I have traded for this thing and believe I ended up on the loosing end.

I had to rebuild the carb to get it running and now I find it will not move. Sounded like a chain jumping in forward but nothing in reverse.

I spent the day pulling the transmission and it looks like the chain is stretched.
Also, the shift fork has wear on the forward fork and there's a loose c-clip on the input shaft. It holds the gears in place but appears that it has slipped out of it's groove and messed up the groove. I'm not sure it will stay in place now.

My questions are:

How much slack is allowed on the silent chain?

Is the input shaft available?

Anyone have a good used transmission? Any ATV salvage yards?

This one my not be economically repairable.

Thanks for any advice. I hope I can get this thing going without spending a bundle. If not, I'll have to try and part it out to recover some of my loss.

 

Chain should be fairly tight. This was a common problem with stretched chains especially on sport machines.Chain is cheap and should always be replaced when you tear apart a transmission.
Here's a reasonable place on oem parts.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

I'll plan on a new chain then.

I'm concerned that I may be getting in too deep $$$ real quick on this thing. The input shaft is questionable. The C-clip that holds the forward/reverse gears in position has slipped out of its groove at some point and has been turning on the shaft. that has messed up one shoulder of the groove. I put it back in place and could not make it jump out by whacking the end of the shaft on the wood workbench.

Engagement dogs look OK but, the forward fork has wear. I'll measure the depth with the calipers... it may be usable. The though of building it back up with the welder and filing it down came to mind. That's and expensive part.

I also noticed the engine backfires. I was thinking I just didn't have the carb set up right and figured I'd get back to that after addressing the gearbox.
Now I'm wondering if the exhaust lobe is flat after reading up on this vintage 500. Checking that is next on the list before I dump anymore into it.

Thanks for the reply OPT!

It's getting cold here!

 

Yep if the circlip has slipped and the sprocket has damaged the shaft,it needs to be replaced along with any gears that have well worn spots on the engagement dogs. Back firing could be another problem that you won't like. Before you get too far,remove the cam cover and inspect the exhaust lobe on the cam. If worn down, it and the exhaust rocker arm needs to be replaced. This was a common problem on 96 to 2000 model 500 engines on soft cams. Here's one members pics on what they usually look like when they start back firing,will idle ok,but wont accept any throttle without bogging.http://forums.atvconnection.com/pola...d-cam-pic.html

 

The gears and engagement dogs look fine. The forward drive fork measures .192" and approx .0178" at the worn spot.
Input shaft looks good other than where the C-clip rolled around. It has worn one shoulder of the groove down. I'm wondering if I can turn a deeper groove and install a smaller 'ID' C-clip. Chain will get replaced.

I have the factory service manual and there's not much info regarding this particular gearbox.

VW had the same issue with the '05-06 "BRM" TDI cams. Too much riding on one soft lobe.

What I find puzzling is the previous owner had the factory service manual but must have never cracked it open. The machine looks really neglected.

If the cam is flat then it's getting scrapped out.

 

I agree that the factory manuals don't give too much if any wear specs on gears, shift forks,etc other than chain stretch specs,torque specs and pretty assembly pics. They basically leave it up to the tech to visibly check each part for wear. If the circlip groove is the main thing damaged on the shaft,I don't see why you couldn't turn it down a little enough to accept a new circlip. Main thing the circlip needs to be at least the same out side diameter as the old one that backs up to the shaft washer and gear or you could have the same thing happen again. Would save you about $135 for a new shaft. I've thought the same thing too on the wide exhaust cam lobe running against the pad on the double exhaust rocker arm. Just a big load until the cams were finally strengthened.Keep us updated on how it turns out.

 

It's alive! I replaced the 'silent chain', turned the C-clip groove down just a tad and put the reassembled gearbox back in the frame.

Camshaft looked OK, found the valve clearance on the tight side so, I set them while it was open.

Carb is still not right even though I have had it apart a half a dozen times and cleaned every orifice I could find. It idles like an old hit and miss engine. Placing my hand over the air inlet smooths it out nicely so it's running a bit lean on the idle circuit. Inlet manifold rubber looks good, no leaks.
I cannot locate the 'pilot screw' on this carb. What I thought was a brass plug covering revealed nothing. I'm beginning to think someone has swapped a different carb in the past.

So far I'm out about $50 for the chain and misc stuff. Nice!

 

Glad you got it back together.As far as the fuel adjust screw,it's under a block off plug in front of the carb bowl. #27-30.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

I drilled the brass plug out and got into aluminum shavings so I stopped. Never did find a screw under there. I'm afraid to go any deeper with the drill.

How thick is the the block off plug and how is it normally removed? I tried a drywall screw in the tiny hole I found in the brass but the tip broke off. That's when I started drilling.

 

Just use a small drill bit as you've used and stop right when you go through the plug. Use a drywall screw and tighten into the plug. Just wiggle the screw around with pliers and the plug should come right out.Plug is only about 1/4"-5/16" deep. Hopefully you haven't gone too far and damaged the fuel screw. Count the number of turns on the screw in to a lightly seated position. Should be around 2-3 turns out on average. Best to have the carb installed,set the screw at 2 turns and after the engine is warm turn the fuel screw out,counting the turns and then turn in to find where you have the highest idle. Then try the throttle and see if if it runs better.