G3 won't start

[Beefheart]

My G3 (1953) had not been run for a few weeks so I thought time for a run, two kicks starts fine leave her idling while I lock up. She cuts out now will not start, absolutely no life not pop spit or kickback. The magneto has been rebuilt nice big spark at the plug New monobloc mixture was spot on, I've checked the timing so many times I could do it in my sleep put fresh fuel in. Compression seems sound even if it was low I would expect it to at least try to start. What to look at next ?

[dave16mct]

Put a new plug in. Clean the pilot jet.
Dave.

[Andy G]

Turn the fuel tap on?

...seriously tho...as per Dave, plus check banjo filter and flow from tank.

[ajscomboman]

I'd say plug, had a very similar issue with a bike on the bench recently. Sometimes it'd start and then nothing not even a cough. new plug and all was back to normal, first time starter.

[Beefheart]

Apologies gentleman for not acknowledging your replies sooner I managed to get chicken pox. At last l got out to the garage , took out the pilot jet it was blocked so stripped and cleaned the carb completely, new plug still no go not even a sniff of fuel on the plug. There were tiny flakes in the float chamber could one of these have resisted my poking and blowing and be stuck in the pilot jet and acting like a butterfly valve? (Shades of desperation there methinks!). So in the same spirit of desperation I will order a new pilot jet size 30 (currently 25).

[SPRIDDLER]

Does the carb flood when you tickle it?
Can you feel suction if you put your palm over the carb intake? ('m only thinking the unlikely possibility that the inlet pushrod may have become displaced from its rocker. Easy enough to take the rocker box cover plate off to check).
If the pilot jet was clogged did you carefully check that the drillings 7, 8, 10, 31 and 32 are clear? I know that's not easy to see but a squirt of aerosol carb cleaner or WD40 into whichever drillings you can access might clear any blockage.

(Purely to eliminate the unlikely possibility that the plug hasn't stopped sparking under compression (rebuilt mag fault?) tip a teaspoon of fuel in the plug hole and try again when it should fire once or twice).

Monobloc Pilot screw side.JPG

Monobloc key to diagrams.JPG

[Beefheart]

Thanks Neville there is plenty of suction at the carb tried fuel into through the plug hole, nothing doing. Which makes me think that despite the good spark it could be the mag. What is the likelihood of the spark failing under compression? Not much I can do myself I guess apart from changing the easycap. Any suggestions most welcome.

[SPRIDDLER]

Thanks Neville there is plenty of suction at the carb tried fuel into through the plug hole, nothing doing.

Hmm, that muddies the water a bit especially as after starting easily enough the engine conked out initially whilst idling on the stand.
Although petrol down the plug hole isn't a conclusive test you should get some reaction, so it could still be either a lack of fuel or lack of a spark.

Which makes me think that despite the good spark it could be the mag. What is the likelihood of the spark failing under compression?

It's a fact that a plug that sparks out of the engine can fail to spark under compression.
When you say it has a good spark, presumably that's with the plug out and resting on a metal part. It's a possibility, although unlikely that the (new) spark plug when refitted and under compression could still be failing or alternatively that the spark may be leaking/shorting via the HT lead onto a metal part before it gets to the plug.
It could also be faulty plug cap. Presumably it's not an incorrect type of cap with an integral resistor.
If I had the bike to hand I'd do several 'fiddling about' things which although long shots and unlikely to be the issue themselves can reveal the real problem: Check the points for any pitting (a sign of the capacitor failing) and give them a clean (whilst still in situ), check that the carbon pickup in the mag was oil-free, wasn't stuck, that the mag slip ring was clean, roughly check that the ignition timing was still correct and hadn't slipped out of adjustment and I'd probably remove the plug cap and connect the HT lead direct to the plug.

Not much I can do myself I guess apart from changing the easycap.

Maybe there's something on Groily's website that can give you a steer?
Link here:

http://www.brightsparkmagnetos.com/DIY/index.htm

As far as the fuel issue goes you can't do much more than check for suction at the carb mouth, ensure that fuel is getting to the carb and that the jets and drillings are clear of obstructions.

Maybe I'm missing the obvious and others may have suggestions.

[Groily]

It's worth taking the plug cap off the HT cable and then positioning the bared end about 6mm from the cyl head, and kicking it over. You should get big sparks that jump that gap easily at kickstart speed. If they are there, every time, then there's enough voltage to operate under any normal compression ratio. If they're NOT there (even if they are when an ordinary spark plug is used to do tests), then it's worth
* taking the contact breaker to bits and resetting everything. This applies to what I guess is an N1 mag, but also to an SR1 if you have one of them on the bike.
* checking that there is no resistance between the ends of the HT cable / plug cap. A badly connected plug cap is a common problem - there can beenough oomph for the spark to jump a gap/faulty connection there and then fire a 20 thou gap on a plug, but not your 6mm gap as above. And
* if you then put the meter across the plug end of HT lead, and the mag body, you ought to see several thousand ohms, which is the HT coil's resistance. If you see an Infinity or Open Line reading, there is a continuity failure somewhere.

If you don't have a meter that will measure capacitance, you can do an 'indicative' (but obviously not definitive) test across the condenser using the resistance scale - it should rapidly climb, almost immediately, to Infinity or Open Line, as the meter's battery tries to charge the capacitor up. If there is a constant resistance across it - with the thing off the mag or otherwise disconnected this is - then there is a fault and the sellers will grovel and replace it (that's me!). It is pretty unlikely, but can never say 'never' with any of these things. It should show a capacitance of 150nF if it's an N1 unit mounted on the cb, or 220nF if an SR1 and mounted remotely from the contact breaker if you have a means of measuring it.

If all tests / looks good and there are still good sparks as per your previous post, then chances are the problem isn't there, but all these things can be capricious (aka darn annoying) sometimes.

Just seen Spriddler's - apologies for repetition in part!

[willowbedone]

As spridder says Maybe the timing has slipped