Weber butterfly angle

[rideaway]

Hello everyone,

I have not been able to find a definitive answer on the forum or on the internet.

I'm having some trouble tuning my carburettors Weber 40 DCOE 31 after an engine rebuild. The transition is poor and causes jerking. Looking at the position of the throttles in the bodies, the first progress hole goes from completely blocked to completely unblocked. Also on the same carburettor the position of the butterflies is not identical. I had previously adjusted the balance with the Airflow syncrometer.
On one of the carburettors, it seems that the throttle cannot close in the correct position. Once closed, some of the throttles show an opening.
I'm thinking, on the one hand, of twisted shafts, but I wonder which butterflies are fitted originally. Mine have an angle of 79.30° with holes (of different sizes depending on the body).

What are your opinions?

I hope the translator has converted my text into something understandable.

Regards
Stéphane

[Craven]

Correct, 79.3 DCOE 31 with air bypass, have only seen one size hole would seem yours have been played with, 78 deg on 151.

[elans3]

Sounds like you have a twisted throttle shaft. You can "untwist" them with some careful spanner application. Carbs need to come off to do it successfully though. If a shaft is twisted, no amount of attempting to balance with the synchrometer will work, until the shaft is either renewed or straightened.

[rideaway]

Hello,

Thank you for your answers.

After having said several ‘’‘%&%@&’' to unscrew the already damaged butterfly screws, I seem to have detected the problem.

The PO had the carburettors rebuilt by a ‘specialist’, which led me not to touch them when rebuilding the engine.
The holes in the butterflies are bigger than the fixing screw, so it's possible to screw them in when they're not properly in place, which prevents them from closing properly.

Best regards
Stéphane

[tonyabacus]

Hello Stephane
Somewhere I wrote to you suggesting this as a possibility having come across this issue several times over the years. Having sent my note it seems to have disappeared somewhere on this site, but where I cannot find out. My experience is that several so called specialists fail to perform this task properly, leading to poor running when the carbs are installed, and owners believing that running problems then can't be due to carburettors as they have been rebuilt.

But for clarity to others that may read your request, what you have found is correct. So for others here is a brief rundown on what to do when rebuilding the carb butterflies,

1. Insert the butterfly in the spindle
2.Replace the screws in the spindle/butterfly, but do not tighten fully
3. Tighten screws just enough to allow very slight movement of the butterfly plate
4.Twist the spindle so butterfly closes and look down the venturi to check the butterfly is positioned correctly.and no light can be seen around the edges of the plate.Snap the spindle back and forth several times with light pressure until satisfied there is no light showing around the edges of the butterfly plate. This action will allow the butterfly plate to move into the correct fully closed position so long as the screws are neither too tight or too loose,
5.Gently tighten the two screws and recheck there is no daylight showing
6. Do not tighten to hard but remember to splay open the ends of the screws with gentle pressure as they can be easily broken off. They only need opening a small amount and will act to stop the screws becoming loose

Glad that you have found the problem Stephane

Tony

[2cams70]

[quote=
6. Do not tighten to hard but remember to splay open the ends of the screws with gentle pressure as they can be easily broken off. They only need opening a small amount and will act to stop the screws becoming loosel

Tony[/quote]

When you do this step be sure to rest the shaft on something solid. I use a brass round inserted into the barrel. Otherwise you risk bending the shaft or damaging the shaft bearings when you start hammering

[tonyabacus]

Opening out the two small screws does not require a great deal of force. Inserting a thin blade of a screwdrver (I use one of those electric testing screwdrivers) into the slot and gently levering to widen the slot a small amount is all that is needed. Also there is no need to tighten the screws too much as a combination of lightly tightening the screw and opening them up will be enough to prevent the screws working loose.

I am not sure what work 2cams70 is talking about in his response, but in my experience sliding the butterflies into the shaft in the venturi usually only needs hand manipulation and then inserting the screws as above. I have never needed a hammer to complete this part of a rebuild

Tony

[rideaway]

Mine were really hard to remove, although they weren't hammered. The slot was already damaged and they were stuck. Even with heating, 2 screws had to be drilled out.
On the other hand, a little fine honing paste on the screwdriver works wonders for screws that resist and where the risk of the screwdriver slipping is high.

I know what 2cams70 means, I think I insure the screws that way. I would hate to have a screw make the journey to the exhaust outlet. And of course it makes sense to put something behind it so you don't bend the axle.
The 60° screw cone should help to prevent the screw from loosening on its own.

[Craven]

Surely self centering ?

[RichardHawkins]

I think Tony and Craven are using different screws. Tony, do your screws have a split end like a split cotter pin? The screws that I bought were like those shown by Craven, which are spread with a centre punch, which of course requires the butterfly shaft to be supported.

Hope this helps,

Richard Hawkins

[tonyabacus]

Richard
Yes you are correct that the screws I was talking about are the original screws with a small slot at the opposite end to the screw head, these are meant to be spread out slightly as a secondary measure of securing the screw.

I can see that those in the posted picture by Craven are different and do not have a slot in the end and would therefore need supporting whilst drilling them out.

There are two different types of butterfly screws commonly used one has a flat underside to the head and the other is countersunk, I think the latter was fitted to most Italian built carbs.

Sorry for any confusion
Tony

[2cams70]

These are the throttle screws I use. Same as original DCOE 31 type except with torx head.

https://www.dellorto.co.uk/shop/dellort ... fly-screw/