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Can you help with my "bucking truck?"

Started by shinnlinger, June 28, 2015, 10:53:16 PM

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shinnlinger

Hello,

I have an '84 F-600 with a 370 4 barrel.  It's an AG truck and doesn't go out often, but this year it has decided that on a hill, loaded or not, it will start "bucking."  I can't power thru it and usually have to downshift.  It will rev freely on flats. 

I did some horse trading for it a few years ago and the story was it has a used "race car carb" put on it.  It's a Holley and seems to overfuel as it will backfire when you let off the gas but that can be controlled by feathering the peddle and not really an issue but I mention it as it may aid in the diagnosis. 

The bucking thing is a problem though as my place is "uphill both ways" and don't feel confident pulling hills with a load.  I suspect the fuel pump may have a pin prick in the diaphragm but don't want to just swap parts for the sake of swapping parts.  I think I have a pressure gauge somewhere but what kind of pressure should I be looking for in a mechanical fuel pump?

I did replace the in line filter and cleaned out the screen where it enters the carb (that wasn't too bad).

As added info, I tried to start it this winter in below zero temps but no fuel came into the filter and I gave up and waited till spring.  The bucking occurs regardless of the amount of fuel in the tank.

Any pointers?

Thanks.

Dave
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

North River Energy

Often when the diaphragm begins to leak, you'll wind up with fuel in the crankcase.
So if there is more oil on the stick then you remember the last time you checked...

Does the truck backfire under load while bucking, or is it more of a surging?
Is the float set to spec? Needle sticking?

Had a problem a few years back where my Dodge would start to skip and miss etc. at under load/very low vacuum.  Turned out the coil was dying.

What's the number on the Holley?


shinnlinger

it does not back fire while bucking, that is a separate issue.   

Coil could be a culprit too I suppose. Will look into gas in oil and number on holleyoin the daylight.  Probably should rejet but he truck has run so well but gets used so infrequently I just lived with the backfiring but the bucking is a problem.

Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

timberlinetree

Might be the floats in the carb need setting or water in the fuel?
If this was in a ford bronco you would have a bucking bronco :D good luck with the fix.
I've met Vets who have lived but still lost their lives... Thank a Vet

Family man and loving it :)

Remle

Just a thought: " As added info, I tried to start it this winter in below zero temps but no fuel came into the filter and I gave up and waited till spring.  The bucking occurs regardless of the amount of fuel in the tank. " This statement would make me think you may have water in the tank, pull the fuel line at the carb and turn over the engine catching the fuel in a clear container and see what it looks like. While on the level water may be distributed over the bottom of the tank and gather at the pickup tube when tipped up going up hill.

Gearbox

7 psi on that fuel pump. That is where I would start . the fuel leval should just show up in the float bowl plug . Is it a 4 barrel or 2 same setting for the back bowl . Some Holleys had a brass filter where the fuel line enters the carb would have a big hex the filter would be behind that . Gearbox
A bunch of chainsaws a BT6870 processer , TC 5 International track skidder and not near enough time

Hilltop366


First thought was wet distributor cap then it reminded me of a motorcycle I was working on years ago, it worked fine until under load and hard acceleration then it would skip and cough, turned out that the float bowl had water in it and would run perfect until the demand for fuel increased then it tried to run on water.

shinnlinger

Thanks for the responses guys, I appreciate it.   It sounds like water in the tank/fuel bowl is suspect and since I recently patched a rust hole that was under the ID tag of the saddle tank, I'm gunna say a strong possibility. 

My wife broke her leg the other day and had surgery today to reset it so it may be a few before I get back to the truck.   I assume I will find drain plugs under the floats and perhaps the tank,and I will try to drain or siphon from the bottom, but is this a drygas situation?

Dave
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

North River Energy

Depending on series, you can probably drain the float bowl by removing one of the two lower bolts holding the bowl to the carb body.  If it's a 'race carb' it's probably a 4150 or similar and that should work.
You may want to pull the bowl and check for any debris/precipitate from old gas etc.

If I have suspect fuel in an older vehicle, I'll run it off an outboard boat tank with clean gas as a diagnostic.

Some of those side tanks have a drain in a sump right in the middle of the bottom.


JB Griffin

Weak Fuel pump maybe? It would have a mechanical pump on the side of the motor, those are low pressure pumps and only put out 5-7psi as Gearbox stated.
2000 LT40hyd remote 33hp Kubota with 6gpm hyd unit, 150 Prentice, WM bms250, Suffolk dual tooth setter

Over 3.5million bdft sawn with a Baker Dominator.

Ford_man

I have a 78 with a 460 and it will do the same thing now and then , I change the screwin filter on the carb and it is good to go. good luck

Hilltop366

Shinnlinger sorry to hear about your wife's broken leg, hope she is doing good.

Take good care of her, the truck can wait.

Gadrock

Holly 4150 type carbs are notorious for "economizer" valve malfunction that may do exactly like you describe. In one or both of the fuel bowl removed, the fuel block that is next has a diaphragm valve down toward the bottom. One backfir can destroy this fixture. One of the competitive companies makes a replacement that does not share this proble. That name starts with "E"...hint, hint.
    The reason the engine backfires is because it runs lean, then out of enough fuel so it resists to continue. As it stops running for this brief time enough fuel returns to the system to allow the cycle to start over. One test is to engage a choke mechanism to see if the engine runs better. I would bet that it will.
    Many time peeps with welders and mowers and such experience the same situation. Pulling out slighly on the choke gives more proper air fuel ratio and allows it to run.
    Chain saws experience the same problems, but hardly ever are noticed because if they run then they soon are operating at a higher rpm band.

David g

carry on
LT40 G18,   bent Cresent wrench,   broken timing light
Prentice 280 loader, Prentice 2432 skidder, Deere 643J fellerbuncher, Deere 648H skidder, Deere 650H Dozer

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