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Low compression GS850GT

  • Thread starter Thread starter Derek Mc Donald
  • Start date Start date
D

Derek Mc Donald

Guest
My old GS850GT has not been starting good lately, plenty battery power and turns over very easy, it fires OK but then dies after a couple of seconds, I go through this procedure maybe 10 times and eventually it runs. After it has been started it restarts no problem. I checked the cylinder compression on a warm engine and found 1 -4 (92,100,100,100 psi) I put some oil into cylinder 1 but this didn't improve the reading so I guess the rings are OK. I don't know the full service history of the bike so I can't say when the valve clearances were last done, could anyone say if poorly set valve clearances would give these low compression readings?
Much appreciate any advice. :cool:
 
First point of call is checking your valve clearances if they haven't been done for a while.

As for testing the compression, did you have the throttle wide open too?

Tight valves can give lower readings. Those seem low for GS spec's but its not far out on all 4. What sort of mileage has she done?
 
It's done 44k, yes I did have the throttle full open. I'll take your advice and check the valve clearances, wanted to avoid this if possible as it seems difficult to get the shims.:cool:
 
Have you gone to the GS Services section and looked up the GS Shim Club yet? Might want to. Then you might want to go to http://members.dslextreme.com/users/bikecliff/ and download the service manual as well as read up on all the great things Cliff put together there especially the Read Me section. Then search out the Top Ten Newbie Mistakes thread.
 
On the 850, normal compression is between 128 - 171 with a lower service limit of 100.

I would definitely check the valve lash before I get too excited.

Cheers
 
Derek Mc Donald said:
It's done 44k, yes I did have the throttle full open. I'll take your advice and check the valve clearances, wanted to avoid this if possible as it seems difficult to get the shims.:cool:
Click to expand...

Much easier to wait and buy new valves, gaskets, stem seals and the rest after your tight valves burn up. :D Wait, you would still need to find shims. :eek:
 
Derek Mc Donald said:
It's done 44k, yes I did have the throttle full open. I'll take your advice and check the valve clearances, wanted to avoid this if possible as it seems difficult to get the shims.:cool:
Click to expand...

Get 'em from Z1 enterprises. Last I checked, that was the cheapest source. They ship quickly too.

To directly answer your question: Yes, tight valves can affect compression. When the clearance goes away, the cam is holding the valve open at least a little, all the time. Combustion gases then begin to burn the valves.
 
DMD

It seems Ed and Tom are off their meds tonight

Valve lash is critical and is the 1st thing that should be looked at when starting is hard

Read your BassCliff, download a manual, order a valve cover gasket and get Steve's spreadsheet. Whether you use the shim tool or the zip ties is up to you.

Then order your shims, easy to get, I see they're expensive in the UK
 
Thanks for all the advice, I have ordered the valve cover gasket so I can get the clearances set then I'll recheck the compression and take it from there.:)
 
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