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Jubilee low compression

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Afternoon all,

I’ve just refitted the heads on my rebuilt jubilee engine and I’m only getting about 35psi compression on each cylinder. Stripped & cleaned all parts, honed bores & fitted new std pistons & rings as bores showed very little wear.

I haven’t yet refitted the idler timing gear and there are clearances under all the tappets.

Any ideas where I should start looking first?

Many thanks

Kieren

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Similar thread on the singles section M50 compression.

Most answers will be similar - WOT, crank speed, temperature  etc,etc.

Fwiw, 35psi is very low and sounds like a closed throttle figure.

George. 

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Thanks George,

Your closed throttle comments have enabled me to realise what an idiot I’m being - 

no timing idler = inlet valves closed = low compression!!

Dunce hat please!

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According to the engine data sheet I have, attached it’s a compression ratio of 8.75:1 which works out to be about 128psi.

once I get a chance to assemble the timing gears properly and then check my compression I’ll update the thread with the results!

Kieren

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Managed to get 125psi on both cylinders finally, but needed to put some eng oil into the bore, dry pressures stayed very low, approx 50-80psi.

 

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Hi Kieran, I would expect the hot engine figure will be close to your oiled figure. Important bit is that they are similar on each cylinder. Personally I would worry about other things on the bike now rather than compression until it has bedded in/run a bit.

Regards, George. 

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Yep, considering it’s only an engine on the bench at the moment - definitely!

now to prep the frame & forks for painting & rebuild....plus the fact that my priority is to replace the headlight on my main ride - melted it with the heat gun trying to split it to upgrade the projectors (2011 Triumph Tiger)

whoops!

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Kieren:

Your engine is way too clean!!!   No staining at all.

What did you use to get it that way?

Mike

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Hi Mike,

It’s a restoration job, engine hasn’t run in 40yrs so I stripped it and had a local guy vapour clean it.

it’s not run yet that’s why there’s no staining!!

 


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