390 backfire problem
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390 backfire problem
Hi everyone. I have a 75 F-250 with a 390. The motor was originally stock, but
I just resently did some work to it. I bought an edelbrock perfomer kit which includes a intake manifold, camshaft and carb (you get to choose the carb size).
Carb size is 750 cfm. At first when I got it running, it was running super rich:
carb was black, spark plugs were black, and tailpipe was black. I bought a calbration kit for the carb and got it running leaner. Spark plugs are no longer black. Problem is when driving it pops and sputters until you put her to the floor
and the secondarys open up. Then she runs fine. I noticed that when you rev it
real fast it backfires through the carb, as a flame shoots out of the carb. So I'm assuming that is what that poping sound is when I drive it. It only happens at part throttle when at cruising speed. I've read that backfire throught the carb is a timing problem, but could it be the carb? Any help would be appreciated.
Thanks
Andy
I just resently did some work to it. I bought an edelbrock perfomer kit which includes a intake manifold, camshaft and carb (you get to choose the carb size).
Carb size is 750 cfm. At first when I got it running, it was running super rich:
carb was black, spark plugs were black, and tailpipe was black. I bought a calbration kit for the carb and got it running leaner. Spark plugs are no longer black. Problem is when driving it pops and sputters until you put her to the floor
and the secondarys open up. Then she runs fine. I noticed that when you rev it
real fast it backfires through the carb, as a flame shoots out of the carb. So I'm assuming that is what that poping sound is when I drive it. It only happens at part throttle when at cruising speed. I've read that backfire throught the carb is a timing problem, but could it be the carb? Any help would be appreciated.
Thanks
Andy
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No there is no smog pump on the truck. I forgot to mention the even when the truck was running real rich it would also backfire through the carb. So it is either running too
lean now or the timing is off.
lean now or the timing is off.
Senior User
Joined: Sep 2006
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From: Newbury Park Ca.
When reved suddenly, you are depending on the accelerator pump to add enough fuel to compensate for the sudden rush of air the butterflies allow in. A short delay before the main jets "feel" the pull and start flowing. It was fat enough before that this transistion wasn't a problem, but now that you've leaned it out, it leans out too far at sudden opening. Check the travel of the acc pump linkage, you may need more squirt.....Different linkage hole, or a "full ****" of the plunger it's self, different nozzle size?? When you look down the carb, (ENGINE OFF), and just ever so slightly move the throttle shaft the squirter should start to squirt, not much, but start nonetheless. If it doesn't, adjust linkage arm until it does..........aMP
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From: Newbury Park Ca.
600?
Originally Posted by Bear 45/70
Carb is to big for the engine air flow requirements. is part of the problem.
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Well let me explain things a little more in depth. I ordered the edelbrock performer kit for my 390 from jegs. I called in and asked the carb size for my application they said to use a 750 cfm carb, model #1411. I at first suspected that the carb was going to be too big for the engine, but when the tech guy at jegs told me to use a 750 i thought well why not. He could have possibly thought that my 390 was a high compression engine with late 60s early 70s heads. I don't recall telling him that it was a truck engine. Anyways the truck is running lean now hence the plugs being white. I have other possible jet and rod combos yet that i have not tried yet. I have to order some rods. Has anybody got this 390 truck engine and 750 cfm carb combination to work? Would this application work better with a 650 avs carb or a 600 performer series carb. Would I lose power at wot from going down from 750 to lower cfm? Please help and thanks.
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
At 6000 RPM your 390 would need a carb rated at 677 CFM if the engine was 100 effecient, which it isn't. The 750 is over kill unless you have a race car. Bigger is never better when it comes to carbs in a street machine. That's twice as true with a truck, as they are heavy. That 750 might work on a light car, but not on the truck. The answer to the power question is NO. Besides just how much time does your engine spend at max RPM? Not much I will bet.
FE "Freakin Expensive"
Joined: Dec 2003
Posts: 6,461
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From: Smith Mountain Lake, VA
Sounds to me like the timing is to far advanced.... The only time your going to get a pop and fire out of the top od the carb,,is when there's an intake valve thats slightly open to allow partial cylinder comprssion and fire back up thru the Induction system, because if both Valves are closed.. the cylinder is sealed...No oxygen to the carb..No fire..the pop..compression coming back up thru the port into the intake and up thru the crab..
Re-static time the motor..check all your plug wires for crossed cylinders....Turn both your air and fuel screws all the way in (but not tight) as you'll screw up the Needle points on them..back em out 1 1/2 turns for starters...Look at the dizzy cap for a good spark trace on the conductor points...and on the rotor..
You may have to have your Distributor re-curved for the new cam.... Good Luck! JMO..
RJ
Re-static time the motor..check all your plug wires for crossed cylinders....Turn both your air and fuel screws all the way in (but not tight) as you'll screw up the Needle points on them..back em out 1 1/2 turns for starters...Look at the dizzy cap for a good spark trace on the conductor points...and on the rotor..
You may have to have your Distributor re-curved for the new cam.... Good Luck! JMO..
RJ
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