ok, so Britt's crazy thread about the mysterious "two-throat webers" got me going on my upcoming winter project: Addition of Dual Carbs.

First to all you FI guys....I know, I know

Now to my question: My 1.8L engine (actually 1.7 heads w/ 1.8 p/c) already has a "carb friendly cam" and a progressive weber. Once again..... I know, I know . Unfortunately I have no specs on the cam other than it was installed to worked with the progressive.

Now I want to install a set of dual carbs. It sounds like most recommendations are of the 40mm variety. My question is about vent sizing. An article on Aircooled.net states that Vent size should be 3-5mm smaller than the Intake valve. According to the manual, the intakes on 1.7L are 39mm, so looks like I need 34-36mm venturi's. Yes / No? Seems too simple.

It also troubles me because I have read several 2.0L guys using 32mm vents (with 42mm intakes)

thoughts anyone? Standby for future questions about jet sizes

sorry for the long post

I would think you can use about the same size as mine,

this is what jake recommended for my 2056:

32mm vents
125 mains
F11 emulsion(stock, 40s)
60 idles
200 airs (stock for 40s)

I can see smaller idles being used though... hell, stock 40s probably have *very* close to the right setup for a 1.8... 28,115,F11,50,200

A GREAT article about jetting

what do you want to do?

smaller venturiis improve port velocity and give you really good throttle response because of the good metering signal. but they tend to starve the engine at high rpm/power levels.

large venturiis give you more top end power. on the track you spend a lot of time at WOT and midrange modulation isn't -as- important.

for a street/AX car, pick smaller rather than larger - it will be much more driveable and deliver better economy.

you, at least, only need to buy 4 at a time if you guess wrong.

and if you buy from aircooled.net:

QUOTE

Choose Application and Size: If you buy these venturis from us, you can EXCHANGE them (no charge) anytime down the road if you change your engine, provided they are in re-sellable shape (you didn't mangle them up during installation or removal).

The majority of 40 IDF's you find have 28mm vents. At least the ones I have seen.
I have 32mm but my 2.0L is a bit of a heavy breather which I want to kick ass from 3500 RPM up to 7000. Hell I launch my car through the start timer at 3000 RPM.

28mm vents as stated above will give you great performance off idle to 4000 RPM (90 MPH in 5th) and still allow you to pull to 5800 red line in lower gears. If you can pull 5800 RPM with a 1.7 in 5th gear Well you da man. cause you are Haulin the booty pretty quickly.

QUOTE (ArtechnikA @ Aug 25 2005, 08:25 AM)

what do you want to do?

hmmm, my bad Rich. Told you lots about the car and nothing about the application.

Definitely Street/AX. About the only time I get above 4K is if I accidentally pullout if front of a soccer Mom in her Texas SUV

I agee with the 28 recommendation.
I had 32's in my old car that was a race only motor with huge valves...

I ran 34 or 36's in that car (can't remember which) and dropped down to 32's.

I wouldn't go that big on a street car.
we had a 6500 working redline in that car.

brant

Way back when my Dad and I were running a 4-cyl with Webbers in our track car.....

2.0 (Euro)
Webcam Cam (I don't know the grind any more but it was what was recommended in the 80's)
4-1 race header (with stub tubes) {I should probably try selling that}
050 and 009 dist. (depending on which one was less wobbly)
40mm IDF carbs
CB Performance Linkage (castings with the hex cross shaft)
6" tall velocity stacks inside tall K&N air cleaners

The car was only used on the track for DE events.

I found that the car would be out pulled by a "stock" fuel injected 2.0 (with Euro P/C).

We had the largest chokes (venturies) in the 40mm carbs (I think that was 32mm), but according to a chart that I found, it needed larger. The winter before the last season that we ran the car with the 4-cyl, we sent our 28mm chokes out and had them custom bored, tapered and smoothed out to about 34mm. (The machinist couldn't re-create the parabolic shape exactly, but made sure all four matched each other, and I think he did a great job.)

This combination worked the best for our car and it would then out pull the stock 2.0 (with Euro P/C) I believe that we could have probably gone a little larger, but we converted the car to a 6-cyl the following winter.


My suggestion would be to go with a little larger carburettor and choke it down with the correct size choke instead of screwing yourself by getting a carb that is too small.


I would also look very closely at the aftermarket FI that is available as opposed to using carbs.

just my $0.02

If you mostly keep the engine under 4K, then 40s with 28 vents are plenty. The power loss at the "top end" for vents that are too small pretty much doesn't even show up until 4K. The aircooled.net article is either wrong, or you're misremembering it. Main jet sizes are related to what vents you use. I'll have to dig up the rule of thumb, but 115s are probably fine, 120s at most. On the street, you spend far more time on the idle jets, so you need to pay more attention to those, really. 50s or 55s, mostly likely.

If you get a set of used carbs, buy a set of rebuild kits first and spend time disassembling and cleaning the carbs very throughly before you try to use them.

All that said, I'd repeat the advice about trying aftermarket EFI instead. A used 1.7 D-Jet system can provide the plumbing and injectors, and you supply an ECU.

If you Autocross and only rev to 4K then ????? how well do you do??? Must be very short tight courses.

My gearing allows me to stay above 4K rpms through 95% of the course. I set my shift light to flash at 5500 or so. sometimes I'll just hold it lit against the rev limiter instead of shifting twice. depends on the feature I am shooting through.

I just got to get a in car video of my runs with sound. and a hot chick in the passenger seat squealing.

Well, I am probably understating it some. My point was that mainly this is a street car (cruising and stuff). I do push it over 4k some in the low gears, I just don't get that many opportunities around town. Put it this way, my car has been back on the road now for just over 1 year. Was pulled over twice (1 ticket) in the first 4 months. That kinda sucked the life out of "driving it like a Porsche".

I will actually attend my first ever a/x in a couple weeks at MUSR-7. I am sure I will have a different perspective after that

Thanks for all the tips everyone.