Bucking, 4-stroking, Snatch, Kicking, Surging, Stuttering

Ray Lovestead · Posted: Wed Mar 06, 2013 7:22 pm Post subject:

Thanks Rod. My DFM is running great by the way.

So I went to the track (IMI motorsports) for 8 hours today. Here is my report.

Symptom: Shuddering and Bucking when holding the throttle at 1/8 to 1/4.

This only occurs when on track. I runs smooth as silk on the stand. So I starting changing things. Air screw (on the track, stop, adjust, retry). Went up and down through the range of needle positions. Then started reducing the idle jet diameter (55, 52, 48, 45).

Nothing. No change whatsoever at 1/8 to 1/4. Only cure is to not use it at the throttle and peg it WOT.

Question: Should the kart carb function work on the stand and the track equally well?

My idea is this. I'm running a PWK 38mm carb with a pump around conversion. Is it possible that the height of the return tube is incorrect? If too high or low (basically setting the 'float height'), will it have these symptoms? The carb seems to be permanently rich at 1/8th to 1/4 throttle!

I've had this problem for the past year and I usually just ignore it and push straight to WOT. I've always had a pump around. So I'm going to try and put it back to a float and see what happens.

Ray
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Tim Pappas · Joined: 25 Jul 2001 Posts: 791 Location: Burkina Faso,

If it is truly rich and only from 1/8 to 1/4 throttle opening, then a slide with a bigger cutaway should help.

Michael Taksa · Posted: Thu Mar 07, 2013 10:39 am Post subject:

Do you have a spare coil you can test with? Most of the time when no adjustment helps on the carb the problem is in the coil or coil wire.

Greg Wright · Posted: Thu Mar 07, 2013 10:57 am Post subject:

What needle are you using? There are many but only a few work in this application.
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Ray Lovestead · Posted: Thu Mar 07, 2013 12:49 pm Post subject:

I'm running a DGH.

I've got a brand new coil, stator and CDI. All new electricals.

How about this for a theory (that I can try tonight). This kart has never had a very good idle. In fact, I usually have to crack it slightly open to keep it running. Q: Is it possible that my idle circuit is totally clogged or non-functioning?

I believe you can operate an engine without an idle circuit if you rely on a slightly cracked open slide. The reason I ask this is because I changed out the idle jet from 58 to 45 and noted zero change.

Ray
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John Matthews · Posted: Thu Mar 07, 2013 1:50 pm Post subject:

http://www.harborfreight.com/25-liter-ultrasonic-cleaner-95563.html
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Heartbeat Power, LLC.

Larry Andrews · Posted: Fri Mar 08, 2013 12:19 am Post subject:

http://www.madsens1.com/saw_carb_tune.htm

If anybody can use it, there's some decent instructions and wav files of what it sounds like to tune a tilly carb with a can motor. Not really what a lot of race tuners would teach, but absolutely useful info if you've got nothing else.

Jim McMahon · Posted: Fri Mar 08, 2013 11:37 am Post subject:

Tom Barth · Posted: Sat Mar 23, 2013 3:49 pm Post subject:

The idle circuit is needed to get mid range started. The little hole (port) near the venturi supplies a shot of fuel to venturi when coming off idle.

Go back to floats but make sure to put Tee in line between pump and carb.

Sorry, if your using Dellorto pump shouldn't need a tee.
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Ray Lovestead · Posted: Mon Mar 25, 2013 9:47 am Post subject:

UPDATE!

I finally got a solid day at the track with an adjustable (pump around) return line depth. Turns out my problem is not something I predicted. After moving the return line height up and down (which basically accomplishes moving the float bowl level up and down), I found a good level that worked perfectly! No bucking and a responsive idle circuit.

Solution? Turns out it was the diameter of my return line brass tube. It was too large a diameter. The float level height (the return) was exactly what it was before, but now I have a much smaller return line tube diameter. The large diameter of brass tubing was making it difficult for the return pump to pull the correct pressure and fuel from the bowl. So now it runs like a champ!

Thanks everyone,

Ray
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Randy Mckee · Posted: Mon Mar 25, 2013 9:48 pm Post subject:

All I can say is really nice job diagnosing that one. I'm curious, how did you determine the tube was too big? Compare it to another one?
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#123 TonyKart - S4 Stock Honda

Ray Lovestead · Posted: Tue Mar 26, 2013 7:18 am Post subject:

I was simply putting a smaller diameter tube into the previously existing pick up tube so that I'd have an adjustable pick up height. I didn't expect that the reduced diameter would impact the performance.

Ray
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Randy Mckee · Posted: Tue Mar 26, 2013 11:02 am Post subject:

Ah, smart.

One thing to consider is a smaller diameter should make the motor run richer as the return may not pull enough fuel out of the bowl. I'm pretty sure that effect is not intended by the pump-around design. I think the original return tube size is probably right, unless you have a hacked version with a larger than normal tube. Think of it this way, pickup height keeps the bowl filled to a certain level and jetting can be consistent based on that level (similar to float level as you mentioned). However, if the return line can't keep up due to a restricted pickup tube, the jetting will run richer, although probably sporadically. Something to play with, though I would tend toward leaving it full size and just set the height appropriately.
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#123 TonyKart - S4 Stock Honda

Ray Lovestead · Posted: Tue Mar 26, 2013 11:57 am Post subject:

I'd done the conversion myself based off the info available on kartweb.com. This wasn't a shop built pump around conversion. It was a garage version...
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