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Old 03-28-2019, 05:09 PM   #1
Cool Hand Lurker
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Default Fresh Flathead bucking

My rebuilt stock flathead has only about 500 miles on it but out on the highway at about 50-60 it will occasionally 'miss' and 'buck' just once, not a continuous series of bucks. If I pull out the choke just a little bit it doesn't do it. Any ideas on what the cause can be? Dirt, timing, bad plug, lean carb, leaking gasket..?
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Old 03-28-2019, 05:18 PM   #2
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Default Re: Fresh Flathead bucking

A lean mixture (like at cruise) is harder to fire. If your carburetor is clean and ignition up to spec, I would try larger main jets.
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Old 03-28-2019, 05:22 PM   #3
oldford2
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Default Re: Fresh Flathead bucking

First thing I would is pull the top of the carb and the main jets. Check the bowl for specks of dirt and blow out the main jets
Easy first step in trouble shooting.
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Old 03-28-2019, 05:24 PM   #4
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Default Re: Fresh Flathead bucking

What model (year range) is the engine and the distributor?
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Old 03-28-2019, 06:18 PM   #5
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Default Re: Fresh Flathead bucking

Sure sounds like one of the main jets is partially plugged or you have a condenser starting to give you trouble.
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Old 03-28-2019, 08:24 PM   #6
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Default Re: Fresh Flathead bucking

Problem seems to be carb related (not ignition) since chocking temporarily fixes the issue.
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Old 03-28-2019, 09:26 PM   #7
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Default Re: Fresh Flathead bucking

Try this it's cheap. Had the same thing. Idled great, but underload bucked.

Pour mmo down the carb at high idle till it almost stalls. Let it sit a couple hours. Do it again and then drive it hard. Will free up valves sticking. Yes they stick on a new rebuild.

mmo is cheap.


Choke added at speed, maybe fuel flow/lean also. But it could be a lot of things... sorry


.

Last edited by Tinker; 03-28-2019 at 09:43 PM.
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Old 03-29-2019, 09:14 PM   #8
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Default Re: Fresh Flathead bucking

I never thought about the valves sticking. That would make it act like it is acting. Just happen to have MMO on the bench so I will try it. If that doesn't work then it is time to check the float bowl and main jets for debris.
Thanks, all
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Old 03-29-2019, 09:43 PM   #9
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Default Re: Fresh Flathead bucking

Could be high speed air bleeds but normally it would be constant and when pulling choke goes better. I had a similar problem and uncle Max said check that, sure enough, blocked high speed air bleeds......worth a look.
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