I think I've got a vacuum leak!

[Motorbike Wanabe]

I have sprayed carb cleaner at the base of the carb cap and at the intake/carb connection. At idle, if I spray it at the intake/block joint, it does nothing different. If I spray it at the carb/intake joint, it stalls also if I spray it at the carb cap joint, it stalls.

I did a WOT plug chop and it was whitish/grey with no hint of tan/brown.

I did check out the 4 stroking video and I'm quite sure that it isn't 4-stroking.

I went out and for giggles, I checked the intake to see if it was perfectly round. It's not one of the cast intakes, it's a tube. I thought maybe it was squished or something. I checked it and it appears to be very round (not oblong in shape like it might have gotten smashed or distorted in manufacture). I put some more teflon tape around the intake and I'm thinking of going for a short spin to try it out tonight. I swear, my neighbors are going to kill me before this is all said and done.

 

[Motorbike Wanabe]

Just got back from a short spin. It was pretty cold out there but, I think the tape on the intake helped some. It was still pinging a little on the hill at 1/2-3/4 throttle. When I closed the choke, just a little, it went away. I think it still wants more fuel. Actually, I think it wants me to find the air leak and fix it so that it gets the right amount of fuel, all the time. Still open for suggestions.

Has anyone ever had a major air leak at the idle set screw? Just starting to think out loud.... er, on a forum. or think out typing???

 

[AussieSteve]

Won't argue - it's definitely lean at WOT and the spray tells the story.
So it's still leaking after you sealed it? (Ensure you're not spraying towards the air cleaner inlet.)
On some steel inlet tubes, the weld at the corner gets in the way, stopping the carby from sliding all the way on. I had to file the weld down on my stock tube to allow the carb to seat properly and prevent leaking via the slots.

(Consider an alloy tube, file a groove and fit an 'O' ring.)

I doubt if it's the idle screw.

... Steve

 

[Motorbike Wanabe]

I have thought of that but, I've not looked for that. I'll do that tomorrow. I have also thought about just getting a cast piece and being done with it. THey're only $3 at Gas Bike but, it's $13 shipping! Egad! I might wait until I get another order of something and then have it shipped along together. Either that or I'll buy about 5 of em.

 

[Motorbike Wanabe]

I think I'm getting closer on this. I found another o-ring at AutoZone. it's a beefier unit with a "shoulder" on the inside. It's made for an oil pan plug but, I figured it would work here too. I checked the 0-ring that we were using and it seemed to be sealing all the way around by the looks of the indentation on the mating surfaces. I took the bike out this AM and at partial throttle, it 4-strokes now. If I accelerate at all, it stops 4-stroking. Now if I climb hills or give it WOT, it will still ping. I think a jet change might do it some good now. By my line of thinking, it sounds like it might be close for just easy driving, and, maybe a little rich. But, when you depend on the main jet, it's not big enough to supply the fuel needed. I'll check with a plug chop before I go drilling or changing jets. Any thoughts are greatly appreciated and thank you for your support up until now.

 

[Motorbike Wanabe]

We took the bikes out for a short spin over lunch today and it ran pretty good. I think we've gotten to the bottom of the problem (or at least solved it until we get the engine broken in better) and the owner of the bike is riding it. Thanks again for all the moral support!

 

[arceeguy]

More random thoughts on this.

The cap that screws on the top of the carburetor (where the throttle cable enters) is not supposed to be sealed because there shouldn't be any vacuum at that point. If there is, the slide is not fitting properly (defective carb). If you "seal" the threads and cap, won't it just draw air though the throttle cable?

That leaning out and pinging at higher engine speeds is the death of any two stroke.

 

[Motorbike Wanabe]

arceeguy,

I agree that there will be some air escaping from the cable. I don't think you can get away from that. But, what led me to beleive that I was loosing vacuum through the cap on the carb and the intake seal was because when I sprayed carb cleaner into that area, the engine stalled. It didnt' do it at the intake/block seal, only at the carb seal and the cap of the carb. It also wouldn't get out of it's own way without the choke pulled on, hot or cold. We will be keeping an eye on the plug to see how the Air/fuel ratio is going. Obviously, we don't want to hurt the engine. Just trying to be sure and figure things out. I do think we're getting a handle on it, slowly.

 

[AussieSteve]

The cap that screws on the top of the carburetor (where the throttle cable enters) is not supposed to be sealed because there shouldn't be any vacuum at that point. If there is, the slide is not fitting properly (defective carb). If you "seal" the threads and cap, won't it just draw air though the throttle cable?

Yeah, that's what I've always felt, except maybe at idle.
The fibre washer is in my top cap to stop bore distortion when the cap is fully tightened. (The slide was jamming.)

... Steve

 

[arceeguy]

arceeguy,

I agree that there will be some air escaping from the cable. I don't think you can get away from that. But, what led me to beleive that I was loosing vacuum through the cap on the carb and the intake seal was because when I sprayed carb cleaner into that area, the engine stalled.....

My point is that there should be a negligible amount of air being drawn through the top of the carb through the slide. If the slide is ill fitting, no amount of tuning and adjustment will make it right.

Did you try swapping carbs from an engine that runs properly?