Any suggestions.....?

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BobbyG
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Location: CHESHIRE UK

Any suggestions.....?

Post by BobbyG »

Out the other day on my trusty 16ms when... chuff chuff splutter splutter … it died. Plenty of fuel, good spark but wouldn't start. NO compression - but blowing back through the carb.. Home, thanks to AA, removed head and tested valves by pouring petrol into the combustion chamber. Exhaust valve - perfect seal: inlet leaked like a sieve. Lapped valve and tested again. Still leaked. Checked contact between valve and seat with 'blue'. Only 1/3rd of the valve was in contact with the seat. Out with the valve seat cutter - 1 5/8"- I've tried and tried but still can't get the valve to seat properly. Valve isn't bent as far as I can see and the guide fit is OK - not perfect but OK. Any ideas as to my next course of action? HAPPY EASTER!
Robert Gallagher
SPRIDDLER
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Re: Any suggestions.....?

Post by SPRIDDLER »

I wouldn't say it's extremely common but it does happen that a valve seat can become loose and turn or tilt in the head when the head is hot resulting in a loss of compression. Once the head cools the seat will be gripped but probably out of line/register with the valve head. The seat only needs to be less than a thou out of place/tilted to prevent the valve head sealing.
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Paul R
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Re: Any suggestions.....?

Post by Paul R »

I would still suspect the valve even though it looks ok . The guide is ok so it can't have shifted in the head ,and the seat hasn't shifted in the head (if it as one) , the only other variable is the valve itself . Are you running an open carb with no filter ? Some thing may have got sucked in and given it a bang maybe ? Either way if you know anyone with a lathe put it in the chuck and run a dial gauge round it to be dead certain one way or another . Good luck.

Typing this when above came on first . Good point about seats coming loose .
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Rob Harknett
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Re: Any suggestions.....?

Post by Rob Harknett »

Had simular happen to a section members 500 single combo. Valve seat had turned, but the bike got him to the Jampot rally site. ( 1995 Pately Bridge, Yorks ) Malcolm Saggers came along asking where the guys head was. I went and got it out his tent. It was put on the Bar - B - Q. The guy showed up, asked why his head was on the Bar - B - Q. Malcolm said we're gonna fix it. When hot enough the seats were turned and a valve seal made. To stop it turning again, it was peened all round. Only problem after that was, some tank fittings were lost, never wired on the bolts. All done with tool box tools. Removed and replaced hairpin springs by hand.
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clive
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Re: Any suggestions.....?

Post by clive »

I would agree with Sprids and Rob, valve seat turning in the head a likely cause. If it has the seat will not be seating flush with the head. Does not need to rotate much. I have had seats rotate in both heads on my G11CS. Luckily in this case the seat has a tiny nick in it to accommodate the spark plug thread making it easier to see it had rotated, and easier to ensure it is back in the right place. Worth checking the valve is not very slightly bent though.
clive
if it ain't broke don't fix
BobbyG
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Location: CHESHIRE UK

Re: Any suggestions.....?

Post by BobbyG »

Yep. There's certainly something seriously amiss with the inlet valve seat. Difficult to say exactly what at this stage so... as the seat is well past it's best anyway...the head is now in the safe hands of Geoff Bain in Widnes for a thorough diagnosis and a new valve seat.

Many thanks for your observations / suggestions.
Robert Gallagher
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Re: Any suggestions.....?

Post by Pharisee »

BobbyG wrote:Yep. There's certainly something seriously amiss with the inlet valve seat. Difficult to say exactly what at this stage so... as the seat is well past it's best anyway...the head is now in the safe hands of Geoff Bain in Widnes for a thorough diagnosis and a new valve seat.

Many thanks for your observations / suggestions.
I believe that the valve seats were "cast in", not pushed, pressed of shrunk in after the head was machined. The inlet valve seat on my 500 AJS came loose enough to turn in the head (but not fall out). T & L Engineering machined the old seat out (it's quite a large chunk of metal) welded up the port, remachined it for a new seat and shrunk it in. They also put in a hardened exhaust valve seat while they were at it.
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