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Old 04-02-2010, 05:33 PM   #1 (permalink)
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Default Engine backfiring at idle.........

Ive got an engine I rebuilt, and its back firing, and spitting through the carb. I put a new timing chain/gear on the engine when I built it. Everything is new including the hyd cam, lifters, aluminum Motorsport heads etc.

Its a 302 engine with stock Ford electronic ignition, including the distributor, coil, and ing. module.

Ive swapped plug wires with some that I know are good as well as another Holley 4bbl carb with no better results. I changed the alternator, and volt regulator today, still popping. It seems to be on the passenger side bank of cylinders. I changed the coil, and module today for new parts.

Also, I've checked all of the vacuum lines, and intake gaskets for leaks, and none were found.

Also, pulled each plug and cleaned them. Reset the float levels, reset the distributor timing, as well as the firing order of the plugs. BUT, the cam is a 351w cam, SVO M-6250-A351. Which I'm assuming is a 13726548 firing order.

I pulled the timing cover today just to verify that Ive got the marks lined up on the gear, and they were. However I think I read some where that this cam needed to be run 2 deg retarded, but I don't think that would cause backfiring, or ?

Lastly, the gas in the tank is over two years old. I pumped what I could out with my electric fuel pump and put four new gallons of gas along with a small bottle of rubbing alcohol since there is probably water in the tank after sitting all of those years.


I'm trying to sell this engine, and I want to work the bugs out so that the people who are interested in buying it can hear it run.
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Old 04-02-2010, 05:59 PM   #2 (permalink)
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Lifter preload?
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Old 04-02-2010, 06:07 PM   #3 (permalink)
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lifters pumped up before adjusting valves.........
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Old 04-02-2010, 07:10 PM   #4 (permalink)
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It sure sounds like a timing issue or incorrect plug wire placement on the cap to me. I know you said you double checked both but maybe you should do more research into the firing order for that camshaft.
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Old 04-02-2010, 11:26 PM   #5 (permalink)
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+1 this cleary sounds like a timing issue. Maybe you got your cylinder banks backwards?
1-4 are on the passenger side, 5-8 are on the drivers side. Easy mistake to make. Also dizzy spins counterclockwise, easy to mix that up as well..

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Old 04-02-2010, 11:49 PM   #6 (permalink)
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First thought is that you are 180 degrees out with the distributor.
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Old 04-03-2010, 07:50 AM   #7 (permalink)
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Your problem will be the firing order I'm sure. The 351 has a different firing order to a 302 and this is determined by the cam.

You need to be 1-3-7-2-6-5-4-8

Good luck, and let us know.
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Old 04-03-2010, 07:56 AM   #8 (permalink)
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Hey, just read your earlier post and you're already doing this. Just to eliminate any doubts about firing order you could try the 302 order which is 1-5-4-2-6-3-7-8
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Old 04-03-2010, 12:04 PM   #9 (permalink)
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I guess Ill have to do the 302 firing order since I don't have the cam spec's card.

The camshaft came out of a 351W Ford Motorsport crate engine that only had 3k miles on it. The engine was from the HP TV Mustang and it was already in the car so thats why I don't have the cam sheet.

However I think while trying to find the cam spec's I read where this cam was supposed to be installed 2 degrees retarded. I have the cam installed straight up, which to me shouldn't make any difference except for moving the operating range of the cam either up or down the rpms.
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Old 04-03-2010, 12:36 PM   #10 (permalink)
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Quote:
Originally Posted by Mtrain View Post
I guess Ill have to do the 302 firing order since I don't have the cam spec's card.

The camshaft came out of a 351W Ford Motorsport crate engine that only had 3k miles on it. The engine was from the HP TV Mustang and it was already in the car so thats why I don't have the cam sheet.

However I think while trying to find the cam spec's I read where this cam was supposed to be installed 2 degrees retarded. I have the cam installed straight up, which to me shouldn't make any difference except for moving the operating range of the cam either up or down the rpms.
If this doesn't solve it, you need to elininate one thing at a time. I would next do a compression check on each cylinder to make sure there are no valve train issues, and to make sure they are all about the same.

Remember to hold the carb fully open when cranking the engine for each cylinder test in order to get best readings.
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Old 04-03-2010, 08:39 PM   #11 (permalink)
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Having the cam 2 deg off is NOT going to cause what you are describing. Either your firing order is off, or your placement of the distributor in relation to TDC of 1 is off.
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Old 04-03-2010, 10:01 PM   #12 (permalink)
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Also, make sure that the wires for cylinders 5 & 6 are not run next to each other or cross each other. Keep them well separated. Did you also check the distributor cap for cracks?
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Old 04-05-2010, 11:16 AM   #13 (permalink)
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13726548...thats the cleveland firing order. i think the W used the 289/302 firing order
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Old 04-05-2010, 11:24 AM   #14 (permalink)
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Pull off the valve covers, turn the crank and watch the valves open and close. Note the sequence.

While you're there...check the lifter preload.

There's no use guessing when you could find out in 10 minutes for sure if you want to.

Phil
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Old 04-05-2010, 02:25 PM   #15 (permalink)
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Quote:
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13726548...thats the cleveland firing order. i think the W used the 289/302 firing order
351W and C used the same firing order - 13726548. If the cam came from a Ford 351W crate motor, then 99% chance you'll need to use the 351 firing order.
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