Pair of Weber's cannot sync

[LaikaTheDog]

Hi
I have a pair of Weber's dcoe40 151

The engine starts and runs ...kind of

I know they are unsynced , I have them set with basic rebuilt settings (as you start out on balancing/tuning), but the left carb is sucking 3kg/hr and the other one is under 1 kg/hr

So I try and get it running at about 2000rpm and the left carb sucks off the scale and the right carb sucks about 3

There is no way I will ever get these synched at this base point, if I try it runs like a tractor/ferrari and stalls

While warming up, a blip of the throttle results in a very odd overrun, almost as if the butterflies are not reseating... it revs away and then settles even though the butterflies are closed.
Floats are setup, all jets etc match
Throttle cable is disconnected
Choke is disconnected
Timing is spot on
Misabs are sealing ok (my symptoms look like poor vacuum on the right carb)

Tomorrow I will swap front to rear and see if the problem follows the carb....

Any ideas?

[ericbushby]

Hi there.
The 151`s have an air bypass screw for balancing the air flow between each barrel.
It is under the white plastic cap at each side. One screw should always be closed whilst the low airflow side is opened to match. Measure the air flow with a snail type meter (synchrometer).
Expected flow is about 3 Kg/Hr. at 1000 rpm
When you say 1 and 3, one and three what ?.
Next, balance the two carburettors with the adjustment on the throttle link between them.
Until the air flow is balanced on all four barrels, there is not much point moving on.
Returning to idle speed slowly is called `lazy idle` and I cannot think what causes it at the moment.
Someone else may know.
I realise you may have done these settings but it is somewhere to start.

Eric in Burnley
1967 S3SE DHC

[mbell]

You need to balance them at low rpm. The point is to ensure balance air flow at very small openings. As the openings increase the differences become irrelevant.

As said you need to check the air bleed screws, on mine they aren't under any covers. Screw+Nut on the side of each carb throat.

Eric's method is the normal correct procedure. I follow a different one of removing the transition hole cover and using the throttle balance screw and idle speed screw to position the throttle plates in the correct/idle location, just to expose the first transition hole to the engine side on both carbs. I then balance them fully using the air bypass screws, using a 4 manometer based carb sync on the 151 vacuum take offs.

[Esprit2]

First make certain the carbs are properly installed on the soft mounts. Any air leaks at the soft mounts will thwart any attempts to balance & tune the carbs.

Remove the progression hole covers and look in. At the bottom of the hole, you'll see several small holes drilled through the floor into the carb's throat. Move the throttle lever, and you'll see the edge of the butterflies sweep past the holes.

When the throttle is closed, ideally all four butterflies will be in the same position relative to the first hole in that throat. If the butterfly positiions (balance) is off between the two carbs, then the two butterflies in each indvidual carb might align similarly with their progression holes, but the two butterflies in one carb will not match the two butterflies in the other carb. That's a throttle linkage adjustment issue, and relatively simple. Turn the screw on the linkage adjuster as required to 'eyeball' match the butterfly pairs carb-to-carb.

However, if the two butterflies on a single carb don't align equally with their respective progression holes, then the throttle shaft is twisted... and that's bad. It usually happens when you use two wrenches to tighten the nuts on the front and rear ends of the shaft... putting all the tightening torque load through the shaft. NEVER, ever do that!

If the shaft is twisted, the 'book' fix is to replace the shaft. However, at that point you have little to lose by trying to untwist the shaft by carefully (!) applying a similar but reverse torque. If that turns out to be the case with your carbs, we can get into more details later.

Adjust the idle speed as slow as possible consistent with smooth running... no missing or spitting.

With all the air bleed screws fully closed and seated, measure the airflow in all four throats. If the two carbs are grossly different, adjust the linkage coupler screw to bring themas close together as possible.

Adjust the Idle Mixture screws for peak airflow... or peak manifold vacuum using a manometer, or peak rpm using a diagnostic tach (not the car's tach), or loudest intake hiss by ear. All those characteristics follow the state of tune/ balance, so pick one. I just strongly recommend that you NOT choose 'by ear'.

With each adjustment that improves the 'tune' or 'balance', the idle speed is likely to increase. So with each adjustment, re-adjust the idle speed as slow as possible consistent with smooth running. You will be constantly re-adjusting the idle rpm down.

With all the air bleed screws fully closed and seated, measure the airflow (or manifold vacuum) in all four throats. If the two carbs are different, adjust the linkage coupler screw to bring them as close together as possible. Re-adjust the idle speed.

Then, in each individual carb, if one throat is "significantly" stronger than the other, re-visit 'twisted shaft' above. If there's a minor mismatch between the two throats in a single carb, then open the air bleed screw on the stronger throat only as much as is required to weaken that throat enough to match the weaker throat... no more. Ideally, less than half a turn will be required, but 3/4 turn is reluctantly acceptable. If one full turn or more is required, then you should be taking another look at 'twisted shaft', above.

When you're done adjusting balance, only one air bleed screw per carb should be cracked open. The other should be fully seated.

Re-adjust the Idle Mixture screws for peak airflow (manifold vacuum/ rpm/ hiss).
Re-adjust the idle rpm to as slow as possible consistent with smooth running.

Each adjustment affects other adjustments. So repeat the adjustments over and over until the last round results in no further change... continuously re-adjusting the idle speed as slow as possible as you go.

When you're done, adjust the idle speed to the normal running rpm.

Yes, it's putzy work, and it requires patience. But it's not rocket science, and not difficult. Just do good work.

The Synchrometer is a very good airflow meter, if that's the way you choose to go. But a 4-tube manometer makes the task much easier, and the CarbTune Pro is the best 4-tube manometer. The UniSyn is best used as wall art. The rubber tube used to listen to the 'hiss' by ear is best used to hang said wall art.

