Any idea why my engine completely bogs out when I floor it?

If I'm going like 15MPH and just rolling and I mash the gas the gas bogs really bad for like 3 seconds then goes. Do I need more accelerator shot? Do I have to much?

I had the car at a dead stop at one point a while back, I mashed the gas and the car made not a noise, the engine just shut right off. Didn't even try to rev then stall, just shut off.

Response when I roll the gas pedal is perfect, if I stall the car to 1500RPM then floor it it's perfect. Is it because my car is hooking and it's bogging the engine down or is it a fuel problem?

 

If that is the 670 street avenger carb, then the stock spring shouldn't cause that much bog on a healthy motor. But assuming you have no vacuum leaks then ye, you need to start tuning the pump shot and secondary opening rate. If it is the avenger it should have come with springs. But like I said, the one that comes in it shouldn't be a problem. Usually you need to move to a softer spring to let the secondaries open sooner.

I would think you would want to add more pump shot before slowing the secondary opening though.

 

It's a 670 Street Avenger, I didn't touch anything, just set the floats/idle mixture/idle and.

It's a really bad stumble/bog/hesitation. Only when I go from 0% throttle to WOT. I can roll it quickly (like when I'm at a stop sign and taking a turn and want to get into traffic quickly) without problems.

I can't imagine it's the pump shot. My buddy is running a standard 600cfm vac sec holley and he doens't have any problems just mashing the gas at a dead stop and smokin his tires. My engine just shut right off...didn't make any sound, you would have though I just turned the key into the off position it was so dead quiet.

Today I was leaving work and one of my work buddies gave me the ol "spin'em up" signal, I was only doing 15-20MPH and my foot was off the gas, I mashed the gas and for like 3 seconds the car bogged really nasty then started reving normally. I've taken the car to 5500 and the motor screams...so the secondary must be opening.

 

Ye, that's the problem. The secondaries opening letting in a high volume of air and not enough gas is coming in at the same time. You tune that by either stiffining the spring in the vacuum diaphram to slow the opening of the secondaries or adding more fuel from the accelerator pump with cams for the rod on the accelerator pump. Or a combination of both. Your buddies 600 cfm carb doesn't have problems because he has fewer cfms of air coming through the barrels than you do. Normally a street avenger is not suppossed to need tuning for that. But I would bet your motor is capable of pulling more air due to the vortec heads and decent cam than the motor holley sets the carb up for. So you will need to compensate for it by making your accelerator pump shoot more gas in or reducing the opening of your secondaries with stiffer vacuum springs. Performance wise you would probably want to try adding more initial fuel shot.

As an example my brother's 383 ci, 282HR cammed, 750 cfm holley, 200 cc headed motor bogs slightly with only the yellow spring and the out of the box accelerator pump arm cam. And the yellow spring is just slighly lighter than the stock one. So it is not suprising that could be happening to you. But I would rather add more gas by adjusting the shot from the accelerator pump than slowing the opening of the secondaries.

 

I've got a 1" carb spacer as well if that matters.

Pump shot is pretty easy to replace isn't it? Just curious, why would you suggest replacing the pump shot before making the 2ndaries opening slower?

I appreciate all the help.

 

Slowing down the secondary opening would be easier since replacing the springs is easier but if your motor is wanting that much air/fuel then slowing the opening of the secondaries to stop a bog may be de-tuning it. So I say add more pump shot first just in case that is what is happening. A dyno tune would be the way to know for sure.

Anyway here is a website about pump shot:

http://www.holley.com/HiOctn/TechSer...fo/TI-221.html

and here is a thread discussing it a little on the second to last post:

http://www.holley.com/HiOctn/TechSer...fo/TI-221.html

and ye, the spacer probably will make it need more since a spacer is basically just making the plenum effect larger by adding more volume. Problem is you adding more air volume but not fuel volume initially.

 

I have the orange pump shot cam, it only has 2 settings, 1 & 2. It was already set @ 1. Does changing the spring tension affect the pump shot?

I don't see how changing which number the screw is in affect anything. Both holes in the cam line up with the 2 holes in the carb bracket. Right now the screw is in the #1 hole going through the #1 hole in the pump shot cam closest to the front of the carb.

 

The number 1 position is the one that delivers fuel the quickest so you are maxed out on that cam. Keep in mind the nozzle itself may be to small also and not allowing enough fuel to be delivered. Here is some good info from the holley site.

"Holley accelerator pump nozzles are stamped with a number which indicates the drilled pump hole size. For example, a pump nozzle stamped "35" is drilled .035". Pump nozzle sizes are available from .025" to .052". NOTE: When changing the pump nozzle it's best to jump three sizes. For example if there's currently a off-line hesitation with #28 (.028")
pump nozzle, try a #31(.031") pump nozzle."

With the bog you are describing you might want to make sure it's working at all. with the motor off and air cleaner off look down into the carb and give it full throttle. You should see, smell, and hear a nice shot of fuel. If you don't then something has gone wrong with the pump itself.

And keep in mind the spring I have been talking about is not the one on the accelerator pump. It's the one for the secondaries. That spring is is inside the vacuum diaphram on the rear passenger side of the carb. That's the one that controls the secondary opening. But I wouldn't worry about that one until you have tried to get the pump shot right.

 

I have a pump shot, I've seen it on many occasions. I am getting a shot...but maybe not enough? I can't imagine my motor needing a 50cc shot...

Just seems odd....


EDIT: The reason I mentioned the spring that is part of the pump shot linkage is that someone I know told me that when you change the cam position that you have to adjust the spring tension as well. Does that spring have any effect on the pump shot?