bluegrassz

iTrader: (3)

I just checked on my 3310s 750vs carb. It has an orange accelerator pump cam. I have read were the blue cam might be a better choice for the street.

I do run at the track some. The carb seems to sputter between 2500-3500 rpm when Im in the throttle. If I continue to stay in the throttle, it clears up. It also does this during decel. If I do a WOT pass, I doesnt seem to do it. Only with moderate acceleration.

Would you guys suggest a different pump cam? or should I just buy the kit and see what works best?

Oh yeah, A break down on the carb:

3310s 750vs with rear metering block, notch floats & jet extensions. Quick fuel adjustable secondarys, 8.5 pv, 35 shooter, 72 primary & 74 secondary jets. Removed choke flap.

I get 17" of vaccum at idle and the plugs look like new after a WOT pass. No tan/red or black.

thanks guys for any advice.

bluegrassz

iTrader: (3)

anyone have any advice.

RB83L69

In my book, moderate accel = put the gas down just a little way and hold it still

Is that what you mean?

If so, the pump cam won't have anything to do with it, because the gas pedal is not moving.

Does the same thing on decel = your foot is off the gas, and above all, certainly not in the act of mashing down on it

Again, the pump cam is not active under those conditions, and so cannot have anything to do with the behavior.

The pump is only active during the time you are moving the gas pedal downward, and if you move it real far real fast, for the length of time (fraction of a second) aterwards that it takes for the fuel in the pump to finish squirting out the squirters. Therefore, the ONLY problems you can affect with a pump cam, are those that occur within a fraction of a second of opening the gas pedal. Not moderate sustained accel; not decel; not sustained WOT. The act of opening the throttles only.

I see alot of stuff on these boards about changing the pump cam. Now I've only been tuning carbs for about ..... well, since right after they invented them; but in all my days, I think I've changed pump cams ONCE that I can remember. And that was because it was a used carb and some goob had stripped the screw hole out that I wanted to use (the minimum one). In my experience the more accurately the carb is set up in the first place, the less need there is for a pump; the purpose of the pump, by definition, is to cover up inadequate fuel delivery from the main system at the instant the throttles are opened. Calibrate the carb right to begin with, and the pump will have very little work to do in order to accomplish its mission.


So with all that in mind, apply some logic. You say you have 17" of vacuum, and a 8.5" PV.... that means that in order for you to open the PV, you have to give the car enough throttle to drop the vacuum all the way to 8.5". There's a clue in there somewhere. Moderate accel... problem. More accel.... works OK. Sounds like the PV opening point is too low. Because at any vacuum between 17" and 8.5", the carb is progressively too lean. It's at its worst at around 9-10".

Measure your vacuum at idle in gear. Change to a PV about 2" lower than that vacuum.

bluegrassz

iTrader: (3)

Ok, I understand what you are saying. When I am saying moderate accel, I mean accerating, not just holding it at one position. Then on the decel, my foot is still on the pedal, but I am letting up on the pressure.

I was going to go with a 10.5 powervalve, but I thought I was suppose to split the vaccum reading. The 17" reading is in gear at idle. When I went from park to drive, it didnt change any. Still stock L98 cam.

So I am probably leaning out with the 8.5 power valve? What about the plugs being clean and white? Is my jetting to close from front to back? Like I said, it does fine (or masks the problem) during a WOT pass. Oh yeah, the timing is set at 10 or 12* base.

thanks