Still about Weber jetting (heres mine and gotta ask)

[Zendalar]

As some recall I have a 2.4T, stock.

I tried of changing my idle jetting from 45 to 55, but when I put in jets of size 55 I can't no longer adjust mixture. The mixture screws simply won't affect anything. So I had to change jets back to 45.

What Should I change in order to make backfiring and popping go away? Here are pretty much all the values I found on the carbs.

Venturies 27mm
Emulsion tubes F26
Idle jets 45
Main jets 120
Air correction 185
Atomizer 4,5
Pump nozzles 50 EW

Now I know 55 idle jets won't work with this combo.

[john walker's workshop]

32 venturis, 55 idle, 130-135 main, 170 air. that should get you started. F26 should work on a 2.4 with stock cams. F3 is generally for cammed engines. what you have now, except for the 120 mains, is basically for a 2.0.

[Zendalar]

I wonder how much has the performance been hit with those jets and settings? 2.0T ?

[TimT]

seems like you should try 50 idle jets or get some jet reamers and open them up until they give you the Idle you are looking for.

I know for a fact that ignition improvements make some of these carb problems DISAPPEAR!!!!!!!!


Lose the points, get a hot coil and a crane or MSD ignition box


and get new wire to fire it all

[Zendalar]

Heh, I know some of the problems disappear. They did when I got rid of Bosch CDI box and put in Crane/allison XR700, matching coil and the pointless thingamob.

[T Bird]

Zendalar
Have you attempted to set up these carburetors? Is vacuum balanced on each carburetor? Is vacuum balanced side-to-side at idle and at 3000 RPM? After balancing vacuum, have you tried to set idle mixture? Are the idle mixture screws turned out more than 3.5 turns?

You have to set them up first before you can make judgments about your jetting. Throwing jets at a Weber that has a case of the hiccups without eliminating all of the variables is never going to solve any problems.

If you set them up properly and you can't tame the popping with the mixture screws open more than 3.5 turns, then it is time to step up a size on the idle jets. When you installed the bigger jets, did you replace the o rings on the idle jet holders? Did you oil them first? If not, they can be drawn into the threads and break the seal.

As for venturi sizes, I posted the Weber formula for sizing a venturi in a response to Fishcop. Here it is again, this time for a 2.4T.

Venturi Size -- Weber formula to calculate the size of the venturi.
(1) - Divide the volume of one cylinder in cc's by 1000.
(2)- Divide the rpm where peak power is made by 1000.
(3)- multiply the number derived in (1) by the number derived in (2).
(4)-Take the square root of the number derived in (3).
(5)- Multiply the number derived in (4) by 20.

The numbers work out like this for a 2.4T.
(1) 2400cc / 6 = 400; 400 / 1000 = .4
(2) 5600 / 1000 = 5.6
(3) .4 X 5.6 = 2.24
(4) sqrt of 2.24 = 1.49666295
(5) 20 X 1.49666295 = 29.93325
So, a 30 mm venturi is the correct size for a 2.4T.

As for jetting, if your engine is stock with 7.5/1 compression ratio, here is what PMO suggests:
30 mm venturi
130 main jet
180 air correction jet
55-60 idle jet
F1 emulsion tube

Has the engine ever run correctly with the existing jetting? If so, I would suggest to you that your problem is that the carbs are out of adjustment- not deficient idle jets or restrictive venturis. Anyway, you must first eliminate that possibility before bumping up idle jets, or rejetting the carb as per PMO above.

Good luck.

[Zendalar]

Yes the Webers are set up properly. I have done it so many times that I have already lost count. Airflow has been adjusted and so is mixture and balance.

Yes, new o-rings were installed. Since all the jetting are actually for 2.0 liter car, I am gonna change all of them as they should be in 2.4.

[Chuck Moreland]

Some great information in this post.

Since I am first-time weber owner I am looking for just this type of stuff.

Can someone recommend a good source for sizing the weber bits?

John's formula is great for the venturies but I'd like to learn more about sizing other items.

[911pcars]

Zendalar,
Assuming the carbs are clean (idle ports, etc.) with evenly adjusted baseline adjustments, have you checked the throttle shafts for excessive wear? Spitting and popping may be signs there is some air bypass somewhere.

If the spark is good, I wouldn't convert until the air/fuel delivery system is okay. Did it work okay before Webers?

Sherwood Lee
http://members.rennlist.org

[Zendalar]

Well, the whole story.

