dcoe idle help please

[2ma2]

Just had twin 40s installed by a "reputable " shop. Problem is idle is rough and transition circuit is a little boggy. So I fiddled with the idle jets and screws and on one of the carbs the idle is not affected by seating the screw all the way down. I know this means another circuit is kicking in early but need to know what causes this. Thy are older Italian series 18 carbs where you can look at the progression holes through a removable plug. the butterflies are in the same position relative to the holes and are synchronized. Floats are also set the same. Is there anything you guys can think of to check or remedy this situation before I pull the carb to check for a throttle shaft/butterfly problem?

Thanks in advance

[BigDog]

A couple of ideas...

1. Jetting, what's your compete jetting profile? Perhaps the mystery lies within, post here so people can compare. (Main, Air Corrector, Aux Venturi (choke), Emulsion tube, idle jet, pump jet).

2. You're probalby idling with the main cicruit partially engaged. Not that this is completely evil...

3. The settings on the air corretor screws for the idle are explained ad. infinitum in the Weber tuning boook, you should probably own one.

4. Synchronized? Use a sync tool to really make sure they are.

BD

[M3M3]

the reputable shop installed them, but did they take them completely apart, clean all the jets and refresh all the gaskets? without doing that, you have no baseline to start with. did the shop do anything but install them? did they do any work to actually get them to work on the car?

lots of books and articles and threads on initial setup/tune of these carbs. second the suggestion to dig in and read/study if you have not already.

but if they have not been rebuilt, take them back off and do it.

btdt.

[TobyB]

yeah, you need to get the throttle plates closed further.

oddly, many things including ignition timing can affect how much

the plates are open, and that 'sweet spot' is not huge.

the only jet that can be in play is the progression- the smaller pair of

the jet holders under the mickey mouse cover.

If the're waaaay too tiny, that can lean the whole progression,

including what the idle screw gets to play with.

So if you find out what's in there, that might help.

Also, if there is any sort of soft mount, check very carefully

for air leaks- that will play hell with any attempt at getting

an idle.

hth,

t

[2ma2]

Yes they supposedly rebuilt them but who knows what that means. All the mounts and linkage are new and there is no vacuum leak.

Venturis 34

Aux venturis 45

Idle 55 F9.....runs worse with 50s

AC 170...runs worse with 190s

ET F16

Main 130...runs worse with 120s and 125s

increasing the mains is helping a lot up to 5000 rpm then it goes flat, I will go to 135 as I dont want to drop the AC jet any more. Car seems to be ok 2K to 5K

Car has tii pistons, pertronix and a rebuit distributor.

Have the books and get the theory but the mains are getting a bit big and the air correction jet a bit small from what I have read here and in the books. I might be heading towards 140 mains and 170 air correctors.

I suspect I am in the ballpark but that one carb is sending fuel into the manifold somewhere it aint supposed to. Will order a unisyn tomorrow and check flow in all 4 venturis. Is this the best tool for the job?

It should tell me if one or both of the throats on that carb are flowing too much air when the plates are shut, is this a good way to proceed

[M3M3]

the dcoe tuning link in the faq actually has a good process to follow. gotta have a synch tool of some brand to get anywhere. i use the one from pierce manifolds.

[TobyB]

unisyn sucks- get a synchrometer.

yes, it might help find idle air and fuel problems.

if you're having trouble at 5k, go smaller on the airs. that will richen it.

go as small as you need- i carry everything down to 130 and sometimes use 'em.

don't fuss with mains if it's good at 4k, wot- they arent your problem.

wbo2 helped me a ton.

t