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The Compression Test


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#1 wvbuzzmaster OFFLINE  

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Posted September 28, 2013 - 11:01 AM

The compression test yielded 75 psi on the left cylinder and 85 psi on the right cylinder. So why doesn't my opposed twin Briggs start? Nothing has changed since it last ran. We sprayed starting fluid at it and not even a pop, nothing. There is absolutely nothing left to try. Help me!
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#2 boyscout862 ONLINE  

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Posted September 28, 2013 - 11:17 AM

Either timing or ignition are off. Good Luck, Rick


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#3 HDWildBill OFFLINE  

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Posted September 28, 2013 - 11:24 AM

Rick is right.  If you sprayed starter fluid in the carburetor and nothing happened, then the next think I would check is ignition.  If you don't have one already get a cheap ignition tester from HF they are great.  

 

Keep in mind that a compression check on these small engine's are not exact and only good for reference.  But 75 & 85 PSI seem awfully low to me.


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#4 bread320i OFFLINE  

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Posted September 28, 2013 - 12:25 PM

I am not sure if I missed any earlier conservations about this topic. 

 

Have you got spark? 


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#5 wvbuzzmaster OFFLINE  

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Posted September 28, 2013 - 12:43 PM

Yes to spark. Last this tractor ran was last weekend. Hasn't started since I parked it....

#6 boyscout862 ONLINE  

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Posted September 28, 2013 - 01:14 PM

I've got to suspect the flywheel shear key. If it partially sheared the timing would be off. Good Luck, Rick


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#7 Guest_gravely-power_*

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Posted September 28, 2013 - 02:39 PM

I've got to suspect the flywheel shear key. If it partially sheared the timing would be off. Good Luck, Rick

Everything being equal, that is my bet. Especially with a Briggs.


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#8 KennyP OFFLINE  

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Posted September 28, 2013 - 03:01 PM

What twin is this? Does it have points? If not, pull the shroud and clean the rust off the flywheel! That can keep it from firing!


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#9 John@Reliable ONLINE  

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Posted September 28, 2013 - 03:45 PM

air,fuel,ignition   >>>  can't have one with out the other

 

Pulled my Gravely til arm was about to fall off last weekend, ( was running when parked :D ) anyway checked the points, and they were stuck closed why? who knows readjusted them and started second pull :thumbs: :beer:


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#10 wvbuzzmaster OFFLINE  

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Posted September 28, 2013 - 03:51 PM

This is the 422437 Briggs opposed twin that powers the Caterpillar. A tractor that parks indoors and is used at least a few times a month if not more. I used it about 3 times just this past weekend. The engine had its magnets cleaned when I replaced the magneto about a year and a half ago. Every electrical component on the tractor was new when input them on in the last couple years. I can rule out everything. The spark plugs are E3 and new this past year. The fuel pump is electric, new this past Spring, as well as the fuel lines. The carb was cleaned two years ago and the tractor has never sat since, always idles low enough you would swear it could die. It cranks nice and smoother sine I oversized the starter wire and battery cables from factory. It got a new car battery this year due to the winch. I mean there should be no reason it doesn't start.......

#11 wvbuzzmaster OFFLINE  

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Posted September 28, 2013 - 03:52 PM

air,fuel,ignition   >>>  can't have one with out the other
 
Pulled my Gravely til arm was about to fall off last weekend, ( was running when parked :D ) anyway checked the points, and they were stuck closed why? who knows readjusted them and started second pull :thumbs: :beer:


There are no points on this engine. If there had been I would have put a coil pack on it.

#12 KennyP OFFLINE  

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Posted September 28, 2013 - 03:58 PM

Clean the magnets again and try it. If it's not firing , that's the first thing to check on these!


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#13 wvbuzzmaster ONLINE  

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Posted September 28, 2013 - 05:01 PM

That's the thing, it is firing, clearly visible spark, very good too. So it isn't spark, it has fuel, and it is surrounded by air. What is it?

Edited by wvbuzzmaster, September 28, 2013 - 05:01 PM.


#14 bread320i OFFLINE  

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Posted September 28, 2013 - 05:28 PM

Pull the flywheel to make sure the key didn't shear.

If it did shear that means the flywheel moved on the crank causing your spark to fire at the wrong time.

 

BTW, Briggs flywheel keys are known to break.


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#15 HDWildBill OFFLINE  

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Posted September 28, 2013 - 06:28 PM

How do you know it is firing under compression?  I agree with the others pull the cowling and check the fly wheel.


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