Low compression, possible causes?

I'm considering purchasing a 2003 sv650s as a project for the remainder of the winter. The bikes only major problem is it has little to no compression in the rear cylinder. The two causes that popped into my mind were intake valve(s), or rings. Can anyone lend any insight into other potential causes? A ballpark figure for the cost of parts would be nice as well.

thanks,
phil

[Sid Squid]

If the compression is low due to valve sealing problems, the more likely area of trouble is the exhaust valves, not the inlets. Before looking to address any troubles in that area, ensure the valve adjustment is correct.
The SV650 in all it's forms is a popular bike, there is therefore a plentiful supply of used engines and parts thereof, if any significant amount of engine work is necessary investigate the costs carefully.

[northwind]

Further to what Mr Squid says SV engines with bad bottom ends are plentiful, which means lots of scrap engines with decent tops. Considering that you can get a rattly engine for £50, I'd be looking at that as an option- though of course, it can be a gamble too since you never really know what you're getting. If there's no compression, suggests a valve really.

Surely bad compression (or lack of any) can only be 2 things, assuming the head itself isn't cracked it must be badly seating valves or bad piston/ring/bore wear.

One thing to bare in mind though, in the past I've seen valves so badly adjusted that they never fully closed at all.

Good luck.

[jambo]

Originally Posted by tapemonkey

Surely bad compression (or lack of any) can only be 2 things, assuming the head itself isn't cracked it must be badly seating valves or bad piston/ring/bore wear.

One thing to bare in mind though, in the past I've seen valves so badly adjusted that they never fully closed at all.

Good luck.

Valve clearances make sense as a 1st check. After that it could be valves or piston rings as mentioned above. Also a shot head gasket's a possible cause as well (Though often you end up with "milky" emulsified oil in this case but I'd imagine it's not always the case).

In short you won't know until you're in there but careful shopping can keep the costs down if you're ok with a set of spanners & feeler gauges

Best of luck!

Now i had this problem...once the valves had been done the compression was fine.

Originally Posted by Sid Squid

If the compression is low due to valve sealing problems, the more likely area of trouble is the exhaust valves, not the inlets. Before looking to address any troubles in that area, ensure the valve adjustment is correct.
The SV650 in all it's forms is a popular bike, there is therefore a plentiful supply of used engines and parts thereof, if any significant amount of engine work is necessary investigate the costs carefully.

i completly disagree with you on that one as i have just stripped my klr250 top end down due to low compression, (after being told by bike shop it needs a re-bore) one inlet valve was so worn it had gone up past its seat, therefore no gap at all, and the second inlet valve was on the same way out. both exhaust valves were in top condition. two valves later and runs like a dream.

[Sid Squid]

Originally Posted by tapemonkey

One thing to bare in mind though, in the past I've seen valves so badly adjusted that they never fully closed at all.

True, as mentioned above.

Originally Posted by madmartyn

i completly disagree with you on that one as i have just stripped my klr250 top end down due to low compression, (after being told by bike shop it needs a re-bore) one inlet valve was so worn it had gone up past its seat, therefore no gap at all, and the second inlet valve was on the same way out. both exhaust valves were in top condition. two valves later and runs like a dream.

Disagree all you like, my response is based upon quite a number of years experience and many bikes, not on one bike that isn't an SV which is what is being asked about.

Originally Posted by Sid Squid

True, as mentioned above.

Disagree all you like, my response is based upon quite a number of years experience and many bikes, not on one bike that isn't an SV which is what is being asked about.

i,m sorry, my response was also based on a fair few years of experince. and you are right a klr250 is not an sv but it does has valves as do a lot of engines that are not sv,s but can also have same problems. i realise the question was about an sv but i was just responding to your comment that it is more likely to be the exhaust valves, not the inlets. as it could be either.