Carburetor Circuits

[keithb7]

I am interested in learning more about how fuel is delivered at a specific point. Just off idle.  1938 Chrysler. Stock carb. 
 

My understanding is at idle when the throttle valve is closed, fuel is delivered thru the small orifice just below the butterfly valve. On the bottom side of the valve, closer to the intake manifold. Fuel delivery here is adjusted by the idle air fuel mixture screw.  
 

Just as the throttle valve starts to open up, the low pressure point below the throttle valve lessens. Now fuel, I think, must be introduced into the venturi at another place higher up the carb.  Is this via the main jet at this point? Or is there some type of intermediate jet just off idle, then later as the throttle opens further, then the main jet takes over? 
 

At this point just off idle, I think I am starving for fuel. If I just wiggle the throttle up from idle and hold it there steady, then engine runs rough. Held there long enough it will stall. Blipping the throttle past and his point it will run on and rev up. So I can drive, but it ain’t right. 
 

This lean symptom can be lessened if I partially close the choke. To me that means I am cutting air flow down, forcing a richer mixture. A band-aide for the issue. 
 

This fuel delivery issue creates surging when cruising around town at low speeds, just a hair off closed throttle. For example 3rd gear 25-28 mph or so. Tweaking the idle A/F mixture screw both in and out offers no improvement. 
 

Dwell is bang on, 38 degrees. Brand new spark plugs installed. Removed spark plugs looked good and even across all 6.  Vacuum advance is working. Verified with a vacuum gauge and witnessed with a timing light. Timing set at spec at 0 deg TDC.  Adjusting and tweaking timing offers no improvement.  Timing is jumping around a little bit, not too bad. Witnessed with the timing light. I suspect maybe due to worn distributor parts. I am not convinced my symptoms are related to my he ignition system. Maybe I’m wrong. 
 

Maybe I need to pull and clean/rebuild the carb?  Its not seen much use in years. I’d like to understand fuel delivery at this very point just off idle. 
Float was set at 5/32. Not correct. I fixed this and set it to spec, 5/64. This had no effect on the off-idle stumble. 
 

Engine starts well without touching the throttle. From outside the car, I can lean in thrunthe window, hit the starter button and it fires right up. Engine pulls good up the hill home under load. Runs good on the hiway. (Higher rpm, not just off idle)

The car has exhibited this symptom since I acquired it. Just takes a while to get past the honeymoon stage and start identifying little issues.  
 

Your comments are appreciated. 

[RobertKB]

I like my timing set 4 deg BTC, especially with modern fuels. Not sure if that would be your issue. Others more knowledgeable I’m sure will chime in. 

[keithb7]

I have been tuning/ tweaking and test driving. Including tweaking the timing, both advanced and retarded, locking it in place and driving. As well as the A/F mixture screw. Process of elimination. I haven’t nailed it down just yet. I just keep testing and removing variables where possible. 

Edited May 29, 2021 by keithb7

[Merle Coggins]

What carburetor is on your car? Some carbs, including the Carters, use a metering needle that helps adjust the fuel metering at partial throttle. Maybe your metering needle is not functioning properly. 

[Sniper]

Typically, there is a transfer slot right above the throttle blade when it's at idle.  It's essentially a fixed orifice.  As the throttle opens it exposes the transfer slot and fuel is drawn into the air stream.  It's a small opening and can get plugged.

 

You can see it in the right side graphic below, it's not labeled.

 

[keithb7]

Thanks Sniper. At the green arrow area? I’ll take a look at this area. I suspect the holes would be quite small. Easy to varnish up if left for years, and engine started very little. 
 

I forgot to mention that I had the bowl cover off earlier this week. The accelerator pump appears to be working fine. 
 

@Merle Coggins I see my carb marked: Carter E6M1. 
 

Edited May 29, 2021 by keithb7

[kencombs]

Agree with the transfer slot being a probable cause.  In order to clean that orifice the plug covering it must be removed.  Most I've seen have a little plug driven in, even though illustrations may show a screw in type. 

 

Metering rod or piston issues normally show up in situations requiring more power than your description.  

[Dodgeb4ya]

The "Story of the Carburetor" Mr Tech is a very good watch if available online.

Sorry for the sideways pics.

[allbizz49]

When you say adjusting the air mixture screw has no affect, are you referring to your off idle condition or at idle?  

[keithb7]

3 minutes ago, allbizz49 said:

When you say adjusting the air mixture screw has no affect, are you referring to your off idle condition or at idle?  

Adjusting the screw has no effect on the off idle condition. At idle, the engine rpm will falter as the screw is adjusted in or out.

[chrysler1941]

Connect a vacuum meter. If mixture screw does not change idle vacuum, passage is blocked. Use ultrasonic cleaner.

