Technical Automatic Choke Adjustment

The Rambler stalled once at the beginning of a trip into town today. I decided I ought to see if I could adjust the automatic choke a bit, and make it work better.

The carb is a Holley 1904, which is a relatively crappy carb. It's also kind of tired, not everything works as it should, but the engine usually starts right up, and it only dies at stop signs occasionally.

The basic operation of the choke is that there's a flat coiled spring inside the round black housing, which applies torque to the choke shaft, and the choke butterfly in the top of the carb blocks off incoming air more-or-less depending on how far closed it is.

There is also a fast-idle cam, which the idle speed screw rests against. On this carb, there is only one idle speed screw, and it rests on the smallest part of the cam when the choke is fully opened, and on the larger parts when the choke is more closed. I noticed that when cold, the fast idle cam was not moving far enough to get full fast idle speed. So, I'll see if I can adjust the choke a bit more "rich" (more closed), and then see if it runs better when cold. If I go too far, it will be too rich, and the engine will tend to "blurble" and make black smoke, which is not good.

Here's the choke butterfly circled in red, in the mostly closed position. Way down in there.

The fast idle cam is circled in yellow, it's hard to see from this angle, but the idle speed screw is below it, resting on the second highest position.



The round black cap contains the spring, and to get it more "rich", I need to turn it counterclockwise. There are notches on the top of the housing, and a notch in the cap, so you can tell where in the adjustment you are. With everything new, the marks were probably aligned to the "index" location in the center, but when they get old, you have to adjust it so it works, not to spec.



Loosening the three screws lets me rotate the cap, to adjust the choke. I ended up moving the cap about 2 to 3 notches, that got it so the idle screw would rest on the highest part of the cam, and the choke is just about fully closed, when the engine is cold.

In this picture, the choke is being held open by the idle screw, but opening the throttle will let the choke spring closed, and set the fast idle. As the engine warms, the hot air from the exhaust manifold tube will heat the spring, which will relax, and let the choke open. There's also a vacuum cylinder inside the housing, which pulls the choke partly open once the engine starts.



Getting all this stuff to work properly takes some patience, and tinkering, and usually takes some repairs, and even then, on a 60+ year old carb, you might not ever get it to work just right.

 

One of the biggest problems with the chokes on AMC 6’s is the heat tube that runs through the exhaust manifold. It gets a hole in it and then sucks in exhaust gas into the choke assembly. Also the manifold bolts become loose. And then it doesn’t idle well, as it sucks air. We were always re torquing the manifold bolts on those 6’s to get them to run right.

 

I know how much you like that old Rambler and I can understand keeping it as original as possible but, wouldn't it be easier to just put a manual choke on it? I mean, you fixed the clutch linkage with barbwire. Just sayin'.

 

No, I'd rather tinker with the automatic choke. Vehicles that came with a manual choke, I'll use the manual choke. But the ones that get adapted from automatic to manual, never seem to work right.

 

The vacuum pulling thru the choke housing is probably one of the most overlooked issues.
Sometimes even just flipping a gasket during a rebuild can block this off. Also often there is a small felt gasket between the carb and housing that gets left out.

 

making sure the housing is getting vacuum is important also. it must pull warm air from the heat tube into the chamber to open the choke quicker. there is a port that can get carboned up

 

Instead of just making a mechanical choke , The more u tinker with & watch how it works, carryover to other task. you gain Knowledge & understanding.

 

If an automatic choke is working and adjusted properly, it beats a manual choke every time.

 

Cars that came with a manual choke had a spring loaded valve in the choke plate that let air in when the car started. When you convert an automatic choke to manual it doesn't have that valve so it floods almost immediately when the engine starts. That's why a manual conversion from an automatic doesn't work like a factory manual choke.

 

That's one reason...another is that the conversion kits they sold for years, were really crappy, and didn't fit, and were too flimsy, etc.

 

yup

 

Class, boys and girls, I'd like everybody to thank Mr. Jim for taking time out to speak to us.

Class...
CLASS...
CLAAAAASS...

 

(random internet pic)
Was tinkering with one of these. (Well, I was watching anyway) Never messed with one with an automatic choke.
Choke spring is working (tested with heat gun) and on the correct side. Choke doesn’t seem to open like it should when it cranks.
It’s been sitting for a while. The heat riser seems to be working.
The vacuum cylinder is free but doesnt seem to be working correctly.
I guess the next thing would be to check the vacuum tube that feeds it. Probably do a vacuum check anyway.

Manual choke conversions? Maybe with a manual choke carb.

 

Isn't there a main idle speed screw somewhere, usually opposite of the auto choke body. Usually faces back, slot fwd. Fast idle cam and idle adj screw are usually proprietary. Sometimes theres a tin shield inside behind the bi-metal coil and it can muller up heat management and leak vacuum thru the carb body. Being a Rambler it may be more rudimentary.

 

I haven't quite hit thw Goldilocks point on mine just yet, though Carter seem to have had bleed-through problems in mind because the choke draws in between the throttle and choke plates on my WCD.
Does mean also it's quite quick to pull the choke off as it draws heavily from the manifold with the choke shut.