Changed oil - All ok, of course
Ran with it for a while - All ok
Switched gas - Spit spat pop (99 leaded to 98 with additive)
Tune tune tune - Spit spat pop
Switch idle jets to 55 - BRoo Brooo. Pow! Pow! (Mixture screws not effective)
Cleaned carb, tuned carb - Broo Broo, Pow Pow! (same as above, running with 4 cylinders, about)

Switched back to 45 idle jets, tuning - Spit spat Pop
Added some more fuel - Spit spat pop, but wroom wroom.
Disassemble carb - check all passages, float levels, clean it thoroughly, new seals, new silicone o-rings (won't rip). Assemble.
Spit spat pop
Ignition - adjusted teeny weeny 1 degree earlier

Wroom wroom, but Pop Pop spittily spattily pop at deceleration..

Well, I am just gonna drive the thing, until I decide what to do with jetting.

[jluetjen]

Maybe the attached will be helpful. I've collected this information from the sources that I could find which listed carb jetting examples, as well as the results. Zendalar, notice that you are already listed, but without comment...

[911pcars]

John,
Thanks for compiling the data. Seems like many combinations out there doesn't it? Here are my specs:

Engine: 2.7
Venturis: 34mm
Mains: 145
Air corrections: 185
Emulsion tube: F3
Idle jets: 60
Sec. venturis: tall
Idle CO: 4%
Runs: Fine

Sherwood Lee
http://members.rennlist.org/911pcars

PS: Actually runs a little rich @ high end. Will lean mains down to 135-140 and retest.

[Chuck Moreland]

My 2.4 S spec setup was

venturies - 30
Mains - 125
Emulsion - F26

The car spit a bit at idle and was erratic at high rpm. Kinda on and off.

On recomendation from PMO for a street setup I changed to:

venturires - 32
Mains - 135
Emulsion - F3

The car still spits a bit at idle but at high rpm it just rips. The spitting may just need a bit more tuning. But off idle it sings and the power is impressive.

PMO also gave a race recommendation as follows:

Venturies - 34
Mains - 140
Emulsion - F3

Although it would help the top end, PMO cautioned that a flat spot would emerge at 3500. I am so impressed with the power improvement with the current setup I am tempted to try the race configuration to see what it can do. "if a little is good.... more must be better". But I will not accept a big flat spot.

Is anyone running this race setup and does it flat spot?

[Evans, Marv]

Zendalar:
I had the same problem with my '72E a long time ago. I don't remember if they changed any of the jets, but at the time, they changed my venturis from 27 to 32 and the popping stopped. I think John Walker is right on this.

[Porsche_911s]

Originally posted by 911pcars
John,
Thanks for compiling the data. Seems like many combinations out there doesn't it? Here are my specs:

Engine: 2.7
Venturis: 34mm
Mains: 145
Air corrections: 185
Emulsion tube: F3
Idle jets: 60
Sec. venturis: tall
Idle CO: 4%
Runs: Fine

Sherwood Lee
http://members.rennlist.org/911pcars

PS: Actually runs a little rich @ high end. Will lean mains down to 135-140 and retest.

I hope no one kills me for bringing back such an old thread but I was wondering if Sherwood ever tested his mains to see if a 135-140 ran better. I did sereral searches to see if I could find it later but came up empty.

Sherwood do you still run the 2.7??

Thanks,

[RoninLB]

I run 34 vents, 60 idles, 135 mains, F11 e-tubes.
E-cams and 8.5:1 pistons.

My mains run cooler EGTs over 4k than at lower on idle jet rpms. With 140 mains the high rpm EGTs were too cold. I could probably run 130 mains but am pleased on what's going on now.

I'd say that JW info is perfect or a 2.4.

IMO
the mains should be dialed in as if you're looking for steady high rpm cruising with good gas mileage and any acceleration should be dialed in from accelerator pump adjusting. Also consistent power at each piston should occur with a good engine and carbs when each idle screw is adjusted exactly the same as all the others. That's been my experience and I have have hard info to prove my act. A 1/4 idle turn different from others is maybe 25F. A 1/2 turn difference from the others is about 50F. 50F is a very noticeable difference in power if all are adjusted 50F colder than what the driver considers optimum. Kinda like drive in 4th gear at 4k and hold the rpms steady and see if it surges on the mains. If not decrease the mains till it surges and then increase till it doesn't.

all above is a prejudicial rant

[Porsche_911s]

on a 2.7

34 vents, 60 idles, 145 mains.
sealevel

from what I am seeing doing research, 145 mains appear be too large.

[RoninLB]

2.7 what?
A signature with details helps.

[Porsche_911s]

'74 2.7 CIS "S" CAMs - CIS pistons - RS Distributor - Stock Ignition

[RoninLB]

maybe try 55 idles and see if that's ok. Try 135 mains as that should be plenty of gas as compared to mine. Then adjust the acceleration pump squirt lean till it boggs when stepping on it then adjust the acc squirt a bit richer till it doesn't bogg. [I adjusted my RS dizzy at 10deg advance with the vac disconnected.]

all this assumes the carbs are air flow adjusted and equal between each carb. Excess gas than required cools and lowers combustion power.