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Old 04-01-2013, 05:36   #1 (permalink)
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351 backfire at 5000 rpm

I have just recently rebuilt my 351 Cleveland. I had an issue with her popping out the left exhaust and after pulling it down found she had a stuffed no. 5 piston. So I purchased a short off a mate that has flat tops and a mild cam, put a set of 2V closed chamber heads on it, they have been ported and polished some what, not over the top at all and they haven't been flow tested. I have an elderbrock performer on top with some porting done also, 1 inch open spacer and a Holley 600 vac on top. She has also got 4 into 1 extractors, electronic ignition, top loader and a 9 inch running 4:11's and locker.

She was running ok but would break down at 4000 rpm and found my float levels were not good so adjusted and she was sweet. She would pull hard in all gears to around 6k after that fix.

Then I started having a high tension miss, which I didn't know what it was so I changed the carby to a Holley 850 dp I have but was way too rich and poured juice out the exhaust. I also tried another Holley 600 vac from my dad but same issues. I then installed a billet full adjustable crane ignition and dizzy. But she still had the same issues. I then found a main wire from my alternator to starter had earthed. I fixed that, put on a new alternator as the other wasn't charging and the high tension miss was gone but now I have another issue.

Im running the 600 vac from my dad still and she pulls hard till 5000 rpm then I get a backfire through the carb. I have played with the curves of the dizzy and have it set to 12 idle and around 36 at wot. I have also changed the plugs and leads. Not sure what would be causing this. I have played with the carb some, checked all vac lines, played with the timing but still get this backfire. The best I have got is 5500 rpm before it happens but can't get rid of it. After the backfire she breaks down a tad also and feels like it did with the fuel bowl issue earlier.

Sorry for the long book but any help would be greatly appreciated as I have no idea what it could be. It has been suggested on another site that it may be my valve springs not coping but seems weird as it originally didnt have this issue.
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Old 04-01-2013, 14:11   #2 (permalink)
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Re: 351 backfire at 5000 rpm

Hi syco. I'm glad you wrote a good amount of info first as at least we get some idea of what has been happening.

Do you know what caused the piston problem. What had happend to the piston. Ring lands broken or a hole or something?.

At first I thought valve bounce as well but then you said it pulled 6k earlier no problem.

It sounds like you have done a bit already regarding the problem.

It could be leaning out at high rpm so it could be worth checking carby jetting.

As you have new leads and dizzy which is set up then I am guessing thats all good there.

I know this is a long shot but have you checked the plugs heat range.
I have had this trouble before and pulled my hair out trying to find it. Car would be good under normal running but a good rev would cause breakdown and then it would progressively breakdown at lower rpms. I finally tried a cooler heat range plug and it solved the problem.

If it is too lean then you will need to sort that out as it could lead to another piston problem if left.
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Old 04-02-2013, 05:18   #3 (permalink)
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Re: 351 backfire at 5000 rpm

Thanks for the response. The piston had cracks between the bottom two rings. As soon as I touched it all the cracked pieces fell out. Was very lucky as when I found the low comp in pot 5 I pulled the heads and the valves were burnt out a bit so I threw on another set if heads that were close chamber and not open chamber so it would have upped my comp ratio. She then started puffing smoke out of the rockers so I pulled the motor and found the stuffed piston. So I guess I saved my block and heads. Luckily. I'll use that block later down the track for another project.

As for the plugs they are just standard NGK BP6F S. The problem doesn't seem to get worse as it gets hot so not sure if it would be the plugs.

Not sure what jets my dads Holley 600 vac has which is on the car now but mine has 62's in it. I have 72's in the front and rear of the 850 dp I have. Think they would be too big for the 600. I do have a 650 spread bore sitting there that may have jets in between that I could try out.

One other thing it may be is the coil, possibly. I did have a Bosch MEC723 coil on my last electronic ignition set up but there is a warning on the side that is nearly worn right off about not using it on a modified ignition so I used a standard style Bosch GT40 coil with the Crane dizzy and module. Not sure if I could use the MEC723 or if I should purchase a crane unit that matches the module and dizzy.
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Old 04-05-2013, 08:25   #4 (permalink)
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Re: 351 backfire at 5000 rpm

NGK ??? That is your problem right there. No, Good, Krap.
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Old 04-05-2013, 18:20   #5 (permalink)
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Re: 351 backfire at 5000 rpm

What brand and style would you recomend?
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Old 04-06-2013, 14:02   #6 (permalink)
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Re: 351 backfire at 5000 rpm

Ford or champion but not autolite or A.C. NGK has never proven it self in an automobile
engine. They are perfect for 2 stroke but don't work well with 4 stroke. Use what your
engine manufacture calls for on the sticker under the hood. It will not say NGK. Don't
use the " E-3 plugs either. They are not as good as claimed. They are a cheap replica
of an aircraft plug. An aircraft piston engine uses a magneto not a distributor. There is
a hell of a difference.
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Old 04-08-2013, 23:44   #7 (permalink)
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Re: 351 backfire at 5000 rpm

Thanks for the reply. I will have a look for one of the other brands. Most auto shops here only seen to stock NGK or Bosch plugs but will look for one of the above brands. I can't go off any sticker on my car either as it is not the standard car any more and has a paint job and all the factory stickers have been removed.
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Old 04-11-2013, 12:07   #8 (permalink)
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Re: 351 backfire at 5000 rpm

call a ford dealer. Give them your engine info and year of engine if possible and they will
tell you what plugs it takes. It's that simple. No offence.
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