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Old 04-20-2019, 04:55 PM   #1
Rand
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Default Stumble

Probably a dumb question for most.
But what would cause my stock Holly 50 Merc to stumble shifting between gears? Almost a jerk
Thanks Rand
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Old 04-20-2019, 05:57 PM   #2
cmbrucew
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Default Re: Stumble

Vacuum leak ?


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Old 04-20-2019, 06:18 PM   #3
34PKUP
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Default Re: Stumble

If overdrive equipped there is a momentary engine kill to aid in shifting.
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Old 04-20-2019, 10:12 PM   #4
Rand
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Default Re: Stumble

Cmbrucew wouldn't a vaccum leak be noticeable at idle as well?
I'm going to spray some starting fluid around and see what happens
34pkup are you referring to the kick down switch that takes it out of OD while passing?
Thanks for the ideas...anymore?
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Old 04-20-2019, 10:51 PM   #5
philipswanson
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Default Re: Stumble

Loose motor mounts.
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Old 04-28-2019, 01:02 AM   #6
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Default Re: Stumble

Rand, how's it going? Vacuum leaks are not as problematic at idle verses load. Same with sticky valves or fuel issues. Put a vacuum gauge on it, you can run one to the cab so you can monitor when driving.


Could be electrical also. Do you have a 3 on the tree OD?

Last edited by Tinker; 04-28-2019 at 01:23 AM.
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Old 05-02-2019, 09:56 PM   #7
Rand
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Default Re: Stumble

Tinker..yes 3 on the tree with OD
But all OD electricals disconnected
Its a stumble between gears. If I ride the clutch it does not do it.
Im thinking it's something in the Holly carb? IDK? Daytona parts in Fl. Rebuilt it in 2015
Not many miles on it since then.
Finally getting back on the road after being parked for 2.5 years due to messed up steering column and shift linkage. But always ran it regularly to just hear and gun the flathead.
I think it's in the carb? Maybe
Motor mounts are good.
Sprayed starter fluid all over intake with no change?
Ideas Im missing?

Last edited by Rand; 05-02-2019 at 10:01 PM.
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Old 05-02-2019, 10:05 PM   #8
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Default Re: Stumble

So if you keep the rpms up it won't stumble when shifting?


But going from higher rpms in a lower gear, to a higher gear it bucks till it reacts a decent rpm, then runs well.


Not sure it's fuel, but could be. What carb? if I missed it. Shifting to a Higher gear would lower engine vacuum a bunch at shift point reducing fuel flow and engine vac flow. Which the pv should kick in.


just a guess... could be a lot of different things too. Maybe a bad condenser. But I agree on the carburetor. Engine,carb,valves, and condensor issues don't show themself idled parked in a garage.

Last edited by Tinker; 05-03-2019 at 12:27 AM.
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Old 05-03-2019, 12:34 AM   #9
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Default Re: Stumble

You let off the gas when you push the clutch in to shift, then throttle up as the clutch comes out. If the accelerator pump was not working it might cause a stumble as the throttle opens with no accelerator pump squirt.
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Old 05-03-2019, 10:42 AM   #10
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Default Re: Stumble

Check the OD cutout switch under the accelerator pedal.
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