Spark plugs into the oven - does it really work

jac

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I seem to recall hearing / reading / seeing someone advise that otherwise non functioning sparkplugs could sometimes be encouraged to work by spending a few minutes in an oven. Is there any truth in this as have lost count of the number of times I have changed spark plugs that have been in an engine that has been used for only a couple of hours then sat idle for a couple of months.
 

lw395

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There are two sides to this.
1) putting them back in the engine warm will help vaporise the petrol and help avoid oil contamination, so may aid starting.
2) heating them seriously may burn off contamination which shorts out the spark. This probably means a few hundred degC at least, so putting them in a domestic oven won't do that.
I have resorted to heating them with a gas flame.

It may just be a displacement activity which gives the excess fuel to evaporate?
But it sometimes seems to help.
 

BobnLesley

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...putting them back in the engine warm will help vaporise the petrol and help avoid oil contamination, so may aid starting...I have resorted to heating them with a gas flame...

I've never found it necessary with our outboard, but years ago on a couple of highly-tuned two-stroke motorbike engines, both of which were buggers to start, the application of a zippo-flame for 10-20 seconds, plus a coating of graphite from a soft-pencil (no idea how/why that helped) ensured that they invariably fired on the first bump off the start-line
 

jac

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I've never found it necessary with our outboard, but years ago on a couple of highly-tuned two-stroke motorbike engines, both of which were buggers to start, the application of a zippo-flame for 10-20 seconds, plus a coating of graphite from a soft-pencil (no idea how/why that helped) ensured that they invariably fired on the first bump off the start-line

Graphite does conduct electricity IIRC - not sure if that would help make a bigger spark??
 

sarabande

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graphite across the two electrodes ? Will short the circuit. You need a gap.



It was always fun to draw a pencil line from the HT cap down to the metal body , and see how long one's fellow bikers would take to figure out the problem.
 

theoldsalt

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Heating the spark plug electrodes in a naked flame was common practice years ago when trying to start a typically stubborn Stuart Turner engine. We also needed to do the same with Seagull outboards sometimes. It was magnetoes that needed drying out in the oven before giving a decent spark.
 

NickRobinson

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Don't use the oven. Place the plugs gap-in to the gas ring flame. They should be smoking when removed after five mins (use a cloth). Replace while still hot-

bodge,- I know,
 

BobnLesley

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...fun to draw a pencil line from the HT cap down to the metal body...

I never knew that. Years ago we were camping at Silverstone for the British Motorbike GP weekend and on the first night some hoon 30' away thought it would amuse everyone if he started/gunned the engine on his Dunstall-exhausted Honda 550/4 at 03:00. Similarly, whilst having a leak at 06:30 the following morning, I thought it might amuse him if I switched the plug-caps around on the two central cylinders of his bike? Saturday night I was undisturbed, but when we were heading across to the track on Sunday morning, the hoon and his mate were clearly 'investigating' the starting problem that they'd encountered in the night; I only began to feel guilty when we returned at 17:30 and discovered that they'd not seen a single race, having spent the whole day working-on/checking the bike. I didn't fancy telling them directly (both were pretty big) but I offered to have a look for them, surreptitiously righted the HT leads during my inspection and when it started first-try, they were singing my praises; the owner even offered me a fiver to 'buy yourself a couple of pints with', but I had to decline as I felt that would've really been taking the p1ss.

I've just realised: That 550/4 owner may like us have moved onto boating? So if you're now reading this, it was someone else entirely, not me that did it, honest.
 
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NormanS

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It's all a long time ago, but I used to have a small gaff-rigged ships lifeboat, into which I installed an ancient JAP Vee twin engine. It was terrible to start. It helped to take out one plug, and start it on one cylinder, often heating the plug in a small dish of burning petrol. It was fun putting in the second plug, and particularly attaching the lead, once the brute was going.
Conversely, the JAP was replaced with a Stuart Turner, not a marine model, but something fitted with a centrifugal clutch, which I think had been used in some form of fuel bowser. Anyway, as Ronnie Corbett would say, I digress, but this engine would start reasonably easily when cold, but when hot, often the only way to get it going was to take out the plug, and put in a cold one. Strange.

I also carried oars, which were much more reliable than either of these engines.
 

skodster

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I seem to recall hearing / reading / seeing someone advise that otherwise non functioning sparkplugs could sometimes be encouraged to work by spending a few minutes in an oven. Is there any truth in this as have lost count of the number of times I have changed spark plugs that have been in an engine that has been used for only a couple of hours then sat idle for a couple of months.

I think if that were me, a new sparkplug would be the solution and sod the expense... :)
 

Heckler

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I've never found it necessary with our outboard, but years ago on a couple of highly-tuned two-stroke motorbike engines, both of which were buggers to start, the application of a zippo-flame for 10-20 seconds, plus a coating of graphite from a soft-pencil (no idea how/why that helped) ensured that they invariably fired on the first bump off the start-line
My old man taught me the same about the pencil, it seems to work, although getting the electrode red hot works as well!
S
 

Heckler

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No not bridge the gap, just smear both the electrodes, if you turn it over with the plug out the graphite appears to burn.
S
 

jac

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I think if that were me, a new sparkplug would be the solution and sod the expense... :)

WHAT _ A WHOLE £2!!!! ARE YOU MADE OF MONEY!!!!!

I was thinking more for those times when you're on board - have used the last spare and need to get ashore to go and buy one. ( Although in such cases you're probably out of gas anyway)
 

DownWest

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Modern CDI ignition systems, with their much fatter sparks, have made plug warming passé. I say modern,but they have been around for over 30yrs, so I suspect we are dealing with pretty old engines here.
 

lw395

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Modern CDI ignition systems, with their much fatter sparks, have made plug warming passé. I say modern,but they have been around for over 30yrs, so I suspect we are dealing with pretty old engines here.

My 100cc Suzuki was CDI, but still prone to starting problems now and then.
I suspect it was quite worn and the compression down a bit?
After that, as a student I had a range of cheap old bikes, some of which had points or even 'proper' magnetos.
 

blxm

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Its not uncommon with old race engines to heat the plugs with a flame prior to starting, weak spark no choke, and if were real posh you would put in different plugs to start and warm the thing before putting in the race plugs, not while running!.
Probably better to cure the cold start problem in the first place, but where's the fun in that!
 

Norman_E

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WHAT _ A WHOLE £2!!!! ARE YOU MADE OF MONEY!!!!!

I was thinking more for those times when you're on board - have used the last spare and need to get ashore to go and buy one. ( Although in such cases you're probably out of gas anyway)

I used to heat up motorbike plugs (with a blowlamp) when they only cost five bob!
 

rob2

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Well, it certainly works and never mind the cost, you can't fit a new plug for each start! I believe it helps partly by drying off the plug and perhaps works like a glowplug in a diesel if hot enough, improving the flammability of he charge by warming. I used to have a Ford Anglia with near terminal loss of compression. I put the plugs under the grill once I'd made my toast... It's just so difficult explaining the oven glove in the toolbox.

Rob.
 

burgundyben

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this engine would start reasonably easily when cold, but when hot, often the only way to get it going was to take out the plug, and put in a cold one. Strange..

That makes sense to me, electrical resistance increases with temperature, a cold plug would have lower resistance, hence more current flow and greater spark. I think.

Perhaps removing and heating the plug is more about cleanliness than temperature.
 
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