Tuning my webers part two....
 

Fighting brain cancer can really take a lot out of you. After Rick made some suggestions I made the changes to the carbs and what a huge improvement. Then I had to go get sick again and have been out of it. Feeling good now though. Anyhow in my latest experience with my Cobra it transistions really well from the lower RPM to the uppers with no stutter. But one thing that I am not sure if it really is an issue or I am hallucinating is that it sounds very faintly like the car wants to stall during cruise from 2000-3000 RPM but it never does. Could I just be hearing things? Anything I should look at??

Thanks

what changes did you make?

I went from

Main Venturi: 37
Aux Venturi: 4.5
Main Jet: 155
Emulsion Tube: F7
Air Corrector: 175
Idle Jet: 75F10
Idle Jet Holder: 120
Pump Jet: 50
Exhaust Valve: 000
Needle and Seat: 2.00


to


Main Venturi: 37
Aux Venturi: 4.5
Main Jet: 155
Emulsion Tube: F16 <-- Different
Air Corrector: 175
Idle Jet: 65F10 <--- Different
Idle Jet Holder: 120
Pump Jet: 50
Exhaust Valve: 000
Needle and Seat: 2.00


Thanks

At 395 cu in & therefore 810cc per cyl I would suggest that '70' idle jets might be a better choice. sounds like there is a slight lean spot at light throttle cruise conditions @ those RPM where the carbs are probably at the top end of the transition point from idle to main circuits. As the F16 is richer than the F7 you replaced its unlikely that this is the cause.

The f16 tubes are leaner than the f7 tubes. The f16 are 8.2mm in diameter and I think the f7's are 5.25? The set up I tried was:
40 chokes
120 holders
65 idle jet
f16 tubes
170 mains
225 airs

The set up was too lean, a lot of popping through the pipes. I switched to f7's with 165 mains and 200 airs. Popping was gone but above 2500 rpm it ran like crap. My guess it is still a little lean. I'm going to try 170 mains and see what changes.

I would stay with what you have with the holders and idle jets, if it is working for you upt to 2500 rpm.

Your transition comes in around 2500 - 2800.

If you are having problems with transition I would lower you air corrector from 225 to 200.

This will help brake the fuel for the transition.

Here is a test.

pull the main circuit out and take it for a drive and see of your still expeirencing the problems up to 2500. Dont step on it just drive normal and you will see the idle jets will take you to 2500.

This will prove your problem is with the main circuit.

So I finally got a chance to look at this again this morning. I had some 70's in a bag so I thought this would be an easy test. I put them in and it seems to have improved the situation a bit. I did the ritual synchrometer adjustment and then the idle with the LM1 attached and then drove it around a bit. Each change seems to be making it better. Thanks all.

I think that the problem is the choice of emulsion tube, main jet and air corrector.
Idle can vary, but I am running a 65 idle jet with a 100 holder. Maybe some slight changes up or down, but that is about it on the idle circuit.
On the main, with 40 mm chokes I have now an F15 with some additional holes drilled up the tube, which has been running quite well, with no transition problems whatsoever with a 145 main jet and 100 air corrector. Yes, an .100 air corrector and the lambda sensor does not lie, the a/f ratio (still rich right now) stays the same up to 6000 rpm.
I did have some hesitation when driving part throttle at 3000-4000 rpm, and was trying to modify the emulsion tube when I discovered that the ignition was at fault.
I discovered one of the springs in the MSD Billet distributor broken ( and rusted) , so the advance was on all the time and with the timing light some cylinders were showing misses. I fixed that and then discovered the ignition still missing at above 3000 rpm and even retarding at higher engine speed. I did clean the distributor cap inside (lots of rust and carbon powder all over the place) and closed the sparkplug gap to 0.045. Ignition is ok now with no misfires now and under the timing light absolutely steady up to the max advance of 38º.
If the weather permits I will take the cobra out tomorrow and see the behaviour of the engine .It is running very, very well and maybe only some mods to the emulsion tube will make it do any better.
Right now I can go from 1000 to WOT in 4th gear with no noticeable bogs in between. That is not bad I think.

Eljaro,

Would you say that you have found that the formula that you should take the choke x 4 = Main , then main jet + 60 is not correct?


How is is it that you are using a 100 air corrector?

Also,

have you thought about running the weber with no choke at all ?

May people feel that the webers carbs do not flow enough for the FE engines and here we are running 40 mm chokes

thanks

I have used the lambda sensor to tune, and with the larger air corrector the mixture gets lean at the top end. The 100 worked best and the A/F ratio stays steady up to max rpm. I started with the 220 air corrector and went all the way down to 80, but I settled for the 100 which I have now. When I put the 115 on to double check that I am not going astray with the tuning I get a lean top end.
Webers were used on smaller engines when they were popular, and the formula may hold true for them.
I do not think running webers without the chokes will work. The chokes have a distinctive shape inside to create under-pressure for the aux venturis to work.
You con go larger with up to 45mm chokes for the 48IDA.
Top end will probably be some better but driveability will suffer.

Two variables swapped at the same time, both towards lean.
I run a F16 in a 1500cc 4 cylinder of mine (DCOEs).

Eljaro is on the money, the old rule of main jet + 60 is out the window.
The air corrector determines main system start point as well as top end.
Just imagine a 300 main and a 360 air, won't work.

FYI,

I have found that some highly modified VW guys are running Webers with no chokes installed. I guess it does work.


Elgaro,

Can you post a data log of your setup.

Also, are you able to achieve a clean burning plug?

Tuning my webers pt two...
 

You can run Webers without chokes. What you need to consider is that with modern cams, plus cylinder heads and so on, there is more than enough metering signal strength to accurately meter fuel with the auxilary venturi alone.

I've run 47mm chokes in 48mm carbs on a road course / street car running a 408 cid engine (in a customers car) to good advantage.

I didn't remove the choke, but I am certain it would have benefited from the increase in area.

Troy Patterson