18A Zenith downdraught eratic running

[Sunnysoutheast]

Originally Posted by Rustinmotion

To run on you need a source of ignition and as the coil is off something is hot enough to ignite fuel and keep the engine running, usual suspect for this is a carbon build up that glows red hot
Can you do a plug cut, find somewhere safe to drive the car under load and switch the ignition off and dip the clutch then safely pull the car to a stop, take the plugs out and check the colour, this will tell you how it’s running under load rather than at idle
Can you blow out the pipe between tank and pump, that might be partially blocked
I’d still verify the float setting it’s not unusual for it to be wrong on a new carb

I shall check the plugs, rotor arm and re-test.
I could also try the "plug cut" you explain so well.
I did blow clean the front to back pipe before the new tank was installed.
And re-check the float.
Cheers.
Dave

Got family stuff this afternoon so might have to wait till next weekend.

[Derek UK]

You might want to check the connections on the small low tension wire from the coil to the dizzy. Check that the crimps are clean and tight. The nut on the dizzy connection might also be loose. I don't think the Zenith is causing the misfires. Stick to E5.

[Sunnysoutheast]

The over run has disappeared with the E10 today. I drained and replaced the fuel with E5. 👌

I ran around the block and there was a marginal improvement but the engine still hiccoughs and misses.

I swapped out the new rotor-arm for the original old one, because it fits in in better fashion (and someone mentioned that was a problem for them). However it’s not improved the hiccoughs either.

It’s dark now and I have to go to work. 😟

The next step will be the plugs. I have new NGK BP6HS in at the mo.
The originals were Bosch RO 871 Super plugs? Has anyone a preference or advice? (18A - not a sports model)

There is a Bosch Blue coil on its way to me which I will Hotwire, as suggested by another kind replier “Othen”, to check if the coils the problem.

So keep firing suggestions, corrections and advice at me; I won’t be beaten. 😉

[142 Guy]

With respect to your erratic misfire:

- I know you said the wires and cap are new; but, carefully check the ends of the wires (pull the boots back to expose the wire) and the caps for any signs of carbon tracking which would indicate a flashover (which results in a misfire). I had brand new IPD silicone plug wires on my 142 and the termination at the coil tower was flashing over to ground resulting in an erratic misfire. If it misfires at idle, a slightly painful way to locate a wire end flashover is to touch the outside of the termination boot with your fingers. A flashover under the boot can generate a voltage gradient that you may be able to detect with your fingers. I accidentally / somewhat painfully discovered the flashover under my coil boot by feel and subsequent removal of the wire displayed tracking on the end of the wire. Flashovers may also generate a 'snap' which you may be able to hear with careful listening. However, the auditory environment around a running B18 is not conducive to this type of investigation.

- The other place to look for flashover problems is where the coil negative terminal connects to the ignition points in the distributor (as per Derek UK). The nominal voltage of that connection is 0 or 12 volts. But, when the points just open there are transient voltage that can reach around 400 volts and cause a flashover on marginal insulation. Make sure that the inside of the distributor is assiduously clean. Make sure the distributor through connection, condenser and moving point connection are correctly insulated with respect to ground. Condensation inside the distributor is notorious for bad behaviour.

- Dodgy ignition condensers can cause misfires and it seems that new condensers are not exactly what they used to be.

As a last resort, a Pertronix ignition module (or similar) will eliminate problems associated with dodgy condensers and insulation flashovers on the low voltage components inside the distributor. Won't do anything to address high voltage flashovers inside the actual cap.

[Sunnysoutheast]

Originally Posted by 142 Guy

With respect to your erratic misfire:

- I know you said the wires and cap are new; but, carefully check the ends of the wires (pull the boots back to expose the wire) and the caps for any signs of carbon tracking which would indicate a flashover (which results in a misfire). I had brand new IPD silicone plug wires on my 142 and the termination at the coil tower was flashing over to ground resulting in an erratic misfire. If it misfires at idle, a slightly painful way to locate a flashover is to touch the outside of the termination boot with your fingers. A flashover under the boot can generate a voltage gradient that you may be able to detect. I accidentally discovered the flashover under my coil boot by feel and subsequent removal of the wire displayed tracking on the end of the wire.

- The other place to look for flashover problems is where the coil negative terminal connects to the ignition points in the distributor (as per Derek UK). The nominal voltage of that connection is 0 or 12 volts. But, when the points just open there are transient voltage that can reach around 400 volts and cause a flashover on marginal insulation. Make sure that the inside of the distributor is assiduously clean. Make sure the distributor through connection, condenser and moving point connection are correctly insulated with respect to ground. Condensation inside the distributor is notorious for bad behaviour.

