Topic: I think I all ready know the answer but....you guys are smarter than me

[greenjeans]

Got a chance to buy a bunch of CB750 motors, frames etc.
Hopefully with all the parts and a bunch of elbow grease, it would yield 1 good resto-project.

There are 3 engines.  All are out of the frames and all spin freely.  

I'm hoping there is one of the motors that I would not have to tear into.  I can shift them through the gears and everything seems ok (wishfull thinking - I know)

I know that compression checks or leakdown tests should be run on a warm engine.   I have a set of guages that I can check compression with, but no leakdown tester.  

Can either a compression check or leakdown test be accurate on an engine that is out of the bike and not able to be properly warmed up ?

Any ideas, explanations or encouragement greatly appreciated.

[jtsmith]

I think all the guys on here are smarter than me too, but I figure I will put in my two cents.

A compression test on a cold engine will at least give you relative numbers. In other words, you can see if all the cylinders are reading close to the same, which is a good sign. You just might not get the correct "warmed up" value you might find in manuals.

Hope that helps (and that I am not wrong!)

cheers,
jt

[Really?]

Put em in a frame and crank em up, lol.  You said you had multiple of both.

[Nikkisixx]

"The only true wisdom is in knowing you know nothing."  - Socrates
I know a little more than a box of hair, but less than my goldfish.  I am wise like Yoda.

That being said, by running a compression test you'll know if the valves are bent or not sealing, if the rings are in the oil pan, and how big the hole in the piston is.  I'd pop off a cover or two and see how funky and sludge filled the case is and run a compression test to select the best candidate.

[Gordon]

If you're looking to find the best motor to use without doing a full top end rebuild, then a cold compression test can give you a good idea of that, but I'd still take the top end apart after deciding which one to go with to replace all the gaskets. 

[greenjeans]

I should know better....time to start sourcing gaskets etc etc....

[GammaFlat]

I think you're on a solid path but.... you might consider this.  You have 3 engines.  Pick a good one and run with it.  As soon as you pull the top end apart you'll get "two-foot-itis"... well as long as I have the head off, I'll work on the valves a little... oh, heck I might as well port a little... and maybe I can find some cheap oversized pistons...  You get my drift.  Save all that for engine #2.  Get something in there and get it running. 

[Gordon]

And then when the head gasket and/or cam tower pucks start leaking, take the engine back out and take apart the top end to put in your new gaskets.

[scrapvalue]

I bought a pair of CB750's that were in a basement for 13 years. One was a 1974 and the other a 1976. Cleaned one set of carbs, put them back on and started it up. Ran great.
Put the clean carbs on the other one and it ran great also.
Built the bike in the picture out of the 76. Have put 5000 miles on it in a year and a half.(Northern Michigan, so that is only about 3 months of ridable weather).
Bike still runs excellent. No problems.
I still have a complete bike to do something else with or just use for parts.
The 1976 didn't have the speedo on it so I have no real idea of how many miles were on it.
The 74 showed 24,000 miles.

[ARANUC]

60 bucks for gaskets for an engine rebuild at www.z1enterprises.com i just got some last week

[dave500]

hey scrap value,,three months of ridable weather?i couldnt stand that.,,,greenjeans,see if you can get one engine going good enough to use,,and maybe tear one apart to learn,,you might have to buy some new tools,,maybe tear two apart and compile the best parts of both to rebuild a complete motor,,you can practise putting one back together with out gaskets and sealer and take it apart again a few times if you want,and do it again.youll get familiar with them that way.