jetting weber carbs

Hello all,

Well I finally finished the intallation of the 2.4S engine in my 70T.
Oil lines and cooler were actually the toughest part (I put in a 930S cooler for track work).

The engine has Weber carbs and Im in the process of dialing it in and was wondering if anyone has a good starting point for me to try?

It had:
34mm venturies
906 tall sec.ven.
F-3 emul.tubes
160 air correction
145 main
60 idle

It ran way to rich and bogged at 4500
So I changed it to:

venturies same 34mm
sec. ven. same
emul tubes same F-3
Air correction 140
main 135
idle same 60

Anybody see anything weird?
I still have a transtitional problem at
3100 and spitting in the carbs.

help anyone?

thanks,
Thomas

It looks to me like the air correction jets went the wrong way ... in fact, I've never seen them that small! I would think something in the 170, 180, or 185 range would be likely to work better. Have a set of jet reamers handy?

The venturi's may be a tiny bit large at 34 mm, per Bruce Anderson's own sample calculations on p. 154 of his 2nd ed. or p. 135 -- 1st ed. ... precisely the right size for a 3.0 SC engine. I know the factory used 32 mm venturi's on 2.0 'S' engines, but my calculator says 31.85 mm for a 2.4 'S' at a power peak of 6500 rpm, and 2341 cc is a lot closer to 1991 cc than 2995 cc's!

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Warren Hall
1973 911S Targa

Warren,

Thanks for the reply.
I had a feeling that the venturies are to small (34mm) it just doesnt whip up enough,well, venturi effect.
I guess Im not clear on the concept of air correction jets.I was assuming that when you step down the main you have to also step down the a.c. jet.

Do you know of any book besides the HP books Weber Carbs.?(forget the auth.)

Its great on history and theory but to broad in the scope of tuning practices.

Also makes the assumtion that your 30+ yr. old 3Cs are brand spanking new!!!!

Thanks for your help!!!!

Best,
Thomas

Thomas, Actually for minor changes in the main jet sizes, you don't even want to change the air correction jets ... that is why I suggested the sizes that were used on the 2.0 'S' engines: 170-185. As far as Weber books go, I don't have any to recommend, but you can check back over the past three months of postings in this forum for anything containing 'Weber' ... there have been some suggested books. You can also check at your local public or college library ... there may be some old books from the '60s in the 'sports car/racing car' section ... don't know the Dewey Decimal number series, but the Library of Congress numbers would be in the TKxxx-TLxxx range, if memory serves me right ...

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Warren Hall
1973 911S Targa

Recently researched the subject myself and came across a book by Pat Braden (Title - Weber Carburettors), which I thought was quite good. Covers full rebuilding, and tuning, of down and side drafts. From memory it is available on amazon, along with reviews. Good luck.

Now I'm not going to say that you are not capable of tuning your carbs yourself, but if you could find a shop locally with a chassis dyno, it would be worth $200 of your time to tune them on the Dyno. Take a freind along to help you change jets quicker. Map out a plan before you go to also save time. If the shop (or you could find) an old SCCA racer around, lasoo him too. You'd be supprised what HP you can find at a dyno with just jetting and ign timing. (5-10%)

I'm going to recommend a simple possibility before getting you all wrapped up in major changes. Our fellow posters have given some great recommendations but after going through the rebuild and tuning process(suicide, but well worth it) recently I'd recommend checking:

1) Make sure when tuning the carbs you use hotter than normal plugs(NGK BP5ES, post tune NGK BP7ES). Temperature is critical.

2) float height adjustment.

3) Injection amount after a single full throttle pump. The quantity injected should be 0.5 +/=0.1 cc for one stroke of the throttle. Can cause bog down.

4) If air correction screws are adjusted and synchronized properly, make sure the idle mixture adjustments(the bottom screws with springs) are properly tuned. Rich adjustments will tend to backfire thru exhaust while lean will pop thru intakes(If memory serves me correct, I'll double check). Read the plugs after 5 miles of hard driving.

Simple stuff, food for thought. Hope it helps. Good luck with it.

Rich