Weber Off-Idle Problems

[Jr.]

Hi, I have been lurking around here for a while to gain insight into the tuning of Weber carbs. Sadly, I am not an owner of a GT40 (I wish I could) but have seen some very intelligent and helpful folks posting here.

I have been working in the automotive field for most of my life and have tuned many multiple carb setups but nothing like the Webers. Recently I have been working on a streetrod with a 5.0 liter stroked to 347. We installed four Weber IDF 44 carbs and began the saga of getting them set. A lot of time was spent getting the linkage properly set and synchronizing the carbs. We are now down to jetting and overall driveability. At the present we have idle jets @ 55, main jets @ 135, air jets @ 170. No other changes have been made.

The engine idles great and under full load runs perfect. Problem comes in when under light acceleration we encounter a lean condition around 2200-2500 rpm. It is not a major stumbling of the engine but firing is uneven, not smooth. I tried using 60 and 65 idle jets to cure it but the 60's made no difference and the 65's seemed a bit too rich.

I am thinking I should change the emulsion tubes. Does this sound like a possible solution? Is there anything else I should do?

 

[Mike Christian]

Accelerator pump squirt adjustment? In my experience, the idle jets are good to about 2500rpm at steady state throttle. Any movement of the pedal then the acc pump is at play. Increase the pump squirt equally if you are going lean then. My guess anyway.

Mike

 

[Jr.]

Thanks for your reply Mike, I really appreciate any help on this.
I should have mentioned that the stumble is there whether I accelerate from a dead stop. The engine will take off just fine and just before it gets to the 1-2 shift point at about 20%-25% throttle is when I notice it. If I accelerate hard it doesn't seem to do it but maybe that is because the engine gets through that particular rpm range faster.

 

[Andrew Robertson]

The engine idles great and under full load runs perfect. Problem comes in when under light acceleration we encounter a lean condition around 2200-2500 rpm. It is not a major stumbling of the engine but firing is uneven, not smooth. I tried using 60 and 65 idle jets to cure it but the 60's made no difference and the 65's seemed a bit too rich.

I am thinking I should change the emulsion tubes. Does this sound like a possible solution? Is there anything else I should do?

Sounds like the main circuit is not coming in early enough. Had the same problem. My fix involved a combination of richer than ideal idle jets, richer E-tubes, and (this made the biggest difference) my float levels were set-too low, raising them and getting them all even made a huge difference to where in the range the main circuit started working. You may be able to fatten the top end of the idle jet range by reducing the size of the air correction in that jet too.

Good luck! Cheers, Andrew

 

[Cliff Beer]

Jr., here's one thing to check as a preliminary matter - are all your IDFs matched? Specifically, do they all four have the exact same progression holes drilled in the bores? If they do they you're right to look at jetting and such. If they're not the same then that's likely your problem.

The stage you're describing (25% power/throttle opening) is exactly the stage of the progression from idle to WOT that the configuration (size, spacing, number) of the progression holes makes a huge difference.

Good luck!

 

[Jr.]

Thanks for the ideas guys.
First off. I did do a check of the float levels and reset each one. They are set per factory specs. It is possible that raising the fuel level would bring on the mains sooner. I do like the idea of using different emulsion tubes, I do not believe we are getting the optimum fuel ratio to the engine under full load. I will give a try using smaller correction jets first.

Second. I assume (trusting someone else here) that the IDF's are matched. They were bought as a set with manifold from Red Line. It would be worthwhile to check them though, after being in this business for many years I have seen worse screw-ups sold to people.

 

[Cliff Beer]

Jr. if you bought the set together from Redline then you probably do have a matched set. However, as you suggest, mistakes are sometimes made, so, best thing to do is take a look in the bores and compare the progression holes. If they're all the same across the four carbs then it's fairly certain you have a base line match.

Good luck and persist!

 

[Rick]

After confirming the carbs are the same the next item to address is the emulsion tubes, they influence part throttle transition more than anything. What emulsion tubes are currently installed?