Spark Plug Failure Under Compression

Farmall Cub Forum -- Questions and answers to all of your Cub related issues.

Moderator: Team Cub

Forum rules
Notice: For sale and wanted posts are not allowed in this forum. Please use our free classifieds or one of our site sponsors for your tractor and parts needs.

Spark Plug Failure Under Compression

Postby lazyuniondriver » Sun Dec 23, 2012 12:47 am

Years ago I watched a technical training video featuring a spark plug installed in a cylinder with a sight glass near the plug tip for spark observation.

A power source was energized producing a visible consistent spark at the plug's electrode. As air pressure in the cylinder was increased, the spark was eventually snuffed out. How many psi I don't recall. As the pressure was released, the spark became visible again.

The focal point of this video was to make you aware that it is possible to visually observe good spark at the electrode of a grounded spark plug during troubleshooting, yet when the plug is reinstalled and energized under compression, it can fail due to power supply issues or if the plug itself is defective.

If the spark is being put out because of a single defective sparkplug, not a big problem as only an engine miss is realized.

However if all the plugs are being snuffed by a weak power supply system, a no start scenario is created.

My theory is the threshold of spark failure under compression from a marginal power source may differ by brand. This may be the reason an engine will not start with one brand of plugs yet will readily fire with a different brand installed, especially after overhaul when compression is the highest.

I have had individual spark plugs that sparked outside the cylinder during trouble shooting yet missed when reinstalled and continued to miss until the plug was replaced. With no way to test the plug under compression I could only assume this was the problem.

I searched YouTube for the original video or a similar one unsuccessfully. I did find a link to a company that markets a tester to ferret out this problem.

http://www.okspark.com/index.php?option ... &Itemid=63
"HAVE ALL YOUR DELIVERIES MADE BY UNION DRIVERS"
Image
lazyuniondriver
Cub Star!!
Cub Star!!
 
Posts: 438
Joined: Tue Aug 07, 2012 1:37 am
Location: Southern Suburbs of Cleveland, Ohio
Zip Code: 44134
Tractors Owned: 1949 & 1951 Farmall Cubs

Several Snapper Garden Tractors
Circle of Safety Award
Circle of Safety: Y

Re: Spark Plug Failure Under Compression

Postby Boss Hog » Sun Dec 23, 2012 10:15 am

That is the reason gas fouled plugs will not start an engine, in the old days you could clean plugs. I do not have great success with cleaning them now. I think it is a combination of lead free gas[ no lead deposits to clean] and spark plugs are not designed for cleaning now. Who knows?
Boss
IN GOD WE TRUST
All others pay cash
Boss Hog
Power corrupts; absolute power corrupts absolutely byJohn Emerich Edward Dalberg
User avatar
Boss Hog
Cub Pro
Cub Pro
 
Posts: 9803
Joined: Mon Oct 24, 2005 8:35 pm
Location: VA. Randolph
Zip Code: 23962
eBay ID: dmb2613
Circle of Safety Award
Circle of Safety: Y

Re: Spark Plug Failure Under Compression

Postby ntrenn » Sun Dec 23, 2012 2:48 pm

And this is part of the reason that modern engines actually modulate spark voltage with throttle position - more throttle, more voltage. I had a coil fail on my Aurora that was just fine at idle and part throttle, but mash the throttle and it missed like mad. Coil would not supply sufficient voltage at WOT.
ntrenn
Cub Star!!
Cub Star!!
 
Posts: 339
Joined: Tue Apr 17, 2012 7:55 pm
Location: Indiana
Zip Code: 47119
Skype Name: ntrenn
Tractors Owned: 63 Cub 221833 in family since '69
65 Lo-boy 19648 with an un-stuck #4 - it lives!!!
144 Cultivators with disc hillers
193 Plow
42C mower
59L mower mounted on the Fcub
23A Disc - lost possession


Return to Farmall Cub

Who is online

Users browsing this forum: chuck ma, johntexas, rustycub, thiggy and 7 guests