Stumble at full throttle

shawnm · Jul 17, 2012

I'm trying to get a bit of a stumble out of my 340. At full throttle it has a miss to it every now and then. Maybe its normal for these old tractors?

Last year at my local pulls it fell flat on its face. It appeared to be a fueling issue but I ended up finding an broken spring in the dist.

Here is what I've done to it this year:

- New Zenith carb
- New points (gapped to .020")
- New condenser, rotor button, cap, Auto Lite plugs, wires.
- New fuel line
- New sediment bowl assembly
- Compression is 170, 180, 175, 160
- Valves adjusted to spec

What else can I check? Here is a video of it running. At 1:40 you can hear the stumble.

http://youtu.be/PQNsrn1myMk?hd=1

davediehl@hotmail.com · Jul 17, 2012

Looks to be running a bit rich. If the engine was up to temp, pull the plugs and see if there is any color on them. I'm going to be they are black.

Dodgeit · Jul 17, 2012

(quoted from post at 11:08:04 07/17/12) I'm trying to get a bit of a stumble out of my 340. At full throttle it has a miss to it every now and then. Maybe its normal for these old tractors?

Last year at my local pulls it fell flat on its face. It appeared to be a fueling issue but I ended up finding an broken spring in the dist.

Here is what I've done to it this year:

- New Zenith carb
- New points (gapped to .020")
- New condenser, rotor button, cap, Auto Lite plugs, wires.
- New fuel line
- New sediment bowl assembly
- Compression is 170, 180, 175, 160
- Valves adjusted to spec

What else can I check? Here is a video of it running. At 1:40 you can hear the stumble.

http://youtu.be/PQNsrn1myMk?hd=1

If the main jet is too big, causing it to stumble you would be able to throttle back slightly and the motor should pick up rpms. If your at mid throttle and it is stumbling, the problem could be that you are in that transition zone between the idle circuit and the main circuit and you have the idle adjusted improperly. If you open to full throttle and then to idle and it back fires your too lean on the main circuit.

Also forgot to mention that your problem may also lie with the compression ratio. There is more than 10% difference between 2 and 4 which can cause problems of this nature, especially at load.

Phil Tibbetts · Jul 17, 2012

MY hearing is not that great but could it be a valve sticking a little? This new gas with the alcohol is s--t. I have had a lot of problems with it even though I use stabilizer. I have found that if I add a little 2 cycle oil or Marvel Mystery oil in addition to the stabilizer I have much less trouble. IMHO

shawnm · Jul 17, 2012

David - You're right the plugs are black, but I have been working on the tractor so much they haven't cleaned out. I have no black smoke at full throttle so I assume I have it adjusted right by now.

Dodgeit - my throttle is good all through the rpm range, idles perfect and runs excellent except for this slight miss. I noticed #3 plug was oily and I presume that cyl is causing the white smoke.

Phil - I've been thinking I have a sticky valve also. I hear a rhythm similar to the spund of a helicopter in the distance. Although I've pulled the rods and set the valves and found nothing out of the ordinary.

Art S. · Jul 18, 2012

Has a similiar problem with my 460. Found I was sucking air at the manifold. Tighten the manifold nuts and problem was gone.

dpski · Jul 19, 2012

I assume the stumble you're referring to is at 1' 04sec, not at 1' 40 sec?

If so, here's my 50cent bet, notice it's a low bet, but worth checking;

Gas cap is not venting properly and you're building up vacumn pressure in the gas tank such that the engine starves for fuel.

Try your test again and exactly when it happens watch the fuel bowl for air bubbling.

Also try immediately shutting the engine off when it happens and quickly move yourself to the gas cap and slowly turn the gas cap and listen for a hissing sound as you turn the gas cap.

Had a similar problem on my SC but only at full throttle. Gas cap was not venting, so bad it caused it to stall. Actually happened more frequently after I put a new gasket under the gas cap, took me a while to connect the dots.

I got another gas cap gasket, trimmed the outside edges smaller so there's was better airflow under the cap, but still sealed the fuel filler neck and drilled out the hole a smidge in the top center of the gas cap that was probably a little quag'd up underneath, problem solved, runs like a champ now.

Stumble at full throttle

shawnm

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Dodgeit

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Phil Tibbetts

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shawnm

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Art S.

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dpski

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