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themanonthebus · Nov 11, 2009, 09:05 AM

Hi team, while mowing a field on my sit-on yardman as you Yanks call it, it cut out & stopped. The engine will turn over but there is no compression. I took the cylinder head off & turned the engine through the stroke. It is clear that the sooted intake valve opens and closes perfectly but the clean but pink-coated (with a rusty texture but NOT rust!) exhaust moves in and out but doesn't fully close shut.

Any ideas or pointers?

Thanks

KISS · Nov 11, 2009, 10:17 AM

First two. Bent valve out out of adjustment. The bent valve would hang up in the valve guide.

crigby · Nov 11, 2009, 10:25 AM

Hi,
You have a decent description of the problem. Some form of obstruction is possible. I also gather the this is a valve-in-block engine which can have the exhaust seats come loose from the blocks.
If you could provide the engine numbers, I could be more certain what to say.
Peace,
Clarke

themanonthebus · Nov 11, 2009, 11:05 AM

Engine number is

MODEL TYPE CODE
252707 0710 01 87120311

themanonthebus · Nov 11, 2009, 02:07 PM

Originally Posted by crigby

hi,
you have a decent description of the problem. Some form of obstruction is possible. I also gather the this is a valve-in-block engine which can have the exhaust seats come loose from the blocks.
If you could provide the engine numbers, i could be more certain what to say.
Peace,
clarke

Model type code
252707 0710 01 87120311

crigby · Nov 11, 2009, 02:37 PM

Hi,
Yours is a 25 cu. in. engine manufactured on December 3, 1987. It is indeed a valve in block engine. To check the valve, you will need to remove the air cleaner, then the carb and finally the breather cover that is behind the carb. With the valve as closed as it gets, take a screwdriver and push/twist the spring keeper to release it. The valve should be removeable and can be checked for trueness by clamping the stem in a drill and turning it at slow speed to check for wobble. The seat can be checking with fingertips by trying to remove it, or by eye for crookedness.
My guess is that the seat has come loose in the block.
Peace,
Clarke

themanonthebus · Nov 12, 2009, 04:45 AM

Originally Posted by crigby

Hi,
Yours is a 25 cu. in. engine manufactured on December 3, 1987. It is indeed a valve in block engine. To check the valve, you will need to remove the air cleaner, then the carb and finally the breather cover that is behind the carb. With the valve as closed as it gets, take a screwdriver and push/twist the spring keeper to release it. The valve should be removeable and can be checked for trueness by clamping the stem in a drill and turning it at slow speed to check for wobble. The seat can be checking with fingertips by trying to remove it, or by eye for crookedness.
My guess is that the seat has come loose in the block.
Peace,
Clarke

Hi Crigby,

... I did as you suggested & took the componenets off and after initial inspection can see that the push-rod that drives the valve from the main crankshaft isn't returning as far as it should into the casing. See...

Valve Problem on Flickr - Photo Sharing!

For confirmation of the part I'm referring too. When I compare to the inlet push-rod I can see the original mark of oily deposits are approx 1/8inch advanced further towards the spring, as if the part in the crankcase if pushing it further out than it should. Does this make sense?. any ideas or a complete strip down?

crigby · Nov 12, 2009, 11:28 AM

Hi,
Yes, it sounds as if you will need to explore it internally. It is rare that a tappet would seize and the cam should not be composite, but something seems to be going on with the cam. On very rare occasions I have seen them split in two.
Peace,
Clarke

themanonthebus · Nov 20, 2009, 10:20 AM

Hey Clarke -

I inspected the valves & push-rods again. I've realised I can manually push the pushrods back into their casing and that indeed, something is causing the exhaust valve to 'stick' open. The spring doesn't have the power to close it shut so it could quite simply be a bent valve stem sticking in the casing.

I've had the screwdrive out & have managed to release the inlet valve spring but the exhaust spring seems to be a whole lot more complicated - any clues as to how to release it?!

Charlie

themanonthebus · Nov 20, 2009, 12:45 PM

Hello again Clarke!

... so valves & springs are all out. I've tested them in a drill & while there is some minor movement its hard to tell weather its coming from the valve or the drill itself. I did the same with the inlet valve & its about the same!

What's the best way of cleaning eveything up so I know its not general grime & dirt?

Charlie

crigby · Nov 21, 2009, 08:26 AM

Hi,
I am guessing you may have had those two semi-circular keepers on the exhaust.
At any rate go to an automotive parts supplier and get some valve lapping compound. If they ask whether you want the water or grease-based, it is the latter.
Put a dab on the valve's sealing surface and put in against the seat and spin back and forth occasionally pulling it out a bit and turning a bit and putting back in to make the distribution more uniform. DO NOT get the compound on the stem or anywhere other than the valve's sealing bevel or the valve seat!
When you are done, there should be a uniform fresh gray ring all the way around the valve that is about 3/32" in width and the seat should be uniformly gray around the sealing area of it. A black spot will be evident on the seat if it is no longer able to seal without cutting.
Cutting will require access to a Anyway cutter, 30 degree on the intake and 45 degree on the exhaust.
Peace,
Clarke