Richness screw not working (weber 151)

[Niklas777]

Hello everyone,

I am facing a problem concerning the adjustment of one of my 2 weber 151 carburettors on an Elan s3 s/e.
The richness screw on a body (left) of the left carb (closest to the windshield) does not work (no effect if you screw/unscrew it), which makes it impossible to adjust. I didn't have this problem before.
I measured the depression at the 4 carb inlets, which is the same, I dismantled and cleaned the left carb, changed the needle, and the problem persists.
These carburettors are barely 1 year old...

Do you have any idea where this could be coming from?

Thanks in advance

[SENC]

2 thoughts -

1, there are different size screws, a longer tapered screw and a shorter tapered screw - make sure all yours are the same (I think the longer tapered are right for 151s, the shorter taper for early dcoes).

2, they can be screwed down to far and damage the carb and/or threads and introduce an air leak. Get some EZTurn fuel lube and coat the screws to prevent air leaks. Some vaseline may be an alternate temporary way to test.

[elans3]

An airscrew not working is normally a sign that one or more of the internal channels is / are blocked with crud. You then have to drill out through the lead "bungs" that give you access to the channels, and then drill out the crud from those channels before resealing them with lead as per original.
I've had to do maybe a dozen in 10 years of buying / selling / refurbing DCOE's from 2009 to 2019. It normally comes about from the carbs standing with stale fuel inside, and doing no mileage.
normally it's the ones underneath close to the carb "sump", and/or the ones that are on the side of the carb body, I used to use a very small drill bit now guessing 1mm or just under. It can be a sod to get through, patience is a virtue, but rewarding when it all works again !
Once the crud has accumulated and set, no amount of ultrasonic cleaning (I had a good one), will shift the stuff, it has to be mechanically removed as above.

[Niklas777]

Thanks all,

I will check if i can do some tests with a bit of grease on the needle to see if a potential air leak is decreased. The screws that are present are the stock screws (except the one i have changed), as the carbs are almost new.
Concerning the crud, fuel seems to go through freely and the carb is not even 1 year old with a car running almost every weekend.

The issue got worse overtime. The first time it appeared i managed to unclog the channel with pulsed air, but the issue came back very quickly.

[h20hamelan]

Has a leak-down test been done?
Otherwise, have you the same flow on each cylinder?

[rgh0]

I would also check that the cylinder is actually firing and you have a good spark.

cheers
Rohan

[webbslinger]

It is very very easy to break the tips off of the adjustment screws if you have the fine brass ones. The tip then stays in the hole plugging it.

[Niklas777]

It is very very easy to break the tips off of the adjustment screws if you have the fine brass ones. The tip then stays in the hole plugging it.

Thanks for this. We do you recommend to proceed in this case?

[2cams70]

Why do you need to suddenly tinker with the carbs if they are barely a year old? What change (if any) to the way the car runs has happened to make you want to do this? Is it a persistent problem ever since they were originally fitted a year ago or is this problem something new?

[Niklas777]

Why do you need to suddenly tinker with the carbs if they are barely a year old? What change (if any) to the way the car runs has happened to make you want to do this? Is it a persistent problem ever since they were originally fitted a year ago or is this problem something new?

Something new of course, otherwise i would not have touched anything.
The car runs not well, especially during progression and iddle. + the fact that carbs cant be balanced due to a richness screw not being functioning.

[2cams70]

Have you checked that the throttle butterflies are all opening the same amount when just cracked open? I know you say you've measured the depression but it's well worthwhile checking it physically. The throttle opening is the starting point for everything else.

[2cams70]

Something new of course, otherwise i would not have touched anything.
The car runs not well, especially during progression and iddle. + the fact that carbs cant be balanced due to a richness screw not being functioning.

The reason why I ask these questions is because some people do suffer from OCD when it comes to their cars (and maybe other things as well)

Have you checked that the throttle butterflies are all opening the same amount when just cracked open? I know you say you've measured the depression but it's well worthwhile checking it physically. The throttle opening is the starting point for everything else.

[Niklas777]

Yes the throttle butterflies are perfectly synced, thanks.

[webbslinger]

Regarding your question about what to do about a broken idle adjustment screw, I sent my carbs to Pierce Manifolds. At the time I didn't know what the problem was, but they identified it and removed the broken bits. They sent the carbs back with steel screws. I've had no issues with them since. Don't let anyone talk you out of fiddling with them; as one member said on here a while ago, "Webers are a tinkers dream".

[Niklas777]

Putting grease on the screw like suggested in the first answer above it works...now i can use the screw without issue...
Now let's see trying the car to see if it stays like this!
Anyway, thanks for the help! Those Spanish Weber seem a bit special...