Backfire through Carb., car hesitates. HELP! - Ford Forums

Shelby289/351 · 09-02-2008, 13:50

Hi all,

I have a Shelby Cobra replica with a 351W motor. Before I put the car away for the winter last year, it started to hesitate upon acceleration and backfire every so often through the carb. The carb is a Holley 750. Car has 15k miles on it and has been perfect until this! I garaged the car for the winter and took it out this year and it's doing the same thing. HELP! I've had it parked all season. :( I have replaced all of the fuel filters and this did not help. The fuel pump is a Holley racing pump so I heard these don't die at all! What else can it be?

When I try to rev the motor, it hesitates as if there is a delay. When the car is off and I push the gas, I get a steady stream of fuel in the carb so I don't think it's accelerator pump.

PLEASE HELP. I WANT MY CAR BACK!!!

Thanks.

Shelby289/351 · 09-02-2008, 16:46

Someone PLEASE help!

**jwko** · 09-02-2008, 22:11

Might be time for ignition tune up.

Shelby289/351 · 09-03-2008, 06:57

Thanks for the reply jwko. I was thinking that. I have the MSD setup. I pulled a few plugs and they seem fine...Not to white so I don't think it's running lean. You still say go with a tune-up? You don't think it's carburetor related?

Thanks again for the help.

**jwko** · 09-03-2008, 07:12

If the plugs & wires look good then I would have a serious look at the carb.

Your not running solid core wires with the MSD are you?

Shelby289/351 · 09-03-2008, 11:32

No solid core wires.

I may throw some new plugs & wires in it and see if the problem persists. Change cap & rotor too?

Is there any way to test to see if it's the carb or spark? Don't really want to throw parts at it.

**jwko** · 09-03-2008, 14:32

I guess you could try replacing the plugs & wires and see if that helps. If the cap & rotor looks bad replace them also. About the only thing thats left is the coil & the carb.

Shelby289/351 · 09-03-2008, 15:17

haha...yeah i am thinking the same thing. Could it be a broken valve maybe? I was thinking because it happens under load on acceleration, that it could be an intake valve. What do you think?

I'm still thinking plugs and wires may be best bet but isn't 15k miles kinda early to have to do tune-up?

Thanks again for your help.

**jwko** · 09-03-2008, 16:12

I don't think it's a intake valve problem

This is from the Holley website Holley Performance Technical Service Department

Quote:

QUESTION My vehicle has a stumble from a dead stop. What can I do to cure this?
ANSWER The most common cause of a stumble is not having an adequate accelerator pump shot. The first thing to do is to look at the discharge nozzle and make sure you are getting a good strong pump shot. If not then you need to inspect the pump diaphram for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever. If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size.

Shelby289/351 · 09-04-2008, 07:20

Thanks a bunch! I think i'm going to try a tune-up and see where that gets me. I really appreciate your help. I'm new to this forum so I'm not sure how it works. I see that you are the moderator. Can other people post on a thread as well or can just the moderators? I posted the same question on another section of this site and have about 100 views but no reponses. Thanks again for the help.

I think i'm going to call Holley as well and hear their 2 cents. It sounds like it's not too big of a problem though right? No timing issue or cam or anything??