Dynamics of a 2bbl. (Carb)
 

This doesn't apply to just first gen RX-7's, but I'm assuming every vehicle equipped with a 2 barrel carb.

I noticed when I first added my Weber that if I mashed the pedal to WOT off the line, it would choke and spit and sputter, than catch it's breath and BRRRAAAAP!

First I figured it had something to do with the jet/circuit transition being off, than I read some where that it was just the dynamics of a 2 bbl carb.

Can someone explain why this happens?

I'm curious.

Thanks.

The big gapping hole that opens and the lack of fuel for a few seconds is normal on a 2bbl. You could try to adjust the ap circuit to rectify this. My 2cents anyway.
You have to rember when your throttle plates open they are alot bigger than the stock nikki primaries.

do you have a 12a, or 13b? what size venturis? you can adjust the acceleration pump to dump in a little more gas thats what it is for to stop what you are describing, however, if the venturies are to large for the engine's porting (or size) it will do it regardless. do you have a 48 IDA or IDF? if so tell me what your porting is (atock, street, bridge or PP) what your exhaust is like, and I can dig up my old data for webers to find out which venturi/fuel air jets you should have.

kenn

It's actually a 40 IDF. Stock port 12a, with a header to dual 2 inch exhaust all the way back to a muffler.

Not sure about the venturi size, I never had them out, and didn't pay much attention to them. The accelerator pump isn't adjustable its the older..or is it the newer...style.
But the accel pump works really well, and 2 really nice powerfull streams of fuel shoot down the carb throats.

can you put spacer gaskets in there and make the ap shot bigger?

It's not just 2 barrel carbs. A mechanical secondary 4 barrel will act very much the same way.

You need two things to make power with an engine. You need flow and velocity. You need flow because at the top end your engine will need to get as much air/fuel mix in as quickly as possible, since the period of time to fill the empty combustion chamber is very short. You need velocity though to *pull* the fuel and atomize it into the air as the air rushes through.

At lower rpms your engine doesn't have enough pull, or suction, to pull a large volume of air through at high velocity. So you open the butterflies (throttle) a little bit and what you get is high velocity (which pulls fuel and atomizes it) but appropriate flow. As your revs increase, so does the flow requirements of the engine and you can "Stomp" a bit more, though really you should be making a smooth transition from closed to WOT throughout the powerband.

When you just stomp open a 2 barrel or mechanical secondary 4 barrel carb, you open big gaping holes. Since the engine isn't sucking that much air in at low rpms, the velocity through those holes is too slow to adequately pull fuel. This is somewhat countered by the accelerator pump, which acts like a squirt gun to squirt a bit of gas into the throat of the carb when you do that. It helps a little but you'll still "bog" if you go too quick.

The reason the stock carb doesn't bog when you stupidly just STOMP it is because only the two primary barrels open. The two larger secondary barrels aren't controlled by your pedal at all on the stock carb. So you can slam it to the floor, and you'll never end up with 4 big gaping holes creating a very low velocity situation. The car will acceleate and as the suction of the engine (or "vacuum") increases, there's a diaphragm that pulls the secondaries open gradually.

So the stock nikki is built for people who just want to drive like 16yr olds and go stomping it. The mechanical secondary 4bbl carbs such as the Sterling and mech-sec Holleys put the control of the primaries and secondaries both on the pedal. Press the pedal halfway, secondaries are wide open. Push the whole way and all four are wide open. With 2barrel carbs there's no such thing as a "Vacuum secondary" so they can't make it "stupid-user friendly". So you just have to use finnesse.

In time, you will learn to drive the carburetor properly. With a little finnesse a properly tuned mech-sec 4 barrel, or even a 2 barrel, can be faster than a vacuum secondary carb because vacuum secondaries can't open as quickly or fully as is optimal for best top-speed climb.

yetterben, yes you can make the ap shot bigger, but that doesn't solve the problem. That just shoots more liquid fuel in, which still has to atomize somehow because liquid fuel doesn't explode, it just burns. The way that fuel gets atomized is the air rushing past.


In a fuel injected system you can stomp on it because the engine will suck as much air as it can, and the computer will use the "mass air flow" sensor to determine how much gas to shoot directly in.


Finnesse, and a bit of understanding of flow goes a long way. If you need a good example, pick up your garden hose and put your thumb over half the end. Notice how the same amount of water picks up a LOT of velocity when going through a smaller orifice. That's what you want when you're taking off. But of course you don't want to *always* have a small orifice, because then you couldn't flow enough when your car needed it at the top end.

Jon

Please forgive any errors in that last post. It's bloody late here and I just got back from the Sens game. I'm a little... off.

Jon

just a little.. :aroused:

Great information there Vipernicus, but I hope you arn't making personal attacks on me with such comments as, and I quote.

"The reason the stock carb doesn't bog when you stupidly just STOMP it is because only the two primary barrels open"

and

"So the stock nikki is built for people who just want to drive like 16yr olds and go stomping it."

I am 17 years old, and I don't drive around stomping it, it was just an observation, and after I noticed it a couple times I learned to work the throttle to get the most out of it.

I didn't drive my car a whole lot until after I put the weber on, so obviously I was gonna go out and "test" the performance of my vehicle. Sorry if I "stupidly" stomped on my pedal off the line to see what my car could do. It happened a couple times and I noticed the "phenomena" and I learned to avoid it. I was just curious as to why it happens.

It's not like my post was.

Hey, I'm trying to learn how to do burn outs in my RX-7 with a 2bbl and I can'y just mash the pedel and light up my tires, cause my engine bogs" Can some one help me tune my carb so I can roast my tires off the line?

Than I could see why you made the comments that you did.

He was just generalizing, I'm pretty sure that wasn't a direct attack on you personally.

so how do you fix the bogging issues.

My apologies. I didn't mean to make it sound offensive. I should learn not to post after a night of hockey and beer!

That's the line that made you sound like a ricer teenager to me, and ricer teenagers really annoy me because they're the ones who give us a bad name. Your car looks respectable though, and reading back over your posts, the fact that you actually know what an accelerator pump and transition circuit are suggests that you're willing to get in there and learn, so you've got my respect for that.

Sorry about that.

Jon

Drive it properly.

There's only so much you can do to help make the "problem" less pronounced. You can't change the science behind how a carburetor works, just the carb and the driver's driving style. Now of course if there is a problem with the accelerator pump, or the carb isn't properly sized for the engine it will make the bogging worse, so it's always good to look at those things, but when he says "the nature of 2barrels", it's a pretty accurate way of looking at it.

Jon

Yeah, thats what I figured, no hard feelings ol' buddy!

I share the same feelings for ricers.

I didn't take vipernicous as offense, just informative. You just need tuning to get yours working great. Listen to all the advice given and don't take any offense. The group will fill you head with lots of good advice and a select few will fill your head with b.s. Just pick through the info, try a few things and you'll get there.

It's been several decades since I've seen 17 and I still 'mash/stomp' the throttle, so what does that make me? :rlaugh:

Good info Jon, tired or not you conveyed the general idea quite well.

I have to disagree with this 100% and you dont make sense. Bogging can be cured but my evil side says make him search. Experience pain so you dont forget.