First off, this happened on an xr500. Even though this is an atv forum, I figured an engine is an engine. My friend let me borrow his 1980 xr500 dirt bike. The problem is, the auto-compression release is stuck open, so it has no compression. The lever going into the head is moving back and forth like it should be whenever I kick the pig over. I disconnected the cable that triggers the compression release, and it still has no compression

Does anyone know what could be causing the compression release to stick. I don't really feel like tearing the top-end apart, but i will if i have to. My friend is currently unaware of the problem and I would liked to get it fixed before i give it back.

Before I borrowed the dirt bike, it was run with no air filter, half of the needed oil that had the consistency of water, and the valves were never adjusted and the chain was never oiled. I have since changed the oil, oiled the chain, bought a filter, and was going to adjust the valves. The last thing I want to hear from him is that I didn't take care of the thing. If anyone could help me, I would be very greatful. Thank you for your time.

JaMeS

 

Just pull off the cam cover and the little spring connected to it is probably broke.

 

KeyRacing

Thanks, i'll look at the spring tomorrow


JaMeS

 

Was just wondering,did the engine sound noisy before this happened?

 

I checked the spring and it is good condition.


truck- It sounded like someone was tap dancing in the head before this happened. I was two days away from taking it to the dealer to have the valves adjusted.

I looked at the spark plug and it was fould out. So I know I am running rich or the rings are bad. But like i said, it had plenty of compression the day before.

Thanks for replying

JaMeS

 

Watch the cam & rock the engine back and forth.The cam should move as soon as the crank is moved.If it doesn't I would be looking at the tensioner.

 

Thanks truck.


JaMeS

 

Over time the valve shoulder and the valve seat wear. This is normal wear and tear over time. Only gasoline lubricates the valve shoulder and seat as it opens and closes each time, so it wears. As the valve wears, it then recedes upwards. The valve seat is worn- made bigger, and the valve shoulder is worn- made smaller. So the valve can now recede further into(up) into the head body reducing the clearance between the valve stem end and the rocker arm.

This is why valve adjustments are done. To keep the clearance between the valve stem and the rocker arm at an acceptable clearance. Typically the valve will always recede, not the other way (clearance increases) and cause the valve to not open fully. Even then it would only be a couple of thousandth difference if the clearance were on the upper end of the tolerance for clearance. The valve stem is hardened and the rocker arms typically have hardened faces. Additionally they are lubricated rather well from the engine oil, so they wear very little during "normal" use(10K RPM start-up can cause galling of the rockers and cam)

Also the clearance is provided to account for heat increasing the dimensions of the parts. The valves swell a tremendous amount at they are exposed to hot combustion gasses. Especially the exhaust side valve, it literally is bathed in fire, that's why the exh valve will always have a greater clearance spec than the cooler intake side.

A couple have things may have happened in your case: I have seen it several times before that a 4 stroke begins to lose or has lost compression due to the valve clearance coming shut from wear and lack of adjustment. The rocker arm holds the valve open. The other thing that it may be is that the compression release lever, which inside looks like a cam lobe somewhat, has over centered for some reason and is holding the exhaust valve(s) open. The head cover is easy to pull and doesnt require too much fiddling. Most dont not use a gasket, just gasketing compound like HondaBond, so to pull it apart is just your time to do it.

Ask the stealer, I mean dealer, for the proper valve clearance for that machine and perform that adjustment. Likely to be somewhere like .004 for intake and .006 for exhaust. As mentioned just a tad loose, while sometimes too loose can be noisy, is better than too tight. The procedure to adjust valves is straightforward and requires only minimal tools. A feeler gauge is your most odd-ball tool required.

 

Backcountry

That was a very helpful post. THANK you for taking the time to write it out.

 

Here's an update of the compression release problem.....IT WENT AWAY by itself. This piece of junk has a mind of its own. It decides when it wants to run like a champ and when it wants to kill me. It still won't start, but now it has alot of compression. It only sputters and backfires(not fun when kicking and it backfires) alot. The kid i borrowed it from wants it back so i am gonna give it to him and tell him it has not started in 3 weeks. Talk to you later.


JaMeS