Driving today, 998 loses heaps of power and sounds like a lawn mower. Pulling leads to 3 and 4 makes no difference to idle when running. opening up the throttle on the SU makes the carby piston oscillate up and down rapidly causing a quite loud rattle like air is being forced back through the inlet on compression. Could this most likely be a flakey inlet valve, or on further thought maybe gasket failed between 3 and 4? I will pull head when time permits but just chasing some thoughts on these symptoms. Cheers

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Steven

Why don't you check rocker clearance first see if the gap has closed up.

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Are the plugs fouling?

I'd say blown head gasket. How about doing a compression test?

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sounding like a lawn mower or VW I would say its a head gasket also.
easy to check, take a spark plug out and put your thumb over the hole. They should be all similar but I'm betting to will be low

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Stop Licking the Dog...I Don't Care Who Started It

Only way an inlet valve will burn is if you have little or no clearance, but then you'd have no compression, either..
Same story if you bend one due to foreign objects getting into the cylinder.
But then you would have extra problems.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks everyone. Probably won't have time to pull the head till next weekend. I don't know much about cam timing but would it be conceivable that if the head gasket failed between cylinders 3&4 that on the compression stroke it could push the charge into the opposing cylinder and up the the inlet? To my mind that would be the reason the carb piston is being forced up and down violently. I'm being hopeful it's just a gasket job and not a head recon.

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Steven

Yep that can happen, sounds like a head gasket to me.
IMO the best gasket to use these days is an ACL Monotorque. But ignore the blurb on the packaging that says only tighten once and forget.
I'd recheck it when hot, and again after 1000 miles or so. And recheck the rocker clearance each time.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Awesome. Thanks all will track down an ACL gasket and go from there. Thanks everyone.

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Steven