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73 Mach 1: New Fuel System with Firing/Idling Issues


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So over the last year or so I've slowly replaced the fuel system on my 73 mach 1 (351C 4V) - new gas tank, fuel line, fuel pump, fuel filters and I re-built the carb. Since then I've been having issues getting the engine to fire up, sometimes it'll turn over but generally it needs the gas pedal to the ground and when it does fire up the idle is pretty rough and it quits after a few seconds. I've noticed there's pretty rich fuel sprayed behind the car from the exhaust as well. Is this just carburetor adjustments that are needed between the fuel mix screw and choke or something else? Figured I'd ask before trying any adjustments, I have no carb adjustment experience so it's all new to me and I'm trying to learn as I work on the car.

Carb: Autolite 4300D

Thanks for any help!

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A 4300D probably isn't the best carburetor to learn rebuilding on, but if you get it running you won't have to worry about anything else.

You should be able to see if the choke is closed when you crank the engine and if it opens some when the engine starts, and fully opens when the engine is warmed up. You can rotate the choke thermostat housing and open the choke some, to see if that helps your startup. If the choke is set too rich the choke blades won't be able to be opened by the choke pulldown and will cause the engine to flood out. Check the pulldown and dechoke clearances.

If that doesn't help I would start by removing the carburetor and placing it on a stand so it's elevated above the work bench. Pour some fuel into the fuel bowls and see if you have a leak. I suspect you have a leak in something like a power valve or gasket that is letting fuel run into the intake manifold, flooding the engine. The only adjustments you have that will cause it to flood is the choke. The idle mixture can cause the engine to stall when set too rich or too lean, but shouldn't cause a major flooding like you seem to be experiencing.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Very good input from Don, as usual. I don't know if you changed the float during the rebuild or not. If you did, getting the float level right and the operation of the primary and auxiliary needle correct can be tricky. Do not put any pressure on the plastic part of the float. If the float level is too high it will run rich and will contribute to fuel percolation during heat soak after shutdown. I have had some success setting the float a bit lower than factory specification. If you open it back up, check to see that plastic retainer that holds the power valve spring in place is intact. Chuck

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Posted (edited)

Thanks for the info guys! So I think the main issue was the choke assembly being kind of sticky because the choke adjustment wasn't actually opening or closing. So I sprayed some lubricant in and blew it out, now it adjusts correctly. I also pulled the carb and adjusted the floats slightly below the factory specs I had it at previously and double checked the the seals for leaks. 

With all of that re-installed and the choke set pretty lean (about 3 ticks past the even mark) it fires up, however the idle is about 2500rpm's, which is obviously really damn high. I've got the idle mixture screws turned out 1.5 turns from just being set but haven't turned them in further since firing it up, in case I'm missing something and if anyone has any advice/experience to share. From what I've read and what's been said above, it sounds like the 4300D carb was scrapped on most cars for being a pain in the ass and most people haven't looked back since. I can afford to buy a new carb if that's the way to go, and from what I understand it sounds like a stock 351C 4v should have a 650cfm carb with a new intake since the 4300D is a spread bore.

*Edit/Addition: I've been looking at the Edelbrock 600 and 650 CFM carbs with electric chokes and they seem well regarded and easy to use.

Edited by Millertime
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4 hours ago, Millertime said:

Thanks for the info guys! So I think the main issue was the choke assembly being kind of sticky because the choke adjustment wasn't actually opening or closing. So I sprayed some lubricant in and blew it out, now it adjusts correctly. I also pulled the carb and adjusted the floats slightly below the factory specs I had it at previously and double checked the the seals for leaks. 

With all of that re-installed and the choke set pretty lean (about 3 ticks past the even mark) it fires up, however the idle is about 2500rpm's, which is obviously really damn high. I've got the idle mixture screws turned out 1.5 turns from just being set but haven't turned them in further since firing it up, in case I'm missing something and if anyone has any advice/experience to share. From what I've read and what's been said above, it sounds like the 4300D carb was scrapped on most cars for being a pain in the ass and most people haven't looked back since. I can afford to buy a new carb if that's the way to go, and from what I understand it sounds like a stock 351C 4v should have a 650cfm carb with a new intake since the 4300D is a spread bore.

*Edit/Addition: I've been looking at the Edelbrock 600 and 650 CFM carbs with electric chokes and they seem well regarded and easy to use.

If your current problem is only idle speed, look at the linkage for an idle speed adjustment screw.

 

By the way I have the same intake, but changed card to a 650 Holly. Had to create an adapter plate and its been fine.

Edited by Ryunker
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Locate and adjust the fast idle screw. It is close to the choke. Chuck

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Something else you might want to consider is welding a bung for an oxygen sensor onto your exhaust system.  A high quality Autometer Air-fuel ratio gauge can be had for less than $200.  For me it isn't something I want in the car all the time, so a little tie wraps and tape does the install well enough that I can utilize it for tuning.  I made it so it is powered off a cigarette lighter plug.  That way you are not guessing if you are rich or lean.  When you are done just uninstall it.  Might even be able to get a case of beer out of a buddy for letting them borrow it for tuning.

As to your carb, I have no experience or input.  Only input is that no matter what carb, having an AFR gauge gives you real time info that makes tuning easier.

Last note on the very high idle - sometimes a high idle can be caused by a vacuum leak, like forgetting to connect a hose.

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Right on Bentworker,

I have one in my toolbox with alligator clips for the wiring leads and a bunch of weld in bungs and plugs.

this helps greatly when trying to tune someone else's car.

