sudden onset of open throttle stumble syndrome

[baileyman]

A stumble has shown up and I'm a bit over my head on it.

The symptom is that mere toodling around town is just fine. Constant RPM is just fine (any constant RPM is the same). But accelerating more than gently from any RPM causes a stumble, where until yesterday afternoon was vigorous acceleration on demand. Although on startup, it still blips nicely.

The car has 45DCOE's and magnetic ignition. Everything is in as-new condition.

My first feeble thought was fuel starvation, since the stumble correlates with open throttle. Swapped the fuel filter. Checked the carb strainers. Checked the Hurco flow to the front.

Since it seemed maybe it was just one carb having difficulty I found fuel in the float chambers is the same height for both. All the plugs have the same good looking color.

There were a couple instances of good running during a short test this afternoon, but mostly poor.

And that's just about as far as I can go, so now I am watching Weber vids to see if anything shows up. Thoughts appreciated! John

[Chrispy]

Assuming your ignition timing is correct I would be looking at the acellerator pumps in the carbs.

What are the plugs like?

[baileyman]

How to test accelerator pumps? John

[elans3]

Take one out at a time & wipe it clean, then (and this is not a recommended way, but has worked for me for 40 years), place it in your mouth, close your lips over the centre "saddle" of the jet, with the bottom outlet hole pointing upwards. Blow down the jet. If it's not blocked, you will feel the air blowing up or past your nose. If you can't, it's blocked.
Alternatively, you can suck from the bottom end. If you're able to suck air through with your lips closed, then it's clear. You're more likely to get any residue of petrol into your mouth (unless it's completely dried out) this way though.
I'd agree with Chris though. Pump jets most likely..

[2cams70]

I'd suggest isolating one cylinder at a time by shorting out (short it out don't just leave it hanging especially if the car has electronic ignition) each spark plug lead in turn. Take the car for a short run each time and try and make it stumble. By noting the effect you may be able to determine whether one cylinder only is a problem or whether it is associated with a more general issue.

[nmauduit]

How to test accelerator pumps? John

you can have a look through each venturi with the help of a mirror, engine off. If on e or several plugged, you may want to check the filter at the banjo fuel port of each carb.

Also, there is the cuve bottom valve in the pump circuit (jet to control the amount of fuel leaked back as a way to vary the amount pumped per stroke), one per carb.

[Craven]

Accelerator pump jets can be checked when removed by using an aerosol can of penetrating oil or WD 40 with straw. Just hold the straw against the open end of jet, quick squirt produces a good jet of fluid from the side facing jet hole. A dribble indicates a partial blockage. You will soon get the idea.

[baileyman]

Thanks guys. I now have today's assignment. John

[powellsmail]

If you have one to hand, try replacing the rotor arm. There have been bad batches and the failure is a known issue. My car had similar symptoms and gradually got worse until it would start fine when cold but when warmed up just stumble under load and die. Some rotor arms have the central rivet through too far which under load can cause a short through the rotor arm straight to earth. Initially it causes a stumble but as there are more arcs to earth the rotor arm just breaks down.

[baileyman]

Okay, we may be on to something.

Son James noticed that probing the passages for the main circuits, #2 did not go as deep.

The kink is the depth the others went to.

Then using a mirror he found #2 was missing two screws under the throat and the locking tabs piece.

This so far is consistent with the sudden onset of stumble and the stumble coming on at about 5 deg of throttle.

John