Thread: Carb question

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Hi all,

I have a carb question. I need help. I have 70 Mustang 351c. It has a Holley 650 4bbl.

I let the car sit from 1998 -2003, and then got it up and running and have been driving it ever since.

It seems that when I start the car, the idle really hunts and surges. It will flucuate from 400rpm to 800rpm. Sometimes it will cut out, then it becomes hard to start. I let it warm up, and it will do the same thing at stop lights until I have driven it for about 10-15 miles. Then it idles ok. Not so much hunting. It will vary between say 600 rpm and 850rpm.

A couple of times the car has backfired through the carb. The good thing is, once it backfires through the carb it runs spectacular. The idle is steady at 900rpm, no stammering off the line. Smooth acceleration.

Here is my question: When the carb backfires what does that mean? I realize the fuel isn't being burned or mixed, but which is it?
Is the carb on ts last legs?
Could my fuel line be clogged?

Is there a danger of fire?
Is it something else?

The motor and carb are about 8 years old total. I had the motor tuned about 6 months ago - that is when this problem started. It ran fine for awhile until I let the car sit for about 2-3 weeks between drives.

Help!

Brustang

Take it back to where it was tuned & tell them that it backfires & surges since they tuned it.
It's a good chance that you got some bad gas also.

If the car sat for that many years, one possibility is that your carb is varnished up. When gas sits in the carb it turns to gummy varnish, and plugs up jets, etc.

I work at a Marina, and 1/2 of our business comes from people who let their boat sit for a while, and when they go to use it, the boat will usually idle fine, but when you try to give it some gas, it bogs, spits, etc. This can even happen in a several month layup.

If you rule out everything else, have the carb soaked and rebuilt. Spray carb cleaners don't even touch the varnish I am describing, you have to soak it overnight, or use an ultrasonic cleaner.

Holleys have lots of little passages that clog up with crud when they sit any length of time. You'll need a carb kit and some cleaning solvent to fix it. Next time use some fuel stabiliser, or just run it out of gas when you store it.

Thanks for the info. Someone told me about that regarding Holleys. I wonder if it would be cheaper to just buy a new carb rather rebuild?

Originally posted by brustang
Hi all,

I have a carb question. I need help. I have 70 Mustang 351c. It has a Holley 650 4bbl.

I let the car sit from 1998 -2003, and then got it up and running and have been driving it ever since.

It seems that when I start the car, the idle really hunts and surges. It will flucuate from 400rpm to 800rpm. Sometimes it will cut out, then it becomes hard to start. I let it warm up, and it will do the same thing at stop lights until I have driven it for about 10-15 miles. Then it idles ok. Not so much hunting. It will vary between say 600 rpm and 850rpm.

A couple of times the car has backfired through the carb. The good thing is, once it backfires through the carb it runs spectacular. The idle is steady at 900rpm, no stammering off the line. Smooth acceleration.

Here is my question: When the carb backfires what does that mean? I realize the fuel isn't being burned or mixed, but which is it?
Is the carb on ts last legs?
Could my fuel line be clogged?

Is there a danger of fire?
Is it something else?

The motor and carb are about 8 years old total. I had the motor tuned about 6 months ago - that is when this problem started. It ran fine for awhile until I let the car sit for about 2-3 weeks between drives.

Help!

Brustang

If it did back fire it is possible you could have blown your power valve.

Since the car is backfiring, that leads me to believe that the timing is off. If one of the valves is open durring compression it will back fire and shoot out of the carb. Otherwise it does sound like a carb problem.

On Holleys, it seems once the backfire, the power valves are gone. Time for a rebuild, which by the way is a bunch cheaper then a new carb.

Originally posted by Dave Severson
On Holleys, it seems once the backfire, the power valves are gone. Time for a rebuild, which by the way is a bunch cheaper then a new carb.

Agreed. Jegs sells a power valve protector kit.

http://www.jegs.com/webapp/wcs/store...tegoryId=19879

Great information! I will check the timing, but I am pretty sure its set at the right spot. I will also look at the Jegs site.

Thank you to all!