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Old 10-10-2005, 11:14 PM
amish_dan amish_dan is offline
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backfires and misses when i'm accelerating

ok, i thought i had her running like a champ... but... whenever i try to accelerate it stalls out and backfires?!
heres the specs on my truck
74 f250
360, 4bbl, new plugs/wires/coil/cap/rotor....
4x4
auto trans

it's seems kinda weird cause it idls great and if i stop on it, it sounds smooth but takes a long time to get up to speed. crusin on the freeway is kinda rough too. i'm loosing serious power to this problem and i've checked the whole ignition system a dozen times. any ideas? carb stuff maybe? i havn't messed with it and it worked before so it doesn't seem like that would be it. it is my driver so it's kinda embarasing putting around town in a big old monster truck. thanks to anyone who can help
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Old 10-10-2005, 11:44 PM
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SwOkcOffRoader SwOkcOffRoader is offline
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whats your timing at?? what kind of carb do you have?? we need a little more details on what has been done or hasnt been done.....sounds like a timing issue off hand, but thats just speculation.....
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Old 10-11-2005, 12:41 AM
yield2s yield2s is offline
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My truck did the same thing and it was a clogged up jet in the carb. It may be time to open her up and do a lite rebuild. Mine would idle and rev fine in the driveway but as soon as i took it out it would buck and stave for fuel.
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Old 10-11-2005, 05:16 AM
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I would check for vacuum leaks first. Easy to do and takes very little time. Just spray or use an unlit propane torch around your fittings and mounts. If you get a change in idle at any of your locations you have found where it's sucking air.
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Old 10-11-2005, 07:31 AM
holland501 holland501 is offline
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Backfire through the carb means it's too lean. Stumbling while trying to accelerate means your accelerator pump isn't discharging enough fuel. I would increase the stroke on the pump and see if that helps. Also moving up a couple of jet sizes won't hurt anything accept mileage. Carb tuning by the sat of your pants can take some time. Keep checking plug color. You wan a nice chocolate brown color. Good luck.
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Old 10-11-2005, 09:06 AM
amish_dan amish_dan is offline
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i'm having a hard time figureing my timing. if i set it where my chiltons says it should be i cant accelerate hardly at all. i have it advanced quite a bit so it runs better, i know thats bad for it.
also my plugs have a white color. i suppose that means it running rich. so either its running rich and getting to much or it's running lean and not getting enough.
one last thing. it's a holley 4 bbl. and i'm not sure where my mix ajustment is. is that just the float ajustment or is there another one somwhere?
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Old 10-11-2005, 09:52 AM
holland501 holland501 is offline
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White plugs means you are lean. Holley makes a lot of different carbs. Usually the mixture screws are located on the sides of the mereting block. I would buy a book on Holley carbs. You will gain a world of knowledge from it. Also the mixture screws only control fuel at and just above idle, then the run circuit comes into play. You say it's running fine at idle. As for the timeing, are you sure the advance is working? Does the timeing move when you increase the RPM? Again, check the accelerator pump. You need a good shot of fuel when you tip open the throttle because you loose all your vaccum which draws the fuel. Also double check your float level. Good luck.
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Old 10-11-2005, 10:23 AM
alhersch alhersch is offline
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had a backfire through the carb one time, found a idle adjustment screw almost all the way in, and the other one out to correct setting. Also the cough sputter problem sounds like an accelerator pump to me as well..

Good Luck
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Old 10-11-2005, 03:53 PM
JohnnyBench520 JohnnyBench520 is offline
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--- Just a thought, fire it up in the dark, and see if you have any plug insulation that has gone bad.. I just had that problem, after changing the same parts you did, and it turned out that I had a cracked insulator on plug no 2.

Have a gooder!!

JA
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Old 10-11-2005, 06:17 PM
yield2s yield2s is offline
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You're staving for fuel hence the no power and white plugs. It could be a partially clogged passage in the carb, or maybe just the fuel filter/pump. I started with the easiest and finally worked my way to the carb and when I opened it it was obvious that the chunk of brown crud sitting in one of the jets was the culprit.
good luck and let us know.

BTW it would also help to know if your problem just started right away or if it's always been like this
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56 F100 restored and shining

77' F-150 custom 4x4 351M/C6
3.5:1, 4"lift, 35" BFG T/A KOs, roll&push bars w/off-road lights

98 Explorer XLT 5.0

02' Acura TL "Type S" (Lemon, bought back)

Last edited by yield2s; 10-11-2005 at 06:21 PM.
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Old 10-11-2005, 06:17 PM
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