MTD Tractor Starting problem

[Arkie]

Anyone gave any thought that it maybe a worn cam lobe on that 793880. I just replaced my first one where the engine actually ran although not good where the exhaust was worn nearly round.

Yes, that is what I'm thinking. That is why I hinted to check the valve's lift. (check both intake and ex lift)
DID you put the Aluminum push rod on the intake valve side and the steel rod on the EX valve side when re-installing??
Do some more tests before just pulling the head for a eyeball test.

I've had it happen to me on Automobile cams on the intake and lose the cylinder.
On the Auto cam worn lobes I could compare the valve lift (push rod lift or valve stem stroke) on the weak cylinder to the good cylinders before I removed the cam for the eyeball inspection.
Seen couple automobile engines that had only one worn lobe, low mileage engines and oil changed regularly. Wear likely due to the cam lobe not being heat treated correctly. (soft lobe)

[lefty]

The gas through the spark plug hole proved effective for starting.

Regarding the travel of the valve and amount of time open, would the exhaust and intake valves have the same duration? If so, I can try to compare them without knowing what the proper figure, if they are supposed to be the same.

The pushrods went back in the proper space as I kept the parts separate. If I remember correctly, one was more silver, aluminum I think, which came out of and went back into the intake position.

[KE4AVB]

Yes both lobes have the same profile; hence, the same amount of rocker movement.

[bgsengine]

The pushrods went back in the proper space as I kept the parts separate. If I remember correctly, one was more silver, aluminum I think, which came out of and went back into the intake position.

You can also detect the difference by weight - the aluminum will feel much lighter than the steel one.. also with a magnet (although the aluminum one has steel tips) Both are hollow.

[Arkie]

I suspect (just a guess) the reason that the intake push rod is the lighter aluminum is so as the weight was kept down so as the intake valve would not float open due to the additional weight of the push rod when the rpm's start rising on the engine??? (the intake valve spring is also wimpy and weak on those OHV engines)

Anyway needs the aluminum push rod needs to be on the intake.
Some older model auto engines went to lighter push rods because the heavier steel push rods even though they were hollow would float the valves open.

[lefty]

Using a small square and the head as a reference point, I can verify that the tail end of the rocker, the end closest to the rod, moves identically between the valves. Assuming that both rockers are identical, it appears that both valves are actuating the exact same amount. It also appears, using the same method, that both springs are compressing the same amount. I just tightened up the lash on the intake, which was suggested earlier, and plan to see if I get any different results.

Not sure if I reported this back but I did remove and inspect the intake manifold yesterday and there were no signs of failure. I pressure tested it as well (held my hand over one end and blew in the other). A highly technical process.

[38racing]

I suspect (just a guess) the reason that the intake push rod is the lighter aluminum is so as the weight was kept down so as the intake valve would not float open due to the additional weight of the push rod when the rpm's start rising on the engine??? (the intake valve spring is also wimpy and weak on those OHV engines)

Anyway needs the aluminum push rod needs to be on the intake.
Some older model auto engines went to lighter push rods because the heavier steel push rods even though they were hollow would float the valves open.

My understanding is that Briggs uses aluminum on whatever valve has the compression release. Mostly it's intake but apparently there's one with it on exhaust. Technician once told me her boss said she was mixing the push rods up. She pointed out to him that the engine model she was working on had aluminum on the exhaust and she had things right.
Read a theory about metal expansion affecting compression release.

[lefty]

Assuming for a moment that the intake valve timing is correct and open time adequate, what else might impact the suction of fuel?

My guess would be the exhaust valve also opening slightly during the intake stroke, or air slipping by the intake valve from the crankcase during the intake stroke? You had mentioned that the springs can get weak. Since I'm getting such good numbers on the leak down, I'm assuming that the cylinder and rings are in good shape, also verified by inspection when it was apart. If the spring was weak, and the vacuum adequate, could enough negative pressure be created to allow air to get by the exhaust valve during intake? Seems unlikely with the intake valve open all the way but what do I know?

Leaving only air slipping by the valve itself, robbing the manifold enough low pressure to draw the fuel? But then again, when I had the head off and inspecting the valves, I didn't notice any slop in the guides. Are the valve guides in these heads even replaceable or would you just get a new head, if that was it?

Or maybe I'm way off.

[Arkie]

You say:
Since I'm getting such good numbers on the leak down,

Is this another RECENT leak down test with the ENGINE LOCKED at 1/4 inch past TDC??

Did you gently open the intake valve by using finger pressure numerous times and check for leakage?????
Of course you have to do this with no high pressure air injected because the air pressure will be holding the valve shut then slowly start re-injecting air and even start wiggling the valve stem as you are increasing the air pressure on the leak down tester and listen carefully to the carb manifold. You are making sure that the valve is seating good all time that you are moving it by hand or take a rubber handle screwdriver handle and push on the rocker. (you are making sure that the valve is constantly re-seating good)

Them valve springs are naturally weak and wimpy even when new. It's a OHV design.
The valve face/seat guide has to be really good for a good seal consistently seating due to the wimpy springs.

Go ahead and give us a compression test. Yes I know that the leak down is the best test but,.
Reason I ask for a comp test is you never did reach a point when you were increasing the intake valve lash to where the engine tried to lockup on the compression stroke when cranking. (or you never did indicate to us any comp lock when you increased the lash previously ) If the engine don't try to lock with the intake valve set in vicinity of .010, something is causing a loss of compression. Normally the compression release won't operate if the intake valve lash is too great. I'm not sure but I think the very minimum compression with the valves lash set correctly is 70 lbs. (anything less than 70 is suspect of)

Let us know the results of the Comp tests with the valve lash set at specs????
Do the leak down first by wiggling the intake valve while slowly injecting air pressure and listening?

[lefty]

Will do. Haven't had much time for the actual work but have been thinking about it. Going to have to grow a 3rd arm for the test manually opening the valves as one of my hands is busy on a monkey wrench holding the pulley. My last leak down was at 1/4 past.

I never did get that high on the valve lash (.010) but I can do that, hopefully, today.