Carburation problem

[worzel]

Hi all

I'm trying to adjust a friend's S2 on Type 18 Weber 40's. After doing something pretty brutal to the throttle spindles I've at last managed to equalise all 4 airflows but- number 3 cylinder refuses to be weakened down. Using a Colourtune I cannot achieve blue, with the slow running screw right in it still shows yellow.

I've tried the following-

Checked all jets sizes etc are matching
Checked float heights.
Checked for blockages by blowing thru with jets etc removed
Checked for leaking floats.
Cylinder is sparking properly
Compressions all around 200 psi
Valve clearances all to spec

I've also swapped over the idle screws but to no effect.

Any ideas? The only conclusion I can come to is that perhaps in the past somebody has overtightened the slow running screw and damaged the seating thereby allowing too much fuel to enter. I've even removed the small spring in the hope that it was becoming coil bound before the screw reached the bottom- didn't work!

Regards

John

[gjz30075]

John, when sighting down the hole with the progression holes, is the throttle plate forward of the holes?

That said (asked), I too had the same problem with my type 18s and just couldn't achieve idle for that reason. I eventually got the later type 151s. I really hated the brass spindles on the type 18s, as they continually twisted.

Greg Z
'72 Sprint

[bill308]

I've had similar experience with my type 18's. The throttle shaft is just not stiff enough in torsion. I think it is still possible to replace it with a steel one.

The other issue was a misplaced progression hole, the first one to be uncovered when the throttle plate tips in. In this case the cure was a small, local chamfer of the plate edge, but only having to go half the way through it. Now this cylinder came on at tip in and an annoying low speed driveability issue was cured.

John,

It seems likely that the needle seat in the affected carb may be damaged. It may be possible to restore it. I'd look for a Weber specialist, maybe an old one or a shop that's been around for a long time and has worked on a lot of Webers. Weber at one time made available specialized tools for working on their carbs and at least one tool was available to recondition the valve seat. I believe the tool gently reformed the defective surface by tapping on the end of the tool and rotating it. A repair like this and maybe a new needle?

How are you measuring your air flows?

Bill

[worzel]

Hi- and thanks for the input so far.

Bill- I use one of BurtonPower's airflow meters. Very accurate but doesn't look much for the ?50 it cost me.

I think once "somebody" gets hold of these carbs and "overhauls" them problems occur. These carbs were given a going over and when I checked the airflows there were wide differences within each carb. I managed to equalise them by the "twist and see" method and three of the mixture screws work as they should but number 3 cylinder refuses to lie down- in fact it runs slightly better with the screw turned fully inwards.

Years ago I had a S4 on Type 18's and that never gave any trouble- but- the carbs had never been "overhauled".

Fortunately the owner has another pair of carbs so it's a case of substitution and see what happens.

Regards

John