Need help with driveability problem...
 

My Superformance 351W based engine starts fine and under mild acceleration runs strong and great. When I floor the car it kicks, spits and stumbles really badly though.

I've been through the brand new Holley carb and made sure that the secondary float is adjusted correctly and verified that the secondaries are flowing fuel just fine. I have also replaced the Holley secondary vacuum spring to a lighter spring but this has not made any difference with the basic problem. I've installed the MSD distributor with the the Superformance factory stock Duraspark II ignition module. I've carefully double checked all of both the primary and secondary ignition wiring against the factory wiring diagram and even replaced the Ford factory coil to no avail.

I used the (MSD) two 2 blue (light) springs and the blue bushing which should be giving me about 21 degrees mechanical advance coupled with my 15 degrees initial advance or 36 degrees total, all in by about 3,100 RPM.

Here are more specs on the very recent engine build.

408” 351W based stroker motor
Late model 351W factory roller cam block.
Crank: Eagle 4.0” Stroke Rods: Eagle 6.200” Pistons: Probe Industries Forged Aluminum SRS 4.030” bore, Reverse Dome 22cc (Compression ratio should be about 10.09:1 with these pistons and the Edelbrock heads with 60cc chamber volumes)
Cylinder Heads: Edelbrock Performer RPM 2.02” Intake Valves, 60cc chambers
Intake manifold: Edelbrock RPM Air Gap 351W
Carb: Holley Model 4160, 750 CFM, manual choke, vacuum secondary’s #0-3310C
Fuel pump: Summit Mechanical 110 GPH

Camshaft: Mike Forte Custom Grind 510/534 Lift, 222/232 duration at .050, 114 LC Camshaft Hydraulic Roller Lifters: Ford M-6500 R302
Roller Rocker Arms: Competition Cams #1131-8
Hydraulic Roller Lifter Retainer and Spiders: Ford factory

Crankshaft Damper: Ford Racing Nodular Iron, external balance #M-6311-K351
Oil Pan: Milodon 8 quart
Timing Chain Set: Edelbrock #7811 Performer-Link True Roller chain set
Distributor: MSD Pro Billet Small Diameter, Magnetic Trigger #8478, with vacuum advance
Distributor shaft Steel Gear (#85834) and installation by MSD factory
Superformance stock Ford Duraspark ignition module coil
Spark Plug Wires: Ford Racing black 9MM
Spark Plugs: Champion RC12YC (Recommended by Edelbrock for use with their alum heads). Spark plug gap: .045”

Again, when accelerating at partial throttle the engine runs just fine and has plenty of power but as soon as I really kick it the engine falls flat on its face kicking, spitting and refuses to run right until I get back off the throttle partially, where it continues to run just fine.

Please let me know your thoughts on where the problem is here. Thank you.

I suspect it's the accelerator pumps. Check to see if there's a gap; otherwise actuation should be instantaneous from idle...

Check the ground wire on the MSD....My 2 cents

Re-check your primary and secondary bowl levels. If in doubt, just raise both levels a bit and take a test drive to see if it changed anything. It sure sounds like you're losing fuel @ WOT. Check fuel pump pressure and fuel filter condition too. You can duplicate the condition, turn the car off immediately and read the plugs too. My bet's on loss of fuel.

Go back to the start before the problem
 

tim sullivan Tim if the car was running right at one time without and drivibilities go back to there. It sound like a fuel problem. Start with the basics, what fuel pressure are you running? is your fuel pressure regulator working right? You have 4.5-6.5 pound of pressure. If yes
Does your car any rubber fuel lines on it?? The new gas is slowly eating the rubber up and turns the inside of the lines into black balls or slime. It will clog the ports and jets in the carbs over time. Yes it some how gets through the fuel filters.
If you can check the fuel filter, pull apart and clean it or replace.
Pull the carb off the car and pull it apart and make sure that the jets are clean, the ports in the block are clean. the cross over tube is clean( becareful with the "o rings on each end" lite grease before installing. Check and set the floats on both bowls if you have them. Basic setup and cleaning done, reinstall the carb. If you have glass levels check them. The motor should run. Start with this then go to the Msd setup.
Do one thing first and pull the cap, check the center post in the cap for the carbon piece to be there, make sure the rotor has no burn holes in the end and the spring end has a little pressure against the carbon center POST. If the post is gone or doesn't look right, get a new cap and rotor. MSD is having problem with these. This will get you started. It sound like a carb problem. Go back to what ever springs where in the distributor to start and work from there. You need to see how fast or slow the timing comes in on the motor, It should be all in by about 2,400 rpms on most motors. Check with a mustang group and see if some one has the same setup as you. Rick L.

One quick way to narrow down the source of the problem is to borrow a working carburetor from a friend. They're easy to swap out and one quick ride will tell you if that is the problem.

I think the problem could be the accelorator pump as well. Cheep and easy fix. Second thought would be your jets or floats. I have been down this road before with an 850 Demon and I finally bought a new Holley 4150 750 DP Race Carb that performs flawlessly. Good luck and let us know how you resolve the problem. Another thought might be your inline fuel filter may be restricted.

Clois

sounds like weak ignition due to some problem to me.


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I had the same problem with my brand new 750 Holley, big stumble when I a got on it.
Cured it with the 50mm accelerator pump kit, now it gets that extra big shot of fuel when my foot goes to the floor, no problems for two years now.
Bill

Is the stumble a momentary thing that clears up after about a second after you floor it, or does it continue to break up the whole time it is floored. if it breaks up the whole time it is floored and under load, it is not an accellerator pump.


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The stumble, kicking and spitting continues as long as I have the throttle opened up. When I get back off the throttle it clears right up. Therefore I'm thinking it is not accelerator pump related?

First 3/4 movement of the gas peddle and the motor runs real strong and solid with no hesitation. The problem only occurs, and continues to occur, when I get past the 3/4 range and really floor it.

when you're in the 3/4+ range you are maybe in the secondaries? secondaries related?

what's the setup on the secondaries, pv, secondary main jets compared to the primary main jets, you get everything back together correctly?

unhook the secondaries and see how it runs.

Vector: It is a brand new Holley 750 CFM 4160 with vac secondaries. Good idea about disconnecting the secondary linkage and seeing what happens. I'll try it this weekend.

I believe you have a secondary problem. Since the carb is a vacuum secondary, they don't open up at any set throttle application. They open as demand increases.

Could be an obstruction in the secondary main jet. Did you take the carb apart and clean it prior to installation? If not, suspect tiny shaving in an orifice it doesn't belong.

Otherwise, possibly a sticking distributor advance. Easy to check with a strobe light.

If my car, replace carb with a known carb and see if problem disappears. If not, then go to the distributor and verify that it the advance is not sticking.

Sounds like a fuel filter/delivery problem to me.

Great ideas for things I need to check out. Thanks for sharing all your thoughts guys.

-Tim