there is no telling what the jetting combo is until you pull one of the idles and the main stack. simple with the carbs off the car. remove the engine grill when you have them on the car and start sorting them out.

kevin

Is there a chart somewhere that recommends jetting based on engine size/cam size or is jetting based on best guess and experience.

I am interested in jetting Dual Weber 40 IDFs on a stock 1972 1.7L. Good street performance is my goal. Not a racer (it is a stock 1.7 after all).

Thanks!

Yes, but... You want them to be warm enough that the fuel going into the throat atomizes well, but not so hot that the fuel in the float bowls boils. (Say that ten times fast!! ;) )

With a single center-mounted carb, you get almost no heat at all. The gas that does evaporate after getting sucked in will lower the temp of the carb further, and then you have those long manifolds for the fuel to puddle in. To get to the "good" range, it needs more heat. It is very unlikely that a single carb will get too hot.

With duals, you have a short path from the hot head to the carb, and the path is straight up--the direction that heat flows best. So you can sometimes get the carbs too hot, out of the "good" range. It is very unlikely for a dual carb to get too cool after the engine has run for a few minutes.

--DD

Mike,

What size venturis are you planning to use? 28 or 32?

For 1600-1800 cc with 40 IDFs with 28 vents the CB Weber Manual recommends: 120 mains, 210 air corr, F11 emulsion, 55 idle.

Mike

Mike (maf) thanks for the data.

For my RPM range I think 28 venturis will be better. I imagine a stock 1.7L develops peak power at a fairly low RPM?

That being the case I think your listed jet sizes will be right.

Peak power just under 5,000 RPM; peak torque at 3,500 RPM in stock form.

--DD

Here's some info that might help you http://www.racetep.com/webjettune.html .

DD,

Thanks for the documentation. I found a chart that lead me to believe that 28 were best for peak power at 5000 RPM

Chart:

http://www.prosalesreps.com/crap/914/venturi.gif

Alfred,

Thanks for the link to the data - very helpful. Although 1.7 wasn't listed based on the chart at your link I still think 28 choke tubes are correct.

These engines were listed:
Porsche 914 2.0 120-130 HP with Dual 40 IDF use 32mm choke tube
Porsche 914 1.8 110-115 HP with Dual 40 IDF use 30mm choke tube

So working the chart I still think 28's are best.

There is a formula that says choke tubes multipled by 1.25 should equal barrell size (28 x 1.25 = 35). But I have seen a lot of 1.7L with dual 40 IDFs......