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Subject: 8N hesitation when going wide open throttle.

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Robert Betz    Posted 07-19-2022 at 22:48:47 [URL] [DELETE]        [Reply] [Email]  
  • 8N hesitation when going wide open throttle.
  • Greetings. I am new to this site but I have owned (or has it owned me?) a 1948 front distributor 8n. Last winter after years of frustration I changed charging system to 12 volt, added electronic ignition and replaced the carburetor. Now however, when i crack open the throttle there is a hesitation as if it had a bad accelerator pump, if you ever drove a Dodge Omni in the 80's you know what I mean. Any ideas? I had the engine rebuilt 8 years ago but do not have alot of hours on it. Thanks for any suggestions.

    John in Mich    Posted 07-21-2022 at 20:41:35 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 8N hesitation when going wide open throttle.
  • I'm with Dean on the carb adjustment. If adjustment is too lean, engine will stumble. If adjustment is too rich with today's gasoline, fowled plugs.
    My test is with a warm engine:
    1. slap gas lever to wide open throttle and back to idle. If it stumbles, open main jet 1/8 turn and try again. Repeat until no stumble.
    2. after eliminating the stumble, close the main jet 1/8 turn and re-test to wide open throttle. If it stumbles, open the 1/8 turn. If no stumble, close another 1/8 turn. Re-test.

    Dean    Posted 07-20-2022 at 07:45:00 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 8N hesitation when going wide open throttle.
  • Hesitation upon acceleration is usually the result of lean mixture.

    Since your tractor carburetor has no accelerator pump the only thing one can do is richen the main circuit by opening up the adjustment a bit. Start with 1/4 turn, test and repeat.

    Of course, one should never turn to carburetion until sure that ignition is spot on.


    Ed Gooding (VA)    Posted 07-20-2022 at 05:16:15 [URL] [DELETE]        [Reply] [Email]  
  • Re: 8N hesitation when going wide open throttle.
  • Did you rebuild your Marvel-Schebler carb or buy a new replacement? If you bought a new replacement, I have heard this complaint from a number of buyers. It seems to be a crap shoot - some are happy with the performance, and others suffer the same symptom as you. A mechanic friend ended up drilling out the main jet just a hair (I "think" it was a size M or N bit) and reported that it solved the issue.

    JMOR    Posted 07-21-2022 at 14:06:23 [URL] [DELETE]        [Reply] [Email]  
  • Re: 8N hesitation when going wide open throttle.
  • Huuuummmmm.....according to the chart, N & M are near 0.3 inch......may be a "tiny bit" on the large size. :)

    Ed Gooding (VA)    Posted 07-21-2022 at 16:27:37 [URL] [DELETE]        [Reply] [Email]  
  • Re: 8N hesitation when going wide open throttle.
  • OK, I was going from memory, which is sketchy these days. Rather than a snarky response, do you have a constructive suggestion for a better size?

    JMOR    Posted 07-21-2022 at 17:05:20 [URL] [DELETE]        [Reply] [Email]  
  • Re: 8N hesitation when going wide open throttle.
  • Sorry, Ed, I didn't mean to be mean/snarky. No on better size. I would likely just go with stock size and recommend that he verify that he has that size, which I believe to be 0.043 for main. Guess I should have followed up with a smiley face?

    Mike Maiman    Posted 07-21-2022 at 20:29:21 [URL] [DELETE]        [Reply] [Email]  
  • Re: 8N hesitation when going wide open throttle.
  • Robert, here's what I just did with my carb. First mine is an original Marvel carb off a 51 8N. I don't know if my procedure would work for an after market carb. My carb was totally full of rust and corrosion and I've had it on and off 3 times now. In the "How To" section of the 8N web site there are great articles on cleaning and rebuilding Marvel carbs. The one article that finally got me success is titled Carburetor Maintenance Tips. I enlarged the idle port above the throttle butterfly to .040" (#60 drill) and left the idle jet alone at .038". I enlarged the main jet to .046" (#56 drill). I made sure I surgically cleaned (again) everything and reassembled. It now idles at 400rpm and I can gag the throttle to 2100rpm with no cough or hesitation. Amazing! Note, that none of this will work if your engine is in need of repair, your valves out of adjustment, your manifold leaks or has cracks and your ignition timing is off. Sorry for being so windy. Good luck.

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