This has been something I've been fighting for months now with my setup. So here are some details. Hoping some of you guru's will chime in and see what people think. 306 ( flat tappet ) flat top pistons Anderson mid range cam gt40 3 bar heads crane 1.72 roller rockers ( shimmed and proper geometry ) 4.10 gears 3 speed toploader Torker II intake Holley blue fuel pump regulated to 6psi fuel pressure mallory hyfire ignition blaster II coil Petronix unit it stock dizzy Initial timing set 16 all in @ 2500 rpm 36 ( tried everything from 10/30 to 18/38 timing ) Vac advance adjusted every which way and currently plugged off. Tried 3 carbs all with the same result..... car runs great and ildes great. Only issue I'm experiencing is a cutout when the I romp on it hard ( does this in nuetral just sitting and under load ) 18hg vac at idle Tried a 4150 650 double pumper mechanical secondaries 600 holley vac secondary with and without a quick fuel secondary metering plate and with and without the quick fuel vac canister. New street demon 625 tried every pump arm setting and went from stock squirter ( .33 ) I think to a 40 and as low as a 25 squirter and I get the same results every time. Valve train and motor seem to be completely fine. If I run it up without absolutely crushing on the gas it accelerates fine and sounds really healthy. Seems weird that the only issue is when I romp on the gas real hard it cuts out / hesitates and then eventually gets over itself and starts accelerating. Only thing I haven't tried is to replace my ignition box but thats not exactly a 20 dollar part so looking for insight before I spend any more money hunting a problem. If needed I can post a video of an in car shot of the vac guage and the tach and demonstrate what I'm having issues with.
Not enough fuel? What are you running for a fuel pump? Maybe can't keep up when put under a sudden demand, but does ok as long as it's slow and steady. Check fuel pressure when you romp on it.
your on the right track. the ignition system has got to be the problem here. you can remove the mallory box from the system entirely. the pertronix will work with out the box. so no cost to see if its the box or not. if the problem still happens with the box removed then your down to the coil or the pertronix. i used to work for super shops and they were owned by the same guy that owned mallory so we sold lots of mallory. i have to say it was not that great of a product. i would put odds on it being your problem. unless you are running nitrous or boost you really dont need an ignition box. the pertronix and coil will do the job just fine.
I'm going with the weak fuel supply, easy enough to read fuel pressure under load, I'll bet it's falling to a couple pounds or less...
i doubt its weak fuel supply because it does it in neutral. when you romp the throttle to the floor thats when cylinder pressures are the highest and require the strongest spark to be able to light it off. weak coil or ignition box could cause this. as i think more about it, a weak coil seems very likely.
I'm running a holley blue with the holley regulator. I will romp on it from the carb and watch the fuel pressure but I doubt that my holley blue is struggling in any way shape or form to keep up with this little 302 lol. I've got it regulated to 6.5 psi if that helps at all. Bryant is there any off the shelf way to check to see if the coil is weak? I'd love for it to be the coil but if my memory serves me I think the coil is only a few months old. Would suck to have to break down and buy an msd box. I'm kind of stuck running the box as my future plans include nitrous.
Have somebody watch the exhaust when you do this. If black smoke comes out the exhaust, your initial pump shot is rich and you need to slow it down or go to a smaller squirter. The car will stumble a bit then clean out and go, that's when you see the smoke (right after it cleans out) If there is no exhaust smoke when you romp it, it needs more initial shot/bigger squirter. The car will hesitate then take off. I have always kept going up or down (as needed) in fairly large jumps of 5 to 7 squirter sizes. If you go from no smoke to smoke when you romp it, you need to be between the last 2 you tried. You should see/hear/feel it getting closer as you change squirter sizes. I rarely mess with pump cams and choose to do most of this type tuning with squirters. If you get over a 40 squirter, you need to add a hollow screw. SPark
The demon comes with a hollow screw and I've used a .25 27 31 35 37 and 40 with no help. With the 40 and 35 37 I got a puff of smoke after romping it.....went down with no improvement.
Just check your fuel pressure when it acts up. Would have to be very poor supply to do it in neutral though.
Well no loss in fuel pressure but even with a 25 squirter I had a spotter watch exhaust and when it cuts out its bogging......will swap coils and go from there.
How is your accelerator pump arm set? With the engine shut off, I pull the throttle open and hold the pump arm down with the tip of a screwdriver to check play. You are looking for .015 clearance between the throttle linkage and pump arm. Retarded timing will also make it bog. What do plugs look like. Too big of a jet will make it do this also. No amount of tuning will get rid of the wrong jets. SPark
Hmmm, I wonder how resistance in a winding can increase by just 6 ohms? Usually a coil will decrease resistance due to shorted windings or show infinite resistance due to the winding opening up. Maybe some internal connection at one of the terminals is burned or something...
