Low compression on engine looking to rebuild

[BeardedGinga]

I bought a 1978 XS400 it has 80 on one cylinder and 60 on the other the numbers get a little better when I put some oil in the cylinders I haven't done a leak down test on it yet so don't know about the valves yet. I was going to make sure the valves are adjusted properly before I did the leak down test and rechecked the compression. Now my question is, is there anything else I should be looking to check before I fully commit to rebuilding it.

 

[xschris]

Make sure the valves are adjusted to spec. But with numbers that low I would think a top end rebuild is in order.

 

[BeardedGinga]

I will I have already downloaded and am reading the manuals

 

[Scorpio1963]

What was the condition of the bike when you bought it?Had it been sitting outside in the weather or just tucked away in the corner of someones garage?Good luck with engine rebuild.Hopefully it doesn't need pistons!

 

[BeardedGinga]

It has been sitting in a garage for the past 8 months the previous owner wasn't real clear about how it was stored by the owner before him.

 

[BeardedGinga]

So I adjusted the valves and the points and I was able to get it running now I'm taking the engine out to rebuild it.

 

[Scorpio1963]

Did you do a compression check afterwards?Did it burn oil and how well did it run?

 

[BeardedGinga]

The compression check produced the same numbers as the first post. It was running very rich couldn't tell if it was burning oil. It revved up to 8 grand no problem throttle response was quick. I didn't hear any clinking or clunking or ticking.

 

[xschris]

At 60-80psi the bike would not run. Do you turn the motor over and open the throttle fully when testing it? It will also take a few turn of the motor to get the full amount of compression to read. The bike will need around 115-120 to run.

 

[Scorpio1963]

I was thinking the same thing last night when I read his response.Also if it runs that good maybe adjusting and syncing the carbs is all that is needed and a topend rebuild isn't neccasary

 

[BeardedGinga]

No I did not hold the throttle open when I did the test

 

[xschris]

That may be why the numbers where low.

 

[BeardedGinga]

I'm getting ready to go to my buddies house which is where the bike is and I'll try it again. I took the carbs off last night to get started on cleaning them so I don't really need to put them back on do I?

 

[xschris]

Nope.

 

[OleDoc]

Nope.

Exactly.don't need to hold open the throttle nowmy Vulcan would read 60psi with the throttle closed and 150psi when opened

 

[BeardedGinga]

Can I still twist the throttle and make engine noise while I do it?

 

[OleDoc]

Can I still twist the throttle and make engine noise while I do it?

Of course.that's how it's done

 

[BeardedGinga]

I just tested it again and there was no improvement

 

[xschris]

If the bike starts and runs I would think it's the gauge.

 

[OleDoc]

X2 on that Chris