351w backfire at high rpm...WHY!?!?!?!

[1987cp]

If he's not getting into the gas much, secondary jetting should be completely irrelevant. Primary metering should be as intended providing the stepup plungers move freely and there are no major internal vacuum leaks - unless the wrong rods and jets were installed at the factory for some reason.

Speaking of which, we still don't know for certain what model of carb this is because that info hasn't been forthcoming either, nor whether he's made the almost-always-mandatory switch to a bigger pump shot. :p

[import convert]

true, i hope its nothing bigger then a 700cfm. anyhting that big or bigger is too much for a mostly stock 351.

[91_351police-special]

its the 1406 carb.....600 cfm electric choke. ive disconnected the vacuum advance and plugged it and changed the spark plugs and still no change. the old plugs have a tan orangeish white color to them and the very tip of the center electrode is black. they were all still at proper gap but did have sliight wear on the electrode. and i will read how to up the accelerator pump shot right now. the calibration kit has to wait till next weekend though. when i only make 150 bucks a week and almost 100 of it goes to gas and the rest to insurance, its hard to find the extra cash for other stuff. and if it helps it is all stock other than valve covers, the edelbrock aluminum 351W performer intake, and the 1406 carb. and i cut the mufflers out and put two flex pipes in the place of the mufflers since idk how to weld lol

[91_351police-special]

ok....just moved the accel. pump shot from the middle hole to the higher shot hole. theres only 3 though so its at its highest shot now. it seemed to accelerate stronger and doesnt start studdering untill higher rpms. so im really leaning towards its not geting enough fuel. that would explain why it runs pretty much fine during normal driving but runs shitty when giving it some gas.

[import convert]

id try a new fuel pump next if youre sure that its not getting enough fuel.

[1987cp]

ok....just moved the accel. pump shot from the middle hole to the higher shot hole.

'bout freaking time. Nearly every Panther application requires this because the car's comparatively heavy and the engine revs pretty slowly. Goes along with the fact that you'll in all likelihood be installing the heaviest metering-rod stepup spring in the near future. Failure to increase the pump shot results in lots of fun popping noises and general lack of acceleration when you try to drive normally.

Now it's high time to break into your nice shiny new EDL-1487 tuning kit you have sitting on the shelf in your garage ....... you have already purchased the tuning kit, right?

[91_351police-special]

Now it's high time to break into your nice shiny new EDL-1487 tuning kit you have sitting on the shelf in your garage ....... you have already purchased the tuning kit, right?

nope...dont have it yet....like i said i jus threw the carb on and it ran fine other than the rare backfire out the carb so i just left it. i will be buying the tuning kit this weekend though. cuz it seemed to really like the increase in pump shot so im leaning towards it running lean even though the plugs dont show the white residue they should when an engine is run lean. i asked my instructor at NTI what could cause it to studder under load and backfire at high rpm and he said the ICM could be going bad....but i thought they just up and quit? and if it was the fuel pump not providing sufficient fuel pressure.....wouldnt it just not work too? or can it still work but just barely enough to keep it running? and also what about the mechanical adavnce....could the springs be going bad?

[gadget73]

Fuel pumps, especially mechanical diaphram pumps, can get weak. What happens with them is the valves internally start to leak, and some of the fuel is pushed back toward the tank instead of to the carb. The diaphram also gets stiff and won't pump as efficiently.

ICM, as in idle control motor? There is no such thing on a carb motor.

[91_351police-special]

ICM, as in idle control motor? There is no such thing on a carb motor.

naw, ignition control module for the duraspark ignition. do those just up and quit or slowly die off?

[import convert]

the icm wil act like any other ignition module. they all have a low and high rpm side so all you need is one side not to work as well as it should. in your case the high rpm side may not be working like it should. they can just completely crap out but not always. with the extra air and fuel youre running now id deffinitely look into change the mudule at least to make sure youre getting the right advance and spark. the thing is though there could be other things you need as well. your coil could be goin bad, you dizzy could be going bad or either could be bad. you wont know until you have htem tested or just replace them but id start with the ignition module

[1987cp]

Pretty sure Durasparks usually die by just not working anymore, but I could be wrong on that. I've ended up with HEI modules on two cars and a Crane Hi-6 on the third (installed by the previous owner). Some people have stories of going through Duraspark modules at an alarming rate, and other folks' modules just never quit. I'm not sure I'd bother worrying about the ignition if it runs reliably.

As for the distributor, the only thing electronic in it is just the inductive pickup. I'm pretty sure those also either work or not.

But before going haywire about ignition modules, let's wait and see what happens once you break out the tuning kit!

[91_351police-special]

But before going haywire about ignition modules, let's wait and see what happens once you break out the tuning kit!

yeah! its arriving by noon tomorrow at advanced autoparts for me to pick up. i also got a new water pump for it since its been leakin and the bearing has become noisey. now what exactly am i gunna be changing? both jets on each side of the carb and both the metering rods? and like how much....i think someone said it might need to go as rich as possible?

