A strange one

mike in idaho · Post by **mike in idaho** » Fri Jul 20, 2018 3:19 pm

"Here's an interesting one, when running (& mis-firing) if I pull off & hold the plug-cap away from the plug, a large spark jumps the gap & it instantly picks up & starts running perfect, put it back on & it mis-fires!!"
That plug is gas-fouled, toss it and install a new one.

modelman · Post by **modelman** » Fri Jul 20, 2018 3:48 pm

They are new plugs.

mike in idaho · Post by **mike in idaho** » Fri Jul 20, 2018 5:38 pm

How long have they been in the motor? It takes literally seconds to foul one with a slug of raw gas. Try a new plug anyway(they don't cost that much).

rrietman · Post by **rrietman** » Sat Jul 21, 2018 12:43 am

OK so sometimes when you are diagnosing you go thru a bunch of spark plugs, just to zero out that variable. here's an old time trick. chuck the suspect plug in a vise, get out the oxy torch and heat it red hot, the carbon will jump off and you have a good plug.
goodluck
Randy

modelman · Post by **modelman** » Sat Jul 21, 2018 1:33 am

I can accept the scenario of a dud plug, I can also accept a fouled plug, but can't get my
head around the fact that it fails/mis-fires when connected, but then runs fine if the plug-cap
is pulled off & held a short distance away!

I even removed the cap & held the end of the (new) HT lead on the plug, same thing & when pulled away there is a massive 1/4" spark to the plug & it runs great.

Is it possible that a duff plug can somehow be 'induced' to fire when a ginormous spark is applied at the cap-end??? I must admit to the fact that it seems plug related.

Last night I removed the condenser ( that was fun ) & this morning i'm going to fit a replacement & we'll see if that makes a difference.

G-Man · Post by **G-Man** » Sat Jul 21, 2018 4:40 am

Modelman

All that the long spark does is increase the voltage at the plug points. You're just storing the charge up for a bit longer to build up a more energetic spark. The biger (and more vigourous) spark jumps across the gap and hey-presto.

G

modelman wrote:I can accept the scenario of a dud plug, I can also accept a fouled plug, but can't get my
head around the fact that it fails/mis-fires when connected, but then runs fine if the plug-cap
is pulled off & held a short distance away!

I even removed the cap & held the end of the (new) HT lead on the plug, same thing & when pulled away there is a massive 1/4" spark to the plug & it runs great.

Is it possible that a duff plug can somehow be 'induced' to fire when a ginormous spark is applied at the cap-end??? I must admit to the fact that it seems plug related.

Last night I removed the condenser ( that was fun ) & this morning i'm going to fit a replacement & we'll see if that makes a difference.

modelman · Post by **modelman** » Sat Jul 21, 2018 7:17 am

So what are you saying Graham, the plugs are dud, or another electrical problem??

I'm at my wits-end just now, fitted a brand-new condenser this morning, & have the same results, SIX new plugs, new leads new caps, tried another coil, a little video shows just whats happening, & i'm beginning to suspect the plugs as I tried a couple of old 14mm plugs & they fired/sparked just fine.
Anyone else had bad experiences with 'new' plugs?? as ALL worked for a day or two, now none do, & i'm wondering if there's a load of fakes out there.
BTW, stripped the carb again, it DOES have the voids in both spacer & carb, found the needle had a slight bend halfway so I fitted a new spare I had,(middle groove)

https://www.dropbox.com/s/c00gv661ir92a ... 3.mp4?dl=0

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A strange one