428 stumbles/misses after warmed up ?????

[wicked fast]

Hey,

I now have about 350 miles on the 428. You can go 10 miles get some heat in it and it runs great, turn the car off and let it sit for 15 minutes. It starts right up, you go 5 more miles and then it starts to stumble when lightly going through the gears or cruising at 25 to 35 mph. Then it will be fine for a mile or two then it will randomly start stumbling again. Not fun.

Changed the plugs, new 8mm wires. Timed it with 12 at crank, 36 total. Fine
tuned the mixture screws (3/4 turn out). Pertronix II in the distributor with their epoxy filled coil. No MSD box. 750 Holley DP with milled air horn. Carter
steet-strip fuel pump.

So what do you think ?? Fuel or ignition ??

Thanks,
BC

[392cobra]

Turn the mixture screw out 1 1/2 turns.Tune the carb with the aid of a vacuum guage.Highest,steadiest reading.

Check fuel pressure after warmed up.

[Ron61]

I agree with Fred. I would first check the fuel and Carb settings as you said it has to warm up first. And if the stumble goes away when you get on it then it is almost certain a fuel problem.

Answer resourced from the Ho Sisters How To Do It In A car.

Ron

[jhv48]

Does this only happen AFTER you have warmed up the engine and turned it off?

If so, might be percolation of the fuel in the float bowls. Will run crappy on restart until you drive a few miles and cool down the fresh fuel. Might need a turkey pan or plastic spacer under carb to insulate it from heat.

[Bill Bess]

If it's a fuel related problem and your using a Holley 750 with the little Acelerator pump, you can install the 50mm pump and related cam fitting and get rid of that stumble. This is what I had to do to tune my crap holley....works good now..gets plenty of gas when you give it just a little pedal.

Check the Holley manual and it will provide you with numerous suggetions and fixes.
Bill

[JJCobra]

I had the same problem with my 428. It would stumble when warm and when getting on the gas and it never really ran as well as I thought it should. I'm not usually one to swap out parts but I replaced the stock 4150 with a custom built Pro Systems carb and now it runs like a top. The throttle response is great and it feels a bit quicker too. I'll never buy another stock carb.

[Jerry Clayton]

You have a hand full of mis matched components and tuning areas

First off, I would say that your driving the car as if it was a modern, auto trans, computor controled electronic fuel injected vehicle---you can not lightly go thru the gears or cruise at 25-35 mph

With the milled off airhorn your carb will be excessively lean on transission from off idle to light throttle

Your ignition timing isn't aggressive enough---change the curve to around 18-20 initial and 36-18 all in by 1800-2000rpms---mainly just make sure it returns to initial at idle so you can shut the car off without it dieseling or running on

K&N used to make a stub stack for the top of the carb that inproved airflow but I don't know if they still do ----otherwise you are going to need a new carb main body---if you don't want the choke horn you will need to get a body designed without it

[trularin]

So, what you are saying is the engine runs rough after the choke cycle.

Check:
float level
route of fuel line to carb
idle mixture screws
leaky power valve
Primary jet passage.

These are what I would look at first.

[Jerry Clayton]

Doesn't have a choke tower

[Dinobyte]

What about the fuel pump...add an electric to supplement the manual. Also there may be some vapor lock occurring...if the lines are heating up because of location, they may need to be rerouted.

[Jerry Clayton]

lightly thru the gears and a cruise speed of 25-35????

No timing advance at those speeds and no smooth airflow ----needs a carb with unaltered carb body and some dist timing curve work along with a more aggressive driving style

[wicked fast]

Thanks for the input.

I think jhv48 nailed it.

Found this on a fellow Cobra owner in my area. www.chuckbrandt.com

"At this point I think I have to assume I have a winter fuel or fuel system heat problem that is causing fuel to boil in the bowl. If you read this article http://eps-hane.com/techtips5.html and follow this thread winter fuel formulation it's almost exactly what I'm experiencing. These guys are at higher elevations but it sure sounds like the same issue.

So after work today I went and bought 5 gallons of 100 octane race fuel to mix with the half a talk of (winter) ultra in the car. At $4.70 a gallon I won't want to use this all the time but it seemed like it would be worth a try. I adjusted the float level on the 750 Holley slightly low and put it back on. We drove it around the neighborhood without problems and had a little more success with a hot start test, no geyser. "

So I am going to try some VP Fuel 110 race gas and see it if fixes it.

Best,
BC

[jhv48]

If that's the problem, try a plastic one inch spacer under the carb (hope it fits under the hood) should help somewhat but won't completely cure the problem. Mine is there for the first few miles till fresh fuel is circulated and the bowls cool down. Just don't try and blip the throttle at first. Embarrassing!

Some have installed electric fans, others have installed electric fuel pumps with return lines to keep the fuel cool. Others live with it.
Good luck.

[patrickt]

Originally Posted by jhv48

If that's the problem, try a plastic one inch spacer under the carb...

... or wood. No, I'm not kidding.

[Jerry Clayton]

or curve the ignition and fix the carb