[1987cp]

yeah! its arriving by noon tomorrow at advanced autoparts for me to pick up. i also got a new water pump for it since its been leakin and the bearing has become noisey. now what exactly am i gunna be changing? both jets on each side of the carb and both the metering rods? and like how much....i think someone said it might need to go as rich as possible?

What you change out and by how much is up to you. You'll be referring to the #1406 calibration charts provided in your owner's manual. I downloaded the full manual in PDF and keep multiple copies of the charts handy here and there. Analyze your symptoms and decide what to do. From what you've said and from my experience so far with 1406s (seems they're usually too lean as-delivered), you will probably start by going one stage richer, and then two or three as needed. Your first goal will be to give it enough fuel to eliminate all popping under all driving conditions so that you at least know you're safe. If you go too rich and start bogging and fouling plugs you can always back off slightly, such as the one car in which I'm pretty sure I went back to the middle pump shot position because it was all it needed once the other stuff was set. In almost all cases you'll start out with just a rod change and probably change to a heavier stepup spring so you get "power mode metering" sooner in your pedal travel (even if the rest of your calibration is spot on, a too-light stepup spring will cause an annoying bog and/or popping noise whenever you try to accelerate and the engine needs the extra fuel enrichment NOW and can't get it). If you're lucky, the you can solve your problems with just a rod change and won't have to remove the top of the carburetor to change jets.

By contrast, my 1404 (now converted to 1403) was delivered too rich for my needs, and so I was able to just bolt it on and drive it for quite some time with the only penalty being extra fuel usage. But once I started leaning it out - I went a full 3 stages in both power and cruise for one application - I found a lot more power lurking around. Now that carb's on a much thirstier engine, so I fattened it up to probably one stage lean or maybe even one stage rich (don't remember offhand).

Speaking of jets, I have a pair of secondaries I need to play with one of these years to see if I can't cure a flat spot around 4500 rpm .... I've never messed with secondary jetting before, so it'll be interesting.

[91_351police-special]

ok, well never fucking mind anything with my car. on my way to school i was going down the highway and it was raining pretty hard...i was doing about 60 and it hydroplaned and i never gained control it drove home and is fine other than its rear end is all fucked up and the front is fucked. im alright though. id rather be all fucked up and broken then my car being all fucked up.

[import convert]

What you change out and by how much is up to you. You'll be referring to the #1406 calibration charts provided in your owner's manual. I downloaded the full manual in PDF and keep multiple copies of the charts handy here and there. Analyze your symptoms and decide what to do. From what you've said and from my experience so far with 1406s (seems they're usually too lean as-delivered), you will probably start by going one stage richer, and then two or three as needed. Your first goal will be to give it enough fuel to eliminate all popping under all driving conditions so that you at least know you're safe. If you go too rich and start bogging and fouling plugs you can always back off slightly, such as the one car in which I'm pretty sure I went back to the middle pump shot position because it was all it needed once the other stuff was set. In almost all cases you'll start out with just a rod change and probably change to a heavier stepup spring so you get "power mode metering" sooner in your pedal travel (even if the rest of your calibration is spot on, a too-light stepup spring will cause an annoying bog and/or popping noise whenever you try to accelerate and the engine needs the extra fuel enrichment NOW and can't get it). If you're lucky, the you can solve your problems with just a rod change and won't have to remove the top of the carburetor to change jets.

By contrast, my 1404 (now converted to 1403) was delivered too rich for my needs, and so I was able to just bolt it on and drive it for quite some time with the only penalty being extra fuel usage. But once I started leaning it out - I went a full 3 stages in both power and cruise for one application - I found a lot more power lurking around. Now that carb's on a much thirstier engine, so I fattened it up to probably one stage lean or maybe even one stage rich (don't remember offhand).

Speaking of jets, I have a pair of secondaries I need to play with one of these years to see if I can't cure a flat spot around 4500 rpm .... I've never messed with secondary jetting before, so it'll be interesting.

ive thought of checking my holley and see if i can lean it out a bit and maybe save some fuel and yea, maybe find a little power hiding behind the the excess fuel. course it doesnt have a rich smell to it and its not bogging or anythin so i wont know till i mess with it, and when i get a vacume gauge

ok, well never fucking mind anything with my car. on my way to school i was going down the highway and it was raining pretty hard...i was doing about 60 and it hydroplaned and i never gained control it drove home and is fine other than its rear end is all fucked up and the front is fucked. im alright though. id rather be all fucked up and broken then my car being all fucked up.

that sucks man. id be pissed to if that happened to me. im sure you could find another box or maybe fix youres. any pics of hte damage