This problem has stumped me for a while.

I inherited a 2005 Vinson 500. When I got it it never went more than 20mph. It needed a lot of work but I still can't get anywhere near the mfr. top speeds. I should at least Max out at 50 mph.

I rebuilt the carb. Replaced fuel petcock. Adjusted the valve clearances. Adjusted Cam chain tensioner. Inspected the belt, removed clutch faces and thoroughly cleaned the rollers - no evidence of slippage. Everything is stock, so putting in a bigger jet might make a diff. but I'd like to know why I'm not getting top end power.

The only "odd" symptom is it backfires just once Everytime I turn the engine off. Will the mixture make that big a differences? It idles smoothly & the plug isn't black, or white.... Kinda just right.

Any ideas? When I adjusted the valves I noticed they didn't seem to be fully open when I was at TDC. I still adjusted them for TDC but thought it odd that they opened a bit more just past TDC notch. Does that mean the timing is off?

 

Compression test would be first priority

 

Valve timing can sure be messed up by someone not installing the chain correctly. Pull the cover and check the marks against
TDC mark on flywheel.

Also, a clogged air filter or muffler can adversely affect RPM and power. Since you have rebuilt carb, check muffler.

If you do a compression check, there is a decomp mech on the cam, but you should get a good reading if you crank fast enough.

 

I'm unclear on how to check timing based on the TDC notches. When. I adjusted valve clearances I lined up the TDC notches, but how do I tell if the timing is off by looking at TDC notches?

 

There is a line on the cam sprocket that indicates where it should be when flywheel mark is at TDC. You will need to have the cover off to see it.

 

Do I remove the valve cover to do that?

I tried adjusting them again, taking care to make sure I'm on the compression stroke - and to see if indeed the Piston was all the way up I used a socket extension put through the spark plug hole to measure piston height. Once I was sure it was all the way up I compared it to the TDC line & the line next to the TDC viewing hole.... It was pretty damn close, almost hard to even say it was off at all. If anything it was just millimeters of the marks put it could arguably be chalked up to an imperfect technique.

So assuming my method proved the timing is still accurate to specs., what else might cause this power loss?

Sometimes when im in gear & rev the throttle to start moving, it almost feels like there's resistance, as though the brake was on or something - though it's not.

Yet, when I'm going up hill and require more power it seems to bog way down - not like it's starving for fuel, but lacks the power to climb. I'm guessing that rules out belt slippage, since I'd be hearing the engine tach right out otherwise, right?

 

Yes, you will need to remove the valve cover to see the cam sprocket.