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Message started by spacepirates on 05/18/11 at 15:09:22

Title: Low Compression -- Thoughts?
Post by spacepirates on 05/18/11 at 15:09:22

I've been getting pretty bad gas mileage on my bike (~35mpg) recently, so I did a compression test and consistently got around 90psi :-/ . As far as I know, the compression should be around 140-200psi.

The bike is a 2002 with just under 19k miles on it, for what it is worth and it does go through a bit of oil (though not too much). valves were adjusted 3k miles ago.

Any thoughts on what I might be causing this? bad gaskets? worn rings?

Title: Re: Low Compression -- Thoughts?
Post by EJID on 05/18/11 at 15:27:37

Could be the "wicked wrist" syndrome ;D mine suffers at times too ;)

Sorry, I can't help you with anything more than that.

Title: Re: Low Compression -- Thoughts?
Post by verslagen1 on 05/18/11 at 15:34:26

Your excessive mpg may be washing down the cylinder walls causing excessive wear.

time for new rings.

Title: Re: Low Compression -- Thoughts?
Post by rustysavage on 05/18/11 at 15:37:57

Add a teaspoon of 10w oil to the cylinder, do a compression test. If the compression go up problem is with your piston, rings and or cylinder. If it stays the same problem with your valves. Decompression valve staying on?

Title: Re: Low Compression -- Thoughts?
Post by spacepirates on 05/18/11 at 16:44:56

0C03000D490 wrote:

Could be the "wicked wrist" syndrome ;D mine suffers at times too ;)

Sorry, I can't help you with anything more than that.

yeah... i don't exactly ride easy, but it can't be THAT bad...

Title: Re: Low Compression -- Thoughts?
Post by spacepirates on 05/18/11 at 16:46:12

54554B060 wrote:

If this is to be believed, then it is the valves.

Put a teaspoon of oil in the cylinder, compression test came back with the same 90psi. I'll double check that the valves are adjusted properly and see what i find....

I'm not sure I follow what you mean when you say decompression valve is staying on (decomp solenoid?). care to explain a bit more?

Title: Re: Low Compression -- Thoughts?
Post by BurnPgh on 05/18/11 at 17:10:58

the selenoid activates the cable that pulls on the lever which manually opens one exhaust valve on start up. If its not adjusted properly you can possibly be riding around with one exhaust valve open constantly. Bad news bears right there. You also have about the same mileage I had last spring when i took the head in for valve work. Valves were pitted and not seating fully allowing compression to be released. Im geussing this condition actually happens in the first few thousand miles when original owners general either - keep stock everything including stock lean jetting or - change mufflers without rejetting making the stock lean condition worse, and then it just gets worse from there once its started happening.

Ive been in school for diesel tech this past year, and wish id known last spring what i know now. If i can borrow some tools from my instructor I can fix it for cheaper than your probably going to find elsewhere. If nothing else I can help you take it apart and see whats what. Sounds to me though like you need new valves (exhaust probably). Unfortunately our valve faces cant be cut. Clymers insists that if you replace valves you also replace the guides. Could go either way IMO. Need to do some measuring to be certain. Id get valve stem seals as long as you/we're going in anyway. My numbers still the same, give me a call.

Title: Re: Low Compression -- Thoughts?
Post by BurnPgh on 05/18/11 at 17:38:24

5655444640554C5744514056250 wrote:

[quote author=54554B060 link=1305756563/0#3 date=1305758277]Add a teaspoon of 10w oil to the cylinder, do a compression test. If the compression go up problem is with your piston, rings and or cylinder. If it stays the same problem with your valves. Decompression valve staying on?

Title: Re: Low Compression -- Thoughts?
Post by spacepirates on 05/18/11 at 18:38:57

6E595E427C4B442C0 wrote:

Also...did you turn the engine over by hand a few times before you did the second compression test? If not go ahead and try it again.

Re-checked compression after doing this, same result. Checked valves, they are adjusted properly.

Looks like Burn and I are going to tear into this issue this weekend and see whats what. At a very minimum good times and beer will be had.

Title: Re: Low Compression -- Thoughts?
Post by BurnPgh on 05/22/11 at 17:04:02

verdict is - head gasket believe it or not.. Burned, discolored and pitted at the front lower head bolt. A little oil seepage there which we at first assumed was part of the head plug leak as the two leaks converged. Took it apart expecting valve issues but the chamber held water perfectly. Checked ring end gap in the cylinder at multiple positions with the worst measurement being .45mm, still well below the factory limit of 1mm. Didnt even really look at the head gasket at first. Just removed it and set it aside as the forum as a whole generally scoffs any time a newbie comes in claming a shop replaced a head gasket instead of a fixing a plug leak. Well...rest assured, these head gaskets do fail. And yes all the head bolts were torqued. New head gasket will be ordered, head/cylinder deck will be checked for warpage.

Title: Re: Low Compression -- Thoughts?
Post by spacepirates on 05/23/11 at 07:43:07

Ordered the gaskets today from a local cycle shop (they are all closed on Sundays so I had to wait till today). should be in by Friday, I hope.

around $50 for cylinder head and base gaskets. Feels kinda steep given what they are, but at this point i just want a bike that runs.

Of course, working with BurnPGH, he has me thinking about all sorts of performance mods and what not. "Oh, well for only XX hundred dollars and XX time spent on your bike you could REALLY have a bike that goes". Carb... Cam... Piston...

hmm... winter project perhaps?