Carb guru's, need to bounce something off you....

[SSMAN]

Got a Holley 4150 HP series 750. (been a great carb till now)

Symptom:
-Surging at cruising speeds
-When you shut the car off, smoke rises from secondaries
- hard to start and has a good miss till it is warmed up

I don't think it is ignition related. My car usually will have a flutter on take off if it is ignition related.

I have replaced the gaskets, power valves, accelerator pumps and needle and seats. All holley parts for that particular carb.

Both floats are at or below the sight hole.

I have 12 psi vacuum at idle.

My thought? Feels like the 4 barrels are seeping gas while cruising and possibly dripping when the car is shut off.

IF my thought is correct, what the hell would cause that?

Any input would be appreciated, really don't want to buy another carb.

[Kidd]

Poor needle and seat in the rear bowl.

[SSMAN]

Poor needle and seat in the rear bowl.

Well, the old seat and needle surged. I just put a brand new on in last saturday an I am just a tad below the sight hole on fuel level. You can shake the car a little and it sloshes up in the hole. So the bowl levels are good or a tad low.Nothing that would affect the cruising speeds.

I will tell you that while sitting , the rear bowl is bleeding fuel into the engine. The fuel level is dropping with the motor off. Which causes a nasty miss till the fuel gets burned off on cold start ups.

[Blackpony]

I am having a similar problem with my 600 vs. When its cold and I'm at or below 1/4 ish throttle its cuts out like you killed any fuel flow. Float level is fine, air bleeds are set good. I'm planning on pulling it apart soon to rebuild it.

[SSMAN]

I am having a similar problem with my 600 vs. When its cold and I'm at or below 1/4 ish throttle its cuts out like you killed any fuel flow. Float level is fine, air bleeds are set good. I'm planning on pulling it apart soon to rebuild it.

What about at idle? Is is smooth? Go get you a lazer thermometer from Harbor frieght. Shine the lazer on each primary tube while holding at or as close to 1/4 throttle. . You can do this on jack stands in gear. Read the temps. You will be surprised at what you can learn by doing this. Also is there an electric choke? It could be releasing too quickly when cold.

Could also be bad ignition wires. The lazer thermometer will let you know this. One of more of the cylinders will be a lot colder than the rest with a bad plug or plug wire. Once they warm up, they conduct a lot better

I am just babbling now. Helped my son with his homework and it kicked my ass. LOL. Need more info on your problem.

Is this a holley?

[Blackpony]

Yeah Holley. It's nice and choppy at idle. Has a small cam. Only does it under load but quits after everything warms up. It acts like a lean miss but it cuts out. Just haven't had time to really mess with it.

[SSMAN]

Yeah Holley. It's nice and choppy at idle. Has a small cam. Only does it under load but quits after everything warms up. It acts like a lean miss but it cuts out. Just haven't had time to really mess with it.

During this time, does it seem like the car misses so much it is almost like a flutter from the engine when trying to gain speed? Or does it just die?

If you pull plugs during this time and read your ground strap, see where your burn line is. If you have one or two plugs soot black, then that will be a dead or misfiring cylinder. If you were closer, I would let you use the lazer thermometer. Great for detecting dead cylinders.

[Blackpony]

During this time, does it seem like the car misses so much it is almost like a flutter from the engine when trying to gain speed? Or does it just die?

If you pull plugs during this time and read your ground strap, see where your burn line is. If you have one or two plugs soot black, then that will be a dead or misfiring cylinder. If you were closer, I would let you use the lazer thermometer. Great for detecting dead cylinders.

Its a flutter but a hard cut out, enough that it kicks the truck like a hard shift kinda feel. i have a lazer thermometer. Ill check it out and see. It doesnt feel like ignition or mechanical related. Almost acts like if for example you were flipping a switch off and on constantly. but if while its doing this, i push the gas farther 1/2-3/4 throttle it quits and takes off.. really weird..

[SSMAN]

Its a flutter but a hard cut out, enough that it kicks the truck like a hard shift kinda feel. i have a lazer thermometer. Ill check it out and see. It doesnt feel like ignition or mechanical related. Almost acts like if for example you were flipping a switch off and on constantly. but if while its doing this, i push the gas farther 1/2-3/4 throttle it quits and takes off.. really weird..

Check the primary temps, and let us know if there are any major temp differences. Like 100-200 degrees difference.

[CatDaddy]

Are you running power valves in the primary and secondary? What size are they? You may be making less vacuum at cruising speed due to engine load and dropping below the power valve range, allowing it to open and add more fuel. If you have 6.5 power valve, try dropping to 4.5.

[JETFAST]

I would check the float level. after all you did just replace the needle and seat. If the float is to high, it will drip fuel out of the boosters.

[SSMAN]

I would check the float level. after all you did just replace the needle and seat. If the float is to high, it will drip fuel out of the boosters.

The front float level is right where it should be, the rear is probably a centimeter below where it should be.

Jet, I have been battling this for a couple weeks now. I keep wondering if it could be ignition related. But all the primaries are at a good temp. So I am not finding a week or dead cylinder. Been spraying around the carb to find vacuum leaks, nothing yet. Even sprayed the carb shafts in case they are worn. Nothing....

Can only be one of three things:
- carb
-ignition
-vacuum leak

Am I missing any thing? Messed up valve or maybe one has got the lash set a cunt hair too tight?? Really no stumble off the start tho.

Oh, and from a dead stop, it will roast the tires good. This whole time I am thinking it is leaking gas into the intake at cruising speeds, now I just don't know.

[SSMAN]

Are you running power valves in the primary and secondary? What size are they? You may be making less vacuum at cruising speed due to engine load and dropping below the power valve range, allowing it to open and add more fuel. If you have 6.5 power valve, try dropping to 4.5.

Yes, power valves front and back (6.5). 12 psi vacuum at idle

[SSMAN]

Its a flutter but a hard cut out, enough that it kicks the truck like a hard shift kinda feel. i have a lazer thermometer. Ill check it out and see. It doesnt feel like ignition or mechanical related. Almost acts like if for example you were flipping a switch off and on constantly. but if while its doing this, i push the gas farther 1/2-3/4 throttle it quits and takes off.. really weird..

A lot of what you are describing sounds like a dead cylinder. But it is hard to know without being there.

[CatDaddy]

Yes, power valves front and back (6.5). 12 psi vacuum at idle

The only way to find out what the vacuum is at the cruise rpm that you are experiencing the surging is strap a vacuum gauge to your car where you can see it while driving. I've been seen more than once driving around my neighborhood with a vacuum gauge strapped to my windshield wiper I had an issue similar to yours and ended up having to go to a 3.5 PV to clean it up during part throttle.