I have a 78 F-100 with a I-6 300 with a 1 bbl mastercraft carb. My engine dies when I have the carb adjusted for a slow idle when I put on the brakes. So right now I have to drive around with the thing idling way to fast and its annoying and wastes gas. The carb has been rebuilt before, but that didn't seem to help. Anyone have any suggestions?

thanks

 

Well you are obviously sucking the vacuum out to operate the brake booster.

Before toy set the idle you need to make sure all the other adjustments affected are within their specs.

1) The entire vacuum system is sealed (no leaks what so ever)
2) Timing set at correct engine rpm (with vacuum hose to distributor plugged)
3) in NO CHOKE ...normal operating temp mode...Air/fuel adjustments screws set for smoothest engine idle with Air cleaner assembly fitted on.
4) Set Idle to spec


Inspect under hood for Vacuum reservoir .. if you don't have one ..get one.

One last check..

Spray some carb cleaner or the like around the carb butterfly shaft on the outside to see if that alters the operation of the engine.
I suspect the shaft is worn out and sucking air.

 

Thanks for the advice, I'll try this stuff out this week and see if I can get it running better.

 

This may be a dumb question, but I don't have power brakes. So I wouldn't think that I had a brake booster...

 

You don't. We need more info.

Does the truck stall if it is standing still and then you step on the brake pedal? If so, I would look under the dash for a frayed wire or a wire being stretched by the pedal.

Does it stall when stopping from a certain speed? or any speed? In this case, I would first check the level of the fuel in the primary fuel bowl.

 

Do you have an automatic transmission?

 

Im gonna throw this out there cuz its happened on my Bike and my Truck:
I have several keys and a brass clip on my key chain. The weight of the key ring Combined with a worn ignition switch, Hard braking in the truck (manuvering on the bike) generates just enough inertia to jiggle the switch enough to shut the engine down....
just something to think about.

 

Cool one!

I had an old '67 bug that'd do something similar to that, took the ignition switch apart several times over the years "re-re-fixing" that. Old VWs... talk about fix or repair daily... gotta like working on them or forget 'em IMO!

Finally removed that old junky ignition switch and replaced it with a hidden toggle switch and a push button ...where the key used to be. YMMV on any of my crazya** ideas!

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Did get a few drops of water in the vent pipes on my '75's 2100 carb once tho.

Took the lid off and fished around in the gasoline and didn't see any water or trash, so put it back on. The water had reacted with something in the fuel tho and turned it into clear jelly that couldn't be seen that way (because of low density or refractive differences).

Not until like a week later did I take the carb off and poured the fuel and jelly out did I find the problem. I had washed my engine with a rubber banded on plastic bag.

The wet plastic bag dripped a little water in the 2100's large vent pipes.

Easy as that.

Got a new Blue Streak coil in the process of trying to figure out what was wrong. I like my new coil tho, threaded terminals instead of that ugly clip-on plastic cap thing. "Hotter than stock" too, see?

Alvin in AZ

 

I quoted:Before toy set the idle

It appears I am going nuts

I typed a T instead of Y and a Y instead of a U to get TOY instead of YOU.

Apparently I need more toys

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Topic at hand.

I with a PB I am now going to have to assume this is a braking motion problem affecting "fuel in the carb" or something like the KEY thing mentioned.

For carb operation I revert back to my first reply

For the possible KEY problem...anything is possible

 

Ok, its a 3 speed manual, and it would die when you were moving and hit the brakes, but not when It was still I believe.

 

Also, when the engine is warmed up and I come to a stop it idles high until I rev the engine and then it settles back down to something normal.

 

Carb linkage binding (needs cleaning and oiling?)

Vacuum advance mechanism is messing with you?
Could be the carb's fault there too since the carb's port effects the vacuum motor controlling the advance.

What do you think?

Alvin in AZ