Mysterious Engine Failure - Ideas, please

[Hugh Barnes]

80% of fuel problems are electrical related

[Duncan Grimmond]

My 4-cylinder BSA trike (that I sold in 2011) suffered from this problem. It would start well and run just fine until it suddenly died after a random number of miles. Eventually while I was looking at something else I noticed that the coil was incorrectly wired. It was a positive earth set-up and when I fitted a new coil it had been wired incorrectly, a quick swap of the terminals and it was as right as rain.

[Graham Barker]

My Ruby is going again now.  I mentioned in my modified original thread that I had replaced the contact points to attempt to correct slight misfiring.  The old points were set at 25 thou gap.  I set the new points at about 12 thou gap because that was recommended in the Austin manual.  This is when the trouble started.
I checked the points gap a number of times during all my fault-finding steps and they continued to be in the region 10-12 thou.  But the performance of the engine was worse and worse with every start until it wouldn't run at all - just gave a few backfires.
Today, I set the points gap to 25 thou and hey presto the engine ran well.  I ran it for more than 10 minutes in the garage before turning it off.
When I pluck up courage, I will take it for a long run to confirm that it is all sorted.
Unfortunately it still has the original slight misfire. I will run with that.  This could be a case of not seeking a perfection for sake of reduced stress.
Thanks for all the helpful comments.
Regards
Graham.

[Colin Cooke]

Hi Graham,

Thanks for letting everyone know how you solved the problem. Always good to find out.

I will now apologise if I am asking a silly question (which it probably is).....Is it possible you were using metric feeler gauges 0.10 would be 4thou, while 0.25 would be a more acceptable points gap of 10 thou. Just trying to work out why such a big points gap is working so well.

All the best,
Colin

[Tiger]

Don’t forget the points gap will adjust the timing

[JonE]

could it be that the distributor spindle is worn and the smaller gap was causing the points not to open properly at some sectors on the full rotation?

[Steve kay]

I understand very little about electrics, but I would very much like to hear an answer to Colin's question. The figure of 25 seems an extraordinary gap for the points, so is it metric and not thous?

[Tiger]

Following this post with interest because I am just about to time my Ruby.
Just reread all the comments
I THINK I WOULD FIT ANOTHER CONDENSER
ALSO CAREFULLY CHECK ALL CONNECTIONS IN DISTRIBUTOR

I recently fitted a new dip switch found to be faulty
Fitted a second new switch faulty again
Finally a third one was OK

All switches supplied in Lucas marked box

Trusted supplier commented all from China

[Malcolm Parker]

When I built my Ulster rep I used a new 'Bosch' distributor.  I had to change the rotor arm, points and condenser before the engine would run.  I trust old , no doubt worn, original Lucas distributors rather than new Chinese or similar rubbish.  I also have an early Lucas distributor with no automatic advance/retard for use in an emergency.

[John Cornforth]

Hi

Just to quantify the effect of changing the points gap, here is a graph I once drew after some bench measurements on an unworn distributor.  The standard gap of 12 thou gives about a 50 degree dwell angle (the time the points are closed and current is building up in the coil).  As you set a greater gap, the dwell reduces which will slightly reduce ignition energy at high RPM.  The timing also shifts, so best to reset timing after re-gapping.

Once the spindle bearings wear, this is all thrown out of the window and the timing and dwell will be all over the place as the cam wobbles depending on revs.  Opening up the points is probably the best short term move, as it reduces the sensitivity to wear at the expense of a slightly reduced dwell and energy at high RPM, something you can probably live with.  In the long term, a new or reconditioned distributor is called for.
  Points Gap Effect.jpg (Size: 46.37 KB / Downloads: 118)