The basic question is this: shouldn't the vacuum pull-off just open the choke plate far enough for the motor to start and run, and then drop back to the second step on the fast idle cam when you tap the throttle?

the long version:

I put a new YFA on my 300 I6 and cannot get the choke to work properly. My understanding is that when you start the engine cold it runs at fast idle for a few seconds and then you tap the gas pedal and it goes to the second step on the fast idle cam.

I have already bent the connecting rod for the vacuum pull-off so that it barely opens the choke plate as seen in the center image, but when the motor is running it seems that the air flow through the venturi pulls the choke plate open more as seen in the image on the right, so that when I tap the throttle it jumps down to the third or fourth step on the fast idle cam and it stalls. After a time or two it is warm enough to run and is ok after that.

The image on the right is just after cranking on a 65 degrees F morning, and I disconnected the power to the choke heater to make sure that that was not letting the choke open too fast. When the wire is connected the plate is open even more at this point, which I suppose is because the bimetallic coil heats up fairly quickly. My carb does not have hot air to the choke; it's purely electrical heat.

My second question is how much voltage should the choke get? I've read that the systems that use hot air and electric heat should be powered by the 7 volt wire, but carbs like mine should get 12 v which is what I use. Even with that it takes several minutes for the choke to fully open.

 

If it's an OEM Ford choke assembly on the carb, then you want/need AC voltage connected to it. This AC voltage comes from the Stator post on your alternator.
My truck has a McParts store reman carb, and I'm still trying to get the choke dialed in...

 

Thanks for the reply. I'll hook it up that way and see how long it takes to warm up. But as I said, it screws up with no connection at all, so, somehow it wants to kick way down on the fast idle cam as soon as the engine is running.

It seems that just the engine air itself tends to open the choke plate because the axis is off center, but if that is my problem, how to overcome it? I tried turning the choke housing an excessive amount so that the spring would push harder, and that did not work, and would have kept the choke plate partly closed all the time anyway.

 

You mean kick down to a lower idle speed?

Man, that sounds like mine. Mine seems to lock open, no matter how tight the spring is. Just yesterday I went to check it out, and when I reached around to open the throttle, I bumped whatever is causing the hangup, and the choke closed...
I think that my problem is the bend in that metal rod which runs from the choke shaft to the fast-idle cam. Something must be opening over-center.

As far as I can tell, the only way to adjust any of it is by bending that rod. The only adjusting screw is for the fast-idle speed.

 

Yes. I'd like for it to drop down to the 2nd step on the fast idle cam but it drops way down to at least the 3rd step and stalls. After one or two restarts it runs ok at that slow idle speed.

From what I've read, if it is set up properly, after running a few seconds or so, you tap the gas and it drops down one notch. But if the choke plate is open fairly wide, it drops a lot further.

As seen in my center pic up there, I've bent the connecting rod quite a bit so that in a static test with my vacuum pump, 20 inches barely opens the choke plate. But with the motor running, it opens a lot more, and it's not the pull-off doing it, as I can see by looking at the linkage. I think it's the air rushing into the engine pulling the choke plate open, but how is that not a problem for this carb in general? I clearly have something adjusted wrong.

 

Here's the stator wire I hooked the choke heater to. It goes from a little under 4 volts to about 8.5, and the scope says the RMS value is 6.57, so I hooked it to the carb and nothing blew up yet.

 

Wasn't an instruction sheet included?

 

No sir.

I guess I might as well confess the truth, the whole truth, and nothing but the truth: it's a new YFA and it looks great, but it's one of those $90 Ebay wonders from our friends in China.

It runs fantastic, after the first minute of startup trouble.

 

Are you saying the CHI-COMS are now making copies?

Can you give me the brand name and PN?

 

lol I'm not sure you are serious, but anyway, crap I buy from China quite often doesn't even pretend to have a brand. The label picture is 100% of the documentation.

The other pic is of the spare. I figured that a rebuilt roll-the-dice YFA was about $300, so go ahead and get two of these for $215 with shipping. I have another old Ford, and if the one discussed above starts behaving, I'll swap the other bog-monster as well.

If you are serious, just search Ebay for "new YFA carburetor" and you'll see lots of vendors with various prices and shipping details. The one I picked had an american sounding name, not that it mattered, but the credit card bill was to something like Lin Yu ****. As long as they have thousands of transactions and 99% approval, it's pretty safe usually.

 

I think there's an internal vacuum circuit which acts as a choke pull-off. If the choke blade is all the way closed, the vacuum pull-off will open it a tad, once the engine starts, in order to sustain an idle. I know two and four barrel carbs have the vacuum pull-off, but have to admit that I'm not sure about our YFAs.

 

Interesting. Hopefully a carb guru will come by with some info.

I've found some good discussion in other forums, but it seems that carburetors have so many designs, and variants within one product line, that it's hard to get definitive answers. I have repair kits for the YFA and it's clear that it has at least three styles of choke control.

 

It seems to me that you guys are going to have to positively I.D. the OEM CARB on the truck (Asm Tag No) and use the rebuild kit instruction (or shop manual) specs to adjust the choke properly.

What you have (IMO) is just a problem.

If the manufacturer sold the carb with no specific instructions, it is a one may fit all and we hope it works for 'ya.

 

I agree, but since there is no tag, rebuild kit, or shop manual, I suppose nothing can be done.

Thanks to KULTULZ and meangreen92 for your helpful comments.

 

Let's try again.

With a properly operating and adjusted choke, one pushes the pedal to the floor (this sets choke) (and mechanical unloader) and removes foot from pedal completely. Start the engine. Do not touch the pedal. The engine should start and run on fast idle. The choke pull-off will then hold the butterfly open enough as to not stall.

As the choke caps warms, it will lessen tension on the fast idle cam and slightly touching the pedal will allow the fast idle to fall as the position of the fast idle cam recedes.

There is more than the choke to give satisfactory cold air performance, heated air inlet, exhaust crossover, advanced timing, etc.

What I would do if no Tag No to get you into the correct settings chart would be to have a parts person look up the carb no. by application and record all the possible I.D.'s so as you can go to the adjustments chart and get within range of how to adjust on the bench and then fine tune from there.

There may be some of these rebuild/adjustment instruction sheets on the net. I have stacks of them from over the years but it would take an act of GOD to be able to find and dig them out.

Do you have one of these CHI-COM Specials also?