Weber Jetting Help

[Halm]

After a terrible performance on the dyno last Saturday, I am trying to sort out why a less than healthy 2.4 E made more HP than my 2.7 with Mahle RS P&C’s, “S” cams and Webers.

About all I know is that my wrench says ignition can be advanced a few degrees, the Webers were set up rich for the break-in period, and they have 70 idle jets and 135 main jets. I want to inventory what air correction and emulsion tubes are in stalled. However looking at the exploded diagram it looks like I have to separate the upper and lower halves of the carbs. To me, that is a pretty scary task. Any tips on doing this?

Also, if you have a similar setup, what jetting are you using?

Thanks.

[Halm]

Peter, thanks for the quick reply. Am I correct in that there is one air correction jet and one emulsion tube per bank? In other words, 2 air correction jets and 2 emulsion tubes total?

[campbellcj]

It's really not that bad to pull off the carb top; just BTDT a week ago. You need an 8mm deep socket and it is a good idea to have replacement gaskets on hand. It is much easier to get at the air correction jets and emulsion tubes that way; plus you may want to check the size of the primary venturis (chokes) while you're in there too.

I would suggest talking to Richard at PMO. He can offer jetting advice & also supply the parts. Contact info is on the web site: http://www.pmocarb.com/

[campbellcj]

Hal there are 3 of each per carb (also 3 mains, 3 idles etc); one per barrel.

They "should" however all be the same, so you only need to check one.

That being said...I found 2 mismatched idle jets in my Webers! (They may have been reamed-out to match the others, but I replaced 'em anyway.)

[Halm]

Thanks guys. So splitting the carbs in the car is no big deal, especially if I have a new gasket. Think I'll go buy the 8mm socket this weekend and see what I've got.

Any recommendation on manuals? I bought the one PP has. Its illustration made me think that there was 1 air correction jet and 1 emulsion tube per bank so any help here is also appreciated.

[ttweed]

Originally posted by campbellcj
I would suggest talking to Richard at PMO. He can offer jetting advice & also supply the parts. Contact info is on the web site: http://www.pmocarb.com/

Larry Turner has some of Richard's jetting recommendations on his Rennlist pages.

[Halm]

Tom, Very cool. Thanks. It is also interesting to see what Richard says compared to what BA has in his Performance Handbook.

[jtf914]

To remove the air correction jets and emulsion tubes while still installed on the car:

Remove the entire air cleaner assembly. Unscrew the air correction jets and use a wooden skewer (like you would use to make kabobs) to retrieve it. Put the skewer back into the carb, and you'll come out with the emulsion tube. Repeat this for each cylinder.

Make sure the skewer is clean and doesn't have any splinters which could fall off and clog other jets. May not be the best way, but it works when you dont have time to remove the carbs......

-Justin

[Halm]

Justin, thanks for the advice.

Would I be correct in guessing I start with a standard flat bladed screw driver or do I need a phillips head?

[jtf914]

Yup, flat blade.......

[Marankie]

I have a 2.4S race motor with similar specs. I cant tell you the exact jetting as I am e-mailing frm work. As I recall the main jets are 160, and the ACs are 180. The idles are 55. 135 is way too lean for your motor, especially at high load high rpm.
Martin
914-6GTR

[Halm]

Thanks for all the help so far. I am just starting to get a good feel for the new carbs.

Maybe another "stupid question", but how do you lean out Webers? Based on the odor of unburned hydrocarbons at the dyno run and on the street, I am running rich, but I agree that the 135 MJ may be too small. I would have thought that there was a direct relationship. Could the wrong AC jets and wrong emulsion tubes be so mismatched that I am rich?

[Marankie]

I dont know what venturi size that you are running but I use 34mm. 40 mm Webers can run up to 36mm venturis but the signal strenght for tuning becomes more diificult. Also make sure float heights are correct.
Mostly percieved stinky exhausts have to do with the idle jets. As I said mine are 55. Go smaller and it will start to pop back thru the carbs (light back fires) particularly when the engine is cold. Play around with different idle jets untill the popping at idle and low throtle settings goes away when the motor just starts to get warm. By the way most of your street driving will be on the idle jets. The main jets really come into play for the full throtle high rpm stuff. So you could easlily be rich at idle and lean on the main jets. The AC are sort of a smaller adjustment on the MJ. Agood place to start is for the MJ and the AC to be about the same size.
Martin.

[ttweed]

Originally posted by Halm
Maybe another "stupid question", but how do you lean out Webers? Based on the odor of unburned hydrocarbons at the dyno run and on the street, I am running rich

Your dyno run did not give you A/F ratio readouts? This should have been routine. You shouldn't have to depend on your nose to find out about your mixture. Put a lambda meter on it!

Basically, you lean the mixture by letting in more air compared to fuel, no? So bigger venturis and A/C jets will let in more air. Bigger idle jets, transition ports, and main jets will let in more fuel at varying RPM points. The acceleration pump volume is critical for throttle response when you first push down on the loud pedal, squirting in additional fuel momentarily, but then becoming inactive. Make sure they are set at the correct volume.

If signal strength is an issue with bigger venturis at low RPM, then use the tall aux. venturis to get more velocity thru the stacks. The emulsion tubes just provide minor control of the "fuel curve" thru the RPM range. As the engine sucks in fuel, the chamber in which the ETs sit changes in level. The different emulsion tubes have different thicknesses and holes drilled in different places, top to bottom, to change the rate at which fuel is "emulsified" into the air flow as the level in the chamber changes. If you have the F3 tubes, I would not bother with changing them, as they will work with your engine fine.

Make sure your float levels are correct, as someone else suggested. If the needle valves are not controlling the float bowl level correctly, you may be getting fuel overrun that is creating a rich condition, since 135 mains shouldn't be doing it.

Carbs are a dull blade compared to EFI, but they can work well when set up right, albeit without the best fuel economy. Keep after it and "she'll be right, mate", as they say in OZ.

Good luck,
TT

[Steve@Rennsport]

Hal:

Great advice here,...all of it.

Your 2.7 sounds FAR too rich at idle and way too lean with those 135 main jets.

That engine will run much stronger and cleaner with:

60 idles
F3 emulsion tubes
155 main jets
180 air corrector jets

Make sure your fuel pressure is 3.5 psi and set the float levels VERY precisely.

You'll be pleasantly surprised what these changes will make.