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This may be a silly question, but I will ask it anyway. I have problems with a 660 raptor. I did a compression test and only got 80psi. I thought about it later. ( after I had pulled it apart) How can I get an accurate compression test if the decompression mechanism on the camshaft is releasing the compression when cranking the engine. Or does the decompression pin retract under cranking revolution speed? :eek: :eek: :eek:
 

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the pin does not retract under cranking speeds (that's what its there for) but it does retract at running speeds.

chances are your battery will not spin the motor over with the decomp defeated but to do it, loosen the left(as you sit on the bike) exhaust valve adjuster, just back it out until it is flush with the rocker arm (on the bottom of the rocker arm) and tighten the nut back down. You need to remove the slides from the carbs to get a good reading too.
 

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I've always been told I should be getting around 130psi with the decomp in.
 

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I to got 80psi jwhat would be casing it to blow one header off and spiting out oil to?
 

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I to got 80psi what would be causing it to blow one header off and spiting out oil to?
 

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The header shouldn't be a factor, the exhaust valves should be sealing up on the compression stroke. But if it's "blowing oil" from somewhere that could certainly be a problem. Check your valve lash again and look for signs of a leaking valve stem seal or worn valve seats.
 

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Master of the Electron
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80 psi with the decomp operating is about the minimum at which the engine will start and run - you likely have problems. Have you added a teaspoon of oil to the cylinder to see if compression goes up? If it does, this is an indicator of worn rings or cylinder wall.
 

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Try adjsuting the valves first and then retry the compression test, maybe you have tight valve clearance...........
 

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The oil is coming out of my crankshaft cover. i know i looked
 

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Master of the Electron
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I agree with Willy - tight intake valves are common on Raptors - easy to check ... then, if that wasn't it, add some oil thru the spark plug hole and see if compression goes up.
 

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I dont use a normal pressure tester, I use a leak down test set up like this. It is $73.00 from summit racing but they can get pricy.


A leak-down tester is a measuring instrument used to determine the condition of internal combustion engines by introducing compressed air into the cylinder and measuring the rate at which it leaks out.

Compression testing is a crude form of leak-down testing which also includes effects due to compression ratio, starter/battery condition and other factors. Leak-down testing confines the results to cylinder leakage alone.

Testing is done on an engine which is not running, and normally with the tested cylinder at top dead center, although testing can be done at other points in the compression and power stroke. Pressure is fed into a cylinder via the spark plug hole and the flow, which represents any leakage from the cylinder, is measured.

Leakage is given in wholly arbitrary percentages but these “percentages” do not relate to any actual quantity or real dimension. The meaning of the readings is only relative to other tests done with the same design of tester. Leak-down readings of up to 20% are usually acceptable while greater than that requires a repair. Racing engines would be in the 1-10% range for top performance.

In the United States, FAA specifications[1] state that engines up to 1,000 cu in (16 L) engine displacement require an 0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle. The input pressure is set for 80 psi (550 kPa), and 60 psi (410 kPa) minimum cylinder pressure is the accepted standard.

While the leak-down tester pressurizes the cylinder, the mechanic can listen to various parts to determine where any leak may originate. For example, a leaking exhaust valve will make a hissing noise in the exhaust pipe while a head gasket may cause bubbling in the cooling system
 

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