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

[91_351police-special]

ok...its a '91 p72...one out of 1,000 made in 91 with the 351W I got rid of the shitty Variable venturi and intake with egr and put on a edelbrock 600cfm vacuum secondary 4 barrel carb and intake. i didnt realise it at the time but i used the gay cork gasket on the back and it popped out and it has a slight leak back there....idt its a big deal. first time i fired it up with new carb/intake and gave it some gas it backfired out the carb. i did have to pull the distributor out for the intake but it was put back exactly the way it was. since then it would ping sometimes really bad over the summer so i put 93 octane in it which stopped the pinging. ived checked my timing and it was set at 11 degrees advanced so ive set it back to 8 degrees advanced. its backfired out the carb prolly a total of 8 or 9 times since spring...but only when the engine is somewhat cool or just started. now recently the secondaries have been acting up.....itll be fully warmed up and they wont open and the pedal gets real damn hard....i let off and get back on it and they operate. the major problem though is when im pushin it half throttle or more it will almost hesitate...not exactly like it missfires and cuts out but its like it stopps gaining rpms for a spit second then kick back in...especially at the top of the gear right when its going to shift...now its almost a downright studder or missfire. now it backfires i THINK out the exhaust at the top of the gear before shifting.....like a very quick couple pops which i let off by then so it dont blow up or something. but at low rpms, normal driving and just getting on it slightly its fine.

SO....why the hell are the secondaries acting like that and why is it backfiring at the top of the gear and doing all the studdering? the car used to run damn good for 4,000 pounds...but now it just fuckin sucks....i cant stomp the damn ricers anymore

ANY help will be greatly appriciated.

[gadget73]

firstly, whats your total timing set to? I don't mean initial, but how much timing do you get for initial + mechanical + vacuum ? Too much and you get exactly what you're having happen.

Have you changed the fuel filter? Have you verified the fuel pump can supply enough fuel? I've also run into cases where starving for fuel causes this sort of stuff.

[import convert]

along with what gadget has said are there any other mods youve done to the car? the last time my car sounded anything like it was backfiring through the exhaust was when my timing was way off.

[1987cp]

You didn't say anything about adjustments to the carb ... my experience so far with 1406 carbs is that they seem to like it when you go two stages rich in both the cruise mode and power mode metering. I'd get the #1487 tuning kit ASAP if you haven't already.

You also didn't mention adjusting the idle mixture or setting the choke. I tend to assume you have since it's an absolute no-brainer, but if those are wrong and/or the electric choke isn't powered up, all bets are off on the thing running well.

I also don't see anything about whether you're running an EGR system, whether you have your vacuum advance hooked up, and/or whether that vacuum advance is hooked to ported or manifold vacuum. I've read a few recommendations that the vacuum advance should be disconnected on a non-EGR car, and it seems to be working well for me.

In the meantime, I'd definitely back the initial advance back to about 4 degrees before you hurt something, take it easy on the gas, and report back with more details on your combo. I repeat, DO NOT floor the gas or wind it out so long as the current symptoms persist. Once you get it running decently we can talk about the carburetor stepup springs and changing the spark advance curve.

[91_351police-special]

ok, the carb i havent touched other than the idle speed screw which has been set back to where it was out of the box. the choke is hooked up. it has a new fuel filter when the carb was put on. and the vacuum advance is hooked up to the port with no vacuum at idle. fuel pump...idk...it ran fine before but this problem just started probally a month ago. it did have an EGR but its gone since i replaced the intake with an aluminum intake. all the air pump and diverter valve and the air tubes to the cats and heads are still hooked up.

how do you check your total timing?

seriously....i greatly appriciate the help....and ill drive like a grannie lol

[Heretic]

I retract my statement.

[1987cp]

ok, the carb i havent touched other than the idle speed screw which has been set back to where it was out of the box. the choke is hooked up. it has a new fuel filter when the carb was put on. and the vacuum advance is hooked up to the port with no vacuum at idle. fuel pump...idk...it ran fine before but this problem just started probally a month ago. it did have an EGR but its gone since i replaced the intake with an aluminum intake. all the air pump and diverter valve and the air tubes to the cats and heads are still hooked up.

how do you check your total timing?

seriously....i greatly appriciate the help....and ill drive like a grannie lol

Sorry, guess I shouldn't have assumed the initial adjustments had been performed. Don't touch the car again until you've done a little reading, starting with this post.

