Backfire under acceleration how do I fix it?
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Backfire under acceleration how do I fix it?
My truck has a 360 with 390 heads and was rebuilt about 2200 miles ago. I swapped the intake for an Edelbrock performer 390 and I put a freshly rebuilt Holley 4bbl 600cfm on it. Now when I take off it backfires if I give it more than a tiny bit of gas and seems to have no ***** even at full throttle but burns rubber at almost idle. I know the carb isn't adjusted right (I don't have a vacuum gauge) but someone told me the carb was too big for the engine and someone else told me it wasn't big enough. WHO DO I TRUST? Neither is what I guess. Anyone have any input on this? It runs great at steady rpm (on the highway or sitting at idle). It smells like its running SUPER rich! Any help would be great. I want to drive it more.
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Could I be one tooth off when I installed my distributor? I am terrible at timing. Once I installed the dist. I just set it to one of the yellow lines someone had marked on the HB. I have no idea if that is even close. I never did understand how you could set the timing a million different ways and have it still run.
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From: Halifx Pa
I would suggest you go to your nearest Harbor Freight and pick up a vacuum guage ($10), and timing light ($25) so you can take the confusing guess work out of it. These two tools alone will pay for themselves over and over not to mention the aggrivation you will be free of. Also I say they 600 cfm is NOT to big for that 360.
I don't know what your timing should be at, but do a search on this forum and you should be able to find that in short order. If it smells really rich at idle, I am sure it is. I am assuming you mean when its warm it still smells rich. This is somewhat normal when its cold tho.
I don't know what your timing should be at, but do a search on this forum and you should be able to find that in short order. If it smells really rich at idle, I am sure it is. I am assuming you mean when its warm it still smells rich. This is somewhat normal when its cold tho.
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From: Stanley, VA
If it's running okay other than when you "punch" it to accelerate, I would strongly suspect the accelerator pump isn't squirting enough fuel and causing the engine to lean out.
You need to "dial in" your accelerator pump to richen it up. Once you get that squared away, then you can work on dialing in the rest of the carb (running rich).
You need to "dial in" your accelerator pump to richen it up. Once you get that squared away, then you can work on dialing in the rest of the carb (running rich).
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From: Halifx Pa
Very good point Mike, but he doesn't know what his timing is at either. It all works in tandem!! But I am also guesisng he isn't all that far off if he is turning the tires, but he still needs to know what it is and get it set.
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I guess its a good time to learn how to set my timing. How do I "dial in" my accelerator pump? I didn't know you had to do that. I just know how to adjust the idle and the air/fuel mixture screws.
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Someone told me I need to remove the vacuum advance line when setting the timing. Is this true? Heard from a Chevy guy so I don't know but it makes perfect sense.
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More Turbo
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From: Western Washington
When I was running a 360 in my 76 F-100 I believe the original yellow mark on the HB was at 6 degree's. Probably a good starting point for now.
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Hotshot
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From: Stanley, VA
You're right Harold, it all works together and needs to be dialed in. He didn't say anything about "pinging" or hard starting so I wouldn't think the timing is too far off but it DOES need to be checked and set properly.
Hotshot
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From: Stanley, VA
Its been a while since I've worked on a Holley but IIRC, there are two areas that you can adjust. The pump discharge nozzle in the throat of the carb (where the gas squirts out) and there is a plastic cam that works the accelerator pump arm, these can be swapped to change the squirt characteristics. Holley also has a larger (50cc I believe) accelerator pump available.
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Yeah it ran great when I got it but you had to push it and pop the clutch to start it (bad flywheel ring gear). I got those parts from someone and figured while it was down for the flywheel/clutch replacement and trans work I would go ahead and swap in the 4bbl setup. I paid $75 to have the carb rebuilt before I put it in. I also had the intake cleaned before I installed it.
This carb has about 200 miles on it at most since rebuild. I took it on one trip to go wheeling and I blew a tire so I haven't driven it much other than to show people how it backfires when I try to give it gas.
This carb has about 200 miles on it at most since rebuild. I took it on one trip to go wheeling and I blew a tire so I haven't driven it much other than to show people how it backfires when I try to give it gas.
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If I pump the gas once it starts faster than my 06 car does. I could burn my 33 inch tires all day if I don't give it much gas but the second I really try to take off (like anyone would if they didn't want to drive like a granny) it backfires once or twice sometimes 3 or 4 times but usually only once or twice. once I get past the backfiring and floor it I have NO power like I'm driving through 10ft of water.