[knuckleharley]

1 hour ago, chrysler1941 said:

Connect a vacuum meter. If mixture screw does not change idle vacuum, passage is blocked. Use ultrasonic cleaner.

I must be having a senior moment. What is a "Ultrasonic cleaner"? A stereo blasting AC/DC at max volume?

Enquirin mines,and all dat.

[chrysler1941]

1 minute ago, knuckleharley said:

I must be having a senior moment. What is a "Ultrasonic cleaner"? A stereo blasting AC/DC at max volume?

Enquirin mines,and all dat.

It's one of these modern gadgets  Works extremely well with soft metals like carb body. Uses high frequency vibrating dirt off. Like a micro oven

https://www.walmart.com/ip/VEVOR-1-3L-Ultrasonic-Cleaner-Jewelry-Cleaner-with-Heater-Timer-Stainless-Steel/208782464

[knuckleharley]

7 minutes ago, chrysler1941 said:

It's one of these modern gadgets  Works extremely well with soft metals like carb body. Uses high frequency vibrating dirt off. Like a micro oven

https://www.walmart.com/ip/VEVOR-1-3L-Ultrasonic-Cleaner-Jewelry-Cleaner-with-Heater-Timer-Stainless-Steel/208782464

Holy CRAP,Batman!

 

Thanks for the link. I don't remember hearing about such a thing before!

[keithb7]

I'd never heard of one either. I need to do more research. Looks like a deep fryer. I assume you submerse the carb and leave in there a few days? What is the cleaning solution?

Edited May 29, 2021 by keithb7

[chrysler1941]

Just now, knuckleharley said:

Holy CRAP,Batman!

 

Thanks for the link. I don't remember hearing about such a thing before!

You're welcome.

Works also on fuel pumps etc.

Will make plastic brake reservoir look like new.

I used on almost everything. Doesn't work with cast iron.

Just add your favorite alkaline degreaser and water

[knuckleharley]

3 minutes ago, chrysler1941 said:

You're welcome.

Works also on fuel pumps etc.

Will make plastic brake reservoir look like new.

I used on almost everything. Doesn't work with cast iron.

Just add your favorite alkaline degreaser and water

Thanks again for the tip. If I am ever able to work again,you can be sure I will be buying one of those babies!

[chrysler1941]

10 minutes ago, keithb7 said:

I'd never heard of one either. I need to do more research. Looks like a deep fryer. I assume you submerse the carb and leave in there a few days? What is the cleaning solution?

Not days, minutes. You cam literally see crap shaking off. Carb body becomes shinier and all the minute passage get cleansed.

 Alkaline Degreaser or plain soap water. 

 

 

Edited May 29, 2021 by chrysler1941

[kencombs]

Those work great.   But be careful with the strength of an alkaline cleaning solution.  Simple green for instance with badly discolor some white metals quickly. My wife used real in jewelry at our. Store.   I used her cleaner more than she did. 

[desoto1939]

Start the car and take a can of carb cleaner with the thin plastic nozzle adapter in the nozzle. spay around at the base of the carb around where the metal pieces connect to other parts of the carb body to check for any air leaks  Also do you have the correct slotted gasket onthe base of the carb where it mounts to the manifold. If you do not have a govenor on the car you need to have the slotted gasket and one slot needs to be where the small hole is located in the flat flange area on the manifold.

 

 

Rich Hartung

desoto1939@aol.com

[keithb7]

I had the carb off earlier this week. I installed a heat sink base plate. I ensured I had the proper gaskets and routing in the base plate all aligned. The symptom was present before I installed the heat sink plate. It remained after. The purpose of the heat sink was to divert heat away from the carb bowl. I will check for any leaks in the area however I am doubtful. The gaskets were in good condition. I'll re-check the gaskets and everything in the area again to be sure. I think Sniper is onto something with the plugged fuel inlet holes right above the throttle plate. We'll see soon enough.

Edited May 29, 2021 by keithb7

[Sniper]

Yes, the green arrow i where it is.

 

I used Simple Green and water in my heated ultrasonic cleaner.  Took maybe an hour to heat up to 50C while ultrasonic cleaning it.

 

http://www.yourolddad.com/carb-rebuild

[greg g]

2 hours ago, knuckleharley said:

I must be having a senior moment. What is a "Ultrasonic cleaner"? A stereo blasting AC/DC at max volume?

Enquirin mines,and all dat.

I find a mix of black sabbath and the 1812 ovature effective.

[Sniper]

Well, For Those About To Rock by AC/DC does have cannons

[knuckleharley]

6 hours ago, kencombs said:

Those work great.   But be careful with the strength of an alkaline cleaning solution.  Simple green for instance with badly discolor some white metals quickly. My wife used real in jewelry at our. Store.   I used her cleaner more than she did. 

What is "real"?