- Dodgy ignition condensers can cause misfires and it seems that new condensers are not exactly what they used to be.

A Pertronix ignition module (or similar) will eliminate problems associated with dodgy condensers and insulation flashovers on the low voltage components inside the distributor. Won't do anything to address high voltage flashovers inside the actual cap.

Thanks for that. That’s got me thinking. I could check in the dark for flashes as well as how you’ve mentioned.
The condensation and cleanliness is obviously important for the dizzy and cap.
I agree the condensers are a problem for any points ignition as you say and the modern items are quite often rubbish. My Bantam has suffered for years with the same problem.

Pertronix sounds like a step up, and a good fit for my future. 🧐

While I’m attending to the plugs for burn and dampness I’ll double check all leads.

[142 Guy]

If you can pull back the caps so that area where the wire enters the high voltage insulator tower / spark plug connector, that might allow you to see flashes. If the boots are in place I don't think you will see anything.

[Sunnysoutheast]

Originally Posted by 142 Guy

If you can pull back the caps so that area where the wire enters the high voltage insulator tower / spark plug connector, that might allow you to see flashes. If the boots are in place I don't think you will see anything.

All ready for the midnight check, now I’ve pulled back the boots from all the leads I should see something if there’s anything to see.

While I was under the bonnet I took the plugs out and that does look interesting. The image below, is one to four with four being the most sooted up on the left.

So what’s going on? Can I improve this in some way?

Thanks in advance

[142 Guy]

That is a truly interesting set of plugs!

Is it possible that you have a localized air leak on the intake manifold around cylinders 1&2 that is causing those cylinders to run lean? Do 1&2 have a siamesed runner or something like that where a crack or failed gasket could admit air?

If you can post a picture of the carb and the intake manifold some bright boy may be able to spot something in the configuration that might explain the uneven fuel mixture distribution.

1 and 2 are looking white enough that you might be having a lean misfire, not an ignition misfire.

[Sunnysoutheast]

Originally Posted by 142 Guy

That is a truly interesting set of plugs!

Is it possible that you have a localized air leak on the intake manifold around cylinders 1&2 that is causing those cylinders to run lean? Do 1&2 have a siamesed runner or something like that where a crack or failed gasket could admit air?

If you can post a picture of the carb and the intake manifold some bright boy may be able to spot something in the configuration that might explain the uneven fuel mixture distribution.

1 and 2 are looking white enough that you might be having a lean misfire, not an ignition misfire.

There is an oddness about those plugs and their gradiated colour. I might have to check the manifold bolts for tightness.

[Sunnysoutheast]

Originally Posted by 142 Guy

With respect to your erratic misfire:

- I know you said the wires and cap are new; but, carefully check the ends of the wires (pull the boots back to expose the wire) and the caps for any signs of carbon tracking which would indicate a flashover (which results in a misfire). I had brand new IPD silicone plug wires on my 142 and the termination at the coil tower was flashing over to ground resulting in an erratic misfire. If it misfires at idle, a slightly painful way to locate a wire end flashover is to touch the outside of the termination boot with your fingers. A flashover under the boot can generate a voltage gradient that you may be able to detect with your fingers. I accidentally / somewhat painfully discovered the flashover under my coil boot by feel and subsequent removal of the wire displayed tracking on the end of the wire. Flashovers may also generate a 'snap' which you may be able to hear with careful listening. However, the auditory environment around a running B18 is not conducive to this type of investigation.

- The other place to look for flashover problems is where the coil negative terminal connects to the ignition points in the distributor (as per Derek UK). The nominal voltage of that connection is 0 or 12 volts. But, when the points just open there are transient voltage that can reach around 400 volts and cause a flashover on marginal insulation. Make sure that the inside of the distributor is assiduously clean. Make sure the distributor through connection, condenser and moving point connection are correctly insulated with respect to ground. Condensation inside the distributor is notorious for bad behaviour.

- Dodgy ignition condensers can cause misfires and it seems that new condensers are not exactly what they used to be.

As a last resort, a Pertronix ignition module (or similar) will eliminate problems associated with dodgy condensers and insulation flashovers on the low voltage components inside the distributor. Won't do anything to address high voltage flashovers inside the actual cap.

It’s dark and cold but the engine starting up was easy. No flashes anywhere on the ignition even with the boots pulled back. Really good sign but I’ll have to look further into the problem.

First light I’m going out for a spin after replacing all the HT leads boots, and checking the tightness of the manifold fixings.

The plugs colours and sooting are haunting me now. I’m going out on the old Bosch plugs too.

Wish me luck.