Spark Outside - No Spark Inside

Mark De Bernardi /

Hey All,

I'm working on a Peugeot moped that was running great until something happened. The bike was filled with high octane fuel (instead of the regular grade that was usually put in) and ran for awhile, but then ran rough and stopped running at all. I've done a little troubleshooting. The problem seems to be related to spark. If I take the spark plug out of the cylinder and ground it against the head it sparks great. Seems like a consistent, fat spark. But once I put the spark plug back in, it doesn't spark. I have a timing light that you put over the spark plug wire and it flashes if it's sparking. So the timing light flashes when the spark plug is out (and I can see it spark), but nothing when the spark plug is installed. I've pulled off the muffler to make sure it wasn't clogged, and it made no difference. Any other ideas? I'd really like to get it running again. Thanks.

Re: Spark Outside - No Spark Inside

Leon Swarmer /

Have you replaced the plug with a new gapped one?

Or at least cleaned the old plug and checked the gap?

Re: Spark Outside - No Spark Inside

Mark De Bernardi /

Yeah, brand new plug. :(

Re: Spark Outside - No Spark Inside

Mike Lumbert /

Check and make sure your coil is good,

And that your points are gapped correctly.

Mike

Re: Spark Outside - No Spark Inside

Don Pflueger /

if you have good spark outside the cylinder, then you have good spark inside. dont rely on an automotive inductive timing light. the only thing that can cause you to lose spark when installing the plug is either a bad plug wire, or a bad boot. but i doubt either of those are your problem if you have spark outside the cylinder.

automotive plug wiresare made much differently than moped wires are. the timing light will not work the same. moped wires are usually too small in diameter for the timing light to pick up the impulses correctly. keep in mind, car wires are 8mm or 10mm and are usually suppresed for noise interference. moped wires are 6mm non suppresed.

more than likely you have either a fuel or compression issue.

Re: Spark Outside - No Spark Inside

Mark De Bernardi /

I've tested the compression and it's got 160 psi. Which seems incredibly high. I know that higher compression will make it harder to spark. Which is why I think it's not sparking in the cylinder. Perhaps the magneto is too weak to actually spark in such high pressure.

Re: Spark Outside - No Spark Inside

Matthew Scholtens /

Mark,

If you are really sure that the spark is not occuring inside and you feel that your compression is too high, can't you just lightly (i.e. hand tighten) the spark plug to allow a little bit of compression relief?

Just my two cents,

Matthew Scholtens

Re: Spark Outside - No Spark Inside

160psi is just a good, healthy figure - nothing excessive about that. At least the top end is solid!

Yes, its more difficult for a plug to fire under compression than in free air. I'd look for a marginal component somewhere in the ignition. Could be things like:

- ignition coil produces spark, but reads 'open' on a meter.

- bad plug cap

- bad plug wire, or bad connections on either end of the wire

- excessive 'closed' resistance of the points (should not exceed 0.1ohm when closed)

Any of those things, or a combination of several would be enough to tip the balance between firing and not.

Re: Spark Outside - No Spark Inside

Don Pflueger /

what would pressure have to do with spark? nothing.

it either has spark or it dont. the high compression could be caused by a blocked exhaust pipe or port. this will cause it not to start. compression will not affect spark tho.

Re: Spark Outside - No Spark Inside

Don Pflueger /

it would suck air and make the mix too lean.

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