Don't worry about total timing for now. Just dial the initial back to 4-6 degrees and leave it there for now.

Second, set the idle MIXTURE. There are two screws in the front of the base of the carburetor. With the engine off, screw each of these all the way in till they bottom out (gently!), then back each one off one and one-half turns. This is typically a good starting point for the idle mixture. Start the engine and verify that the choke is opening fully (you can just set the choke adjustment way lean for this step), and set the mixture by backing each mixture screw out 1/4 turn and then rechecking idle RPM (or vacuum if you have a gauge). Once you get the maximum idle vacuum and RPM (they will coincide) by adjusting the MIXTURE screws ONLY, reset the idle speed and only then should you move on to driving the vehicle at all.

By the way? If you bought your Edelbrock carburetor new, it should have come with an owner's manual that already contains all I know about tuning this carburetor. I highly recommend reading the entire manual before touching the car again.

DO NOT ATTEMPT TO DRIVE THE CAR AGAIN UNTIL THE IDLE MIXTURE HAS BEEN ADJUSTED PER THE DIRECTIONS IN THE MANAUL!!!

[torquelover]

DO NOT ATTEMPT TO DRIVE THE CAR AGAIN UNTIL THE IDLE MIXTURE HAS BEEN ADJUSTED PER THE DIRECTIONS IN THE MANAUL!!!

http://www.edelbrock.com/automotive_...ers_manual.pdf

:smirk:

[1987cp]

Thanks! I was going to scare up that link, but it sort of got late. :p

[91_351police-special]

ok...i got a 4 day weekend from school at NTI so i will adjust it. i did watch the carb adjustment dvd and read about it but it said it was set up right out of the box so i didnt think anything of it. one problem....i dont have a vacuum guage or electric tach....so just go by ear?

[91waggin]

harbor freight has a vacuum gauge for less than $20 and sears has a diagnostic tach for less than $30. That is if you want to go out shopping on black friday

[gadget73]

Info on the Duraspark distributor. You'll need to know what plate is in yours, and what notch its set to to figure out the total advance. If you read through this and look at the pics, it explains what I mean by that.

http://www.carbdford.com/viewtopic.php?t=5392

http://www.fordmuscle.com/archives/2000/03/timing/index.shtml

This of course assumes you're running a Duraspark distributor. If you're using something else, then you'll need to look up that particular info.

[1987cp]

ok...i got a 4 day weekend from school at NTI so i will adjust it. i did watch the carb adjustment dvd and read about it but it said it was set up right out of the box so i didnt think anything of it. one problem....i dont have a vacuum guage or electric tach....so just go by ear?

This is partly correct, Edelbrock carbs will typically RUN right out of the box, and quite often drive as well. This does not imply that it's at all safe or appropriate to just bolt it on and romp on it without touching anything.

I'll give a +1 on the cheap gauges, but doing initial mixture adjustments by ear is a big improvement over doing nothing. You probably know that trying to set ignition timing by ear is potentially disastrous, but doing idle mixture by ear is a different deal and should at least get you somewhat close - screw the screws in too far and the engine dies, and if you have to back them out something crazy you know you probably have worse problems than poor idle mixture, such as when I had a needle-and-seat alternately sticking completely closed and then completely open and lo and behold, the corresponding mixture screw was being completely unresponsive.

And of course, no special tools are required for setting the choke.

[Pirate]

My thoughts here? I smell a touch of over-revving the motor now that there's a decent carb on it able to allow the motor to rev well beyond the VV's capabilities......and a weak set of valve springs that allow the valves to hang open when he's trying to rev it to the stratosphere. Either that or the Duraspark is worn out, allowing the rotor to bounce all over the place. My first thought is valve-float.

[import convert]

now if it was valve float then hed have almost no problem below 4000 but the main probelm would be after 4k. but i could be wrong.

this may be a dumb question but did you remove the spout connector when you set the initail timing?