Thoughts on the choke breaker & jet

[4wdchico]

I recently helped a friend diagnose and replace a failed choke breaker (CB hereafter) that had a ruptured A diaphragm. His car is a stock '85 t4wd. He had bought a rebuilt carb a couple of years ago and it did not come with the jet that fits in the line that connects to the A diaphragm of the CB. The A diaphragm must have failed soon after he installed the rebuilt carb as he has had a major over rich condition before fully warmed up for a good while now.

After some thought studying the FSM illustration of the CB vac circuit I'm pretty sure that the vac also serves the very important role of preventing the A diaphragm from being destroyed by an engine backfire. Without the jet the A diaphragm would be exposed to the full effects of a backfire as it is directly connected to manifold vac via a short hose.
Please don't assume that your jet is bad because you cannot blow air thru it: put a vac line on both sides of it and then put one end in water and blow thru the other and you will know for sure as these jets don't flow much air. I'm pretty sure that my buddy's CB diaphragm failed due to the lack of the jet on the rebuilt carb.

The choke breaker is available new as Standard Motor Products part number CPA416. If you do some searching you will find out that it goes for about $250 at the cheapest. Yikes! I did see that Autozone has a CB for our cars in their Duralast brand for $99.99. Still kind of crazy. So getting a spare, tested while on the car, CB from a JY might be a good idea.

Changing out the CB is not difficult:

First mark how the black plastic choke assembly cap is clocked to the alloy choke body with a scribing tool. Then remove the three screws that hold the black plastic choke cap that holds the thermal spring and it's heater. Then remove the black plastic choke cap to gain access to the three much larger screws that hold on the alloy choke body to the air horn casting. Those three larger screws also pass thru the CB mounting holes, remove them and the alloy choke body. You now can remove small circlip from the rod that transfers motion from the CB to the choke mechanism. & then remove the CB.

Notes on testing the CB on the car: The car must be cold. Pull the air filter assembly. Then step on the gas once to set the choke. Hook up long vac lines to both of the vac nipples on the CB. First apply vac to the A diaphragm and make sure that it both causes the choke to open slightly and also holds vac. Then while holding vac on the A diaphragm apply vac to the B diaphragm, doing this should cause the choke to open a bit more and it should also hold vac. Note that the B dia will not move at all unless the A dia has vac applied first.

[irowiki]

Hmm. Is this the port on the bottom of the carb right next to the HIC connection that looked really restricted? Compared to the others, it was about the size of a pinhole. I thought that was a "built in" jet, as the diagrams were confusing.

If not, I guess I need to put a jet in there! What color are the jets again? Does direction matter?

[4wdchico]

Hmm. Is this the port on the bottom of the carb right next to the HIC connection that looked really restricted? Compared to the others, it was about the size of a pinhole. I thought that was a "built in" jet, as the diagrams were confusing.

If not, I guess I need to put a jet in there! What color are the jets again? Does direction matter?

I think that we are talking about the same port on the carb base, but I'm not sure.

Sorry, but I have seen the jets in different colors.

As far as direction goes, I have done the water test on the jets and there only seems to be a slight difference, if at all, on the direction of airflow with respect to volume flowed. YMMV. If your jet does show a big difference in directional flow I guess that you could "tune" the CB's action by trying it oriented in both directions and seeing which works best for your climate.

[dlb]

i also learned how important the CB is to cold running a while back. had a project car that ran like crap when cold, found both diaphragms of the CB were ruptured. i managed to replace in with the carb still in place. i posted pics of it here.

https://tercel4wd.com/forums/viewtopic.php?f=4& ... 715#p63661

car ran great afterward!

[irowiki]

So the jet is the one that restricts air to both sides and is about the same size as the orange check valve, where a VTV is like a giant check valve in how it operates?