tom61

6v to 12v conversion for automatic choke

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I have a 1947 Desoto (for sale by the way) and when I bought it, it had a Rochester monojet 1BBL carburetor 213 (i added a picture).

Since it was missing lots of parts and was leaking like a fountain, I decided to buy a brand new one .After, I found out it is not the original one, and it used on 77-79 chevy, gmc and buick.During the summer, the car would start well, and the engine would not stall during the warm up period.

The problem is that during the colder weather, it starts fine, runs for 3 or 4 seconds, and then dies. Also, I think it's using alot of gas, and i think it floods alittle on start.

After trying to adjust the carb, I came to the thought that the automatic choke might be the issue, since it is a 12volt choke and the car is 6volts.

 

I am thinking of buying a 6v to 12v converter and then wire it to the choke (and also to a 12v fuel gauge, since the original one is missing).

What are your thoughts on this?Will it work?Or should I buy/install a non-electrical choke?

 

 

$_35.JPG

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I would get whatever came on a 1947 Desoto. Probably some kind of a Carter, and thats going to be miles ahead of any Rochester.

 

That said, automatic chokes (or at least anything as modern as that MonoJet) all work about the same. It should just barely close at room temperature. It should close harder when cold, and go to a high step on the fast idle cam. Step on the throttle to "set" the choke before cranking.

 

When the car starts, there should be a vacuum device (right front lower in your pic) that pulls the choke open partway. There should be a fairly significant gap. There is some springiness, so it is less of a gap if the car is really cold, but it pulls open a lot. Maybe 3/8 at the bottom of the choke plate. The manual for the carb will have the actual spec.

 

When you step on the gas the first time after it starts, it should drop to a lower step on the fast idle cam, and the idle should come down partway. It will stay on a slight fast idle until the choke is all the way open.

 

When the car is all the way warmed up on the gauge, the choke should be all the way open, unless it is really super cold out, and even then it should be almost open, and get all the way open in the first mile or two after that.

 

If the choke isn't getting open all the way, and staying open, then I guess you really do need more voltage.

 

Otherwise, maybe you need to get the manual for the car the carb came on and go to town setting the choke up. It could be the carb is just a horrible match for the car it is on (wrong jetting and venturi size). I wouldn't know for sure about that.

 

Good luck.

 

 

 

Edited by Bloo (see edit history)

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Why reinvent everything for no net gain or benefit?  The original carburetor and automatic choke (Slsson) were a rather flawless combination that gave long, reliable performance.

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I think the match was ok, as It would perform decently in the summer. Had a 6v to 12v converter ordered, I'll let everyone knnow how it goes.

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These days of computer controlled engines have spoiled us. They don't have a choke as we know it anymore.

Most people don't let their engine warm up anymore (which is really hard on them) as they don't know how to use a choke.

As suggested, a couple of pumps will set the choke,  (I would do this before cranking)  start it up and let it run awhile at high idle before stepping on the gas again.

With the incorrect carburetor you may experience shifting problems as well.

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It worked relatively well in the summer.The only problem was that it seemed to be using up too much gas, but It might just have been due to the need for adjustment of the float, which I hadn't done yet. 

 

Another issue I'm considering is the spark plugs, which showed clear signs of dry carbon fouling.

I looked up the spark plugs, and they are a NGK B6HS (that's whats printed on it) and my research tells me the Desoto S11 would have used a R45 spark plug, which my research tells me is hotter than the current NGK ones.

I wonder if this could be the cause of flooding and hard starts in winter....????

 

I'm thinking that the main problem is the float adjustment.

 

Let me know your thoughts!

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I definitely know there is too much gas coming from the carb.

The only other cause I can think of is excessive pressure from fuel pump

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You are on the right track. If the plugs are soot-fouling, it is too rich. If the plugs are too cold, it will aggravate the fouling, but not cause it.

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Also, could the excessive fuel be caused by the choke not operating correctly?

It occured to me that when I initially installed the carburetor, there was a clear high idle.After some time however, it seems that the high idle completely disapeared, and the car would turn on directly into normal idle.I also don't see an adjustment screw for high idle.

I only see 2 adjustment screws, 1 for acceleration (directly onto throttle/accelerator pedal) and the other must be mixture.

Am I missing something?

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Would the colder plug allow for unburnt gasoline ?

No, they just foul easier. Once fouled, then of course they miss.

 

I wouldn't put too much stock in "equivalent heat range" charts. They used to make sense, then one of the manufacturers, I think it was Autolite, discontinued some heat ranges, and suddenly the plugs they still made covered those applications (in other words, they would screw in the hole). The other manufacturers propagated this wrong information into their heat range charts. Then Champion introduced some plugs with numbers that APPEAR to be in their old numbering system (for heat range) but are not. Those charts are nonsense now. Don't believe them.

Edited by Bloo (see edit history)

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See my reply above. Is the choke opening all the way and staying open all the way? It needs to.

 

There should be a screw for idle speed (this is the stop screw that the throttle hits when the idle is all the way down when the choke is ALL THE WAY open.) If the choke is not all the way open, It will be on the fast idle screw, if there is a fast idle screw.

 

The mixture is a jet. It is for idle only. If there is nothing wrong the following will work:

 

With the car all the way warmed up, choke all the way open, hook up a tach. Back out the idle jet until it is smooth and high. Set the speed with the throttle stop screw slightly higher (maybe 50rpm higher) than it should be. Turn in the idle jet EXTREMELY SLOWLY, watching the tach. Find the fastest spot. Keep going in, super slowly. The speed will get a tiny bit slower and the needle will seem to "wave". Keep going in, very slowly. There is a spot, slightly slower, where it is stops waving. Leave it there.

 

If the idle speed is right, you are done. If it is wrong, back the jet out, change the throttle stop screw (speed) a little and repeat the above procedure until you have the mixture right and the idle speed right at the same time.

 

Of course if the carb is screwed up, this wont work. Good luck.

 

 

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Thanks for the help. How do I adjust the float, considering that the carb is from a different car...?

I might look in to buying the original carb(or any other that works) if i am not able to make the adjustments, but couldn't seem to find it anywhere....any ideas?I'll wait for the 6-12 converter, see if it solves the issue, but I'm looking for the carb,  just in case.

 

I believe its a stromberg BXVD-3  no. 380218

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Set the float the same as it was on whatever car the carb belongs on. If it is plastic, you may need to weigh it (on a grams scale) to make sure it is ok.

Edited by Bloo (see edit history)

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