Ford 8n Marvel carburetor throttle shaft bushing

In my quest to fix my idle problem (stumble), it clears up and runs great with about half choke. This means to me lean or unmetered air getting in. I took my carburetor off again and this time i concentrated on the throttle shaft. When I rebuilt it the first time I did not install the new throttle shaft. I removed the old throttle shaft this time and measured it .247 thousandths just like the new one.
Taking a closer look at the bushing I could see it was well worn and the shaft wobbled significantly in the bore.
I have rebuilt many Quadrajet, Edlebrock and Holley carburetors and a loose throttle shaft bushing can cause a vacuum or unmetered air leak. I think this may be my problem and I found a new bushing online.

Alsp, my float level is correct and my mixture screws and.....Main meter 1 1/2 turns out, idle mixture is 3/4 out. Turning the idle mixture does little, all the way in and still runs better with some choke. Turning it out doesn't do much either.

I will report the findings, if any.

I just rebuilt the engine, and fired it today, it runs fine above idle, so again it leads me to a vacuum leak.
 
The idle mix will only adjust at very low RPM. It needs to be down around 400 or so to be fully on the idle circuit.

When adjusting the idle, turning the screw in richens the mix, out leans it. It adjusts air bleed, not fuel. The idle circuit draws fuel up from the main well through the small hole in the cover gasket. If the cover is warped, that seal will be sucking air instead of enough fuel to idle. There is also an idle metering jet that can be partially clogged.

Also take a look at the distributor shaft bushings and the point gap. If loose the points will not stay set, can cause just what you are describing. So will weak spark in general.
 
Distributor is brand new, .025 gap (side mount).
Idle is very low when attempting adjustment.
I can turn the idle mixture screw all the way in, with little affect.
Two small holes near throttle blade a clear and free.
Cover gasket has witness marks showing full contact all around.
I sonic cleaned the carburetor in heated water with Pine Sol. Removed the jets, probed all passages with fine wire, brake clean then compressed air.
 
One small thing I noticed....the idle mixture screw has a groove around the needle, I'm sure from over tightening at some point. Its not extreme, but i can catch a fingernail on it.
 
I don't think the ring on the screw would effect anything.

Jets can be difficult to fully clean. They tend to collect a microscopic layer of crust that adheres tightly to the metal. Especially when running E10. Just a little will effect the smaller jets. I have found the only sure way to clean them is to start pushing progressively larger gauge pins through until the resistance prevents it from going through. You'll be surprised at how much one will open up, just have to know when to stop without removing metal. A set of micro drills will do the same thing, just push with the round end so not to do any cutting.

Have you tried opening the main jet more? The procedure for adjusting the main is to quickly open the throttle from idle, listen for the results. Start turning the main in, goose the throttle. When it hesitates, it's lean. Back it out a 1/4 turn, repeat the test until it will take throttle without hesitation. A single puff of black smoke is the goal.

How is the rest of the engine? Good compression? Valve lash set? The manifold is siamese, so there could be an internal burn through between the intake and exhaust, but that or a vacuum leak would cause rough idle. A vacuum gauge might reveal something...
 
I just rebuilt the engine, today was the first start. Valves are new, seats cut, set .012 intake, .016 exhaust. New sleeves, 3 ring pistons, bearings, crank ground etc..
Head surfaced, new gaskets everywhere, spark plugs (.025), coil, distributor, wires etc.
Manifold is good, new gaskets.

Main I have opened to 2 1/2 turns out, not much improvement (if any). Runs best at 1 1/2 minimum. No smoke.

I can go through the jets again
 
"it clears up and runs great with about half choke"

That tells the ones of us who have been around these engines and carburetors all our lives that you have issues other than the throttle shaft "bushings".

But it seems you've already made up your mind.

NOT to say that having a better fitting throttle shaft couldn't be a GOOD thing.
 
(quoted from post at 21:33:33 09/07/19) Ok, educate me

I'd look for a blocked fuel passage, and check that the venturi is correct for the application and properly installed, and that the gasket is correct with all the matching holes for various passages and is sealing around the venturi area.

