Total Basket Case

[black02ktm]

I have an ATC 90 that I recently rebuilt with all new electrical parts, piston, valves and springs. My problem is this: It seems way too rich (it blows black smoke, cuts out when you open the throttle, usually won't even come off idle, plug and combustion chamber is all sooty). It seems like the centrifugal advance isn't working or something. It seems like it can't burn the fuel, or is firing at the wrong time. I think the timing is set right, jetting does not help any. It runs the same whether you run a 90 carb and intake or 110 carb and intake or shorty filter or an airbox with stock filter or any jetting combination.

One thing I thought: The current head had the plug hole stripped and resized to a standard sized plug. I thought maybe the re-machining had messed up the shape of the combustion chamber or maybe the flame propagation was messed up or maybe the spark was now in the wrong spot. I bought a stock ebay head, but I have yet to try it b/c I am so frustrated.

FYI, Im running a BBR knock off pipe.

Does this sound familiar to anyone?

[black02ktm]

How 'bout it, anyone?

[darkestskydiver]

i would definatly recheck the timing make sure it is spot on. sounds more like a carb issue to me tho. i had a similar problem with my old 90 i messed around with the jetting and ran a hotter plug and all was well. also remember that the outside temp and your altitude make a huge difference in the carb. gl

[black02ktm]

Like I said, jetting, carbs and air filters don't help. It's almost acts like a rev limiter. It idles great, then you jack with the throttle until you get it to speed up a little, then as soon as it gains some rpms it rhythmically cuts out. You can also jack with the carb screws and make it run away. I agree it seems like it has something to do with timing or spark but the condenser, coil, stator, points are all new.

I even tried flipping the cam gear and turning the points base plate 180 degrees but to no avail.

Is it possible for the cam chain to be off a tooth on the driver gear? The mark on the cam gear won't line up EXACTLY with the mark, when the crankshaft is at the f position. Seems like it is half a tooth off or so.

[shortline10]

Make sure the timing advancer isnt stuck and the springs are working prop .