DCOE tuning help

[AustrianVespaGuy]

For any of you experienced carb tuners out there, I could use a pointer. I'm having a some trouble getting my new sidedrafts to run nicely at idle, and I can't figure out if it's a mixture or vacuum leak problem. No obvious vacuum leaks, no uncovered vacuum ports, can't feel any air leaking around manifold gaskets, etc. Here are the symptoms: It backfires through the carbs some, but not real bad. Mostly only when cold and overrun at higher revs. It hangs a little coming back to idle (hangs worse warm than cold) and at idle it's rough and it searches a little bit. I have advanced the timing some but not scientifically, only to ear. I've ordered a carb syncronizer, but I spent a lot of time trying to balance the two banks and I think they are pretty close. Motor is basically stock and idle jets are 59F21. 59 sounds quite sizeable to me, but I really have no idea what size F21 emulsion holes are. I've had to set the mixture screws at 5-5.5 turns out to get rid of most of the backfiring. Well, that's the jist of it, so any advice as to how/where to hunt for vacuum leaks or whether I should try a different size idle jet? Any and all advice appreciated!

Have a good one,

-Carl

[FL02]

you might me too rich. My car seems to run good super rich, but it idles rough, even though it'll rev to the moon. I've found that here at sea level the stock setting of 2 1/2 turns on my downdraft is too rich and I can lean it out some. I'm like at 2 turns, but I'm running a sync-link setup so it's a little rich to begin with. It is probably the same with your sidedrafts. You should set them inbetween the car backfiring from too much backpressure (too lean) and second gear crunching a little when shifting (too rich). I'm sure someone will chime in on stock sidedraft settings on your idle mix screw but it's seems like you are running rich. good luck.

[dapbmw]

before anything else, can you please post your carbs specs. what size type/tipo, jettings, emulsion tubes, chokes, aux vent, float settings.

cheers,

dave

[AustrianVespaGuy]

Sure thing, here's all the stuff I know exactly:

-40 DCOM sidedrafts, center cable linkage

(rebuilt before install, but didn't know enough to mess with float levels)

-Dual TWM manifolds, no coolant passages

-30mm venturies/chokes, no velocity stacks

-120 main jets

-F47 emulsion tubes

-150 air correctors

-59F21 idle jets

-air bypass screws are all seated

-all (5) vacuum ports plugged

-mixture screws set at ~5 to 5.5 turns out (to minimize backfiring)

Hope this helps, and thanks for the advice!

-Carl

[TobyB]

Your idle jet numbers seem odd to me-

I don't remember coming across anything off evens and fives.

Anyway, I'd agree with your air leak diagnosis...

hey... (thinking on my feet..)

40DCOE's need the stacks to center and seal the aux venturis in front of the chokes-

does the venturi seem to wobble around a bit? I've never looked at a DCOM, but that would decouple the fuel to the aux venturi, and let too much air in. And cause all sorts of little instabilities...

Otherwise, the soft mounts are almost always worth checking- use propane from an unlit propane torch (carefully) and if the revs go up and get less even when you wave it around a soft mount, there's your culprit.

And then, the warm- up circuit can stick open, and...

hth,

Toby

[AlanM]

I am guessing the carbs are off an Alfa. One particular Alfa 1600 has 40DCOEs with 30mm chokes 55F21 idle, and F41 emulsion. You may have misread the 41 as 47. Set the float level at 8mm.

Fuel pressure should only be 2.5-3 PSI.

The motor should run with the idle mixture screws out 2 turns or so, all being right. I simply use a 2 Ft piece of garden hose to listen to the carbs at their mouths at idle to determine the balance of the butterflies.

[dapbmw]

is your motor stock?

your carbs came of an alfa 1300-1600 cc. you had the mixture screws turned out too far because the idle jets are small and leaning out causing you to backfire, try to advance the timing a bit too.

you may want to switch to a baseline setting like this:

f16 or f9 emulsion tubes, 50 f8 or 50 f9 idle jets, 40 pump jets, 200 needle valve, 32,33 or 34 mm chokes (30 is too small for a 2 liter), change the main and air later, get it running smoothly first.

'02's like float levels set from 12-14mm by the hanging method, you can try the nail method with the top cover still on the carb and the jet inspection cover off, a 3 inch finishing nail and make a reference mark of 29mm measured from the sharp end of the nail. now remove the one of the emulsion tubes complete with the main and air corrector. drop the nail in the whole and see where your fuel level sits at. make sure the car is in a level surface.

make sure to sync your carbs, use a sk synchrometer if you have access to one. then post your results back.

cheers,

dave