Carburation

[rlkarren]

I've done quite a bit of reading on this subject and so I think I know quite a few answers but carb tuning still seems to be my nemesis...

Here's the details:

Ace 90 bored .030 over.
Pilot jet = 22.5
Main jet = 75
Elevation is 4400 ft.
All other jetting/needles are stock.
mixing 40:1 YL 2R

Using the tuning method from the manual and following it the best I can, the main jet seems to be right on in all my tinkering so my focus has been on the pilot jet since it always seems to burn rich at idle. When I say burning rich I mean that there is quite a bit of smoke if I take off again after idling for a minute or so. Further, it seems to run better WITH E10 fuel. I would really like to tune the idle circuit properly.

Here's what I've tried:
1. Install a size 20 pilot and reset the air screw to 1 1/4 turns out. Will not start. Adjusted air screw between 1 - 2 turns and still won't start.
2. Reinstalled the 22.5, then attempted adjustments to the air screw, some difference but I can turn it all the way in and the motor keeps running. If I understand correctly, it should sputter and die if turned all the way in.

Every test I use, as I understand them, indicate a smaller pilot. I'm planning a carbon cleanout on the Ace 90 this spring and I'm also building a Wombat which has had the same issue so far, (which tells me I'm missing something). I keep wanting to drop the gas/oil ratio, but I read and understand that that would be a BIG no-no.

and a couple other questions:
1. What would cause it to run better with Ethanol fuel? I'm not kidding, it really does run better.
2. Cold air is more dense so you get MORE air in the mix when you ride in cold weather, which logically means you should be burning leaner. right? My little Ace 90 smokes and loads up at idle like a bugger if I take a ride in our arctic environment.

What am I missing? If left out any details that might help, let me know.

Thanks.

Roger

[Dale]

Roger, Have you done an air leakdown test? You might have a clutch side crank seal with a slight leak???

My recently rebuilt Ace 90 is running great at 3000ft with 85 main, 25 pilot and 1 1/4 turns out on the air screw. Everything else is stock. I am running non-ethanol at 32:1 with YL 2R. I am also at .030 over.

75 main may be good at your altitude. If the bike keeps running with the 22.5 pilot and the air screw all the way in, that tells me that you are not getting enough fuel through the pilot to load the motor when the air is cut off. I would go back to the stock 25 on the pilot and try the air screw again.
Dale

[Bullfrog]

It sounds to me like adjusting the pilot air screw has no effect. If that is correct, a thorough carb cleaning is indicated because the passage which delivers air into the pilot circuit is plugged.

If your pilot jet is close to being correct, then adjusting the pilot air screw in 1/4 or 1/2 turn increments should cause a change in idle speed.

NOTE: Smoke production is at best a secondary indication of proper tuning. The ears are your main diagnostic instrument, not your eyes.

Ed
PS: The question about a leak down test is a very, very good one. A leaking clutch side crank seal could be introducing extra oil. BUT, for now, let's clarify whether or not adjusting the pilot air screw has any effect at all on engine operation. (and yes, a plugged pilot air passage might indeed explain why E10 seems better right now)

[rlkarren]

Thank you Dale and Ed. Very helpful. I suppose it is entirely possible there is a leak since this is the first 2-stroke I've built and didn't know about a leak down test until after I built it. And since it ran "pretty good", I thought maybe I got lucky. anyway...

I think I'll go after the leak down test first to eliminate that variable, and then go after the carb. Thank you.