• Required reading for all forum users!!!

    Welcome!
    Register to access the full functionality of the GSResources forum. Until you register and activate your account you will not have full forum access, nor will you be able to post or reply to messages.

    A note to new registrants...
    All new forum registrations must be activated via email before you have full access to the forum.

    A Special Note about Email accounts!
    DO NOT SIGN UP USING hotmail, outlook, gmx, sbcglobal, att, bellsouth or email.com. They delete our forum signup emails.

    A note to old forum members...
    I receive numerous requests from people who can no longer log in because their accounts were deleted. As mentioned in the forum FAQ, user accounts are deleted if you haven't logged in for the past 6 months. If you can't log in, then create a new forum account. If you don't get an error message, then check your email account for an activation message. If you get a message stating that the email address is already in use, then your account still exists so follow the instructions in the forum FAQ for resetting your password.

    Have you forgotten your password or have a new email address? Then read the forum FAQ for details on how to reset it.

    Any email requests for "can't log in anymore" problems or "lost my password" problems will be deleted. Read the forum FAQ and follow the instructions there - that's what we have one for...

  • Returning Visitors

    If you are a returning visitor who never received your confirmation email, then odds are your email provider is blockinig emails from our server. The only thing that can be done to get around this is you will have to try creating another forum account using an email address from another domain.

    If you are a returning visitor to the forum and can't log in using your old forum name and password but used to be able to then chances are your account is deleted. Purges of the databases are done regularly. You will have to create a new forum account and you should be all set.

Confused about compression after research

  • Thread starter Thread starter MechMessiah
  • Start date Start date
M

MechMessiah

Guest
I'll start with the problem I'm having which is very low compression in 3 out of 4 cylinders after doing a top end rebuild. Yes, I did the compression test with the throttle open. Cylinders 1, 3, and, 4 had very low readings. Maybe 10 to 15 psi but my gauge doesn't start till 20. Cylinder 2 had a nice reading of 130ish though. I did a dry and wet test on the bad cylinders with no noticeable change in the reading. So that's my problem, and here's a link to my rebuild thread for more info http://www.thegsresources.com/_forum/showthread.php?239949-82-GS650G-with-lots-of-problems.

Now for how I got confused. It happened once I started reading other threads on here about other people with low compression after top end rebuilds. It was very common to see responses that basically said 'don't worry about compression on a rebuilt engine till it's got some miles on it'. Now most of those types of threads either didn't give their compression numbers or they were in the 50-70 range which is significantly different from my almost 0 reading but it still confused me as I though these engines needed to be around 100 psi to run.
 
RGM said:
Check your vales are closing
Click to expand...

I'm just going to pull the top apart again. If 3 cylinders have valves that won't close/seat properly then I can't think of a way to fix that with the head still on the engine.
 
MechMessiah said:
I'm just going to pull the top apart again. If 3 cylinders have valves that won't close/seat properly then I can't think of a way to fix that with the head still on the engine.
Click to expand...

Well.adjusting your valve clearances comes to mind
You say you did a top end rebuild, did you lap the valves?
 
Big T said:
Well.adjusting your valve clearances comes to mind
You say you did a top end rebuild, did you lap the valves?
Click to expand...

I should have finished that thought. I can't think of a way to fix that with the head still on that I haven't done yet. The Valve Clearances were adjusted and triple checked and yes also lapped. The link to my rebuild thread is in the first paragraph of this thread and also here.
 
Did you check the valves for leakage after you lapped them in? Have you put new piston rings in? If you did, did you hone the bores! The compression readings are poor on 3 cylinders. Are the valves clearances set correct?
Did you check the cylinder head for warpage?
 
Going right outside the box here.
Might be worth having a close look at how the valves return before taking off the head. Is the return smooth all the way back up and can the buckets be moved easily when the cam comes off them at the top of the return. #2 is your comparison.
Carbs, was anything done to them. If you are going in I would try another compression test with the carbs off before removing the head.
Is the exhaust free and clear ?
 
londonboards said:
Does the engine fire? How does it sound and ride?
Click to expand...


Thanks for the suggestions everyone but that wasn't why I made this post. I'm confused about comments like the one above and trying to get some clarification. I was always taught that a combustion engine needs three things to run. Spark, Fuel, and Compression. With out one the engine just won't run.

However when I was doing research about very bad compression I kept seeing people ask "Will it run/start?". Were these bad questions, people not paying attention to details of the situation? Or have I been taught wrong about compression? Can you run an engine with drastically bad compression?
 
Last edited:
If anyone is interested I found the problem. It's been posted in my rebuild thread here. I'm still curious about this compression quagmire though. Isn't it a fundamental requirement for a combustion engine?
 
So what is the root cause of the problem? Undertorqued head nuts or a bad gasket?
 
You must log in or register to reply here.
Back
Top