N15

I have a 1966 N15cs. Has been running fine but recently it has started misfiring when hot and is very difficult to start when warm. It has a refurbished magneto and a mikuni VM34 carb. I have replaced the cap condenser as fitted by Paul Wolf which has helped with the misfiring but it is a bugg*r to start when warm, ok when it cools down. Any ideas.
Cheers
Giles

Air leak at the manifold flange, and/or a HT lead with a core break or connection fault. Plug caps should be of the non-suppressed type. The HT lead should be of the copper type.

- Knut

Last week I was testing a K2F I was considering using again with resistor plugs at the end of either copper core or stainless core Joe Hunt HT plug wires with plane rubber caps. It works, but the spark is not impressive with the resistor plugs. I put a set of non-resistor plugs on the plug wires and the difference in spark was visually about 2 times as bright and a wider spark. Plus, the magneto never misfired. It did misfire once in a while with the resistor plugs. I was spinning the magneto by hand and looking at the plugs on the bench. Anyway, big difference with all non-resistor wire, caps, and plugs.

Check your timing too. No real reason why it would change, but it never hurts to check. Too much advance can make a warm engine harder to start. So can a magneto getting ready to crap out.

I was having a hot start issue with my BMW R69s- which also uses a magneto for spark. After swapping out the points, coil, and condenser I still only had a weak spark under test, and was worried the magnets in the magneto had lost their mojo. (Alliteration!)

But testing the plug wires for resistance revealed the factory plug caps I had installed some 30 years back were now adding a ton of resistance- far more on one side than the other. Changing them to non-resistor caps restored the big blue sparks and it now starts easily and idles correctly.

….So even if you have non-resistor caps and plugs check them to make sure.

I recently got two free '83 Suzuki GS1100E's that the owner had gone bat **** crazy rebuilding the carbs trying to run pods etc, same old story. I put back to stock, did carb rebuild properly, and it cut out when hot. It ran great on a cool morning, and as the bike and the day warmed up it started bogging. It was coils. I'd go with something electrical in this case. If it was an old car I'd say vapor lock LOL.

Thanks for that, I've put some no resistance caps on but not had a chance to try it yet. I've been riding my old Rudge a lot recently and I noticed that when it is cold I can set the manual advance retard to fully retarded to stop it throwing me over the bars, but when it is hot, it will not fire on full retard, I have to advance it about half way and it starts fine. I'm just wondering if the base timing of the N15 mag is slightly too retarded causing the warm starting issue? I'll try it today with the new caps and see how it goes.

Don’t wonder about ignition timing. Check points gap and ignition timing on both cylinders.

Checked timing and points gap 32deg fully advanced on both sides and points gap in spec. Took it for a short ride between showers and let it idle until it was hot. It restarted OK so maybe the non resistor caps have done it? Non suppressed plugs and copper core leads. If it ever stops raining I'll take it for a proper ride. Thanks for your input.
Giles

Gilesy said:

Checked timing and points gap 32deg fully advanced on both sides and points gap in spec. Took it for a short ride between showers and let it idle until it was hot. It restarted OK so maybe the non resistor caps have done it? Non suppressed plugs and copper core leads. If it ever stops raining I'll take it for a proper ride. Thanks for your input.
Giles

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Always run magneto ignitions with non-suppressed plug caps, as advised by Lucas. See entry #2.


- Knut