YFA Choke pull down piston question
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I have been checking over my Junk Yard carb of an 1981 F150 4.9. It had a manual choke cap conversion in place of the electric choke. When I opened it up the choke pull down piston was rattling loose inside.
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So.. Do I really need the choke pull down piston?
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It looks like there is vacuum on the bottom side of the piston which would pull the piston down after the engine starts. I believe there is a bypass along side the piston, so after the choke is opened a little, and the piston moves down a little, the engine vacuum will then draw hot air from the choke stove bypassing the piston, at least that is how I think it works.
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The newer carbs did not have a piston and the hole were the 7/16 dia piston was is only about .25 inches.
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I was thinking about just using the carb without the piston.
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Jim

 

The purpose of a choke pulloff (pull-down piston in this case) is to prevent the choke plate from completely closing during a cold start. When you press the pedal once before you start the engine, the opening of the throttle frees the fast idle linkage so that the choke plate can snap shut. In that situation, the engine cannot run because there is no path for air to enter the throat of the carburetor. When you crank the motor over, enough vacuum is created to pull the choke piston inward, which cracks the choke plate open. After the engine fires, engine vacuum maintains the choke plate in the pulloff position. As the engine runs and the choke spring warms up, the choke plate opens and is taken off of the pulloff. However, the piston remains pulled inward for the entire time the engine is running. In summary, the choke pulloff serves to fight the tension of the choke cap to prevent the choke plate from suffocating the engine completely.

Your question, however, was if this mechanism is necessary - the right answer is yes, it's necessary to set up the carburetor properly, and its proper operation will make the choke much easier to set. However, people have gotten away without using it. Many people are driving around without their's working and probably don't even know it. Many people even rebuild carburetors without knowing anything about it or setting it up. The problem you'll run into if you set the choke as normal and try to start the engine cold, it might close off the throat of the carburetor and prevent the motor from starting. In some cases, however, air moving past the choke plate is enough to keep it from shutting off completely. Another fix people try is to set the choke such that it does not snap completely shut when the choke is "loaded" by pressing down the pedal, but rather lightly shut such that there is still somewhat of a crack for air to get past. That solution will most likely work, however I think in that case it might be a little harder to dial the choke setting in, because you'll need to find a tradeoff by keeping it open enough that air can get past, but closed enough that the fuel mixture remains enriched.

 

Thank you for the detailed and informative response. I think I will just see how it works without it.

I did some static bench testing on the electric choke to see how long it took to open the choke.
With 14 VDC applied it would open fully in 2 min. 25% in 30 sec, 50% in 1 min.
With 7.5 VDC applied it took 10-12 min to get it fully open. 1 min it was starting to loosen and at 2 min it was open a crack.

In real life the air flow would also try to pull it open.
I am sure a lot of trucks on the road don’t have a working hot air choke stove as most I have see at the junk yard the pipes are missing or rotted off.

Update…

I just went out with my stop watch to time my truck with a good working hot air system.
I started it and let it run for 1 min, it had the choke plate open about .25 inch while it was running.
I shut it off and gave it some gas to reset the choke plate, it snapped fully closed and seem to have the same spring tension as when it was stone cold (45 F).
I then started it back up and in 4 min the choke plate was fully opened but still felt loose to the touch.

Thanks again for the info on the choke pull down piston.

Jim

 

fmc400,
I think I have the opposite problem. I have probs running AFTER the engine is warmed up. It runs sooty, cycles or surges, dies or stalls out/off. It is a 170 cid w/carter YF - orig. thermocoil/bimetalcoil type choke - now unwired up ele choke (orig. 1 rotted off). What can I do to get it to run when warm?
cf
Amherst. MA
chrlsful@aol.com

 

Verify that the choke plate is open once the engine is warm. If you're using an electric choke with no heat source and the power terminal is disconnected, the choke spring won't unwind and the plate will stay shut.

Refer to the following article regarding choke operation; this explains what needs to be going on: https://www.ford-trucks.com/forums/8...ml#post6939116

 

fmc400,
Thanks 4 the post.
<<the plate will stay shut.>> Ahh. OK will check to see if the "throat" stays blocked off. It seems to run better than that, but your idea sure fits. Am I supposed to hook up some wires (off that 'ele choke') to the "stator" on the alternator? How do I identify the stator?
Thanks,
Chad
chrlsful@aol.com
BTW: how many yrs did they make that 400 ( was in a Bronco several yrs, I've heard?)

 

How many terminals on the choke? Do you know if it's a factory choke, or an aftermarket? To hook something to the stator of the alternator, there is a threaded post on the back labeled 'S' or 'STA.'

I believe the 400 came out in '71 in cars and in '77 for trucks. I think it stayed in trucks until '82. I do believe you could get a 400 in 78-79 Broncos. I've got one in my Galaxie.

 

Good Day!
Yes, there are 2 terminals on the plastic/bakelite choke housing. I had an original choke in '82 when I got the rig. The innards (& may b the whole assembly) were removed/replaced with the ele choke. I watched as the ford specialist took out the rotted coil. I think a riser type tube of sorts was also there (wrapped in asbestos?). He put on an "electric choke' but never hooked up the wires. The housing has lines (6 - 8) on it's outside top that R lined up w/a center marker on the metal of the ...is it called air horn?...part of the carb it's attached to. Almost like "adjustment indicators"
I read your detailed post (Nooouice!) of the 25th. I have not checked your recommendation on "C if the butterfly stays closed after warm up". I'll hafta take off the mickey-moused air filter I put on and let it run to see.
I'm wondering if either of us has the "high Boy" a nick name 4 a Carter carb that was desirable (4 what I don't know)?
Yes, the 400 was in the 1st "big bronck" (the 2 yrs. B 4 another model change). It was designated with the "M" or "400M" - for Moline or Milwaukee I guess. Yrs. ago, when 1st on-line (1998?) I got on a forum (early bronco registry or something) and a guy who knew his stuff sang it's phrases. He did a few minor changes (think he called it a "408") & had a real stump puller. Hauled huge trailers 2 as I remember.
Thanks 4 walkin this long lonely mile w/me.
cf