Most Webers don't have the vacuum ports that allow a manometer to be hooked up, but the DCOE-151 and -152 do have them. Take advantage of them.

Regards,
Tim Engel

[pharriso]

Tim, thanks for takingthe time to post this tuning procedure.It's the most succinct tuning procedure that I have seen.

I have had an ongoing issue with the throttles jamming slightly open , raising the idle speed between 1200 & 2500 rpm- I am pretty good at pulling the accelerator peddle up with my right foot.

I am about to take my Webers off to replace the throttle spindle bearings & will take care to center the butterflies in their bores. I'll then follow your procedure to tune them.

The thing I have struggled with in the past is the mixture screw, I do not have vacuum ports & there is no easy way to measure engine speed since there is little room to place an optical tachometer between the crank pully & radiator.

[Donels]

Ref comment on not to set air flow by ear. That is also my understanding and have always used a Synchonometer however it’s interesting to watch 'Tyrell's Classic Workshop' On YouTube. He always sets up multi- carburettor Ferrari’s and Lamborghini's by ear, admittedly he appears to be a genius!

[mbell]

there is no easy way to measure engine speed since there is little room to place an optical tachometer between the crank pully & radiator.

FYI: I've bought one of these:
https://www.amazon.com/AIMILAR-Tachomet ... B07K7LP3PT

To allow me to more calibrate my tacho on the car and get more accurate rpm. Just need to wind the cable around the main HT line and you get engine rpm.

[pharriso]

there is no easy way to measure engine speed since there is little room to place an optical tachometer between the crank pully & radiator.

FYI: I've bought one of these:
https://www.amazon.com/AIMILAR-Tachomet ... B07K7LP3PT

To allow me to more calibrate my tacho on the car and get more accurate rpm. Just need to wind the cable around the main HT line and you get engine rpm.

Thanks for the suggestion Mark, for $14 I'll buy one.

[Esprit2]

Or just buy an old-school Tach & Dwell meter. But use some 'diagnostic' tach that is likely to be more accurate or sensitive than the one on the car's dash. It would still hook up to the coil and ground with aligator clips, so no great hardship or space required.

Regards,
Tim Engel.

[Esprit2]

however it’s interesting to watch 'Tyrell's Classic Workshop' On YouTube. He always sets up multi- carburettor Ferrari’s and Lamborghini's by ear..

I don't recommend tuning carbs by ear, or setting timing belt tension by feel, only because it takes practiced experience to be able to do that accurately & reliably. It's often the first-timers who are looking for a cheap way out who want to tune by ear, or tension by feel. And they're the ones least prepared to do it.

I've been doing this stuff for a very long time, and I'm confident in my ability to tune by ear, or to tension by feel; however, if there's a proper instrument handy, I always use it as my first choice. Then I save the 'bodily functions' methods for times when the correct tool for the job just isn't available... like roadside emergencies.

Regards,
Tim Engel

[pharriso]

however it’s interesting to watch 'Tyrell's Classic Workshop' On YouTube. He always sets up multi- carburettor Ferrari’s and Lamborghini's by ear..

I don't recommend tuning carbs by ear, or setting timing belt tension by feel, only because it takes practiced experience to be able to do that accurately & reliably. It's often the first-timers who are looking for a cheap way out who want to tune by ear, or tension by feel. And they're the ones least prepared to do it.

I've been doing this stuff for a very long time, and I'm confident in my ability to tune by ear, or to tension by feel; however, if there's a proper instrument handy, I always use it as my first choice. Then I save the 'bodily functions' methods for times when the correct tool for the job just isn't available... like roadside emergencies.

Regards,
Tim Engel

Wow, fascinating, see https://youtu.be/gltlKhTvIL0 Love the party trick!

Just lost an hour

[SENC]

however it’s interesting to watch 'Tyrell's Classic Workshop' On YouTube. He always sets up multi- carburettor Ferrari’s and Lamborghini's by ear..

I don't recommend tuning carbs by ear, or setting timing belt tension by feel, only because it takes practiced experience to be able to do that accurately & reliably. It's often the first-timers who are looking for a cheap way out who want to tune by ear, or tension by feel. And they're the ones least prepared to do it.

I've been doing this stuff for a very long time, and I'm confident in my ability to tune by ear, or to tension by feel; however, if there's a proper instrument handy, I always use it as my first choice. Then I save the 'bodily functions' methods for times when the correct tool for the job just isn't available... like roadside emergencies.

Regards,
Tim Engel

Wow, fascinating, see https://youtu.be/gltlKhTvIL0 Love the party trick!

Just lost an hour

Thanks for sharing, that was fun!

[nmauduit]

regarding synch issues, if the butterflies have been taken out at some point, it's easy to reinstall them just slightly off, which may cause slight bind and/or slight unbalance : looking at the fit when they close using a backlight is one way to assess proper centering. Then if perfect fit is not reasonably achievable by repositionning each butterfly (loosening a bit, push fitting to seal, retightening on the shaft, checking, ...), esp. on older carbs or carbs that have endured numerous rebuilds, then what I do is settle for the best compromise and balance them not at full close because of this small residual inbalance but just slightly above (say around 1500 rpm, depending on flow meter used), so as not to induce a running unbalance that would remain in the whole low rev regime.

[LaikaTheDog]

Found the problem.

I bought a new "lotus" lever, but when they had bent over the tab for the throttle stop it had bulged the metal so it was not allowing the butterflies to completely close, even with the nut all the way out the bulge was hitting on the Weber casting and stopping the butterflies from fully closing.

A couple of minutes with a file and now they are almost synched before adjustment.