400 backfiring/carb problem?
Thread Starter
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Senior User
Joined: Mar 2003
Posts: 248
Likes: 1
From: Ottawa, KS
400 backfiring/carb problem?
I need some advice.
The weather's changed around some, and my 77 F-150 has been ruuning poorly. I've been messing around with the mixture screws on the front of the Edelbrock 750 cfm carb because it was stalling, backfiring, and I had spitback through the carb. After messing around with it, there is slight hesitation and some backfiring under load. What should I do? Is the A/F ratio to rich, too lean, or is it timing?
Thanks for the input.
The weather's changed around some, and my 77 F-150 has been ruuning poorly. I've been messing around with the mixture screws on the front of the Edelbrock 750 cfm carb because it was stalling, backfiring, and I had spitback through the carb. After messing around with it, there is slight hesitation and some backfiring under load. What should I do? Is the A/F ratio to rich, too lean, or is it timing?
Thanks for the input.
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
The Mixture Screws on the front of the carb only control the Idle mixture.
Does the motor idle smooth?
Did the motor ever run right with your current setup?
What changed?
What color are the Spark Plugs?
Does the motor idle smooth?
Did the motor ever run right with your current setup?
What changed?
What color are the Spark Plugs?
Elder User
Joined: Nov 2003
Posts: 705
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From: Okla.
I think timing and a/f. The a/f screws on my 750 holley are out 3/4 of turn with 72 jets in it not the 74 that were there to start with. If your off 1/2 a sprag under rotor that is 7 1/2* of rotation on the crank. Pull #1 piston up on compestion stroke and pull destibor cap and rotor and see where its at.
Posting Guru
Joined: Sep 1999
Posts: 2,297
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From: Indiana
BrentKasson
Did you buy the carb new? As mentoned, has the motor ran right with thats set-up?
Thats alot of carburetor for a 400.
If you did not nuy it new, maybe the p/o installed smaller primary jets and or metering rods and has it a little lean.
And it could be the timing as well. Give us a little more history.
Did you buy the carb new? As mentoned, has the motor ran right with thats set-up?
Thats alot of carburetor for a 400.
If you did not nuy it new, maybe the p/o installed smaller primary jets and or metering rods and has it a little lean.
And it could be the timing as well. Give us a little more history.
Thread Starter
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Senior User
Joined: Mar 2003
Posts: 248
Likes: 1
From: Ottawa, KS
The carb is brand new, only about three months old. I bought it with the manifold (Edelbrock Performer) and it was running great up until the weather changed. The truck almost always idles a little rough, but it was fairly smooth before. I havn't pulled the plugs yet, I'll probably look at them today and use my Hayes manual to diagnose if they are miscolored.
Thread Starter
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Senior User
Joined: Mar 2003
Posts: 248
Likes: 1
From: Ottawa, KS
I did mess with the mixture screws some before the weather changed, before that I had excellent throttle response, I was getting good mileage. The engine only has 14,000 miles on it, it was a brand new motor bought about four years ago. The tranny was rebuilt at the same time. The setup on the truck worked pretty good, but I think the ignition timing was advanced too far because when I adjusted it a few months back, it ran good, but it's a little hard to start. If it's warm and you turn it over with no gas, it hesitates and then fires up. If it's cold or any other time, I turn the key and give it one pump on the gas and it fires right up.
Elder User
Joined: Nov 2003
Posts: 705
Likes: 0
From: Okla.
The timing chain is steched a little bit, nothing to get all upset about. You just have to find a happy medem.
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
If it idles rough look for a vacuum leak. Spray carb cleaner all around the carb and intake manifold while it is idling. If it smooths out then you have found the leak.
Check the carb studs to be sure they are tight.
The Edelbrock Performer #2171 has been known to have sealing problems at the mounting flange.
Check the carb studs to be sure they are tight.
The Edelbrock Performer #2171 has been known to have sealing problems at the mounting flange.
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Thread Starter
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Senior User
Joined: Mar 2003
Posts: 248
Likes: 1
From: Ottawa, KS
I checked for a leak, there was none, the spark plugs have good electrodes, but they are sooty. I leaned out the A/F ratio, but now it's like ten degrees outside, and the thing doesn't want to run. I put a timing light on it, and it's so far off I can't see the numbers. The guy that installed the engine a few years back put a white mark on the factory setting, which I think is 12* BTDC, but I didn't even see it. It's getting hard to start after it's been running, so I'm pretty sure the timing is off by quite a bit. Is there anything other than that? I just wished the truck ran like it used to. Thnks for the input.
I'll definately let you know how it goes. Should warm up eventually.