Topic: When compression ratings are out of whack

[eigenvector]

I did my rebuild compression check today.  I'm not happy with it, but I have to fiddle with the valves some more.

Mind you I did my compression check via kickstarter - and honestly my kickstarting technique leaves much to be desired.
#1  100psi
#2  120psi
#3  145psi
#4  120psi

The number 1 compression rating in retrospect probably has to do with me giving up on it sooner than the rest.  It was the first one I did, and I didn't have a feel for when it was time to stop.  I'll fudge a bit and say it was in reality closer to 120psi  145psi seems a bit too high.  Mind you, the manual indicates that 170 psi is the 'standard' compression rating.

I'm trying to decide if I should dig into the #3 cylinder a bit or, since this is a fresh rebuild, get the bike running for a while - seat the rings and all that, and THEN start taking a hard look at the compression.

[70CB750]

+1 on getting it running first.

Why would you kickstart for compression check in the first place?

[chewbacca5000]

The best way I have found is to do your compression test with the carbs off and to turn kill switch on and hit the starter button for 5 seconds or so.  I would do that before "digging it" as your testing procedure is suspect.

[Bailgang]

  and to turn kill switch on

Huh? Why would you want the coils energized when doing a comp test or is that a typo?

[chewbacca5000]

  and to turn kill switch on

Huh? Why would you want the coils energized when doing a comp test or is that a typo?

Yeah typo.  Good catch.  Kill switch to off.

[eigenvector]

+1 on getting it running first.

Why would you kickstart for compression check in the first place?

Didn't have a battery handy at the time.
Kickstarter turns the engine over just the same.

In any case I'll just not worry about this for now.  It gets me a ballpark figure and I can live with that.  I'll revisit when I get a few hours on the engine.

[chewbacca5000]

+1 on getting it running first.

Why would you kickstart for compression check in the first place?

Didn't have a battery handy at the time.
Kickstarter turns the engine over just the same.

In any case I'll just not worry about this for now.  It gets me a ballpark figure and I can live with that.  I'll revisit when I get a few hours on the engine.

If you find comp is low then using 50/50 mixture of atf and acetone in 2 cylinders at a time helps to unstick rings which is a common problem if the bike has had a long sit.  Make sure pistons are all the way down then fill it up.  Let it sit for 2 to 3 weeks, and with plugs out kick it over.  Make sure no one or nothing is in your path it will shoot for 15' for more.  Repeat for other cylinder.  I did this and it brought the comp back up on all 4 cylinders.

[RevDoc]

If you find comp is low then using 50/50 mixture of atf and acetone in 2 cylinders at a time helps to unstick rings which is a common problem if the bike has had a long sit.  Make sure pistons are all the way down then fill it up.  Let it sit for 2 to 3 weeks, and with plugs out kick it over.  Make sure no one or nothing is in your path it will shoot for 15' for more.  Repeat for other cylinder.  I did this and it brought the comp back up on all 4 cylinders.
[/quote]

Hey, Chewie, thanks for sharing that great tip!

[chewbacca5000]

If you find comp is low then using 50/50 mixture of atf and acetone in 2 cylinders at a time helps to unstick rings which is a common problem if the bike has had a long sit.  Make sure pistons are all the way down then fill it up.  Let it sit for 2 to 3 weeks, and with plugs out kick it over.  Make sure no one or nothing is in your path it will shoot for 15' for more.  Repeat for other cylinder.  I did this and it brought the comp back up on all 4 cylinders.

Hey, Chewie, thanks for sharing that great tip!
[/quote]

Also, make sure you do it in warm weather no chemicals work less than 60F very well.  You can get alot done in 80F once a year here is Syracuse.

Make sure you, pets and others are clear when you kick it out.

[RevDoc]

Having grown up in Auburn, 30 miles west of "Syracorpse", I know just what you mean, Chewie!

[Hot500]

If you have just finished a rebuild with new rings and honed or rebored sleeves, then the comp figures you have are fine.
Allow some time for the rings to bed in properly and you will see the comp figures come up towards the stated 170psi.

[chewbacca5000]

Having grown up in Auburn, 30 miles west of "Syracorpse", I know just what you mean, Chewie!

It is a balmy 10f today!  Think I will head to the beach.  Where you at now?

[eigenvector]

If you have just finished a rebuild with new rings and honed or rebored sleeves, then the comp figures you have are fine.
Allow some time for the rings to bed in properly and you will see the comp figures come up towards the stated 170psi.

Thank you for the reassurance.
It's a bit irritating holding down a full time job and trying to rebuild an engine.  You have to wait 5 days in between to see if you're making progress.