group a carb ?

[mk5gtt]

New poster! (less than 10 posts)

just wanted to know if they are any good and what is changed on them from standard or an i better re jetting a standard carb thanks for any advise

[Markey Mark (BD)]

A grp A carb from a tuner is really just a standard carb with the venturi bored out and larger main jet fitted with a smaller air corrector. Terrible for over fueling low down the revs and leaning ot at top end.

Best thing to do mate is to re-jet your standard venturi size carb so it will fuel correctly throughout the rev range on and off boost.

[Spooky]

Exactly what Mark said :agree:

[Jimmy_GTT]

And what about rejetting a Gruop A carb?
The larger venturi will give a gain.
Does anyone use Group A carb with an AFR gauge. Could it be rejetted to get correct values in the rev range?

[olidaviesuk]

Had one of the original ones on my cup - proper renault 11 one at that! and had nothing but grief with it and getting it right, since gone to a standard venturi custom job that Matt5 did for me, and the fuelling is so spot on, and the car has never been better, on and off boost.

Probably worth having a chat with Scoff or Matt - if you want something a bit more custom, there are lots of mods that can be done to improve the flow through and the performance of the carb body. There are also some tricks that can be done with the second stage to really improve on and off boost performance.. The days of the larger venturi and dropping in a massive main jet are hopefully well and truly behind us!

Also see http://www.rtoc.org/boards/showthread.php?t=4743 and http://www.rtoc.org/boards/showthread.php?t=4734


And what about rejetting a Gruop A carb?
The larger venturi will give a gain.
Does anyone use Group A carb with an AFR gauge. Could it be rejetted to get correct values in the rev range?

[Jimmy_GTT]

The venturi is the narrowest part of the flow. By increasing the venturi you will give it a better flow. The only question is if there is a good combination of main, air corrector, 2nd stage for a 27 venturi.
I know what you mean about big venturi and big main jet (150). It is not working.
The main benefit of the EFI conversion is you will get rid of the 25mm restrictor.

[olidaviesuk]

There probably is, I suspect that its never been found or alternatively the mechanics of how the carb controls the fueling just isn't up to the job to do it. The trend with the majority of the big power cars to go back to a standard(ish) venturi or to EFI kinda suggests this.. as its a fairly small community if someone was getting the big venturi carbs to work really well then everyone would be doing it ;)

If someone has got it to work really, really well then i'm sure that a lot of peeps on here would be VERY interested to know how its done :agree:





The venturi is the narrowest part of the flow. By increasing the venturi you will give it a better flow. The only question is if there is a good combination of main, air corrector, 2nd stage for a 27 venturi.
I know what you mean about big venturi and big main jet (150). It is not working.
The main benefit of the EFI conversion is you will get rid of the 25mm restrictor.

[Jimmy_GTT]

But if you check the 1/4 leader board every 2nd car runs on 27mm venturi.

[olidaviesuk]

It would appear the bigger you make the venturi the more problems you run into with getting the fuelling right...., and 1/4 mile times aren't the whole story, how many of those cars are tuned specifically for straight line at the strip – for drag you're on WOT 99% of the time, part throttle response is a factor for real world use...


But if you check the 1/4 leader board every 2nd car runs on 27mm venturi.

[Markey Mark (BD)]

But if you check the 1/4 leader board every 2nd car runs on 27mm venturi.

Alot of those cars which had the large venturi haven't now i bet, i know my one on the blue car come off.

Its not all about the flow past the venturi, by enlarging the venturi you loose the venturi effect i:e the vacumm to pull the fuel through the emulsion tube. This is why tuners then fitted the large main jet (if i am wrong someone correct me. :D)

Yes i bet with some work you can get a large venturi carb to fuel niceish but the issue will still be running rich low down, and the gains of having a larger venturi i doubt there will be much at all over standard one.

[gtmatt]

I would not bother with a grp a carb waste of time and money ,it has been covered alot this subject ,and grp a carbs get a vote of rubbish IMO

[Scoff]

But if you check the 1/4 leader board every 2nd car runs on 27mm venturi.

yes, but that doesn't mean that it is working better than 25mm. I understand what you are suggesting, and I would tend to agree if I had not seen the proof time and time again that shows that 25mm works just as well :)

[Jimmy_GTT]

I have built a 27mm venturi carb about 2 years ago. But I've only used it for some weeks as by that time I haven't had afr gauge in my car. So it was safer to use a standard one. (it was jetted by a carb repair company and as I remember it has a 135 main and 100 a/r)
Now I have afr in my car, but I have set up my std. carb very well (with all your help!). Afr readings are perfect in all rev range, wot, or part throttle. So the 27 carb sits on a shelf. :)
I'm planning to try EFI in the summer. I have megasquirt in my car and ignition is already controlled by MS. Thanks Scoff for the ig. map!!!

[mk5gtt]

New poster! (less than 10 posts)

Another qustion do they take the butterfly flap out for the choke mechanisim or do you leave all the parts in for the choke and do most people still use the choke.

[James5]

New poster! (less than 10 posts)

Another qustion do they take the butterfly flap out for the choke mechanisim or do you leave all the parts in for the choke and do most people still use the choke.

You can remove the choke butterfly bits and retain the high idle cam on the outside to help with cold starts. You will need to block the Choke butterfly hole up.

[Mart]

Remove the flap & spindle, but remember to block the outer spindle hole.

[Mart]

Too slow...