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I hade a misfire in my propane burnisher (buffer). The Exhaust valve is stuck and wont move, as a result the push rods bent. Manufacturer recommended I get a new head. I replaced the head according to the instructions. New gaskets, new push rods, all bolts torqued to spec. The gap between the rockers and valve springs are gapped correctly. Yet I'm getting 120-130psi compression in the cylinder, where spec is 60-70psi. I've checked the head part number and engine model number and it all matches. The old head and new head look identical besides the carbon on the old.

I haven't tried it with any fuel because I'm scared of pre-ignition and ruining the head again.

Any ideas on why I'm getting high compression?

Engine: Kawasaki FS48V-ES10 Burnisher: PE400BU

Edit: For further context, Cylinder 2 is within compression spec. It was tested before and after the head on Cylinder 1 was replaced.

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  • Are there different head gasket thicknesses?
    – Solar Mike
    Feb 13 at 20:31
  • @SolarMike There is only one thickness head gasket from Kawasaki for this engine.
    – Bondark
    Feb 13 at 20:39
  • Did you measure the new head against the old? Is the new head designed for a different stroke? Is the new head designed for a new piston? When you bought the new head what did you check?
    – Solar Mike
    Feb 13 at 20:43
  • @SolarMike New mechanic here :). No I didn't measure anything before putting on the new head. It was a replacement head kit with everything included except the rockers. The FS48V-ES10 is just 4 stroke. Not sure if the same head is used for 2 stroke. When I put the head on it fit exactly like the old.
    – Bondark
    Feb 13 at 20:50
  • Too many things to check - was the old head correct? Seen things like this before... had a 6 cyl which had valves tapping pistons - due to 1cm having been machined off the head over 40 years. Cure a 0.95cm plate with two gaskets...
    – Solar Mike
    Feb 13 at 21:08
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120 to 130 psi compression test result is within the serviceable range of a typical spark ignition combustion engine running on unleaded pump fuel.

However a spark ignition combustion engine powering a specialised machine/tool set up to run on propane with a low compression ratio will have an oil ring designed to manage engine oil that would otherwise be subjected to the solvent effect of unleaded fuel.

If however your engine has been run on unleaded with an inital compression ratio of 60-70 PSI the reason your seeing a test result of 120-130psi will likely be due to a build up of engine oil, unburnt fuel and carbon in the piston ring lands and between the crown of the piston and the top compression ring. a build up of this tacky material is likely the reason the exhaust valve hung open.

Can you provide the source recommending a compression test result of 60-70psi as serviceable?

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  • This isn't a typical spark ignition engine due to it running propane. As stated in the comments to the question, propane is prone to pre-ignition, so the OP's fears are justified. Since this is a propane run engine, it doesn't suffer from "a build up of engine oil, unburnt fuel and carbon in the piston ring lands" ... it burns clean. Probably even too clean. While what you've stated does apply to a "typical spark ignition" engine, this is far from typical, and therefore most of what you've stated doesn't apply in this situation. Feb 14 at 0:28
  • Paulster is correct. No fuel but Propane has been used. Compression specs are found on page 81 of the Kawasaki FS481V Service Manual
    – Bondark
    Feb 14 at 0:56
  • Captivating, I look forward to seeing a diagnosis on this one. Feb 14 at 8:15
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This is only a partial answer:

You could narrow down your diagnosis by comparing the volume of both heads. If both volumes match, then the new head is fine and your problem lies somewhere else.

How to measure the volume of a cylinder head.

  1. Remove head. Attach spark plugs and valves, without rocker arms or camshafts, so that all the valves can be closed.
  2. Insert a layer of grease between head and valves and close the valves. You now should have a fluid-tight "pocket".
  3. Place the head head-down, so that the opening is at the top and the spark plug is at the bottom.
  4. Place an old head-gasket on the head. Bolt on a plexiglas plate over the gasket. The plexiglas plate has two small holes, just enough for a syringe needle, over each combustion "pocket". One hole is for inserting the measurement fluid, the other is to allow the air to escape. The smaller the hole, the more accurate is your measurement.
  5. Use a syringe to fill the entire "pocket" with some fluid (diesel, soapy water etc..) until the pocket is bubble-free full. Write down how much fluid is needed to fill the pocket for each cylinder.
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  • Of course the method can be adjusted to your needs. For example you could prepare and measure each cylinder on his own, thereby the camshaft and rocker arms can be left attached etc. This is more about giving you an idea on how to measure the volume than an do-this-exactly-as-written..
    – Martin
    Feb 18 at 16:03

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