Ariel square 4 misfire

[MARK.WALSH]

Hi all
I'm after a bit of advice
I have covered well over 7 thousand miles since I restored my 1951 square 4, and like most owners I have had the odd carb problem, or more usual - misfires caused by points, or condenser, carbon in the dist cap etc, but the latest misfire is proving hard to eliminate.

I moved over to an accuspark electronic ignition this summer and the transformation was immediately noticeable. Better idling and smoother running plus less demand on the battery at slower speeds reducing the possibility of volts dropping whilst doing 30 mph in country lanes.
All has been more than well, but recently the bike has become a bit harder to start, (not the usual 1st or 2nd kick) and running is poor with missing at all speeds - especially on acceleration.
The only two things I haven't renewed of late are the HT leads and the ignition coil.
Does anyone know how often coils should be renewed or if this sounds like a coil problem? Its been on there for the full 7000 miles but I wasn't sure if this was beyond their life expectancy. I don't think the Accuspark is faulty but I'm loathe to replace the conventional points and condenser at this stage just to check.
Many thanks
Mark

[simon.holyfield]

You can measure the coil resistances and observe it's running temperature. I wouldn't expect the coil to fail after 7000 miles - mine is a 1951 coil and I've run loads of old BMC cars on very old coils.

That doesn't mean it's not knackered though. Does it get hot?

Your HT leads - are they and their connections protected from water and oil? Is one of the plugs giving up?

[MARK.WALSH]

Simes,
thanks for the reply. Will have to check if it gets hot next time I take it for a run. The coil lives behind the battery case so getting a grip of it isn't so easy unless I unbolt the battery strap
All connections on leads are good , just a bit of oxidisation on the coil lead terminal on the distributer cap which I had previously cleaned (I thought that would cure it)
I will check the windings to see how many ohms it produces, I think 2 ohms is the norm.
One thing I might do is open the plug gaps a bit more. Reading some threads on the forum this evening and some owners advises 25thou - mine are only 20 thou. Might make a difference?

[simon.holyfield]

Did your coil originally live behind the battery Mark? Mine does too, I'd be interested to see a picture.

Coil resistances - the important bit is the relationship between the two readings. The primary should be 2-4 ohms, the secondary should be around 2-4 Mohms.

I'd be surprised if the 20 thou plug gap is the problem, but it's certainly not helping you. 25 is more usual for a coil ignition system, but bear in mind that if there's a problem with the coil, opening the gap up will probably reveal it.

You haven't got a fuel supply problem have you?

[MARK.WALSH]

The mystery may be solved!
Just did some tests on the ignition this morning. Coil seems okay but will recheck when hot. Opened up the plugs a bit to 25 thou. What I then noticed was the left hand plugs have a far better spark than the right hand plugs where the spark is weaker. Additionally the right exhaust pipes took longer to get hot when starting
I concluded this must be a rotor arm, dist cap problem.
Swapped the distributor cap over for a new one and noticed that one of the cap retaining clips is extremely difficult to clip on. I think where the Accuspark wires sheath exits the cap is tight and putting undue pressure on one side of the cap, possibly making it sit eccentrically (or the accuspark baseplate is slightly thicker than the original lucas base plate.
After struggling for quite a while to secure the clip (easing the point in the cap with a file where the wires exit) I started the bike up and she sounds more even.
Peeing down with rain so cant test out my theory until the weather improves, but the bike started easier.
Simes - As for the coil mount I made one out of plate alloy to keep the coil vertical. When I very first built the bike it sat under the saddle - horizonal, within 500 miles it leaked out oil and left me stranded in the countryside. I only fall for that once!
Hopefully you can see the set up. The coil is well protected by the battery and bracket and the bike alarm sits in front of it

[simon.holyfield]

Good find there Mark! Sounds like you sorted it.

[MARK.WALSH]

Well well well,
the plot thickens!
After more investigation of the ignition system I have worked out the actual problem. Fake NGK spark plugs. I had replaced my last set when I serviced the bike less than 100 miles ago and all looked/worked fine. The problem gradually became apparent with engine misfires within that mileage but assumed it couldnt be the plugs. After stripping out leads for continuity and looking for damaged outer lead covers I eventually replaced my older plugs. Problem gone!!
I just checked the NKG web site which covers counterfeit copies, and the plugs I removed showed two distinct giveaways.
1 - no batch number on the hexagon flat of the plug and 2, - loose sealing rings on the threads.
Last time I purchase plugs on ebay.
- The seller has now been informed
Just took the bike for a 12 mile test ride - back to its normal smoothness and acceleration with no misfire. (phew!)

[paul.jameson]

Well done Mark. The Square Four is more than capable of creating its own misfires without the aid of counterfeit spark plugs so you did well to find the problem.

[robjameson]

I don’t use NGK plugs for exactly that reason, having had several issues with the plugs over the years, mostly due to the difficulty in finding genuine plugs!

I’ve used Bosch plugs for a long time now and had no issues.

[david.anderson]

Mark
Good news that you have solved the problem.
I will bet that the seller obtained the spark plugs from China and as such would have known that they were counterfeit. Chinese counterfeits have been a problem in Australia for some time, mostly purchased through ebay. If you purchased them with paypal and you provide relevant details such as photos of the plugs and a link to the NGK site regarding counterfeits plugs to show they are not genuine, paypal will probably refund you.
David