The only issue I have is people want to see a specific A/F number at all times and carbs don't generally lend themselves into doing that.

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if you're looking for an easy to install and tune carburetor, that works good out of the box, I would take a look at the Summit carburetor:

Summit Racing Carburetors Carburetors & Accessories Air & Fuel Delivery - 600-699 cfm CFM Range - Universal Parts & Accessories | Summit Racing

I've tried 2 or 3 Edelbrock carburetors, and wasn't that impressed with them, needed tuning out of the box and finicky after getting them running right.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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I played around with the fast idle screw by the choke but the best the idle gets is 1500rpm's for a couple seconds and then it jumps back up to 2700rpm. I'm pretty sure I haven't missed any hose connections that would cause a vacuum leak, although I don't have the air cleaner hooked up on top of the carb? I added a few photos if anything glaring jumps out. Appreciate the help fellas!

Engine_2.thumb.jpg.2de96a4a9a6d38ef05016356cde40706.jpg

Engine_3.thumb.jpg.74dba7c621e149bc2f8448912cb7c90a.jpg

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You may be adjusting the fast idle screw, which is controlled by a cam on the choke mechanism, which is why the idle speed jumps back up, if the throttle plate closes. The idle speed screw is under where the throttle cable connects to the carburetor on the other side of the carburetor. Make sure the choke is wide open when you try to adjust the idle speed. Also, if your carburetor has an anti-stall dashpot make sure it is not interfering with the throttle lever, I would back it off until you get the idle speed sorted out.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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I just noticed the carb is a 4300 NOT a 4300D and the intake appears to be an aftermarket single plane intake of some kind. The 90 degree pipe fitting in the runner for cylinder #4 is also suspect. Do you remember what the top of the intake looked like with the carb off? 4 round holes, two elongated oval holes, or one more or less square hole? Chuck

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1 hour ago, c9zx said:

I just noticed the carb is a 4300 NOT a 4300D and the intake appears to be an aftermarket single plane intake of some kind. The 90 degree pipe fitting in the runner for cylinder #4 is also suspect. Do you remember what the top of the intake looked like with the carb off? 4 round holes, two elongated oval holes, or one more or less square hole? Chuck

I believe it was one more or less square hole. I can pull the carb off quick and double check too if necessary 

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I also see the metal pipe that connects to the heat pipe in the inlet is directly connected to vacuum on the back of the inlet. On mine, the metal pipe just connects to the top of the carby to get air from the top of the carby not vacuum air like you have set up. I am not sure if that is having an impact also.

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@c9zx Another previous owner conundrum, why people do some things defy logic.

@bdennisGood eye, that will definitely affect the choke operation, it will be drawing in cold air into the choke sometimes while the carburetor internal connection will be trying to draw in hot air.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Millertime, Get a can of carb cleaner and with the engine running, lightly spray all fittings, carb base, and hoses one at a time. If engine speeds up it is a vacuum leak. Is there a gasket between the carb and the intake? In the picture the fast idle screw looks to be screwed in too far. If it is one large square hole it is a single plane intake, typically not good for a street intake. You may have an assortment of engine components that you will need to identify and deal with. Chuck

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No rpm increases spraying carb cleaner anywhere. I’ve got the idle around 1000 now but when you throw it in drive it bogs down and dies. Is that a fast idle mixture screw adjustment now? I also pulled the carb half off and the top of the intake is one hole with gasket, gasket should probably be replaced so I’ll look into a new one. Also threw in new spark plug and battery cables that came in today. 7B339975-5CBC-4FED-8F38-EFBD7DFA1B55.thumb.jpeg.5cf9f63eec2667604bd0dbea51984741.jpeg

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Millertime.

looks like you have the Holley street dominator intake manifold with the smaller than 2Vintake ports.

please advise us if you have any oil fouled spark plugs, just because the carb spray did not make an increase in idle speed does not mean it is not leaking

(pulling in oil from the underside of the intake) Do you have 2v or 4v heads ? , if you have 4Vheads the port mismatch is huge.

Looks like you have a throttle kicker solenoid on that carb also called an anti-diesel solenoid, you should be adjusting the idle (curb idle) there and there will be a smaller screw on the carb base plate to get the idle lower so it won't after run, if you leave it @ 1000 rpms it most likely will after run.

Please do make sure you have the choke and fast idle screw fully sorted before setting idle speed's.

Boilermaster

 

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Just a thought but that gasket on the top side looks like it is not compressed and could be causing an air leak under the carby and causing some of the issues you have.

David

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9 hours ago, Aus73Mach1 said:

Just a thought but that gasket on the top side looks like it is not compressed and could be causing an air leak under the carby and causing some of the issues you have.

David

That's what I thought when I saw the gasket. 

OP, be sure to get a gasket that seals well, as some of the generic Holley types won't seal the perimeter of the base plate casting. The 4300 has vacuum passages cast into the base plate, which will cause a high idle situation if they are drawing outside air. You'll also want to check the base plate with a straightedge to ensure it is flat and not bent/warped from over tightening. 

 

 

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I’ve run out of time working on the engine for now, I work 3 and 3 rotations as a geologist in northern Canada so I’m thinking it might be best to invest in a new manifold/carb that hasn’t been fiddled with as much by the previous owner. I’ll check out that carb Don linked and if there’s anything else you guys think I should replace, feel free to let me know! Really appreciate all of the help 

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I used a Holley Brawler Carby. No adjustment was needed for idle, choke, etc. Also has sight glasses for float levels. Works extremely well on the 4V through full throttle range.

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