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spark plug testing and compression testing

6.3K views 21 replies 10 participants last post by  jkustomz28  
#1 ·
I'm planning on doing a huge walk though on my bike and one thing ill be doing is testing the spark to see how far it can jump and how strong it is. my question is how far should the spark be able to jump? ill be using this tool to measure it.
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should just be able to jump just pass the gab of the plug to indicate good and strong spark?

also ill be doing a compression test while i change my plugs to the correct number 6's but the service manual gives me some weird rating. what should the proper PSI be per hole?

thanks guys i know you all love my anal questions :roll::mrgreen::wink:
 
#2 ·
Compression test should be done with a warmed up engine if possible with the throttle held wide open when doing the test. Not knowing what bike you have I would assume 140-190 PSI would be normal. Not ever having used a spark tester I have also found that the spark with electronic ignition does look rather weak, but still does the job as long as it is jumping the Spark Plug gap.
 
#4 ·
Perfect compression on your bike would be 141.12 psi, this is a calculation of your compression ratio of 9.6:1 and an average atmospheric pressure of 14.7 psi, but within 15 percent is the usually acceptable loss.

With your plugs, they should be able to jump a good bit farther than your normal plug gap as with every spark it "eats" a bit of both the central and lateral electrode away and will cause your gap to constantly get wider by every so little bit.
 
#6 ·
And don't get hung up on those PSI numbers. It's more important for the two cylinders to be close to each other than it is for them to be in the normal range. If the numbers were say 110 PSI and 115 PSI then I would not worry about it but if one is 90 and the other is 150 then thats not good and it will run rough being that far apart.
 
#11 ·
Don't look for 141.12 PSI on your bike, there are other variables and that formula is a simpified version, it also assumes you are testing at sea level and under controlled climatic conditions, it also doesn't allow for test equipment variations so anywhere between 120 and 200 psi is fine..if you really want to test the compression you should use this reading as a baseline to use for future readings using the same test equipment, as for the spark, that tester is usually used just to make sure there is a spark..as JPR and thunderglass have already said there is no need to do these tests on a fairly new bike if the bike is running, but if you are the sort of person who keeps a mileage, coolant, oil usage, and tire pressure log, then this is just another set of data to put in the book and won't do any harm..

John.
 
#14 · (Edited)
Actually, it's just as I said an average, also, if even with harsh conditions you would have to be a mile and a half below sea level to get 200 psi out of that ratio. Do not go by extreme outside measurements meant for "all bikes", do your research and do it right and you will be much more satisfied with your findings and results. Also, although loss is not as important as being close to each other when measuring the two cylinder for pressure, loss is also a VERY big aspect to look at as it can tell you if you are needing new rings before you realize it and many more things.

Edit, also, along with loss, you need to have your proper measurements for a GAIN in compression also that could mean carbon build-up in the cylinder as well.
 
#16 ·
spark plugs should be able to jump 0.035"..... Besides the distance the spark is able to travel through air, has more to do with the quality of the ground than the stregth of the spark.

You want to TEST your ignition get a meter and test it.
 
#17 ·
Yes, jkustomz28, as your cylinder and rings wear they will both increase the sizes of your cylinders ever so slightly and this will increase your ring end-gap clearance and allow more gases to pass by the rings into the crankcase during compression, also build-up of carbon and burnt valves/seats will also allow for leakage and decrease your compression. This is all COMPLETELY NORMAL as long as engines run and the abuse they take they will all do this as metal on metal, even with lube/oil, will always wear.
 
#22 ·
the idea of those tester is to see how strong the spark is not if you have spark. the ones with the bulb just tell you if you have spark or not. these ones give you a kv rating as to how strong your spark is you set the length you want the spark to jump and see if it can if not then something is causing weak spark. 2 totally different tools so to call on junk isn't a correct thing to say when you are comparing to totally different tools. :wink:
 
#19 ·
Light bulbs can interfere with your eyes seeing the spark completely as they, as bulbs do, illuminate and cause more light where less light is needed. These are just fine and so are the ones you can get from harbor freight that plug in in-line with your plugs and wires.
 
#21 ·
Because the air is less dense at higher altitudes and therefore can compress more under the same pressure as low altitudes, this is also true in reverse as air is denser at lower atmospheres and compresses less therefore increasing the reading faster. This is also why the manuals describe high altitude adjustments.