Jetting of V6 engine carburettors

[Robert]

Hello Guy,

I need your help. In fact a friend who wants to compete in "Historical Vehicle" races. His engine was with a fuel injection previously but now to be compatible with the regulation he goes back to carburettors.
His A310 is equiped with a V6 Renault 3000cc. The A/F is too lean and the engine looses hp and doesn't work so efficiently than formely. It is difficult to obtain hi revs.

The carburettors are Weber DCOE (verticaly) their dia is 46 and the choke 42. Cams: I 34/69 L: 11.14; E 63/38 L:10.7

So is it possible to indicate a starting point in term of jetting ? Fuel and air (frankly speaking I'm not confortable with carburettors but I understood there are 2 jets one for Fuel and one for air...)

Thank you very much,

Robert

[Guy Croft]

Hi Rob

there isn't a 46DCOE far less a vertically mounted DCOE, that carb is a sidedraft (though it can be mounted up 5 deg off horiz).

Do you mean a 46IDA type? Please ID the exact setup, the link below may help. I imagine being a V6 you have a triple carb setup with the spindles oriented parallel to the crank axis as mounting any downdraft 90deg to the crank would be wrong for the float alignment.

http://www.pbase.com/9146gt/factory_ori ... ts_46weber

If you can give me more info I shall do my best to determine the right jetting but from the outset, whilst the carb size is not massive the primary choke size is miles too big for 500cc cyls with those cams and 40mm would tend to be better. The problem is that with too-large chokes the combined depression generated by the combo of py and sy chokes stacked up in line can be too low to draw-out and atomise the fuel. The bigger the choke for given carb size the weaker the fuel 'signal' - except to say that very radical cams and very high CR can give effective results but generally only at higher rpm = 'soggy' torque curve lower down on a less highly-tuned motor.

G

[Robert]

Guy,

Indeed it is 46 IDA 3C triple carburetors. The choke (42mm) seems too big as I understood. But the set up was used in the past by Renault in Gp4...
Is it possible to use a smaller venturi/choke in this carbs ?

Thank you

Robert

[Guy Croft]

Does this have 4 valve per cyl or 2? The cam setup is not radical for a 2 valve head but on a 4 valve might be OK with 42 chokes.

I do not know yet what is available for IDA carbs, but typically I would treat a 46 IDA as being 'on a par' with a 45 DCOE and - in the first instance - not go bigger than 42 choke.

As far as carb reponse goes a great deal (as ever) will turn on the header layout and system back-pressure, can you tell give me any detail on these?


G

[Robert]

Guy,

It is a 2 valve per cylinder engine. It seems the camshaft timing is not a "war weapon" ! But with injection and the best maping it reached 260 hp on dyno tets.
Unfortunately with carbs the result is that the engine is sputtering and my friend tries to obtain at least a "normal" behaviour.

What is the "header" exactely (sorry) ? The intake dimensions are :
The total lenght of the intake from valve to trumpet mouth is about 350mm with 55mm straight trumpet (he has also bent trumpet giving 410mm total lenght)

Robert

[Guy Croft]

Sorry Robert, mixed you up with Robert Kenny in the USA.

The header is the exhaust manifold. I imagine it's a 3-1 with either separate or joined secondary pipes. I want lengths and diameters. It's quite simple: the more info I get on an engine so the better I can estimate jetting, the less I'm told the more likely the problem is not actually jetting - but in fact something else.

Why should that be? Because I can jet up most engines from memory based on experience but if my jetting doesn't improve the engine then the fundamental problem - ain't jetting.

Give me also a list of jets and emulsion tubes currently in the IDAs, the layout is very similar to the DCOE except of course it is a downdraft carb, items 10-15 incl and 57 (pump jet) on the att link:

http://www.webcon.co.uk/weber/45dcoe.htm

GC

[Robert]

Good afternoon Guy,

Just coming back from the trials with jetting modified. the engine works!
So 46 IDA 3C set up:
choke: 42
main jet : 180 : no more problem at 4000 +
Air jet: 150 : no more problem at hi revs (up to 7500)
Iddle jet: 70: remains a little bit of sacade in the iddle to 2000 range ... So not a big issue.
Emulsion: F24

So now remains only refining the set up.

Sorry disturbing you but the "story has badly began "...
Hoping the above info can help some one as starting point.

Robert