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Grounding Spark Plugs

  • Thread starter Thread starter UncleMike
  • Start date Start date
U

UncleMike

Guest
I've been reading Dan's Online Motorcycle Repair Course, and although I've learned a lot, (not sure if I've learned more about motorcycles or the bible,) I'm still confused on one thing.

It's mentioned a lot in the back posts of this forum too...

Grounding spark plugs to the cylinder head when checking compression.

Please excuse my ignorance, but how is this done? Shall I remove them, put them back in the caps and just hold them against the engine? Seems like it would be annoying with my two-cylinder bike and impossible to hold all four with a 4cyl and still read the compression gauge.
 
I can't even figure out what the spark plugs have to do with checking compression. I know you can ground the plugs to the head when checking for spark but the plug just gets yanked out when checking compression.
 
That's how ignorant I am, and that's my question.

How do you ground the plugs to the head?
 
just lay the plug against the head. When you crank the motor around, by hand, you can watch it fire. A good way to see where the plug is firing or to make sure you are getting spark.
 
Perfect. Just wanted to make sure it was as simple as that.

So, how do you guys with four cylinders lay four plugs against the head AND watch the compression gauge?
 
From what I understand you do one cylinder at a time. Leave three plugs in and check the compression on each head seperately.

I still don't understand why you need to ground the plug when checking compression, because spark has nothing to do with compression.

Satch
 
From what I've read you take all the plugs out, and if you don't ground them, you risk burning out some components.
 
To check to see if the spark plug is firing, you remove the spark plug, put the plug wire back on the plug and hold the threads of the spark plug against any bare metal on the bike (ground) Everyone says cooling fins probably because that is the closest and easiest spot to get a good fround for the plug threads. No matter how many cylinders you have, you can only check one plug at a time because you just plainly dont have enough hands. :-) Checking that your plugs are firing/ignition is working is one test.

Checking compression is a different test/check. Obviously to insert the compression testor, you have to have the plugs removed. :-) Its also easier on the starter and less battery drain if you remove all plugs and just let them hang loose. a compression test should be done after the engine has been warmed up to normal operating temperature. Also, the throttle should be held wide open.

Earl





UncleMike said:
Perfect. Just wanted to make sure it was as simple as that.

So, how do you guys with four cylinders lay four plugs against the head AND watch the compression gauge?
Click to expand...
 
Yeah, but I've been reading that if you don't ground all the plugs while you're doing it, you risk burning out some of the electronics.
 
Its possible that is true with some new bikes having computer chips, etc etc, in the electrical system, but relative to our old GS's, its a bunch of BS.
I havent burned anything out in 35 years checking one plug at a time.

Earl

UncleMike said:
Yeah, but I've been reading that if you don't ground all the plugs while you're doing it, you risk burning out some of the electronics.
Click to expand...
 
Oh. The place I read it was from was an old Clymer manual for GS 400's from 1977-1978.

But I'll believe you guys anyday.

Bottome line is that if I want to be real careful, I can get a friend to ground the other plug.
 
the reason all the plugs must be removed is so the engine has an even cranking rythm

ian
 
If it will make you feel better, you can use a couple of spring clamps and clamp the plug ends to the cooling fins.

Earl :-)


UncleMike said:
Oh. The place I read it was from was an old Clymer manual for GS 400's from 1977-1978.

But I'll believe you guys anyday.

Bottome line is that if I want to be real careful, I can get a friend to ground the other plug.
Click to expand...
 
Can I just get those at any auto supply store?

Would I have to solder them to some plugs or do they just attach to them with tension?
 
Spring clamps are just metal clamps that resemble a giant clothespin. You can buy them at any hardware store or home depot, etc.

Earl


UncleMike said:
Can I just get those at any auto supply store?

Would I have to solder them to some plugs or do they just attach to them with tension?
Click to expand...
 
It's a Good Thing to ground the high (secondary) side of the coils when cranking with plugs out, the secondary voltage needs to discharge to ground and if it can't go through the plug wire they have been known to arc right through the coil case..... electricity will arc 1" for every 1000V, so a coil that puts out 35KV can do some arcing and damage
 
UncleMike said:
Oh. The place I read it was from was an old Clymer manual for GS 400's from 1977-1978.

But I'll believe you guys anyday.

Bottome line is that if I want to be real careful, I can get a friend to ground the other plug.
Click to expand...



Great idea.....it gives you a check on the output of the coil, as well. Just get the friend to hold the wire with one hand, without the plug, and have him place his other hand on the engine as you turn on the starter.


Even better, do this with the love of your life......just wait until you see the 35K spark in her eyes.

Of course, knowing an average woman's temperament, that may be the last thing you see for a while...... 8O 8O :roll:

If you plan to do this, be sure to send us an address for the get-well cards.
 
To be more specific, I think I'll clamp both plugs to the fins when checking compression.
 
earlfor said:
Spring clamps are just metal clamps that resemble a giant clothespin. You can buy them at any hardware store or home depot, etc.

Earl


...or head shop where they are called roach clips.
Another way to ground them plugs is to clip their wings when they're young, right out of the box.

:oops: :oops: I gotta stop this... :lol:
Click to expand...
 
jimcor said:
...or head shop where they are called roach clips.
Another way to ground them plugs is to clip their wings when they're young, right out of the box.

:oops: :oops: I gotta stop this... :lol:
Click to expand...

I feel like I'm taking crazy pills.
 
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