Saying that, overall it works pretty well. A nice feature to have.

 

I have always adjusted the bimetal spring cold adjust the spring until it just will close, tap the plate with a screwdriver to see if it just returns to close there is a little piston that vacuum will pull the choke open a small amount when the car starts in the housing with the bimetal spring.

 

Had an Olds and after a lot of fooling around found the vacuum was lost to the choke chamber. And not wanting to take the Q-jet apart in the dead of winter or put on one of those chintzy manual choke kits, I drilled a hole in the housing and ran a piece of 1/8" copper tube to manifold vacuum. Worked great for several more years.

 

The choke should stay closed while cranking, until the engine starts running, then it should open a little bit. Getting "a little bit" just right, can be tricky...there are adjustments for the "vacuum break", which is how far it opens the choke, while things are still cold. Shop manuals and carb kit instructions have more details. But there is a lot of trial and error involved, most of it is error, so you need to be patient and try different things.

If, after you get tired of playing with the automatic choke, you decided it needs a manual choke, best bet is to find another carb that came with the manual choke. Chevy used automatic chokes in most cars starting in the early 50s(?), but trucks with similar engines got manual chokes well into the 60s.

 

Middle of the winter in the middle of nowhere Ontario Canada, on a highway in the middle of the night (between Huntsville and Bracebridge). Trucking along just fine (at highway speed) until the choke just decided to close on my AMC 258 (in a 1980 Spirit). I was not happy. (I wired it open, in the cold and the dark, and got myself home).

Another winter my wife complained she didn't think the rear brakes on her Cutlass Supreme were working ... turns out it was the choke stuck on fast idle and pushing her when she was on snow, giving the sensation of no back brakes.

Sure, electric chokes work great ... until they don't. I am a firm believer in converting to manual choke and have personally had no issues with the cheapie conversion kits. My current daily has a manual conversion on its Quadrajet that has worked well enough to get me through many Canadian winters.

A perfect working electric choke is a nice thing ... I just don't trust them anymore. With a manual, I know the choke is on when the cable is pulled and I know the choke is off when the cable is pushed.

 

I've been known to forget to push in the choke knob....that alone is a good reason for me to fix the automatic choke.

But there are a lot of guys who don't like tinkering with stuff, and I can sure understand why they wouldn't want to deal with getting an automatic choke working just right.

 

I have found in my SoCal climate even in winter is to turn the the bimetal spring unit just enough to move the side fast idle cam on to the 1st notch or setting.
Depending on how often I use the car 1 or 2 pumps and then foot off pedal. I let the engine run about 30 seconds on the faster idle and back out to the street, blip it once to see if it settles to the 600 rpm in drive and head to where ever I’m going. It may slightly stumble once or twice early in the winter when it’s 50* out but I don’t like a closed choke butterfly.

 

It's been in the 20s in the mornings here....SoCal choke adjustments don't quite do it.

Yes, you have to take lots of things into account. My Chevy II has both chokes wired open, they aren't needed with a blower.

 

Good procedure, that will give you a very good starting spot.

 

I worked for a Dodge dealer for about 8 years and we used to get a lot of small block Chrysler products in with manual choke conversions. As you said they were really crappy. With the Chrysler engines you had to have a working exhaust damper and the choke passage in the intake had to be free of carbon. Our top mechanic said that with a warm engine, the choke stove in the intake should be hot enough to leave skin when you touched it. To get the choke working correctly you had to insure the damper worked, clean the passage, and the critical adjustment was the choke pull off (external on those carbs). Start with the factory setting and open or close it a little depending on whether it wanted to load up or die on cold start up.

 

This link to my website may (or may not) be interesting:

https://www.thecarburetorshop.com/Automaticchokes.htm

Sorry, cannot agree with those of you that favor automatic chokes. I generally convert everything to manual, but not by using the A/M one size fits all, works well on nothing, kits. I modify the carburetor itself.

Showing my age, but I can remember when the two most expensive cars in the world (at that time Ferrari, and R/R) both had both manual chokes, and manual transmissions.

Jon.

 

All the car companies made hundreds of thousands of cars with automatic chokes. And when adjusted correctly they all worked fine. A manual choke is only as good as the person driving the car. Through out the distant past I have removed crappy aftermarket manual choke kits and fixed the factory automatic choke. The customers were always surprised at how well their car started and ran afterward. Manual chokes in the 50's and 60's only came on the cheapest cars or the stripped down models. You get what you pay for, at least back then.

 

That surprises me a bit, as I usually judge a carb guy's prowess by his ability to get an automatic choke working properly.

 

That’s why I have the red 56 and the 57’s bimetal spring working the opposite way keeping the chokes open all the time. About the only reason to stay in California anymore.

 

To be honest Jim it surprises me as well, however I suspect it’s preference not a lack of ability to make an automatic choke work.

 

I get why an expert on carbs with total understanding of how they work would want to control the choke. For the rest of us a working auto choke is the best.