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Old 30 December 2011, 18:11   #1
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Compression tester needed

Hi

Viewing a rib tomorrow with a 40hp yamaha 2 stroke but completly forgot about testing the engine for compression
I can't sea trial so it's vital I get one
Does anyone know where I can buy one tomorrow morning? or does anyone have one that I could borrow for a few hours?

also is this right;
Disconnect fuel line
remove spark plug and screw in compression tester
turn engine over and read compression
repeat for all cylinders

what should compression should I be looking for in a 40hp Yamaha?

Thanks

Nathan
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Old 30 December 2011, 18:19   #2
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Don't forget to remove the kill cord to cut off any spark.

Your looking for the tolerance levels to be within 10 % tops. A reading of 130 would be about right but expect anything from 115-150 dependant on age, use etc. Repeat when warm too

Not local but most car auto shops such as Halfords will stock (but they will sting you for one) it. Just be sure it's either got the correct fitting or adapters to suit.

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007
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Old 30 December 2011, 18:25   #3
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Thanks Peter, how do I know what adapter I need?
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Old 30 December 2011, 18:34   #4
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Nathan.

I've a couple knocking about in the garage, with adaptors for different size plug holes.

If you can get up to Clanfield just up the A3 South of Petersfield before viewing the engine you are welcome to borrow one.

Other than that I'd also suggest Halfords, but also Machine Mart.

I'll PM you a telephone number.

Nasher.
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Old 30 December 2011, 18:41   #5
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130 is way too high, anything above 100 is fine IMO. Just look for the difference between the cylinders as Peter mentioned anything above 10% then you should just invetsigate the reason.

One engine I owned had flooded carb, I could see one cylinder does not spark, and then the compression test on that particular cylinder was higher than the other cylinders.

A compression difference does not necessary mean a defect cylinder or piston ring etc..

Halfords sell tester with both adapters and be careful as the battery may die on you if the battery is not charged.
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Old 30 December 2011, 19:01   #6
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I think you are meant to open the throttle fully when compression testing?
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Old 30 December 2011, 19:05   #7
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Good advice. Keep us posted on what the readings are and how you get on

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Old 30 December 2011, 19:09   #8
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Quote:
Originally Posted by Boats&Outboards View Post
Good advice. Keep us posted on what the readings are and how you get on

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007
Will do!

would this be ok? Halfords | Gunson Hi-Gauge Compression Tester
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Old 30 December 2011, 19:20   #9
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Looks good to me (although as discussed you can get cheaper ones but not so quick/easy check out the pic).

From memory a Yam 40 will have something similar to a BR7HS-10 (NGK) which has a 14mm thread diameter. Might be worth double checking with Yam in the morning though

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Old 30 December 2011, 19:27   #10
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Thanks for all the advice, i would buy online i just don't have the time.

Anyway i've got a early start for a change. no more 12am lay ins...
I'll give yam a call in the morning
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