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Old 12-09-2014, 09:37 AM   #1
07jkx OP
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1982 XL250R - Sudden Loss of Compression

Just looking for some suggestions on things to check. Basically, The bike was running fine. I replaced the petcock and then started it up. Ran fine. Turned it off, went to bed, attempted to start in the morning and nothing.

I am getting spark. I am getting fuel. Carbs are clean. My valves are in spec. My auto decomp is adjusted properly.

My compression is... at best and suddenly around 15-30 psi when kicking.

I disabled the auto decomp and removed the valve cover to take all pressure off of the valves. I hooked up my air compressor and set the piston at TDCC. No air leaks from valves or rings.

My next test is the leave the valve cover off/auto decomp system off and then kick it over while hooked up to a compression tester. Aside from that, I have no idea what else it could be.

Any suggestions, tips, tricks or other things to look at would be greatly appreciated.
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Old 12-09-2014, 11:07 AM   #2
tkent02
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You changed the petcock and it won't run. It's not getting fuel. You said it ran once, that was from fuel that was already in he carburetor. Find out why it's not getting any fuel.

A petcock change didn't cause a lack of compression.

The compression is something else. Poor testing technique probably. What was the compression sat time you checked it? Did you check it exactly the same way?
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Old 12-09-2014, 11:13 AM   #3
07jkx OP
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That was my first thought. I have checked the petcock and it function fine. I can drain the carb, turn on the petcock and watch fuel flow through the inline filter. Each time I drain the carb, the same amount of fuel comes out of the bowl.

Never had the need to check compression until it stopped running. Hooked up gauge to spark plug hole, began pushing the kick start lever until the auto decomp clicks, return to top and full kick through stroke. The same method that I have used thousands of times in the past to crank. This has always been a 1 or 2 kick bike.

Tried push starting and it wont start that way either.
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Old 12-09-2014, 11:23 AM   #4
Onederer
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Have you checked the valve timing? Maybe the cam chain jumped.


Maybe someone played a trick and stuffed something in the muffler.
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Old 12-09-2014, 12:02 PM   #5
07jkx OP
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I will verify the timing is correct. It has an auto chain tensioner and when I removed the valve cover, the chain seemed to have a proper tautness to it where a jump would be very unlikely without teeth and/or chain failing somewhere.
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Old 12-09-2014, 12:27 PM   #6
Gripsteruser
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Keep re-checking the basics. Seems unlikely to be something complex like jumped timing chain while it was sleeping overnight.

How are you testing for spark? Plug connected and lying on the cyl head?
Got a different plug to try?
Recheck plug wire at coil end?
Plug wire positionally intermittent?
Is the plug wet from trying to start? Choke? No choke? same result?
Got a way to check spark timing?

My old man's rule was always - check the ignition. If it still isn't the ignition, check it again because you missed something. Worked to solve a lot of problems we worked on over the years.....
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Old 12-09-2014, 12:41 PM   #7
07jkx OP
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Thanks.. I will check into that. I am checking the spark by grounding it on the head. Get nice consistent sparks every kick. Tried it with several new park plugs. All look good.

I am going to check compression again with the valve cover off to see if by some chance a valve was too tight or the auto decompressor was improperly adjusted. Going to strip down and clean the carb again too.

Also going to check with compressed air with the piston at various heights (BDC up to TDC) to see if there may be a flaw with the jug or head gasket.
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Old 12-09-2014, 12:49 PM   #8
Gripsteruser
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It's a 250 so you could completely disable the decompressor and try kicking it to start.

Engines can still run with poor compression such as chipped rings or worn cylinder - they just don't develop a lot of power.

They can even kind of run with burned valves so a poorly seating valve shouldn't disable it completely. But I guess a stuck-open valve would disable it - but you should be able to see that with the valve cover off.

Put a couple of drops of gas down the sparkplug hole and retry? (another way of asking again if the plug is wet)

Fuel, air, compression, spark. Sounds simple, doesn't it?
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Old 12-09-2014, 12:58 PM   #9
07jkx OP
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Quote:
Originally Posted by Gripsteruser View Post
It's a 250 so you could completely disable the decompressor and try kicking it to start.

