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dyno and the 520

B

bill finn

Member
Well hears one i have not ran into. We pulled the 520 down to rated rpm and saw we only had 20hp. I walked up to the engine to see gas pushing out the top of the carb bowl. Every time the engine fired the gas squirted from the carb. I was concerned for fire and quickly pulled the load off and idled down. The gas quit running out and the tractor and it ran pretty good. Because of the pulsation of gas from the carb I came to the conclusion that this must be compression coming into the intake pressurizing the carb under full load. We started with removing and inspected the intake for cracks, it was like new then verified that we had valve lash, all looked good so next we removed the head. I noticed some darker shades on the copper head gasket between the cylinders and figured it must be cross firing under load. The tractor has not had a load for 20 years, maybe she didn't like getting back to work. Oil looked good(not diluted) and the level was not overfull. The head was delivered to the machinist today and more will be known after he does his work. If any body has any addition thoughts or ideas we sure would like to hear them.
 
Oh one more thing. Since the head is off anyhow. I'd also pull the pistons out because I have seen several of the 620 models with broken top piston rings. Seems these models using thinner rings and aluminum pistons don't hold up well when past "normal" overhaul limits like a cast iron piston with wider rings will.
Pretty much anytime I pulled a head off I'd also pop the pistons out and if nothing else bad found hone it and install new rings.
 
Some diagnostic checks can be handy to narrow down a problem: vacuum gauge; check tappet clearance; compression test; or a leak-down test.
 
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