2005 Foreman 500 - Compression

[Coyote251 3]

Hi all, first post here on the forum, might be a bit long.

 

I just picked up a TRX500FE and am going through resolving some issues from the previous 2 owners. I unfortunately didn't take it for a drive before I bought it as there was no place to go. It started fine, is in great shape, has 6000kms on it and it's a Honda so I figured its got to be good and bought it. Got it home and went for a drive with my son. Well he is driving a 01 250 Recon and I couldn't even keep up with him, the best I could get was 30km/h out of it. I am suspecting old gas and am in the process of rebuilding the carburetor at the moment. It was smoking a bit though, so I thought that maybe the rings were going and that I would need to do a top end. First step to confirm was to test compression, this is were things seem a little strange to me. Without reading the manual, I put the gauge on and start the bike, get a reading of 64psi. This seems very low to me so I go inside to look at the manual. Go back outside after finding out the proper procedure, repeat the test with the throttle wide open and I get 125psi. The manual says compression should be 64psi, which is what it was without following the procedure. 

 

So here's my confusion.... 

 

This lower compression reading than what I am used to seeing is caused by the "decompression" system on the bike. Decompression is used to reduce the starting torque required for the motor, but who would start there bike at WOT? I would expect that at WOT decompression would be off, and I would want all the compression I could get. Am I wrong in thinking that there is a mistake in the manual and that the compression reading is normal at 64psi without the throttle depressed?

 

My main concern is that if I am running double the compression then there is excessive carbon build up and may need to take the top-end apart anyways. If my thinking is correct and the compression is fine, then finish the carb and I am off to the races (well once the tappet cap for the valve adjustment hole arrives that broke coming off when I was setting the valves) . I will probably dump a can of Seafoam and a tank of premium through it anyways just to try and clean it up. 

 

Thanks for any clarification that you can provide.

[jeepwm69 7,619]

I'd get your fuel system cleaned out and get some fresh fuel flowing and see how it does. 

 

Decompressor on the cam might not be working properly, but I'd see if you can get it running right first, then see if the smoking clears up.  Have you checked your oil level to see if it's dropping/ using oil?  Might be smoke due to carb issues.

 

 

[Coyote251 3]

18 hours ago, jeepwm69 said:

I'd get your fuel system cleaned out and get some fresh fuel flowing and see how it does. 

 

Decompressor on the cam might not be working properly, but I'd see if you can get it running right first, then see if the smoking clears up.  Have you checked your oil level to see if it's dropping/ using oil?  Might be smoke due to carb issues.

 

 

 

Yup that's my plan right now. I only picked the bike up on Sunday and drove it for maybe an hour. I knew it wasn't working right after that so it went into the shop and started coming apart. So, no I haven't been able to monitor the oil level yet. That was my thought as well after I got into the carb. The shape that the carburetor was in could be causing the smoking and that could explain the high compression if it was causing excessive carbon buildup. If the carb is the problem, then some premium gas and a can of Seafoam should help clear it up. I'm hoping to start putting the carb back together tonight. I also just realized the tappet cover that I am waiting for, for the valve cover is the same part number as the gear oil plug on the rear diff of my son's Recon, so once its back together I'll be able to test drive it and should know pretty quickly if the problems are fixed. I'll update once a get a little farther along.

 

I still think its very strange that the compression is tested with the throttle wide open and only expected to be 64psi. I would think that the decompression should be disengaged at wide open throttle. I'm going to look into how that all works when I have a bit of time.

[retro 4,048]

The decompressor is on the camshaft and it works by slightly holding the exhaust valve open at starter motor cranking RPMs (resulting in low cylinder pressure), which are usually about 400-500 crankshaft RPMs (camshaft spins at half the RPMs of the crankshaft, so about 200-250 RPMs). After the motor starts the camshaft is spinning at half of the crankshaft RPMs (~1500 idle RPM = 750 camshaft RPM) which results in centrifugal force disengaging the decompressor. There is no return spring on the decompressor cam, so sometimes it can be stuck in the disengaged position while cranking the motor with the starter, resulting in higher than specified cylinder pressure. It's nothing to be concerned about.

[retro 4,048]

You should adjust the valves while you have the carb off. It's possible that the previous owners never touched them.... if the valve lash is loose that could explain why the decompressor isn't working.

[Coyote251 3]

18 hours ago, retro said:

The decompressor is on the camshaft and it works by slightly holding the exhaust valve open at starter motor cranking RPMs (resulting in low cylinder pressure), which are usually about 400-500 crankshaft RPMs (camshaft spins at half the RPMs of the crankshaft, so about 200-250 RPMs). After the motor starts the camshaft is spinning at half of the crankshaft RPMs (~1500 idle RPM = 750 camshaft RPM) which results in centrifugal force disengaging the decompressor. There is no return spring on the decompressor cam, so sometimes it can be stuck in the disengaged position while cranking the motor with the starter, resulting in higher than specified cylinder pressure. It's nothing to be concerned about.

Thanks Retro for the explanation on how the decompression works. Makes a bit more sense to me now, the higher compression then is only be caused by the bike sucking in a larger volume of air when the throttle is open. Where as it has a smaller volume of air to compress when the throttle is closed. Good to know that it is centrifugal as well, eases my mind that it is not stuck open or something.

 

17 hours ago, retro said:

You should adjust the valves while you have the carb off. It's possible that the previous owners never touched them.... if the valve lash is loose that could explain why the decompressor isn't working.

I adjusted the valves on the first night, It was ticking like a clock. They were off quite a bit but are now set at 0.15. The exhaust valve tappet cover was all chewed up by a pipe wrench or vice grips though and broke coming off. I am waiting on a new one to get here now so I can take the bike out for a drive now that the carburetor is all rebuilt and back in the bike. 

[retro 4,048]

6 hours ago, Coyote251 said:

Makes a bit more sense to me now, the higher compression then is only be caused by the bike sucking in a larger volume of air when the throttle is open. Where as it has a smaller volume of air to compress when the throttle is closed.

 

Right, you must open the throttle whenever you are doing a compression test, else the cylinder will not fill with air completely, which gives you a false measurement.