Hi There,

I'm new on here and new to Austin Sevens. Already I need help!

I have a magneto ignition engine which is running on three cylinders and has been since I got it. I firstly fitted a new set of plugs, set at 15 thou, but when I clip my timing light on the plug leads there is no flash from No 2 lead. The other three are fine.

The magneto is a Bosch and I've had it on the bench, cleaned the points, set them to 14 thou, and also cleaned the distributor cap with contact cleaner and a rag. I also cleaned the HT brushes at the other end of the magneto. Then just now I thought I'd found the problem, the No 2 lead was loose in the distributor cap and the screw holding it was loose and corroded. I had some new solid copper HT lead so I replaced all the leads using new screws and before refitting the cap I tested from the brass segment inside the cap to the plug end of the lead and got virtually no resistance on any of the four leads.

Confident that I'd fixed it I put it back together and started her up... still on three cylinders and no flash from the timing light when hooked on to No2 lead.

Can anyone offer any suggestions as to what I should try next?

Any help much appreciated.

Kind Regards
Paul in NZ.

Location: Tauranga, New Zealand

Do you have compression on number 2?

Hi Charles,

Thanks for your reply. No I don't have the compression figures at this stage. However, since writing my question I've discovered that the plug in No 2 had very little resistance from the electrode to earth. The one in No 3 wasn't much better but No 1 and No 2 were fine. The only other plugs I have to hand are the original ones which tested OK so I've put those back in. Unfortunately I flooded it when trying to start it (it has a down-draft SU)so I've had to leave it for now due to other commitments.

Regards
Paul.

Location: Tauranga, New Zealand

Is there a hairline crack in the cap?

Try running it in the dark to see if there is any tracking.

Thanks Ruairidh. That seems likely but I have had a good look and couldn't see anything. I also ran it in the dark but there was no sign of any leakage. However, the distributor cap is showing its age ad it could well be that there's a hairline crack I can't see. Unfortunately I don't have another one to swap it with.

Regards
Paul.

Hi Paul

I wonder if you are getting plug fouling by carbon and oil ? This can be a bit of a vicious circle, as an initial misfire fouls up the plugs, which then makes the misfire worse. It's worse if the engine is run off load and never gets thoroughly hot. It's possible to have a faulty or lightly fouled plug which seems OK i.e. shows open circuit when tested at low voltage (with an Avometer or modern equivalent) but conducts at higher voltages. This can allow the ignition energy to leak away, and the voltage across the electrodes never quite rises high enough to make a spark. You could try swapping the plugs between the cylinders to see if the fault moves, in which case the finger of suspicion points to the plug rather than the magneto or the cylinder. If the fault stays with cylinder 2 then I would suspect a valve or compression problem.

Location: New Forest

Would a valve problem prevent the magneto from sparking on one lead?

Have you tried it with the plug out and earthed to the head?

Location: Devon

If you still have no spark on No2 swap the plugs around and see if the misfire moves with the plug.

Location: Devon

If having done this No2 is still not sparking the problem has to be with lead or Mag.

Location: Devon

Hello John,

I think you might be on to something here. At present I have the car on stands in my shed whilst I do some other work on it and I've been trying to fix this misfire problem at the same time. As a result it doesn't really get properly hot when I'm test running it. Also, and this may be the main point, I've had trouble with the SU's float chamber flooding and petrol running out of the jet. I've fixed the flooding problem but during that period the plugs were quickly fouled. I cleaned them and refitted them more than once but this repeated sooting may have weakened their resistance to earth. I take your point about the vicious circle. It's possible that the lead to No 2 had a bad connection in the distributor cap as well - also fixed when the new HT leads were fitted. This may have been more than one fault originally.

I'll start again with clean plugs and take it from there.

Thank you very much for your advice.

Regards
Paul.

Hi Stuart,

I've had the timing light showing a spark on No 2 when I tried a different plug so I believe it's either Ruairidh's theory of a hairline crack in the distributor cap causing intermittent earthing or John's suggestion of the viscious circle of plug fouling with a rich mixture in an engine that not's fully hot.

I can't spend any more time on it until the middle of next week now but I'll report back when I've had another go at it.

Thanks you very much for your help.

Regards
Paul.