Ballpark compression & Also valve tappet clearance?

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Chris G
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Ballpark compression & Also valve tappet clearance?

Howdy, does anyone know what approx compression small mower engines like A114 should be? I can't find reference to it in any of the docs I have or on line for the Punch.

Cold, Choke off and WOT on one of them I am getting about 105psi on one of them, does that sound ok? Reason for asking is looking to do a head / valve clean / service on a spare motor to tuck away in case needed and could do with an idea of compression idea beforehand. Also for A15 if anyone has any idea.

Many thanks

wristpin
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Compression figures are not

Compression figures are not usually published for small single pot industrial engines as they usually have some form of automatic compression release to ease starting. Compression release may be achieved by a centrifugally operated mechanism that holds a valve very slightly  open until running rpm is reached or, alternatively a cam profile that releases compression at starting rpm but is so slight that it has no effect at running rpm . Some engines use a combination of both systems. 

The most meaningful assessment of engine condition is a “ leakdown test”  where the engine is pressurised with the piston just past top dead centre ie out of the compression release zone. Pressure loss is shown on a gauge and the Mk1 human lug’ole can detect where the pressure loss is coming from  eg  the dip stick hole, piston blow by; the exhaust, leaking exhaust valve ; the carb, inlet valve, or around the head / cylinder interface for a leaking head gasket.

 

 

 

 

Chris G
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Thanks for that reply, very

Thanks for that reply, very informative, your first paragraph re decompression makes perfect sense but I would not have thought of that.

I will measure all 3 A114 I have to give me an idea and the A15.

Leak down gauges on their way, as I don't have one. more tools - I need no other excuse :-)

Chris G
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Hello, on this spare motor I

Hello, on this spare motor I have decoked, surfaced head ready for new gasket and lapped the valves - not too much and to an even colour.

I am getting the following tappet clearances

Exhaust 0.22 book says 0.25 - I don't know what the service limits are but guess that's ok if a bit tight?

Inlet 0.30 book says 0.15 ! - should I lap that a lot more? Not sure why its so high, I did not notice any carbon deposits holding it up after lapping...

Not seen the inlet that far out on any of the others I have done..Many thanks, Chris

 

 

 

Chris G
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Out of interest - mine at

Out of interest - mine at least... did a compression test on 3 installed A114's and all came out around a suspicious 100psi mark. Leak down showed 2 at good end of moderate and one in the middle, but not bad enough that I could hear where any pressure loss was coming from. Trickiest thing I found with the leak test was getting to where both valves were completely closed but ok once gotten the hang of.

Valve clearances on this spare A114 are still a bit of a head scratch, exhaust being a bit too tight and inlet still a lot bigger than spec despite further lapping. The boy is going to take 0.05 off the exhaust stem which will bring it back into installed height spec, but am wondering if the inlet has already been ground - and too much?

Overall length of exhaust valve is 89.36, inlet is 89.15 and 0.15 short of spec, the shorter end taper on inlet stem is quite visible compared to EX. I guess this might be adjusted on original assembly?

It's not a sports car :-), am I being over critical on these clearances prior to installing and test firing up?

 

wristpin
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Both valves shut. In the

Both valves shut. In the absence of any engine specific information I take the piston to tdc on the compression stroke and then continue rotating the engine enough to drop it approximately a quarter of an  inch . Depending on the air line pressure required by your leak down tester you may have to devise a way of preventing further rotation.

If the engine runs ok I wouldn’t worry about the over wide valve gap. By the time you’ve ground away enough seat or valve face to close the gap you will possibly have done more harm than good. 

Chris G
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Spot on re the compression of

Spot on re the compression of the leak down turning the engine over, I had to chock the cylinder.

Noted on the valves, I had given up trying to close the gap via seat/ face, I may also park the idea of taking off the stem and just seeing how it runs.

Thanks for the advice