Weber Carbs 40 IDA Stumble

slowhand · 10-09-2006, 07:02 PM

I have a 1970 911 E with Weber Carbs. I am finding a stumble and loss of power. I have cleaned and checked the Carbs and get them to run better, then at the track I seem to lose power and cant get up to 7000 RPM.
I have a Crank Fired Ignition and that seems to be working fine, but I am stumped.
Anyone out there have a solution.

cstreit · cstreit's Garage

At what RPM range does the stumble begin?

Do you have the ability to get the air/fuel ratio checked?

Have you changed anything recently?

slowhand · 10-10-2006, 05:47 AM

Stumble starts between 4000 to 5000 RPM..This week I am going to put a gauge on the 02 sensor and see what is up at the higher RPMs.
A good Porsche friend said the same thing. Thanks.

cstreit · cstreit's Garage

Hmmm...

Is it in the turns only, or all the time?

slowhand · 10-10-2006, 09:33 AM

All the time.

Steve@Rennsport · 10-10-2006, 10:39 AM

Hi:

A few questions:

1) What is your timing curves?

2) What fuel pump do you have? Filter?

3) Are your fuel pressures (at the carbs) at 3.5 psi?

4) Are the float levels absolutely perfect?

5) What are all your jets in those carbs?

6) What do you have for a muffler? (stock?)

7) What plugs/wires do you have?

Once I have some additional information, I can offer some suggestions,...

slowhand · 10-10-2006, 10:41 AM

Thanks for your interest. I will get that all together and send it to you.

kucharskimb · 10-12-2006, 08:37 AM

the attached thread has my dyno pull on the second page.
2.4L dyno and carb jetting question
I have also run an LM-1 across the RPM range at WOT and have experienced the same curves on my AFR. You will notice that in the 5-5500 range, there is a lean area.

Henry Bieker is looking at this info as we speak. I'll get back with you if we learn anything.

my engine specs

2.2E stroked to 2.4E
aftermarket ignition
BP6ES open to .045"
60 Idle
135 Main
34mm venturies
180 air correctors
F26 emulsions
4.5 secondaries
3.5 psi fuel
half gutted stock muffler
ssi's
jacobs wires
timing 6btdc at idle, 36btdc at 6000

viejopatron · viejopatron's Garage

My T (running same carbs) also has a flat spot, but at approx 3500 RPM. I rebuilt the carb 2 years ago. Acclerator pump shots appear OK, however they've not been measured.

It'll be interesting to see what the boyz come up with.

Standing by.

T77911S · 10-27-2006, 07:04 AM

tell me if i am out there in left field, what about compression ratio, too high for single plug ign. also will the E cams pull that high rpm.
when u said u clean them and it runs better, is that at higher rpm too.
maybe the fuel pump supply is low and cant keep up at the track. is the fuel tank clean, debris blocking the screen, it may be just enough to starve the engine.
$.02 worth.

slowhand · 10-27-2006, 07:38 AM

Thanks for all the info. Each one of you has helped. I stablised the fuel pressure. and have ordered a different fuel pump. My car was always able to go to 7000 rpm before the carbs bogged down. The car was in storage and the fuel went bad. I have cleaned out everything that I could think of and removed the fuel from the tank as well.
It feels like it is starving or going rich. Starts and Idles well. Revs up nicely, but on the track cant go faster than about 95 to 100. Has power then goes flat. I am going to put on a fuel meter then go test again.
Thanks

slowhand · 10-27-2006, 07:40 AM

Also I have been seeing bubbles in the G2 clear filter. I am used to seeing a bubble in the fill portion of the filter, but not tiny bubbles.

cstreit · cstreit's Garage

Quote:

Originally posted by viejopatron
My T (running same carbs) also has a flat spot, but at approx 3500 RPM. I rebuilt the carb 2 years ago. Acclerator pump shots appear OK, however they've not been measured.

It'll be interesting to see what the boyz come up with.

Standing by.

This can also be cams/exhaust. I had a nasty flat-spot at 4000 in my racecar that was solved by a freer flowing exhaust.

BReyes · BReyes's Garage

Rebuild both carbs. There are little brass pieces inside the main body and I can easily imagine yours are clogged.

An unrelated question as it regards to above exhaust comment. For a little motor the heat exchangers are what diameter?, Are they not good enough when choosing exhaust for carbed motor?

T77911S · 10-27-2006, 12:00 PM

when i had my 914-6 i had webers. i would take them off and soak them in one of those 5 gallon cans of carb cleaner for a day or 2. if u put something in there with paint on it, it wont be on there when u take it out...strong stuff. how are u cleaning?