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Old 04-16-2002, 02:52 PM
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Question Pontiac 406 popping above 4000 RPMs

My buddies '70 Formula Firebird has a bored 400 with a couple of thousand miles on it. It has been completely rebuilt, Pertronix ignition, and Jett performance just did the carb. It starts OK, seems to idle OK, but when you get on it, it starts popping around 4000 RPMs. Any suggestions? How about fuel pressure, think it's running out of fuel? The pump is new but I believe that it's stock. The motor is slightly over stock but nothing too radical.

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Old 04-16-2002, 04:51 PM
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This problem can be caused by bad valve float. IF he didn't rebuild the whole engine himself, perhaps the original springs were reinstalled. Tired springs would cause a popping usually above 5000 or 5500 but very weak springs could conceivably pop over 4000. I doubt the valve springs were reused but it's worth a shot to check.

A.J.
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Old 04-16-2002, 05:21 PM
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How are you measuring the rpm? Had the same problem with a 69 stang. Seems after the third performance engine buildup the car did the same thing (it was a little more radical than the first two), only after I measured the rpm with a nonfactory tach did I see that the factory piece of s_it was reading 4500 and the engine was actually doing 7000 according to the SW race tach I installed later. That kind of explained the other two engines I blew out of that car (rods through the block), I was thinking the factory short blocks were just crap. Honestly I couldn't hear any difference inside the car. Not too proud of that one.

Lesson learned.
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Old 04-16-2002, 05:56 PM
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hrf100,howdy! the fact that it has so few miles on it worries me!you say popping?what?through the exhaust,or through the carb?get back to me!jimm
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Old 04-16-2002, 06:27 PM
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Jim, by popping I mean like a mild cough or maybe even a momentary dead spot might be more descriptive. When I first drove his car after it was rebuilt, I noticed that when I mashed it, the car kinda did a forward and backwards rock for only a second but continued to rev. At that time it revved pretty good but something to me seemed slightly strange. Since that time he replaced the points with a Pertronix module and sent the carb to Jett Performance. He has also had a seemingly unrelated problem with the car wanting to kick back when you tried to start it when hot. The motor had a total rebuild done on it, and I believe that everything was gone over top to bottom. The thing should be like a new one.
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Old 04-16-2002, 06:41 PM
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Sounds like timing, check distributor advance and initial timing.
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Old 04-16-2002, 07:02 PM
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Sounds like a lean misfire condition, when a motor leans out at mid to high rpm's it will miss as if it has an ignition malfunction.
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Old 04-16-2002, 08:38 PM
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I agree with jooooooe just my opion it sounds like its in the carb if stumbling out of get go larger accelerator nozzle
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Old 04-17-2002, 04:11 PM
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hotrodf100,hey!check those plugs,you can go to page eight of this forum and dmorriss posted a questio re :p lugs and kultulz gave alink that is excellent for diagnostic help re: plugs.let me know what you discover!,see ya,jimm
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Old 04-17-2002, 04:32 PM
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Everything I've read points to timing issues. Start with static timing, vacuum disconnected. Set it to factory specs but it is probably somewhere around 8-10 deg BTDC. Then connect your vaccum line and your timing light and rev it up. You should see the distributor advance about another 24 or 25 degs. This should be there before 3000 rpm. The advance should be fairly linear, no quick spikes of advance.

The kick back when hot tells me maybe too much static advance.

You can get a motor to start with the timing way off, but it just won't perform well across the rpm range. Another indication of bad advance settins is when you can't get the motor to idle down.
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Old 04-21-2002, 11:13 PM
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I would have to agree with chuck on the tach issue, make sure where your at before you go to far (and destroy somethin) then check your timing. Ive had similar problems with stockcar engines and found that the dist wasnt reachin as far as the engine in the rpms.I dont have any experience with Pertronix but if its anythin like MSD then I would be lookin at the rev limiter as well..........good luck

[ April 22, 2002: Message edited by: critter ]</p>
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