I know this subject has been covered and I tried to go back and look through many post but did not find the exact answer I was looking for. I bought a 3 weber carb conversion from Classic Jaguar quite a while back and am now trying to get them to work. This is for my xk150 which I installed a 1985 4.2 liter rebuilt motor with 9 to 1 compression a little more aggressive cam and headers etc. Not too wild but a few notches above stock. Any combination I have tried- I have not had any luck leaning the car out and continue to foul the plugs. I have tried several different timing advances, A Mallory Distributor, a conventional points dist. and a petronics conversion. The motor is fresh and shows 150 psi per cylinder. I have not tried changing the jets because i know there are a thousand different combination. I have adjusted the idle mixture screw all the way from out to in and still did not lean it out enough. My guess would be to start with the Idle jet. I am using a stock E type SU fuel pump. The following are the jets in my carb.
Main Venturi 36
Aux. Venturi 45
Emulsion Tube F2
Air Correction 195
Main Jet 185
Idle Jet 65 F8
Starter Jet 150
Pump Jet 45
I can start the motor but it runs rough and it is hard to get it to idle less than 1000 rpms. I have ordered a colortune from Gunson but what is the use unless I have it within range to where the Idle mixture screw will make a difference. As I said I bought these carbs new and they supplied the jets with what they thought would work for a XKE 4.2 liter motor.
I was 10 years younger when I thought of converting these carbs. Now I wish I would have stuck with the good ole SU’s. But I am committed and they do look awful nice. Can anybody give me some advice?
That setting group looks way to rich. On my 4.2 XKE race car, I was running 165 mains and 55 idles. Colortune works fine for idle which is adjustable for any combination of jets but only for idle. If the idle screw has no effect, you are idling on the mains. It would be worth it to take it to someone that can sort it out. You will also need a great assortment of jets unless you just guess and by 6 of each item until you hit upon the right combination. To set it right, you will need a air fuel mixture gauge so you can check it under load. Great carbs once set up for your engine. It should idle smoothly at normal idle speeds. I was running large ports, headers and the wildest cam pattern you can do with the stock cams and still idled smoothly at 800 RPMs with good throttle response.
I was running 40 chokes and 3.5 aux venturis. Do realize that each engine setup is different and needs to be set up properly. I have a very large supply of most all of the jets and pieces to tune the webers. Very easy to spend $1000 to have enough pieces as you will need 6 of anything each time you try a different part. Don’t forget the ignition system. I had to add MSD to get it to fire evenly. I added a switch so it could go back to stock. Night and day difference. I was also using a very short advance curve starting at 17 BTDC and total of 35 degrees which gives a 18 degree advance or 9 degrees at the distributor. https://www.youtube.com/watch?v=WQ25_JWUfsM is a YouTube of the car in action on the slalom course. It still holds the all time record on the JCNA slalom.
HI Holden,
The main jets certainly look to be too rich. But shouldn’t be a factor at idle anyway.
There should be some difference in the when you adjust the idle jets.
You say adjusting the idle mixture screws doesn’t lean it out enough but does the engine note change? If you can’t really hear the exhaust then it can be difficult to hear the difference when making a change to each cylinder sequentially. If you have dual exhaust pipes then that does help.
The idle jets assuming they are 45 152’s - should be about 2 to 3 turns off the seat. Don’t screw them down too vigorously as they can jam and break. Try setting each one at say 2.5 turns out and it should start and check the balance between the carbs. It doesn’t matter if it is idling at 1000 rpm. But as Dick noted you should be able to get down in the 800 range. A Colortune is very useful - you can then get a sense of how sensitive the idle screw adjustment really is. Two Colortunes make it easier and then you can always have one cylinder as a reference. basically if you have the idle screws less than 2 turns out and more than 3 turns out then you need to change the idle jets.
Questions - how many progression holes do you have?
Have you checked the float levels? There is a way to check this without pulling the floats out. As long as each one is pretty close to the others this isn’t an idle issue.
Do you have a fuel regulator? Fuel pressure should be less than 3 psi.
If the carbs are too lean they will spit back through the carbs at more or less steady rpm so start at 2.5 turns out and then adjust down to 2 turns and see if you get the lean spit back.
Another metric that is useful is to pull the emulsion tubes out. This then tells you when (rpm) the idle circuit is transitioning to the main circuit. If you do this on the road just be careful as the engine will quit!
Timing is another issue after you get them to run smoothly
Hi Holden,
Mine came from xke’s Unlimited set up for the 4.2 XK engine. I was not totally happy so made some changes, and quickly went back to the original. With some patience, and a known good engine you can get a very nice result.
45DCOE’s
Choke 36mm
Aux Vent 4.5
Main jet 145
Air Correct. 155
Emulsion F16
Idle 55/F8
Idle Mixt. 100
Progressive holes 120/100/80
Needle valve 200
Pump Jet 45.
Hope this helps.
Duncan
I went the weber route once after two years of constantly adjusting a TJ mechanical injection set up, another couple of years of spit and cough until I got things near correct, the one good thing about them is they are easy to sell on, because everyone likes the look and sound, but the gains on track are not there for the road, poor economy and bore wash will eventually lead you back to triple SUs, read Lou Fidenzas notes on the SU, he will say they are more than adequate for the job, and he is right,