ScootDawg Scooter Forum - low compression

essexs4
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low compression
« Thread Started on Jan 9, 2009, 9:43am »

New to Scooters here. A friend gave me a 2007 Eurostrada rx8 50cc scooter with 80 miles on it. I rode it 5 miles, and it stalled a few times. Now won't start at all. Spark plug was very loose. Replaced it. Friend had run outboard engine oil in it to get home one day. Took it to my local garage and they said compression was low- around 70? It is getting a spark, and coil is good. What next??

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90GTVert
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Re: low compression
« Reply #1 on Jan 9, 2009, 10:21am »

If the compression really is at 70psi, there's your problem. Isn't the Eurostrada RX8 a 49cc Jog/Minarelli style 2 stroke engine? If so, the service limit is 139psi and the standard compression is listed as 198psi. You're way out of spec if that number is correct. I have had trouble getting my compression tester to seal on my scoot's head though. I had good compression and it only tested 75psi once. If you pull the fan cover off you can turn the cooling fan by hand. If the motor has enough compression to run you should be able to feel more resistance as the piston approaches the top of the cylinder. The other way to check is to pull the spark plug out and kick or crank the engine over while you hold your thumb over the spark plug hole. Don't stick any fingers in there, just try to seal over the outside of it with your thumb. If it has a good amount of compression it will try to push your thumb off of the cylinder when the engine is turned over.

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Re: low compression
« Reply #2 on Jan 10, 2009, 12:52am »

A better compression gauge to use would be one that actually screws into the spark plug hole, not a rubber tipped one that you just push in.

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Re: low compression
« Reply #3 on Jan 10, 2009, 12:57am »

I use a Snap-On screw in compression tester set. Never had a problem on cars, just the scoot.

« Last Edit: Jan 10, 2009, 12:57am by 90GTVert »

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rallyrrr
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Re: low compression
« Reply #4 on Jan 11, 2009, 10:44pm »

Jan 9, 2009, 10:21am, 90GTVert wrote:

If the compression really is at 70psi, there's your problem. Isn't the Eurostrada RX8 a 49cc Jog/Minarelli style 2 stroke engine? If so, the service limit is 139psi and the standard compression is listed as 198psi. You're way out of spec if that number is correct. I have had trouble getting my compression tester to seal on my scoot's head though. I had good compression and it only tested 75psi once. If you pull the fan cover off you can turn the cooling fan by hand. If the motor has enough compression to run you should be able to feel more resistance as the piston approaches the top of the cylinder. The other way to check is to pull the spark plug out and kick or crank the engine over while you hold your thumb over the spark plug hole. Don't stick any fingers in there, just try to seal over the outside of it with your thumb. If it has a good amount of compression it will try to push your thumb off of the cylinder when the engine is turned over.

Has anyone here ever seen 198 psi on a two stroke? Sounds pretty outlandish to me. Not doubting your looked up figures GT just considering what goes on with a two stroke's effective compression ratio of around 8:1 tops and more like 6 - 7:1 on street scooter motors.

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essexs4
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Re: low compression
« Reply #5 on Jan 12, 2009, 10:21am »

Thanks all. Spoke to my mechanic friend. They did use a screw in tester. What could have caused low compression? Could rings already be bad?

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Re: low compression
« Reply #6 on Jan 12, 2009, 10:52am »

Jan 11, 2009, 10:44pm, rallyrrr wrote:

Has anyone here ever seen 198 psi on a two stroke? Sounds pretty outlandish to me. Not doubting your looked up figures GT just considering what goes on with a two stroke's effective compression ratio of around 8:1 tops and more like 6 - 7:1 on street scooter motors.

It sounds high to me as well, but those are service manual specs.

Piston rings are the most likely cause of low compression. It could also be a cylinder head sealing issue, so make sure your cylinder head nuts are all torqued down to around 15 ft/lb for a minarelli. Faulty or non-sealing reeds can cause a compression loss, as well as the crankcase not being sealed.

My first steps would be :

-Check compression with thumb over spark plug opening as I described earlier.

If low :

-Unbolt the exhaust from the cylinder and move the piston until you can look into the exhaust port and see the rings. See if they are stuck or if there is scarring etc...

-Remove the air shroud. Look for anything obvious. Torque head nuts.

« Last Edit: Jan 12, 2009, 10:54am by 90GTVert »

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Brent
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