Engines can still run with poor compression such as chipped rings or worn cylinder - they just don't develop a lot of power.

They can even kind of run with burned valves so a poorly seating valve shouldn't disable it completely. But I guess a stuck-open valve would disable it - but you should be able to see that with the valve cover off.

Put a couple of drops of gas down the sparkplug hole and retry? (another way of asking again if the plug is wet)

Fuel, air, compression, spark. Sounds simple, doesn't it?
haha... yup ... it does sound simple. I have brought countless bikes back to life from craigslist. Never had one given me this much trouble.

The suddenness of the symptoms is what has me perplexed. There was no hard off road drop or the smell of burning wires, no episode of oil starvation, no airbox submersion, only had ethanol free fuel in it, no issues with idle or lack of power or anything other indicators that something may be amiss. Oh well.. I will figure it out. Trial and error.

Just for confirmation... I have the carb off it now... for compression testing, I assume this is as good as holding it WOT while kicking.
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Old 12-09-2014, 01:11 PM   #10
Gripsteruser
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Quote:
Originally Posted by 07jkx View Post

Just for confirmation... I have the carb off it now... for compression testing, I assume this is as good as holding it WOT while kicking.
Yes. No throttle plate to restrict air inflow.

I agree that the suddenness of the problem is an important clue.

I HAVE seen a worn-out/jumped timing chain on a V8 engine. Everything was right for it to start except that when we put the timing light on it the timing had shifted a LONG ways. That was a head scratcher the first time I saw it. (phenolic timing chain gear had no real teeth left)

If you have the valve cover all the way off that's another thing to check - cam timing. (i.e. jumped cam chain if that's how you check for it) I think that using leakdown test on it would also show shifted cam timing because at TDC there would be valve leakage.
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Old 12-09-2014, 01:34 PM   #11
07jkx OP
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Quote:
Originally Posted by Gripsteruser View Post
Yes. No throttle plate to restrict air inflow.

I agree that the suddenness of the problem is an important clue.

I HAVE seen a worn-out/jumped timing chain on a V8 engine. Everything was right for it to start except that when we put the timing light on it the timing had shifted a LONG ways. That was a head scratcher the first time I saw it. (phenolic timing chain gear had no real teeth left)

If you have the valve cover all the way off that's another thing to check - cam timing. (i.e. jumped cam chain if that's how you check for it) I think that using leakdown test on it would also show shifted cam timing because at TDC there would be valve leakage.
I will verify that as well tonight. Although, the first thing I checked were the valves at TDCC... both had a bit of wiggle and measured out in spec of .002 intake and .003 exhaust. The auto decomp seemed in spec (between 1 and 3 mm freeplay at TDCC).
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Old 12-10-2014, 07:46 AM   #12
07jkx OP
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OKAY... new update. I put compressed air into the cylinder with the head cover off so there was no pressure at all on the valves. No leaks at TDC while stationary.

However, when I manually rotated the motor, I could hear air hissing out of the clutch side (cover off). This led me to believe that rings and/or a worn jug is causing my compression loss on kicking.

I have the head off and everything looks good. There is a ton of carbon buildup on the piston face. Going to pull the cylinder and piston tonight for measurements and inspection.

Hopefully I just need new rings. Do rings typically just fail? As in, running just fine one day and then minimal compression the next crank up?
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Old 12-10-2014, 08:00 AM   #13
1stgenfarmboy
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not normally but I have seen rings break.
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Old 12-10-2014, 08:47 AM   #14
07jkx OP
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Hmmm.. okay. We I guess it is all a guessing game until I get the cylinder and piston out for inspection.
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Old 12-10-2014, 09:46 AM   #15
Gripsteruser
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Overnight failure shouldn't just be worn rings. Broken maybe?
Chipped cylinder wall from something?

One of the tests I should have suggested yesterday was dry vs wet compression test. (a little motor oil in the cylinder)

Too late now.

You might want to test the valve sealing while the head is off.
Turn the head valve faces up and put some fluid (gas?) in the chamber and see if the liquid leaks thru the valves.

We're gonna learn something here! Thanks for sharing!
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