carb backfire under load
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carb backfire under load
I need some help. I have an 81 f250 with a 5.8. I recently was having problems with it so I decided to do some tune up work. I bought a rebuilt carb, put new plugs and wires. I also put on a new cap and rotor. Now the truck will start up and runs real strong. I can drive it no problem until I give it more than half the pedal. At this point of acceleration the truck will spit and sputter and then backfire. Any ideas on what I need to do? Thanks
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yes, I think it is a fuel issue as well. I can run it well in nuetral and it does well in reverse too. But in drive it will backfire when I floor it, I think the adjustments are way off or possibly I vaccum issue. Not sure?
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From: Virginia
When you floor the gas pedal and open the throttle plates in the carb, the engine has no vacuum, so it would not be a vacuum problem.
It sounds likeduring high fuel demand, it's not getting enough. Revving the engine in the driveway does not use much fuel. Pulling a hill requires a lot of fuel.
Did it do this before the carb change? It could be the carb, fuel filter, the fuel pump, or a clogged line somewhere.
It sounds likeduring high fuel demand, it's not getting enough. Revving the engine in the driveway does not use much fuel. Pulling a hill requires a lot of fuel.
Did it do this before the carb change? It could be the carb, fuel filter, the fuel pump, or a clogged line somewhere.
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This was the basic problem before the carb change, except no backfire then just died then under load. I assumed the carb needed rebuilt so I elected to buy a rebuilt one. Now it will run good just not when it is under heavy load. I have mech. fuel pump, I think is working because I crack with fuel line off and it pumps fuel. Maybe not enough? I have adj. carb to every possible setting and the best is both screw like 6 turns out, I doubt this should be the case.
Laughing Gas
Joined: Feb 2010
Posts: 781
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From: Sanford, NC
What you are experiencing is the problem with mass rebuilt carbs. The idle mixture
screws should not have to be out 6 turns to run right. They are masking some other
problem.
screws should not have to be out 6 turns to run right. They are masking some other
problem.
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From: Virginia
Since you had the same problem before the carb change, I believe the carb is ok. The idle screws only affect the engine idle mixture, they are out of the picture under higher rpms.
I would check the rest of the fuel system. Since you are getting some rust, I would pull the gas tank and check the sock on the end of the sending unit. It would not be a bad idea to change the sending unit, everyone has problems with their gas gauge not working. If you find the tank rusty, they are cheap also to just replace.
If yo do all that, I would go ahead and replace the fuel pump also. I had a car once that had a rusty tank and lines, and that got inside my new fuel pump and caused it to fail.
It seems like a lot of trouble, but if you get your fuel system clean, it will avoid a lot of problems in the future. One little speck of rust is all it takes to mess the carb up.
I would check the rest of the fuel system. Since you are getting some rust, I would pull the gas tank and check the sock on the end of the sending unit. It would not be a bad idea to change the sending unit, everyone has problems with their gas gauge not working. If you find the tank rusty, they are cheap also to just replace.
If yo do all that, I would go ahead and replace the fuel pump also. I had a car once that had a rusty tank and lines, and that got inside my new fuel pump and caused it to fail.
It seems like a lot of trouble, but if you get your fuel system clean, it will avoid a lot of problems in the future. One little speck of rust is all it takes to mess the carb up.
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The saga continues. Today I put in a new fuel pump and new gas lines from it to the carb. The old ones didn't seem that bad. Anyway, It may be alittle better, but the problem persists. I will drive away fine, but if I push the accelerator down more than half or so it will spit and backfire. I am thinking that I am starving for fuel. Is the next step to try to replace the accelerator pump? IF so how is it done? Any other ideas what is next to try. I do have dual tanks and the problem is happening on both tanks. Thanks!!
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From: Virginia
The accel pump should be fine, since it's a new carb. But you can check it with the engine off and the aircleaner off. While looking down the throat of the carb, push the throttle open. You should see two strong streams of gas squirt into the carb throat.
It's been my experience if you have a fuel delivery problem, it will take a few seconds to cause a problem if you have let the truck idle for a few minutes. The carb has a bowel like a toilet, and it will store some fuel in there. It uses very little fuel at idle, so usually the carb can fill up the bowel some, and when you first hit the pedal there is plenty there for the engine to use. And if you keep using it, with a fuel delivery problem it will eventually not be able to keep up and it will start giving problems.
If you have a carb or timing problem, the problem will always happen right away.
It's been my experience if you have a fuel delivery problem, it will take a few seconds to cause a problem if you have let the truck idle for a few minutes. The carb has a bowel like a toilet, and it will store some fuel in there. It uses very little fuel at idle, so usually the carb can fill up the bowel some, and when you first hit the pedal there is plenty there for the engine to use. And if you keep using it, with a fuel delivery problem it will eventually not be able to keep up and it will start giving problems.
If you have a carb or timing problem, the problem will always happen right away.
Mountain Pass
Joined: Jun 2010
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From: Dunbarton, NH
I had a similar problem and the cause was a clogged fuel filer. I replaced the filter and it went away. But since you replaced the fuel filter and your truck still sputters, next thing I'd do is, like Franklin says, check the fuel tank and sending unit filter sock for rust. Small amounts of rust can wreak some major havoc in the fuel system.
Post Fiend
Joined: Nov 2005
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From: Charles Town, W bygod Va
How about doing a fuel pump volume check? Just unook the line to the carb and stick it in a container then start or crank the engine. If you're getting low flow then persue the in tank sock or pissibly a small leak in the line from the tank to the pump, wich would be sucking air. If you've got good output maybe it's not a fuel problem. One thing that comes to mind, plugged exhaust sometimes feels exactly like you're running out of fuel. Listen for a whistling sound at the tailpipe while revving the engine. Stopped up exhaust will also cause vaccum readings lower than normal.
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Today, I used a boat tank hooked directly to the fuel pump, to see if a had a fuel tank issue. It did the same thing with the boat tank, so I will assume the sending unit and socks are fine. I also took off the manifold to see if it was plugged up, I cleaned all build up and the truck still will run awesome in park but falls flat on it's face driving down the road with anything more than half peddle accel. I now am wondering about the vaccume advance being bad.