2.7 Webers misfire - carb gasket or something else?

[Alexandphoto]

Hi team!

Battling an issue with my 2.7L 911 with Weber 40IDAs.

I have an O2 Gauge to help with tuning with a sensor in each header, and recently have a problem with one side of the motor (4/5/6, or if I’m looking at the engine from behind the car it’s the right side)

At idle it will intermittently misfire, and at low rpm it will stumble. However, at wide open throttle it is happy.

AFRs at low rpm are way too lean on that side of the engine, but fine at WOT. Fuel pressure is fine (PMO gauge reading 3psi, and checked with a separate gauge as well). AFRs for the other side of the motor are where they should be.

I suspect it could be leaking intake gaskets on that side of the motor. Would that make sense for this issue? Are there other areas I should poke around too?

Thanks!

[r lane]

I have gotten an extra gasket between the carbs and manifold. As in 2 on #4 and 1 on #5 and #6. A vacuum leak. Idle jet issue will definitely cause a stumble

[Nditiz1]

Does the mixture screw do anything when you turn it all the way in/out on each of the 4/5/6? How are the temps across all the exhaust pipes? Which bank is misfiring?

[Alexandphoto]

Currently bank 4/5/6 is leaning out. I think at idle cylinder 6 runs cooler so suspect the misfire is there at idle only.

I am thinking to replace all the leads with Clewett leads, new spark plugs, machine the intake manifold surfaces so they are flat, replace the studs and gaskets.

Hoping that solves it, but wondering where else I should be looking too

[john walker's workshop]

Webers and trying to dial them in with O2 sensors is like a dog chasing it's tail. Likely resulting in running them too lean to fill the bill and ending up with flat spots on acceleration and popping and farting. They run best when they produce enough fuel to run the engine smoothly, which is on the somewhat rich side. They're a gas soaked sponge essentially, as opposed to a well tuned FI system.
Love 'em.

[proporsche]

does it shoot into carbs or exhaust?
Did you balance the carbs?Did you check your fuel level in chambers?

Ivan

[al lkosmal]

Check to see If the shafts are leaking and need rebushing

[Old H2S]

Just built my friends 2.7 with the same problem.. and he blew a ring and that took out the piston, broke 2 rocker arms, bent both valves, trashed the head good. I told him to rebuild the carbs. Parts came today and he dumped the job to me. This is what I found, the bottom of the manifolds are not flat enough, got some 240 sandpaper and the surface plate out and recut the bottoms.
Bottom of the cabs are not flat, and this is not from over torquing the nuts. The bottom of the carb bores is canted in the same direction, this was machined poorly.
The cover for the accelerator pump housing was not flat and the inner rubber diaphragm was not closing the 3 nipples for the arms that shoot fuel down the bores. This is a big leak.
One of the Acc. pump shooter arms was loose, this is a big leak.
The butterflies were not all closing fully.
The needle valve top caps were stuck on and I destroyed them to get them off, to get to the clean out wells that had crud sitting there.
The float bowls had crud in them and the breather tubes on top were starting to rust on the inside.
The main jet circuit "hidden passages" were full of crud. These are a main backfire cause.
Worn out "O"rings everywhere.
This is just 1 carbs problems from what I found on the Paul Abott web site on rebuilding the Weber's.
This is the first time I have done one and tomorrow I will do the other. The problems were the same as yours ..backfiring and it took out one cylinder on a new engine. "But the carbs were working fine before we rebuilt the engine". EVERY engine comes out different. Backfiring requires an air leak and execess fuel which with 6 bores and a 60 year old design made for fuel that weighed 11 pounds per gallon and you got a never ending mess to fiddle with. I can see why they charge 3000.00 to rebuild these things, even with the right tools they are a time eater.
I have an O2 on my set up and it pulls like a turbo.. O the purests hate me. Flame on gents.

[Old H2S]

This is what leaking throttle shafts look like..

I did the first cleaning to get the big crud off and now you can see the "clean spot" where the air is leaking past the shaft.

[al lkosmal]

Often, with the engine running, you can grab the ends of the shafts/lever and feel it pulsing/moving if it is leaking....and if you can move it back and forth a bit, the engine running will change....misfire more or less. It's a quick way to check for leaks.