Help on Weber Carbs please...
 

I've got a single DCOE 42 sidedraft , which is like a 40 DCOE except the venturi's are held in by spring clip instead of bolts. Mine has a 27mm venturis. I got it with an engine I bought and it was never really sorted out. It's mounted on my 1500 spitfire which is stock except for headers.

First Question, I just bought some different jets and it is running pretty good now, but I notice the main jet kicks in at very low rpm's, around 1600 or so. There's almost no time on the progression circuit before the mains kick in. I attribute this to the 27mm venturi. These are not readily available to purchase but I can drill mine out to the size i need. Should I do that or leave it alone cuz it's working. I'm thinking 33mm would get the mains out to 2500 or so

Secondly, one site (i think it was CB performance) says you should drill a 2mm hole in the throttle plate when using as a single carb. I presume this is for VW applications but what is the purpose of this and would that apply to my setup also?

Advice would be appreciated. Thanks

Well venturi's do affect both the metering signal and (to some degree) the progression of the carb.

How did you determine the car is "on the mains" at 1600 RPM? Wideband O2? ...or seat of hte pants?

I aimed a flashlight down the throat of the carb. You can see the gas coming from the center of the aux venturi when i rev the engine. And it's just off idle.

I suspect what you are seeing is just the accelerator pump pushing gas through. On the Weber IDA's any time you move the throttle linkage gas shoots from the aux venturi as the accelerator pump diaphram is pushed.

Try doing the same thing with the car off. I think you'll see that it's not the airflow/mains. The only way to really diagnose this is with a wideband O2 sensor.

Let me know.

well, i'm pretty sure it's the mains because of the location. the acc. pump jet is at the top of the carb. plus it's not a squirt but a flow if i keep the engine speed up.

reduce the float level a few MMs and retest

Venturi size does affect the metering circuit progression, larger venturi's will probably help, but then you will probably also need to increase the size of your main jets. Here is a useful read:

http://www.teglerizer.com/dcoe/inglese/inglesetuning.htm

I've read the Tegler stuff a dozen times. It's good information for 1 throat per cyl but is vague with regards to a single carb setup. All the info I read pointed to a 27mm choke on a 1 throat/cyl setup and bigger chokes on a 45DCOE for a single.

I recently rejetted putting in a leaner idle jet and a richer main jet. That improved things a lot. It actually runs pretty good, no bogs, good throttle response etc.

I think I'll take an extended test run today looking for problems. If it works don't fix it

The venturis are the same as IDF's and IDA's and Porsche IDA's. There are a few companies that will enlarge them, aircooled(dot)net does them for $15 each IIRC.