Compression Testing Smaller Non-Electric start motors

[Putzaroni1]

I wanted to clarify this one as I see mostly larger electric start motor instructions on this forum and elsewhere.
tashasdaddy has great instructions in the Top Secret file but again it's for larger motors. Please have a look at these instructions for a smaller non-electric start motor and let me know if I'm on track or they need any adjustment.

If these are good, I will try making a video of this procedure as there are lots of questions about this including mine ;-)

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1. Start engine and warm it up (not completely necessary but will provide higher compression numbers)
2. Remove all spark plugs and disconnect fuel line
4. Pull the emergency kill switch on rope pull start motors if there is one. If not be sure to ground the spark plug wires to the block. To do this, put the plugs back in the spark plug wires and attached a grounding wire to the spark plug and then ground it to the engine block. Do this for all plugs. This will insure that any current coming out of the coils will go to ground rather than potentially fry something back in the ignition
5. Screw guage into #1.
6. Pull start engine 5 or 6 times watching the compression gauge to confirm the needle is no longer moving up
7. Repeat steps 5 & 6 for all cylinders.

All psi readings should be within a 10-15% span.
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[Bosunsmate]

Re: Compression Testing Smaller Non-Electric start motors

Sounds good to me but i normally keep the fuel line attached

 

[F_R]

Re: Compression Testing Smaller Non-Electric start motors

I suppose a lot of people are curious about the warm compression readings, but generally speaking compression tests are done as a first step before proceeding to do other repairs. That avoids beating a dead horse. Therefor, I'd say to test it cold. If you later want to see if it gets any better after running, go ahead.

 

[JRegier]

Re: Compression Testing Smaller Non-Electric start motors

I always set mine up WOT before testing. I'm sure one of the pros will chime in on that.

 

[racerone]

Re: Compression Testing Smaller Non-Electric start motors

There is absolutely no need to open the throttle when testing compression on a 2 stroke motor !---------Intake and exhaust ports are wide open before compression starts.

 

[bonz_d]

Re: Compression Testing Smaller Non-Electric start motors

What I would add is to repeat the test 3 to 4 times on each cylinder and then take an average. It's harder to get a consistant pull with a rope or even a consistant number of complete turns per pull. I have found that rope start models generally show lower numbers because they are not spun as fast as an electric start model.

 

[Putzaroni1]

Re: Compression Testing Smaller Non-Electric start motors

So I'll try this again one more time with adjustments

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1. Start engine and warm it up (not completely necessary but will provide higher compression numbers)
2. Remove all spark plugs and disconnect fuel line from tank or fuel source
3. Pull the emergency kill switch on rope pull start motors if there is one. If not be sure to ground the spark plug wires to the block.
To do this, put the plugs back in the spark plug leads and attached a grounding wire to the spark plug and then ground it to the engine block. Do this for all plugs. This will insure that any current coming out of the coils will go to ground rather than potentially fry something back in the ignition
4. Screw the compression gauge into plug hole #1.
5. Pull start engine 5 or 6 times watching the compression gauge to confirm the needle is no longer moving up
6. Repeat steps 5 & 6 for all cylinders.

All psi readings should be within a 10-15% span.

Notes:

- For a more accurate reading ,repeat this test 3 to 4 times on each cylinder and then take an average.
It's harder to get a consistent pull with a rope start or even a consistent number of complete turns per pull. I have found that rope start models generally show lower numbers because they are not spun as fast as an electric start model.

- Throttle position does not matter as intake and exhaust ports are wide open before compression starts.
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Question:
Is what's described in Step 3 the best way to ground the spark plug leads or does anyone have a better way ?

Again, thanks you guys, you're awesome !

 

[phillnjack2]

Re: Compression Testing Smaller Non-Electric start motors

no the reading should not be with 10 to 15%
they should be within a maximum of 10%.

a 155 difference would be terrible, and could mean an engine of 100psi on 3 cylinders can have 1 cylinder at just 85 psi.

if the engine is less than 4 cylinders it should be that all cylinders are above 100 psi to get it running half decent.

when percentages are used its very misleading, the original percentages used were for comparing a new engine to a worn out one.
a engine that had been run in correctly and would say give 130psi on all cylinders would of been ideal, and a engine of a maximum of 10% less would be usable.
anything below 10% of the newer run in engine would be on the point of worn out and needs re-building or new rings etc.

 

[boobie]

Re: Compression Testing Smaller Non-Electric start motors

You're speaking of checking the MTR warm. What if it won't start when you get it and it doesn't have any spark ? You fix that and still find out it won't start. Best thing is a cold compression test first and go from there. That way you'll know what you have to work with.

 

[Putzaroni1]

Re: Compression Testing Smaller Non-Electric start motors

So unless someone else pipes in with other recommendations, this is my final cut at this.....
Please speak up if any of this could be done better or differently


-------------------------------------------------------------------------------------------------------------------
1. Remove all spark plugs
2. Pull the emergency kill switch on rope pull start motors if there is one. If not be sure to ground the spark plug wires to the block.
To do this, put the plugs back in the spark plug leads and attached a grounding wire to the spark plug and then ground it to the engine block. Do this for all plugs. This will insure that any current from the coils will go to ground rather than potentially fry something back in the ignition
3. Screw the compression gauge into plug hole #1 (make sure it's about as tight as you would screw in the spark plugs)
4. Pull start engine 5 or 6 times watching the compression gauge to confirm the needle is no longer moving up
5. Watch to see if it holds or drains out. If it drains insure the gauge is in tight enough but do not over tighten
6. Repeat steps 5 & 6 for each cylinders.

Readings must be within 10%
ie. If it's a 2 cylinder, if one reading is 100 PSI, the 2nd one needs to be less than 10 PSI different lower or higher

Notes:
- For a more accurate reading ,repeat this test 3 to 4 times on each cylinder and then take an average.
It's harder to get a consistent pull with a rope start or even a consistent number of complete turns per pull. I have found that rope start models generally show lower numbers because they are not spun as fast as an electric start model.

- Throttle position does not matter as intake and exhaust ports are wide open before compression starts.

- A warmed up engine will provide higher compression numbers
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[Jon Boat Jim]

Re: Compression Testing Smaller Non-Electric start motors

After you pull the "kill switch" on a rope start engine, why do you have to ground the plugs? Doesn't the kill switch prevent spark (current) going to the plug?

 

[racerone]

Re: Compression Testing Smaller Non-Electric start motors

The kill switch disables the ignition system ( grounds it out ) and so there is no need to worry about the sparkplugs .--------Plus I have not seen a warning in an OMC manual that states to ground the ignition to prevent damage.

 

[oldboat1]

Re: Compression Testing Smaller Non-Electric start motors

not a technician, but I've never seen reference to that caution either (OMC manuals or service manuals like Clymer and Intertec). I rarely play with anything except '50s motors, so thinking maybe a warning out there somewhere applies to modern ignition or charging systems. One way around concerns, maybe, would be to hook up a spark tester while testing compression. Might be useful, for example, in testing a motor you are thinking of buying.