Topic: Questions re: Basic rebuild of '78 CB750K motor

[eastbloc]

Our '78 CB750K has just clicked over 48,000 miles.  It suffers from leaky head gaskets, low compression in cylinder #1, less than spectacular (but mostly within spec) compression in the other three.  After long rides, she seems to pee a bit of oil down the sidestand, leading me to suspect that the output seal is going, too.

In short, I think she'd stand to benefit from an overhaul.

We're touring riders.  The bike is outfitted in vintage 70s highway style, with a Windjammer and full Bates bags.  She's no beauty queen, and above all I'm looking to have a functional, pleasant, go-anywhere tourer that won't break the bank.  She's just about there, but would be a lot closer to my ideal if the motor was refreshed.

I'm assuming new gaskets, seals, rings.  Maybe valves and a hone job, in the worst-case. 

My hometown is New York, where I have other bikes.  The CB lives in Denver, in my friend's garage, otherwise I'd tear her down and do the work myself.   

So I have a few questions.  First of all, what can I expect to pay to have someone do this work?

Second, does anyone know someone local (Colorado) that knows these bikes and would take the job on for a reasonable price?  We could deliver the motor or the whole bike, whichever made more sense.

Finally, are there any mods or improvements that I will absolutely be kicking myself for not getting in there during the rebuild that are cost-effective?

I'd hope to be able to do this job for $500 or less.  Does that sound reasonable?

P.S. I just finished a thousand-mile tour on the old beast and was very pleased with her.  I'm hoping to do many thousands more!

[SohRon]

Where in Colorado?

[Gordon]

I'd hope to be able to do this job for $500 or less.  Does that sound reasonable?

Double that is more like it, if you want it done well. 

[eastbloc]

The bike is in Denver.

I have experience doing top-end work mostly on pushrod machines -- BMWs, Urals and old Harleys.  It's not that bad of a job there, boxers are obviously easier, but even Harleys don't require you to pull the motor to do this task.   

I assume the CB does, and this must add to the labor cost.  My friend whose garage the bike lives in wouldn't be up for a top-end job, but he could certainly pull the motor if it would offer a significant savings to getting this done.

Appreciate your thoughts, keep them coming please. 

[eastbloc]

One last thing, given the costs you've estimated for rebuilding my stock mill, is there any reason I wouldn't want to find a low-mileage motor to simply swap in?  They seem to be available for reasonable prices.  Granted, I might be taking a chance, but it might still cost me a quarter of the rebuild.

Also, would a CB750F motor fit, and would it offer any advantages?

[JohnG]

You need to also consider the possibility that your pistons are worn significantly.  The key measurement is the vertical gap between the top compression ring and the groove in the piston.  If it is too great, from heat and use, then new rings will still loose oil and compression.  You might find the gap  (with a feeler gauge) to be .010" whereas the spec is more like .003" or less.  This often goes unchecked, with people focusing on the skirt diameter instead.

I used to do alot of touring and I don't think any touring rider likes a motor that they just rebuilt to use oil...