Weber throttle by-pass settings

[Foxie]

I fitted a set of Spanish made Weber 45s some time ago.

They are running very well.

I have just been trying to fine tune the idle speed. The idle mixtures are perfect, the throttle return springs are new, the linkage rod adjustment is correct, the idle speed adjuster is off the stop, and the throttles plates are fully closed.

But the idle is still too fast, around 900 rpm. With previous carb sets I could get it down to a much lower speed

On this set of carbs there is a throttle py-pass port on each barrel, with an adjustable needle-valve and locknut, located just below the pump jet, see pic I cannot find any information on setting these, I seem to remember seeing something about these being factory settings and not to be adjusted.

Before I start experimenting, has anyone any ideas ?

[EPA]

The additional adjustment is to compensate for any discrepancy between the two barrels of the carb when adjusting
the air flow/ balance. They would normally be screwed fully in and only adjusted if there is a slight difference in the air flow.
I would check the throttle linkage to see if the throttle is being held slightly open, it doesn’t take much.
Ed

[Foxie]

I would check the throttle linkage to see if the throttle is being held slightly open, it doesn’t take much.
Ed

All checked already !

[h20hamelan]

I have adjusted mine, not sure if it helped mine much-but it did a tad.
There are some good Weber docs, one just recently posted. See if I can find it, also a video from the UK guy does Mini and Elise.
I cant recall exactly, but 1/4 turns at a time. Release the lock nut, out gives more air from what I recall.

Will try to track down his DCOE YouTube, and find the Weber doc.

Do you have o2?

——-edit

DCSshooter YouTube

https://240260280.com/Tech/Carbs/Weber/ ... uning.html

[mbell]

They are for balancing the air flow at idle between the two throats of the carb. It could be that they are by passing too much air and letting it idle higher.

Not sure if they are set by the Weber factory or not. I would expect the balance between throats to vary a bit between engines based on various engine characteristics so don't think there is an issue with adjusting them.

Personally I use them (DCOE151) to balance all four throats on both carbs rather than using the adjuster on the linkage. This allows aligning the throttle plates at consistent position between the carbs (before balancing), so the progress holes are exposed at the same time on both carbs.

[elans3]

As per EPA, these screws are ONLY to use if you have an imbalance of airflow in both barrels of ONE carb. (likely a slightly twisted throttle shaft). Otherwise they are to be lightly tightened down and locked with the locknut.
Adjusting these for any other reason is only masking a different problem, and you'll be chasing your tail.

[ericbushby]

Hi Foxie.
I have the similar but 151 type Weber carbs.
To set the air bypass screws correctly you will need a scroll type air flow meter, such as the Synchrometer.
Expect about 3.5 to 4 Kg/Hr.
Make sure both bypass screws are closed, and then running at idling speed check the airflow on both trumpets.
If one barrel is lower than the other, then open it`s bypass screw (anticlockwise) until they are equal. you may have to keep adjusting the idle speed back to the original setting.
It is important that one screw is closed and only the lower flow side is adjusted.
When satisfied move over to the other carburettor and balance that one too.
Now balance the two carburettors with the adjusting screw on the shaft between them.
This, and setting the idle mixture with a Colourtune plug gave me smooth and steady idling.
Mine is set to idle at 900 to 1000 RPM, which I always thought was correct for this engine.
Some say this can be done by measuring the vacuum level in the inlet manifold. My opinion is that if you want to balance airflow, then airflow is what you should be measuring.
Hope this helps,
Eric in Burnley
1967 S3SE DHC

[richardcox_lotus]

At 900 rpm I also think you have it pretty much spot on. You may be chasing a problem that doesn’t exist.

[Foxie]

Thanks Eric

That procedure makes a lot of sense ! I'll try it in a few days when I have the engine back in the car after bursting the clutch last week-end !

Thanks also for the many helpful comments, and also the post with the great Weber graphics.

[vxah]

I was thinking, you did say 45’s? That’s quite a big carburettor for a road tuned twincam. What I mean is that the throttle bores are large so a small opening with the stop screw would allow more air flow than with a 40, coupled with balancing screws that may not be fully home (set for a bigger engine that wouldn’t be affected so much) and this could be your issue?
I would check them balance screws and close them fully if they are not already, taking note of where they were?
I’m also interested in what chokes you have and how you find the 45’s? This is because I have a spare set of Dellorto 45E’s that I was considering fitting but, people say they are too big and may cause poor response etc?

[elans3]

I'd agree with the above. way too big for road use.
45's only needed for out & out race cars. A few years ago I had an ex hillclimb Elan with 165 bhp (Big-bore T/C), and it ran 40's with 33mm chokes, and it was the fastest thing to 100mph I'd ever had. It was 1825cc, and was as sweet as a nut driving around town too.

[Foxie]

I was thinking, you did say 45’s? That’s quite a big carburettor for a road tuned twincam. What I mean is that the throttle bores are large so a small opening with the stop screw would allow more air flow than with a 40, coupled with balancing screws that may not be fully home (set for a bigger engine that wouldn’t be affected so much) and this could be your issue?
I would check them balance screws and close them fully if they are not already, taking note of where they were?
I’m also interested in what chokes you have and how you find the 45’s? This is because I have a spare set of Dellorto 45E’s that I was considering fitting but, people say they are too big and may cause poor response etc?

I just replied to this, but a notice popped up saying there was another reply, did I want to edit my reply, then my post disappeared !

So here we go again:

Yes, they are 45s, with 38mm chokes. I have been developing my engines(s) over more than 20 years, and now it's an all-steel 1860cc, with QED big valve head and QED 450 cams.

It's a road car that I use for historic racing and hillclimbs.

I am really happy with how smoothly it runs after reading so many stories over the years about big carbs and cams.

Not a lot of power under 2K rpm, but that doesn't bother me.

The carbs are out on the bench at the moment, and I can see the by-pass needles are partially open. As I said previously the linkage, springs etc. are all good, but when I "lean" on the main shaft the idle really goes down, "ticking over" nearly like a big marine diesel.

So downwards and onwards !

[elans3]

Ha ! Now you've filled us in with the rest of the engine spec,, and what you use the car for, I can see the reason for the 45's.