Poor Running 1985 ATC 200m

[Troy T]

I will start by saying that I've never had an issue I couldn't figure out and resolve on a motor until now.

Background;
I have a 1985 ATC 200m that started to run poorly one day a couple months ago. I put it in my shop and ran a compression test I got around 50psi of compression AKA not a good result. I then replaced the piston and rings with a new WYSCO 10.25 to 1 piston. I put everything back together and started my trike, it ran poorly.

I then performed another compression test and found that my compression was now at 200psi. With compression fixed by replacing the rings a and piston I moved on to the ignition.

I replaced every ignition part, coil, cdi box even though my old stuff tested fine I thought this is all fairly cheep just replace it. replacing all of the electronics in the ignition system and set my timing correctly it made no difference, it still runs poorly.

I then replaced the carburetor with two different carburetors both cleaned, inspected and working properly and still it runs poorly.

I then performed a leak down test and at 100psi there is less than 1% leak I say less than 1%.

I may be misinterpreting what my machine is doing but to me my machine acts like the intake valve isn't sealing even though my leak down test would say it's sealing fine so I replaced the head with a known good head. I then ran the leak down test again. My results were the same little to no leak, less than 1% at 100 psi.

I started my machine and it runs exactly the way it did when I started working on it, poorly.

Description of "poorly"

my machine is running poorly in a very particular way.

It starts fantastic, one revolution and bam she's running. It only runs poorly when you start rolling on some throttle. I would say it runs not that bad up to a quarter throttle but anything more than that it makes more noise but makes no more power and doesn't really increase in RPM's. It also seems to mist some air/fuel back out of the carburetor onto my air filter when you give it throttle. It acts like it has a bad intake valve seal but my tests would say other wise.

Any and all help would be appreciated because this one has me stumped.

[Jim mac]

did you check the springs on the centrifical advance behind the pulse generator? jim

[Troy T]

Yes Jim,
The springs are good and the timing advances as it should.

[kb0nly]

Exhaust.. Check that, i had a few do the same thing as you described and after removing the muffler and running just on the header pipe they cleared right up, put on a new muffler and problem solved. They rot out inside and get restricted, also if it has its original spark arrestor they get plugged up and can be removed and cleaned and put back in. Spitting gas out the carb means its running richer then rich, and since you tried multiple carbs, the only thing i can suggest is you have a stuffed up exhaust. Engines are just a big air pump, if the output ain't flowing the input is gonna be messed up also.

My 200ES did this, fought it for a month, rebuilt carb, tore engine down and put a fresh top end on it, bored and honed, new piston and rings, valve job, compression was good, leakdown was good, pulled the exhaust off because i was about to pull the motor again, decided to run it quick with just the header pipe stuck out the side because i wanted to triple check things before going through the trouble of pulling it again and it ran great!! I ran it with just the header pipe and a straight pipe for a few days, even got a little lean because of zero backpressure, so i put a new DG exhaust on it, adjusted the carb and its been good for more then six years now.

[Troy T]

Amazing, somewhere in the header pipe where you can't see there must be a restriction. I take off the header pipe and she runs like a top. That was the exact problem!!!!

[ATC200X4716]

had a mouse nest in a pipe before....Clean out the carbon and any junk in the spark arrestor. Pass a piece of welding rod or coat hanger to make sure the pipe is clear. Nice find.

[Footy]

I have had a similar issue with dirt dobber nests inside of the exhaust. I have an atc110 with a propipe on it and I think that one is stopped up as well. It will start but will not take throttle at all without stalling.