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Fri Oct 18, 2019 5:13 pm

Hello everybody,
I have spent several hours trying to find this issue on the dellorto thread but I couldn't . I run a pair of 40 drla carbs on a 2l engine and one cylinder affects idle less than the rest. This makes it rather hard to tune since the change in tone is less noticeable and it sometimes cause an idle variation. Compression is ok , valves are ok , the gage shows that all throats pull the same amount of air. I use 55 idle jets(not original ones, purchased from alfa 1750 on eBay) .
Has anyone experienced this issue before ?

Fri Oct 18, 2019 6:54 pm

There are lots of common reasons that dual two barrel carbs don’t idle well on one or more cylinders

To help me move in the right direction I like to spray some carb cleaner down the barrel with the lazy responding mixture screw. If the cylinder starts running normally it is lean, if the engine slows down that cylinder is rich. If there is no difference at all that cylinder is possibly not running at all and it could be due to ignition, compression etc

Common lean problems:
Intake manifold leak, most common at the head to manifold due to worn/torn gasket or the manifold or carb are hitting the tin
Clogged fuel passage in the carb, pull idle jet and see if the well is dry
Clogged idle jet
Mixture screw is damaged or missing washer/o-ring
idle jet holder o-ring damaged
Leak in the intake port due to excessive porting
- Sometimes intake manifold leaks show a small positive response to winding out the mixture screw more.

If the cylinder slows down when you shoot a bit of carb cleaner:
Float over running due to damaged needle and seat
Sticky float pivot, missing the e-clip?
Float adjusted too high
Fuel pressure too high
Internal lead plug coming loose
Mixture screw is damaged or missing washer/o-ring
Cold start enrichment system activated
butterflies are opened too far exposing too much of the progression circuit
burr or junk in the idle well causing the idle jet not to seat

- Bent throttle shaft will show lower on the snail gauge. Try opening the air bypass screw to compensate.

Other options:
Ignition component related, use a spark tester to verify or switch wires/plus around to see if the problem follows
Mechanical problem leading to low compression like a burnt or miss-adjusted valve

For me, it is almost always an intake manifold leak. Even when I initially don't think that is the problem. I have had little luck finding a leak with the commonly suggested methods of spraying carb cleaner, starting fluid or using map gas at the intake to head area.
Check to be sure the bottom flange of the intake manifold is flat with a straightedge. If it not perfectly flat or you have any doubt tape a piece of fine sandpaper to a piece of flat glass, spray some wd40 on the sand paper and sand the bottom of the intake manifolds until the color/texture is even all the way across. Use new paper gaskets and put grease on both sides of the gasket to delay them from getting brittle and for easier disassembly.

Float adjusted too low won't matter at idle, this will mainly delay the transition.
As as starting pointy make sure that at idle the butterflies are just barley exposing a small bit of the lowest progression port hole.

I am sure others will chime in with additional points.

Fri Oct 18, 2019 9:32 pm

Panayotis Nikas wrote: Hello everybody,
I have spent several hours trying to find this issue on the dellorto thread but I couldn't . I run a pair of 40 drla carbs on a 2l engine and one cylinder affects idle less than the rest. This makes it rather hard to tune since the change in tone is less noticeable and it sometimes cause an idle variation. Compression is ok , valves are ok , the gage shows that all throats pull the same amount of air. I use 55 idle jets(not original ones, purchased from alfa 1750 on eBay) .
Has anyone experienced this issue before ?

Silly question but the motor doesn’t idle well? I’m trying to figure out if you’re just irritated with the lazy mix screw or if it legitimately won’t idle well.

The other guy posted great answers and I might have missed it but it’s possible someone screwed the mix screw in too tight and messed up the seat in the carb (or the screw itself but I think the screw is a lot harder than the carb).