Head problems....

westonboege · Full Member Joined: Jun 03, 2008 Posts: 30

Hey guys... been a while since i posted... I pulled my motor from my 72 xs 650... tested compression and it was making 90 psi on both cylinders... decided i need a valve job...

What im wondering is I may have access to a good head off of a 78 motor... Are the heads interchangeable between motors? Or no?

Retiredgentleman · Posted: July 23, 2009, 8:40 pm Post subject: Re: Head problems....

I believe they are inter-changeble, but if you use the 78 head you need to have the matching 78 rocker cover. The head and the rocker cover are machined as a matching pair in the factory.

You can't swap the 78 camshaft into the 72 motor. 72 motor has 17 teeth on the crankshaft and 34 on the camshaft. 78 has 18 teeth on the crank and 36 on the camshaft.

yamaman · Posted: July 23, 2009, 9:20 pm Post subject: Re: Head problems....

You can change sprockets on the camshaft if need be, I do believe!

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

I would just refresh the 72 head with a complete valve job and be done with it. If the head gasket surfaces are exact,then swapping out is no problem.

tacoswild · Posted: July 23, 2009, 9:28 pm Post subject: Re: Head problems....

the head gaskets are different, at least they're supposed to be. That raises some red flags. From looking at pictures of the two I think the camchain tunnel is a slightly different shape at one end at least.

yamaman · Posted: July 23, 2009, 9:32 pm Post subject: Re: Head problems....

What made you decide it was the head anyway? Did you do a leakdown test? If both pistons were the same comp reading, you either have the same problem with both sets of valves or normal even ring/barrel wear (more likely)!

If your pulling it down, it's what you find that defines what you need!

I hope it all works out, keep us in the picture as you go, cheers

westonboege · Full Member Joined: Jun 03, 2008 Posts: 30

Well my problem is like most peoples probs.. money.

I checked comp dry, then with a teaspoon of oil... then ran the engine and checked again... all came back with around 90psi per cylinder... with the throttle wide open...

I was told if the compression didnt change between wet and dry readings its likely the valves... When the motor was running there was alot of valve noise from the right side of the motor...

If i rebuild this motor im gonna end up spending upwards of about 500 bucks....

Will prob need new valve guides, stem seals, cam chain, gasket set, needs new clutch, oil feed tube, rings, couple of other odds and ends...

stuff ads up fast...

I found a complete bike with 8k on the motor for 300 bucks on craigslist... so me new plan is to try and get that bike... swap motors...

Then slowly rebuild mine put it in the new bike, and sell the bike...

if you guys have any shortcuts or other insight let me know... i kinda just wanna get the bike back on the road for whats left of the summer... then tear it down again this winter and do it right...

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