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Old 11-22-2002, 09:55 AM
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Post air not hold the valve up

I'm trying to change valve seals with the head on the engine. I have the air fitting in the spark plug hole, and the cylinder TDC. I hear air excaping, and when I compress the valve spring more air comes out the exhaust holes. It looks as if the whole valve is going down, and I have the air compresor as high is it will go. THis is my first time doing this and any help would be appreciated

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Old 11-22-2002, 10:47 AM
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It sounds as if you are not on TDC of the individual cylinder. check that both valves are closed and give the spring retainer a light tap (without hitting the end of the valve) before you compress the spring to remove the locks. Also make sure that your compression tool is not hitting the valve stem or lock.
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Old 11-22-2002, 10:57 AM
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You could be on TDC, have both the valves closed and still have trouble.

Just went thru thi a few weeks ago. Be sure to tap the retainer around enough to move it at less 30 to 40 degrees. This frees up the retainer from the valve stem retainers.

Go ahead and remove the rocker arm from the other valve on that cylinder to ensure that it is indeed closed.
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Old 11-22-2002, 11:20 AM
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Another method for doing this is to stuff some cord or rope into the combustion chamber. Its still gotta be at TDC, but it works just as good, but it does take a little longer. Just make sure you use clean/brand new rope, don't let the rope drag all over the frame while you're stuffing it in, and don't shove the whole rope in (obviously).
I personally prefer the rope method, because I dropped a valve into a cylinder once. Not fun.

Have you done a compression check on it? On the other hand, if you can't get the valve to stay seated with air pressure, it might be time to pop the head off anyway.

Another thing to think about is ring wear. Piston bores wear most at the top, so the rings might not be holding enough pressure at TDC. You can try it at bdc, but if you do, be prepared to take the head off. But i5t does sound like you may have a bad valve seat.
One more thing to try. On the off chance a piece of carbon is stuck between the valve and seat, apply air pressure and tap on the valves with a rubber mallet to try to blow it out.
Good luck
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Old 11-22-2002, 03:31 PM
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The only way to do it with air is at bottom dead center. Unless you have a manual trans. and have it in high gear and a good emergency brake. If you are concerned with air loss, or your compressor not "keeping up", do the rope method.
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Old 11-22-2002, 05:04 PM
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I have never had any problems with this set-up. I always make sure I remove all the rocker arms to ensure all valves are fully closed. Then one by one I remove each spark plug, hook up air to cylinder, and remove springs. With a decent compressor you shouldn't have trouble. Are you making sure that both intake and exhaust rockers are off so both valves are completely closed. Have you tried a different cylinder to ensure it's not a bad valve in that cylinder or bad rings.
And by the way BstMech, I have done this one engines all the way around the engine without ever setting any of the cylinders to TDC or BDC. Just shut the engine off and started my way around with the trans in Park. So be careful when you say "the only way".

[ November 22, 2002: Message edited by: dmorris1200 ]</p>
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Old 11-22-2002, 07:41 PM
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And the motor turned itself to bottom dead center didn't it.
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Old 11-22-2002, 09:03 PM
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What am I missing here? what difference does it make where the piston is if you take all the rockers off? every valve is closed and the only way your going to lose air is if (1) you have a bad valve and (2) your rings are so bad that its time for the whole engine to come down anyhow...
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Old 11-22-2002, 09:13 PM
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It doesn't, but I have never had one stay anywhere when I shot air to it, they ALWAYS turn to bottom dead center (of course with both valves closed).
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Old 11-23-2002, 10:54 AM
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Very good everyone. Thanks a ton. I turned the engine to bottom dead center and a few taps to seat the valves and the job is done! Thanks for all the tip and help.
John
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Old 11-23-2002, 12:39 PM
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Way ta go jazzguitar!
The first time I did that job, I let the piston go to bdc, but I was using an old lever type valve spring compressor, and somehow while fiddling with the retainers and keepers, and trying to hold the spring down, I knocked the valve loose from the seat and down it went. My only set of wheels and I had a date that night. That really sucked.
So the few times I've done it after that, I used a ratchet and cheater bar against a frame rail to hold the crank still so I could keep it at tdc.
But I also worked on aircraft engines, and there weren't a lot of air compressors out on the flight ramp, so I use rope.
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Old 11-23-2002, 06:45 PM
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No the engine stayed pretty steady both times I've used that method. Maybe because I remove all the rockers at the start making it more difficult for the pistons to travel up and down, remember the other spark plugs are still in only one plug is taken out at a time. If all the plugs were out the crank would turn more freely. Another determining factor could be amount of air pressure used to fill the cylinders, I use a restrictor in the line. Any variations can affect what will happen.
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Old 11-23-2002, 07:08 PM
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I guess, but I never use a restrictor/regulator because I aint chancin' droppin' a valve. In fact, if my compressor put out 300 psi, I would use 300psi.
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Old 11-23-2002, 07:29 PM
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Old 11-23-2002, 08:53 PM
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Right on! Kinda' like "If ya' aint livin' on the edge, yer' takin up too much space!"
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