Hey guys, killed my new to me '83 CB 750 today out on I-10. had it 5 friggin days. I guess I was asking too much of it to cruise at 7000RPM for several hours in 110F weather.
Preliminary thinking is I holed or completely destroyed a piston(2). Its only running on three cylinders, exhaust on piston 2 is cold and it its spurting oil everywhere (breather and random seals) which means positive crank pressure. Oil is burnt. BUT it got me 15 miles like that to the next town ....no way in hell was I stopping in the middle of the desert. So I'm pleased with it in that regard.

We'll see when I rip it apart next week.

Anyway. Where do I get gaskets, pistons, rods etc? I understand I can get it all from the dealer, I also understand dealer prices on such parts ..ebay is sparse (surprisingly...)
this is my first bike, and I'm from the car world where you can just hop on summit racing and get everything you need....

Thanks,

Mike

 

Try to find a complete used engine first. If you plan on a rebuild and know what you need (got a service manual?) then bike bandit and ron ayers will get you what you need.

Good luck.

7000RPM on the highway? How fast were you going?

 

not sure, I don't have a working speedometer lol. However, I was still getting passed by people. Speed limit is 75MPH, working speed for that stretch of road is about 85. We have no truck speed limit either, so they run up there as well. Anyway, i was getting passed ALOT doing 6500RPM(couple bikes too), so I kicked it up to 7000RPM - This bike seriously needs a 6th gear.....

EDIT: ron ayers only goes to 1984 doh!

 

find out if the 83 and newer cb750's share the same hard parts (gaskets, rings, pistons, etc.). Knowing that will also help you source a replacement engine.

I know what you mean by riding a slower bike (my KLR650 for example) but I just learned to slow down and keep out of the way.

 

yeap... what duy said.

Also... yes, 7k on that sohc is a bit much, those bikes are very much different than the modern engine and the way they deliver power.

 

its a DOHC (well at least it looks like it from the head), and it seemed perfectly happy and smooth at 7k....its a CB750SC I guess the Nighthawk....and they only made that in 82-83 according to bike bandit. Even looking on ebay this thing is hard to track down.

http://www.bikez.com/motorcycles/hon...thawk_1983.php

 

Ok guys, took it all apart - very large hole in piston #2. Now what would you guys recommend? I am on a severe budget. I can get used pistons on ebay for about $20....throw the old rings on a new piston stick it in and call it day.....and replace a couple gaskets... I really don't have the time or money to get the cylinders bored and buy new pistons for it....

or get a new engine. I found one out of a CB750Custom in Cali for $350, and it looks to be the same engine, do they mount out the same though?

 

not sure if they mount the same, but it'd probably be easier to just swap the engine, drop out bad, bolt up the good, ride away. On older bikes that have had motor troubles its almost easier to just swap the motor than to do a rebuild.

I can't imagine what bad would happen to that engine with just a used piston and the old burned up rings.

 

Well I've decided to go ahead and rebuild it. Getting a new engine would be much easier/faster/and more convenient yet more expensive...I'm a college student, and the one thing I have is plenty of time, and the one thing i do not have is plenty of cash lol. The upper end has also been recently rebuilt, and lower ends....well hopefully the lower end doesn't fail. lol.

I just ordered 4 pistons and cylinders off Ebay for $50 off a '82 DOHC. 30k miles on them. Mine have 48k+ (that was when the odo stopped turning) on em.
I'm going to hone the cylinders, put new rings on the pistons, and throw them on my connecting rods......I just need a gasket set and rings, I guess.

Anyone know where to get those? ServiceHonda says my bike doesn't exist, so I dont know how to get parts from that site....and I'm somewhat leary of ebay gasket sets....though they are cheap. $100 for an entire engine gasket set. Also, rings? where do I get em? cheap(er). I'm seeing around $100 just for rings on ebay, that seems awefully expensive. $100 for 8 rings on a Chevy 350, sure...but 4 little rings for a honda?

 

A piston has a hole in it but the head is OK? Your engine has had a catastrophic failure. I hope you don't think you can just pop in a new set of pistons and rings and ride off. You're going to want to go through the entire engine looking for aluminum shavings.

If you try to do this rebuild on the cheap, you're going to end up doing it again soon. My $.02

ps: you may want to look here:
http://www.advrider.com/forums/forumdisplay.php?f=35

As a group, these guys have a lot more experience with old bikes than HT does.

 

+1 engine internals are no joke. if you can get a replacement for $350 jump on it and save yourself the headache