Unless you know for SURE that this carburetor is worked properly on this engine in the past, it's POSSIBLE that someone has mixed and matched upper and lower bodies or perhaps other parts.

Also, once you get down to slow idle, for everything to work properly, it's important that the throttle butterfly be centered in the bore.

With carb apart, back off idle speed screw so butterfly can fully close, then hold the upper carb body towards light and look for light passing the closed butterfly. If there's a gap at one area, loosen the two butterfly screws slightly, and reposition the butterfly so it closes of the bore completely when closed.

(The idle mixture ports and transfer slot are next to where the edge of the butterfly closes at one side and if there's more gap at the other side air will bypass the ports and not pick up fuel correctly at idle.)
 
Thanks Bob and Steve for your replies.

When I rebuilt the carb, I did check for correct
placement of the throttle plate.
No matter what position, it never completely shuts
out all light.
It has number 15 stamped on it.
 
(quoted from post at 13:49:55 09/08/19) Thanks Bob and Steve for your replies.

When I rebuilt the carb, I did check for correct
placement of the throttle plate.
No matter what position, it never completely shuts
out all light.
It has number 15 stamped on it.

My Carb is a "B" and my serial number is a 280836. Pretty sure mine came with an "A" carburetor, but the "B" is better "they" say.

Im going by the tag on the carburetor.
 
(quoted from post at 16:46:08 09/08/19)
(quoted from post at 13:49:55 09/08/19) Thanks Bob and Steve for your replies.

When I rebuilt the carb, I did check for correct
placement of the throttle plate.
No matter what position, it never completely shuts
out all light.
It has number 15 stamped on it.

My Carb is a "B" and my serial number is a 280836. Pretty sure mine came with an "A" carburetor, but the "B" is better "they" say.

Im going by the tag on the carburetor.

I can't help you with the meaning/correctness of the "15" marking.

With idle screw backed off away from the stop you SHOULD be able to tweak the plate around until it fits well in the bore.

There may be a little gap at the sides near the pivot shaft, but the outer edges need to fit the bore well.

If not, you will have idle issues.
 
(quoted from post at 15:44:10 09/08/19)
(quoted from post at 16:46:08 09/08/19)
(quoted from post at 13:49:55 09/08/19) Thanks Bob and Steve for your replies.

When I rebuilt the carb, I did check for correct
placement of the throttle plate.
No matter what position, it never completely shuts
out all light.
It has number 15 stamped on it.

My Carb is a "B" and my serial number is a 280836. Pretty sure mine came with an "A" carburetor, but the "B" is better "they" say.

Im going by the tag on the carburetor.

I can't help you with the meaning/correctness of the "15" marking.

With idle screw backed off away from the stop you SHOULD be able to tweak the plate around until it fits well in the bore.

There may be a little gap at the sides near the pivot shaft, but the outer edges need to fit the bore well.

If not, you will have idle issues.

Thanks Bob for your patience. Im thinking this may not be an original Marvel carburetor. There are not cast in words or numbers, only a tag.
The third picture is the absolute best I can center the blade. The picture seems to exaggerate the gap, but its definitely there.Obviously it does not come close to sealing.....
mvphoto42316.jpg


mvphoto42317.jpg


mvphoto42318.jpg
 
That's a lot bigger air leak than your throttle shaft bushings!

I think we've found your problem!

NOT sure how well this will fit all the series of carbs, but it looks like part number is 8N9585, Upon GOOGLING that number, they start @ $3.10.
 

I think I found an original Marvel on eBay.
Im going to rebuild an original and see if that works.

I agree with your diagnosis. The aftermarket carburetors seem to get nothing but bad reviews......Here is a perfect example.

I will let you know the outcome.
 
I received my original Marvel carburetor the other day and rebuilt. The throttle valve fit better than the aftermarket carburetor, but did not completely seal off the bore.

Installed it, started it up, minor adjustment to idle mixture screw and it runs a lot better.

So, ......stick with an original carb it you can and rebuild it. I thought I had an original Marvel but it wasn't.
 
(quoted from post at 19:07:38 09/13/19)
Fairlane, where dd that bad carb come from?

Not sure, it was on the tractor when I bought it.

It looks like an original Marvel, but it does not have any markings on it.
 

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