Hi, can anyone tell me what jetting I need to run on a standard L20B with a side draught 40DCOM Weber. On initial setup it had a very bad dead spot and I changed the Idle Jet and float level and it appears to have fixed it but now it runs rough in midrange, on or just above idle runs smooth and as you go into midrange it runs rough until you get the revs up high and it runs smooth again. I tried adjusting timing and it made no difference, this rough patch is very noticeable whether it's under load or not. my setup at the moment is Throat size 30mm, Main Jet 120, Emulsion Tube F47, Air Jet 59F21 and Pump Jet 35. Don't know much about Webers so if someone can point me in the right direction that would be great.
cheers,
Shane

DCOM is jetted same as DCOE (DCOM has vacuum ports and other modern changes). The pump is quite different however.
Quote:

DCOE Jetting

I'd say if it was a DGEV than your air correction are to big for your main jets. Have you checked your spark plugs? You may be running to rich.

By 'midrange' do you mean:
* cruising at medium RPMs (steady RPM)?
* accelerating through medium RPMs?
or
* throttle open 1/2 way at steady cruise?
* throttle open 1/2 way during acceleration?

Start with venturi/choke size. 30mm is good on twin-carb setups (one barrel per engine cylinder) for max power maybe at 4000 RPM with a 2-liter engine. It will rev higher, but make max power way down low. With a single sidedraft each barrel feeds two cylinders - it is jetted the same way but the carburetor sizing differs. 30mm single is good for maybe 5000 RPM with a 2-liter.

In DCOE/DCOM series, the main jet should start at 4 x venturi size. For 30mm venturis, start with 120 Main Jets (for DGV (32/36) the guidleines are totally different)

The Air Correction Jet should be 1.25 to 1.50 of the Main Jets. For 120 main jet, start with 165 Air Jets.

That much is easy. The trickier part, for which tuning to the engine is called for, is the transitions.
* When accelerating the pump jet and duration is the main factor, I have no idea what the guidelines are for the DCOM.
* Transition between idle circuit and cruise circuit is controlled by the Emulsion Tubes and Idle Jets with minor effect of air correction jets. Do not attempt to fudge this by raising or lowering the float level. The Float Level should be set to the Weber specifications before any other tuning is done.

> Air Jet 59F21

If I'm not mistaken, the 59F21 is a Idle Jet. These are different from DCOE Idle Jets so be sure to use only the DCOM types.

Thanks for the replies, by mid range I mean if you are accelerating steadily as you come off idle say around 1500-1800rpm it starts getting rough and around 3000-3500rpm it clears again ( this is rough estimates no tacho ) so I guess it is probably at around cruise throttle. It doesn't matter if you are holding the throttle steady in this range or accelerating to top end it still runs rough as it is going through this midrange circuit. At the moment this engine is standard with just the weber and inlet manifold changed. I am hoping someone can tell me if I am way out on a particular jet, then at least that will be a starting point for me, bit different to a Holley
Also has a V91 head can anyone tell me if this would be a closed chamber head, I bought a V912 head from someone on this forum that was suppose to be a U-Bute Racing Closed chamber head but it's not. I will possibly get the chambers welded in if the V91 is no better.
thanks again for your help
cheers
Shane

V91 is reportedly open chamber. It comes from Series 2 Bluebird.


> It doesn't matter if you are holding the throttle steady in this [RPM] range or accelerating

If the 1500-3000 RPM roughness occurs when NOT accelerating -- just holding the speed steady -- and the throttle is open a ways, then it is caused by the main jetting. This is the combination of Main Jets + Air Correction jets. With throat size 30mm, use Main Jet 120 and Air Correction Jet 165 or 170.

But if at a steady speed, the thottle is barely open, then it is probably a transition issue. After adjusting idle speed to 800 RPM and mixture for Best Lean Idle, how far out is the Mixture screw turned? For the DCOE if it isn't near one turn the the Idle Jet needs to be changed. Likely the DCOM is the same.