At what point.....

[seabeelt]

Had a blast driving the car this weekend, back roads mostly, but had to get on the highway after stopping for ice cream. When getting up to speed I normally wind it out to 6500 in second and third to get me up close to 70 to merge. When in third this time, I let it get a little past 6500, closer to 7000 and the car seemed to find a new gear so to speak and jumped up to almost 8000 in a blink.

Just wondering for all of you running dual IDFs 13/15s at what point that tube that hangs over the Venturi starts to siphon fuel from the bowl and add it to the mix. Scared the crap out of me because of the jump in rpm and surge forward (a little exciting too ).

All my time messing with tuning and blipping the throttle in the garage, I never had fuel spray from those tubes. I suspect not enough consistent vacuum to siphon any, and I never wanted to really push it without any load just sitting in the garage. So just wondering at what point they should kick in.

R/

[sptcoupe]

The tubes hanging over the bores are the pump jet nozzles, which squirt an extra shot of fuel into the mix when you tip the throttle in on acceleration to prevent the mixture from getting lean when you open the throttles and add more air. This happens because air moves through the carbs faster than gasoline, and it takes a moment for additional gas to get drawn from the fuel bowl and make it's way through the carb circuits before it gets added to the air mixture. The nozzles are attached to the accelerator pump circuit, which pumps that gas in immediately and directly into the venturis.

The surge in power you describe is related to your cam profiles. On the typical IDF set ups on out TCs, the idle circuit is in play from idle to about 4000 rpms. Somewhere around this point, as the throttle continuers to open, the vacuum becomes great enough to transition from the idle circuit to the main jet circuit - which uses only the main jet and air correctors to mix fuel in the emulsion tube. For 40/80 cams, this is around 4000 rpms. For 42/82s, it is around 4200 rpms. For the GC 3A cams I run, it is around 3800 rpms. People commonly refer to this point when the power comes on as the motor "coming on cam".

[RoyBatty]

Hey sptcoupe,
He is not talking about the accelerator pump discharge nozzles.
Rather, he is referring to the high speed enrichment circuit which discharges from the j shaped canes above the top of the carbs inlets.
These are plumbed right into the bottom of the float chamber. And at some point during wide open throttle, fuel is drawn out of these canes to further richer the mixture.

[seabeelt]

RoyBatty Is Correct, not the accelerator nozzles. I have seen some IDFs- the newer version that do not have these tubes. I don't remember seeing them on the EMPI carbs either.

[sptcoupe]

Yeah - just noticed he's running 13/15s.

[bradartigue]

http://www.artigue.com/fiatcontent/carbs/40_IDF.pdf

Read the description of operation for the "mixture enriching device" under "Normal Operation," devices 2, 7 & 8.

From what I know there is no set point of operation, these nozzles work like most things in the carburetor, a pressure drop draws fuel from the nozzle. The position of the nozzle here is important as it is quite high in the barrel - it will only function at high speeds when the pressure drop is significant that high in the barrel.

This is not a common device after about 1972. I've only seen it on the 13/15 carburetors. The later (and other models) don't seem to have this tube sticking up like that, though they do draw fuel at high speed from a similar device.

[RoyBatty]

I'm left wondering if your sudden surprise surge in power and rpm, is from a mechanical distributor advance curve breaking loose and giving you timing advance that wasn't there before?

[seabeelt]

I don't think so,. I'm running ing Mark's Computronics ignition. I'm thinking I finally developed enough consistent vacuum at a high enough air flow that the carb actually siphoned fuel from the bowl. I have never been able to duplicate it in the garage. I guess I'm trying to figure out if it actually takes that long to get a good siphon flow, or should it be at a lower rpm.

Are you running 13/15s or 70s?

[RoyBatty]

My spider has the unlabeled Chinese knock offs that I think are equivalent to the 70's.
The coupe has the 13 and 14.
Yeah I know it should be a 15...but the coupe does not seem to know the difference.

[seabeelt]