Backfirein Through the carb?!!?
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Freshman User
Joined: Jul 2011
Posts: 47
Likes: 1
Backfirein Through the carb?!!?
Truck is a 77 250 with the 351m 2bbl. took it out the other day and it started backfireing through the carb under load. It idles rough and hesitates when you snap the throttle in park. timming seemed right when I put the light on it. thinkin a carb rebuild may be in order and a new cap rotor to top it off. Any thoughts on what else it could be?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
How does that help him understand what's happening?
This sounds like a vacuum leak, and possibly a faulty accelerator pump on top of that. Pull off the air cleaner (truck OFF), look down the throat of the carburetor and pull back the throttle - you should see two good streams of gas shoot onto the venturi's.
To check for a vacuum leak, lean out the idle mixture so that the engine barely runs, and spray around suspect areas (intake manifold, carb base, power brake booster, etc) with carburetor cleaner as the engine is running. If the idle jumps, you've found a leak. You can also use a vacuum gauge to tell if there's a vacuum leak in the first place.
This sounds like a vacuum leak, and possibly a faulty accelerator pump on top of that. Pull off the air cleaner (truck OFF), look down the throat of the carburetor and pull back the throttle - you should see two good streams of gas shoot onto the venturi's.
To check for a vacuum leak, lean out the idle mixture so that the engine barely runs, and spray around suspect areas (intake manifold, carb base, power brake booster, etc) with carburetor cleaner as the engine is running. If the idle jumps, you've found a leak. You can also use a vacuum gauge to tell if there's a vacuum leak in the first place.
Senior User
Joined: Dec 2010
Posts: 117
Likes: 0
From: Vancouver, Canada
How does that help him understand what's happening?
This sounds like a vacuum leak, and possibly a faulty accelerator pump on top of that. Pull off the air cleaner (truck OFF), look down the throat of the carburetor and pull back the throttle - you should see two good streams of gas shoot onto the venturi's.
To check for a vacuum leak, lean out the idle mixture so that the engine barely runs, and spray around suspect areas (intake manifold, carb base, power brake booster, etc) with carburetor cleaner as the engine is running. If the idle jumps, you've found a leak. You can also use a vacuum gauge to tell if there's a vacuum leak in the first place.
This sounds like a vacuum leak, and possibly a faulty accelerator pump on top of that. Pull off the air cleaner (truck OFF), look down the throat of the carburetor and pull back the throttle - you should see two good streams of gas shoot onto the venturi's.
To check for a vacuum leak, lean out the idle mixture so that the engine barely runs, and spray around suspect areas (intake manifold, carb base, power brake booster, etc) with carburetor cleaner as the engine is running. If the idle jumps, you've found a leak. You can also use a vacuum gauge to tell if there's a vacuum leak in the first place.
Thread Starter
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Freshman User
Joined: Jul 2011
Posts: 47
Likes: 1
Was at work today messing with the truck. Checked for vacuum leaks and hadn't really messed with the Carb Much. Decided too check the air fuel screws and it seems the PO had them out 6 turns each. Still has a rough idle but throttle response seems ALOT better. Truck is parked in one of the shop bays for the night I'll see what else I can find wrong on lunch tomorrow. I should mention that I just bought this truck so it's all new too me.
New User
Joined: Feb 2012
Posts: 9
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Mine was doing the same thing only its an edelbrock 600. The PO also had the air fule screws out four and a half turns and the idle set screw was three turns from touching anything.
I ended up taking it off and rebuilding the entire thing as it leaked more fuel than it shot down the intake. It also was so gummed up and black from popping that most of the moving parts would get stuck.
Once I rebuilt it it starts right up. Now my problem is that it don't have much for throttle response and it loves to back fire through the tail pipes. I am thinking that I now need a timing set put in it.
I ended up taking it off and rebuilding the entire thing as it leaked more fuel than it shot down the intake. It also was so gummed up and black from popping that most of the moving parts would get stuck.
Once I rebuilt it it starts right up. Now my problem is that it don't have much for throttle response and it loves to back fire through the tail pipes. I am thinking that I now need a timing set put in it.
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Junior User
Joined: Dec 2011
Posts: 94
Likes: 0
From: Powell Wyoming
ive been having the same problem with mine since i put on an edelbrock performer 400 intake and a edelbrock 650 thunder series carb, and it was orginally a timing issue which was corrected by.adjusting the timing and putting in a refurbished distributor and however it started doing it again like 2 days after i got it back from the shop so now i think maybe my float or something is sticking in the carb...which sucks because its only like 2 months old and it already needs to be rebuilt....(i guess next time i will get a holley carb or do fuel injection)
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Junior User
Joined: Dec 2011
Posts: 94
Likes: 0
From: Powell Wyoming
it happened to my truck after i put on a edelbrock performer 400 intake and a 650 thunder series carb, mine did it because the timing was off and the distributors springs inside went bad, so the timing was adjusted and the distributor was replaced with a refurbished one and now its doing it again 3 days after i got it back from my mechanic so now im thinking its my carbs float or something so that gonna get rebuilt and we will see if that does the trick.
Cranky Old Guy
Joined: Mar 2008
Posts: 3,562
Likes: 6
From: Raphine, Virginia
A vacuum gauge is one of the most useful tools you can have in your collection. You can spot troubles, tune carb, time engine all with one tool. I don't get hung up on timing numbers. When I'm thru tuning, I'll put the timing light on it just for a reference.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
When an engine backfires, the combustion is taking place with a valve open. In most cases there are two possibilities:
1) Spark does not happen when it should because the timing is off.
2) Spark happens when it should, but actual combustion is delayed because of an extreme lean condition.
You mentioned you played with the idle mixture screws and that seemed to make it better, but you really need to be using a vacuum gauge to dial in the idle mixture.
When you checked for vacuum leaks, did you lean out the engine so that it barely runs? You won't be able to find leaks if the engine is running strong enough.
When you checked timing, was this with the vacuum advance disconnected, and the source plugged? You did this with a timing light and white paint? Where did you put your marker?
Fleet Mechanic
Joined: Feb 2007
Posts: 1,326
Likes: 48
From: Middle TN
If you open the throttle and it starts to die then it pops through the carb. and has a short burst of energy..... This is classic lean fuel mixture.
The carb is dirty, fuel filter is stopped up or both.
The carb is dirty, fuel filter is stopped up or both.
Thread Starter
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Freshman User
Joined: Jul 2011
Posts: 47
Likes: 1
New fuel filters and rebuilt the Carb today. Power valve was shot and the accel pump was going no more hesitation when I blip the throttle. Still need to hook up the vac guage and get everything dialed in.
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