequate maintenance for an extended period in order to do what it did.

The other motor might be the one to fix. This one has been nuked and has suffered quackery. I would be reluctant to trust much on this motor that's apparently been run with lots of metal shavings in it. Who knows what damage that did.

Posted: Tue Sep 29, 2015 3:46 pm

I've been told that the bottom end of the donor engine is bad from sitting in water. not sure how salvageable that would be.

Posted: Tue Sep 29, 2015 3:50 pm

I really think I'd look for a runnable salvage motor then. You could end up throwing good money after bad on that chewed up one, and the other one should have been stripped and cleaned immediately after being submerged... even then, most of the bearings would need replacement from dirty, sandy water contamination. By now it's likely a rust factory.

Posted: Tue Sep 29, 2015 4:32 pm

Crap. I was afraid that would be the answer. i suppose the next questions should be "will only the cb750 motor be plug and play?" and " any good sites other than craigslist or ebay that i could find some?"

Posted: Tue Sep 29, 2015 5:34 pm

You probably only want a DOHC 750 motor. A DOHC 900 or larger chain drive motor will fit with some motor mount and exhaust conversion issues, but no other motor type will be anywhere close to a plug and play. Other motors would probably require cutting and welding.

Craigslist, ebay, other bike guys at internet forums, maybe salvage yards.

Posted: Tue Sep 29, 2015 5:51 pm

Yeah, if you can find a good used motor that would be ideal. I wouldn't want to open up a questionable motor. Just opening up a good one costs some $$$.

Posted: Wed Sep 30, 2015 8:40 am

I would just take the head off of the donor motor and check it out. If everything is in good condition and you don't need it for the rebuild, you can sell it to help fund the project.

Posted: Fri Dec 18, 2015 1:58 pm

So I found a 750 dohc motor with a compression of (LEFT TO RIGHT) 1- 135PSI, 2- 155PSI, 3- 155PSI, 4- 140PSI . Is this acceptable for these bikes? also, are there parts that i should get before starting the swap?

Posted: Fri Dec 18, 2015 2:32 pm

It's enough to run. It's not really optimal, but that may improve with some running too. If I was doing it, I'd pull the head and de-carbon it, check how well the valves seal with solvent and lap if necessary, put Viton valve stem seals in it, and put a Timesert in the B cam chain tensioner hole, at a minimum. Also check all the tensioners, chains and such.