1978 2F Carb Help!

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I was trying to get a little better mileage out of my 40.....Truck is running awsome but I thought it should have a little better MPG....I removed the number 1 plug to see what is going on and it had some white deposits on it as if it was running lean... I decided to adjust the carb.

As I began to richen up the mixture, I noticed that the RPM would drop..I began to turn the screw in and the rpm's would increase....ALOT!

KNowing the proper way to ensure the mixture is correct, I should turn the screw in to increase the rpm and then idle it down with the idle adjustment screw for optimum performance, I did.....problem is the idle adjustment screw wont bring the idle down anymore...it is all the way in and the engine is turning 1200 rpm and I can bring it higher if I continue to turn the mixture screw in...... What gives?

Carb was rebuilt by Jim Chenoweth last year.....
 

FJ40Jim

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If it is idling at 1200RPM, then air is getting into the engine somewhere.

I think it sounds like the secondary throttle is not quite fully closed. See if twisting on the front end of the secondary shaft (where the diaphragm rod is connected) makes a difference. Bottom right in this SOR pic.

 
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Thanks Jim, I will take a look when I get home.....Question tho, if I am turning the mixture screw in (less fuel) why would the rpm's increase if more air was getting in? (Not doubting just trying to understand)....thanks again




If it is idling at 1200RPM, then air is getting into the engine somewhere.

I think it sounds like the secondary throttle is not quite fully closed. See if twisting on the front end of the secondary shaft (where the diaphragm rod is connected) makes a difference. Bottom right in this SOR pic.

 

FJ40Jim

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When the secondary doesn't snap fully closed, it allows enough air to idle very high, and it is pulling fuel from the secondary slow port. The sec slow circuit is jetted very rich to make sure it doesn't lean stumble when the sec. falls open with no accel pump. By leaning out the pri. idle circuit, it compensates for the sloppy rich sec. idle circuit.

This is also why the idle speed adjust screw does not seem to be having an effect, the carb is idling on the cracked 2nd barrel.

Just an edumacated guess.
 
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Maybe you have the idle mixture and throttle stop screw confused. If the throttle plates are closed properly ( throttle stop screw is correct), then closing the idle mixture screw down will stop the engine. I don't see what you were trying to accomplish anyway. The idle mixture screw only affects the idle. If the truck was running lean, then you need to address the enrichment/run circuits.
 
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Jim, I checked the secondary throttle and it is fully closed....to further confuse the situation, if I continue to turn the mixture screw in, the RPM's will go up MUCH higher........close to 2000 rpm's even with the idle screw all the way in....anything else you can think of???

Oh, I also took a can of carb cleaner and sprayed the outside of the carb in small areas to see if there was some sort of leak.....rpms slightly increased when I sprayed the body ( not down the throat).....not sure what that means but found it very odd.

Your help is greatly appreciated!

Thanks Jim, I will take a look when I get home.....Question tho, if I am turning the mixture screw in (less fuel) why would the rpm's increase if more air was getting in? (Not doubting just trying to understand)....thanks again
 

edwjmcgrath

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Your first mistake is messing around trying to improve the mileage when your truck is running awesome!

There is always a small vacuum "leak" around the throttle shaft because that is not a tight seal, so you will get the rpm increase if you spray near there.

"if I continue to turn the mixture screw in, the RPM's will go up MUCH higher........close to 2000 rpm's even with the idle screw all the way in"
It sounds like you may have the idle mixture screw and the idle speed screw reversed. The idle MIXTURE screw is on the engine side of the carb. Very visible. It is not connected to any linkage, it just screws into the carb base. The idle SPEED screw is on the firewall side and is hard to see. It controls the stop on the throttle linkage. Turning the idle SPEED screw in is like stepping on the accelerator.
 

FJ40Jim

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Doh! You're not turning the base idle screw. :doh:

I think you're turning in the choke high idle screw.

Back that screw out, until it is no longer contacting the linkage (not reducing the idle speed).

Please post pics of which screws are being turned.
Also post pics of the overall carb & vac hose installation.
 
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Thanks Doug.... when I tried to richen the mix, the engine rpm would drop immediatley. I then decided to turn the screw in to see what happens and sure enough....the rpms started climbing ...ALOT! The idle screw would not bring t down and the RPM's were actually over 2k......

Based on what Jim was saying, Air is obviously getting into the carb from somewhere ( thus the lean condition) and he thought the secondary curcuit may not be fully closed.....but it is.


Maybe you have the idle mixture and throttle stop screw confused. If the throttle plates are closed properly ( throttle stop screw is correct), then closing the idle mixture screw down will stop the engine. I don't see what you were trying to accomplish anyway. The idle mixture screw only affects the idle. If the truck was running lean, then you need to address the enrichment/run circuits.
 

FJ40Jim

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pics

These pics of a stock '78 carb are borrowed from SOR.com

In this pic, the choke hi-idle screw is circled in purple.

The throttle positioner screw is circled in orange.
78rearmarkd.jpg
 
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Thanks ed....I have always been taught to read the plugs....mine were lean.......but i believe you are right........I have the two screw reversed.....I am an ass........!

Your first mistake is messing around trying to improve the mileage when your truck is running awesome!

There is always a small vacuum "leak" around the throttle shaft because that is not a tight seal, so you will get the rpm increase if you spray near there.

"if I continue to turn the mixture screw in, the RPM's will go up MUCH higher........close to 2000 rpm's even with the idle screw all the way in"
It sounds like you may have the idle mixture screw and the idle speed screw reversed. The idle MIXTURE screw is on the engine side of the carb. Very visible. It is not connected to any linkage, it just screws into the carb base. The idle SPEED screw is on the firewall side and is hard to see. It controls the stop on the throttle linkage. Turning the idle SPEED screw in is like stepping on the accelerator.
 

FJ40Jim

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mo' pics

In this pic, the idle mixture screw is circled in blue.

The base idle speed screw is circled in red. It is on the back (firewall) side of the carb, hard to see. It is very easy to see and turn the choke and TP speed screws. Much harder to see and adjust the base speed screw.
78RHmarkdup.jpg
 
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Jim, thank you for your help...I obviously have the screws reversed....I repeat....I am an ASS!:whoops:

I have been assuming the red screw in the base by the firewall is the mixture and the screw on the engine side is the idle........:bang:

Thank you all for your help

Doh! You're not turning the base idle screw. :doh:

I think you're turning in the choke high idle screw.

Back that screw out, until it is no longer contacting the linkage (not reducing the idle speed).

Please post pics of which screws are being turned.
Also post pics of the overall carb & vac hose installation.
 

FJ40Jim

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For now, turn the idle mix screw all the way in til it lightly bottoms, then back out 4 full turns.

Start the truck, warm it up and adjust the idle speed screw to get the desired idle speed (500-650RPM).

Turn the idle mix screw in until it starts to drop RPM, then back it up a little to max RPM.

Chock the choke blade open, pull the choke knob and adjust choke speed screw for 1800-2000RPM.

Put everything back, and don't F- with it anymore. ;p
 

FJ40Jim

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Set it wherever you like. That is just the book spec.
FWIW, on a cold morning, that 1800RPM fully warmed up becomes only 900RPM with a crankcase full of frozen 15W40.
 
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