Author Topic: Think my exhaust valves are toast  (Read 286 times)

Offline Minto

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Think my exhaust valves are toast
« on: 04.05. 2020 00:11 »
Happy Sunday y'all
When I removed the cylinder head a couple of weeks ago, I put it away safe and paid no real attention to it as I was more engaged in removing the engine from the frame with the intention of doing the head last. Now I've had chance to look at it, I think I need some help. It is very badly coked up with really thick heavy black deposits, the one exhaust valve in particular, the sealing face looks glazed, and the edge is a bit rough here and there. The seat surface doesn't look as bad as the valve, but I'm not sure really how to proceed. Ive only removed the exhaust valves so far, the inlet valves I'm thinking look better and hopefully won't need as much attention.
Any chance I can get away with just lapping them in to clean up the faces? Or is this a long wait for the local engineering shop to reopen?
The bike has always smoked a bit on the left pot, the same side as the worst valve.
What causes this kind of heavy carbon build up? Rich running? Timing too advanced/retarded?
Another thing is that the two exhaust valve guides are different, one has a conical end protruding, the other is more rounded.
Any and all advice gratefully received.
Cheers
Jase
52 A10 plunger
Aprilia RSVR

Offline bikerboy

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Re: Think my exhaust valves are toast
« Reply #1 on: 04.05. 2020 01:48 »
Sounds like it was burning a bit of oil to me, what colour were the plugs when you took it to bits? Technically it would not matter if the guides looked different but I would personally want them the same. Are the valves loose in the guides? Are the guides loose in the head? Did it need a rebore? How were the pistons and rings? The possibilities for it being covered in crap are pretty endless really. I will say that if an A10 is going to smoke 99 times out of a 100 it will be from the left hand pot.

Why did you take the engine out was it running poorly or were you painting the frame or something?

Sounds like a few quid to me and a top end overhaul, as you have it to bits you might as well get it done now because it sounds like it needs doing.

Offline Swarfcut

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Re: Think my exhaust valves are toast
« Reply #2 on: 04.05. 2020 09:03 »
Jase.   If the truth be known, the motor may not have been apart for a long time. If this is the case, a good dose of carbon in the combustion chambers is to be expected, we are talking an old design and components which don't have the performance expectations of  modern engine development.

  For starters, give the head a good deep clean, reassemble and try a leak test on the valves and seats, they may look crap but actually seal OK.  You can lap them, but be careful as the seats are narrow. If a valve itself is pitted, it can be refaced. If you change the valve for new, then a change of guide and recut of the seat by your engine reconditioner gives you the best outcome.

 Another spare head is a possible, but as usual, may be as bad or worse than the one you have.

  With burning oil, sources are the guides and bore/pistons.  If it was running reasonably well, no knocks or clanks, a good clean up should do for now.

 Piston rings are easy to check, as long as the ring gap is within limits, that's fine.  More important  is the up and down movement of rings in the piston grooves. There should be minimal up and down movement, if any. You may find the compression rings worn to a T section, so compare the inner depth of the ring with the outer. New rings will do in old pistons, as long as the piston grooves will only allow up and down ring movement within the specified limits.

 So all is not lost, it's just a matter of how far and expensive you want it to be.

  Surprisingly, just cleaning up worn components will produce almost a good results as new parts. Pistons with carboned up oil drain holes and stuck rings are a typical example.

 Swarfy.

Offline Minto

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Re: Think my exhaust valves are toast
« Reply #3 on: 04.05. 2020 11:13 »
Thanks lads
Bent valve!  Thought it had somehow become oval but on further examination it's bent and not seating.
Cheers
Jase
52 A10 plunger
Aprilia RSVR

Offline Minto

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Re: Think my exhaust valves are toast
« Reply #4 on: 04.05. 2020 14:07 »
Upon even further examination, and after a panic call to SRM, to see if I could catch them and order a new valve and guide before they shipped the rest of my order,( just caught it.) It seems it was the build up if crap under the valve head and on the stem stopping it from seating. Cleaned up and leak tested both ex valves, sealing pretty well it seems.
I'll keep the new valve for when I need it.
Thanks again guys, this place rocks!
Jase
52 A10 plunger
Aprilia RSVR

Online Triton Thrasher

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Re: Think my exhaust valves are toast
« Reply #5 on: 04.05. 2020 17:32 »
Rich mixture causes buildup of coke on the valve and so does oil burning.

In your smokey case, I suspect oil.  It’s not all that hard to fix.

Offline Swarfcut

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Re: Think my exhaust valves are toast
« Reply #6 on: 04.05. 2020 18:56 »
Bit late now, but putting the valve stem tip in the chuck of a drill and giving it a quick spin will sort the straight and true from the bent and bowed.

 Swarfy.

Offline Minto

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Re: Think my exhaust valves are toast
« Reply #7 on: 04.05. 2020 20:12 »
Cheers TT, I suspect it's oil too, especially since dismantling and finding a bit of a little puddle of oil on top of the piston.
Swarfy, I did think about that but was concerned about marking the valve stem in the chuck, I searched my garage for a piece of rubber tube to slip over the stem but couldn't find anything.
Going to do a bit more of a clean up on the head before calling it good enough.
52 A10 plunger
Aprilia RSVR

Offline Swarfcut

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Re: Think my exhaust valves are toast
« Reply #8 on: 04.05. 2020 20:33 »
 Holding the valve stem above the collets is fine for this quick test. As long as the valve can be held just firmly enough to be driven, any error will  show. Typical bowed stem is one that is hard to push down the guide. A stem bent above the collets acts in a similar way, then suddenly eases.

 Swarfy.