Tuning webers

[kent olsen]

Once again I'm going to try to tune out the weber stumble between idles and mains.

3.0L twin plug, 9.5:1 pistons, Dougherty GT2-102 cams, 1.5" SSI's.

40mm webers with 60 idles, 175 air, 160 mains, 36mm chokes and F3 emulsion tubes.

This engine was tuned on J&B racings engine dyno and runs perfect from 3800rpm to 7200rpm, power, torque and Air Fuel Ratio.

I have tried to tune out the stumble transistion from idle to mains. I end up with 60 idles but there still is that stumble. I tried 55 idles and there is a hole I can put my hand thru, no good. 65's are about the same. I thought I should try to open up some of these to try something inbetween.

Then I talked to Mark Jung the resident Oregon MFI expert and he suggested changing the emulsion tubes. Hmmm, does anyone know if that would do anything about the transistion stumble or just move it around?

Kent Olsen

[Steve@Rennsport]

You might try F26 E-tubes since those cams are mild.

[kent olsen]

Thanks Steve

I just so happen to have some of those. What will they change?

[Steve@Rennsport]

Originally Posted by kent olsen

Thanks Steve

I just so happen to have some of those. What will they change?

They alter the transition between idle & main jets.

[kent olsen]

Well that didn't help. I put the 60 idle jets back in and replaced the F3 emulsions with F2.6. All it did was slightly lower the transistion RPM and it seemed to lengthen the transistion to the main jets.

By the way these webers where rebult by Harry Becker a number of years ago so they have modified throttle shafts so no air is leaking there.

I have tried 55-60-65 idles with the 60's seem to be the least stumble. I've been reading Paul Abbott's technical data and he does say that since my Weber's are 40 IDT models that I might have to settle for some stumble in the transistion

[Steve@Rennsport]

Originally Posted by kent olsen

Well that didn't help. I put the 60 idle jets back in and replaced the F3 emulsions with F2.6. All it did was slightly lower the transistion RPM and it seemed to lengthen the transistion to the main jets.

By the way these webers where rebult by Harry Becker a number of years ago so they have modified throttle shafts so no air is leaking there.

I have tried 55-60-65 idles with the 60's seem to be the least stumble. I've been reading Paul Abbott's technical data and he does say that since my Weber's are 40 IDT models that I might have to settle for some stumble in the transistion

Ah HA,...........having ALL the pertinent & detailed information makes very big differences in getting the "big picture",...

That little fact affects everything here.

IDT's are emissions carburetors (as you already know) and tend to run considerably leaner in the idle & transition than the IDA's, all things being equal. You will need to "cover" that lean spot by running richer-than-optimal E-tubes and idle jets.

The fix would be to seal up your current transition ports and drill the IDA pattern ones, not a DIY job. Paul would be THE man to do this.

(sorry I missed you today,....I was swamped until late)

[kent olsen]

I wondered if you were taking a long weekend?

I saw something about that on Paul's website. I'll call and talk to him next week while I'm gone to work.

[Steve@Rennsport]

Originally Posted by kent olsen

I wondered if you were taking a long weekend?

Not me!!

I saw something about that on Paul's website. I'll call and talk to him next week while I'm gone to work.

Good plan,...he's very helpful.