# Air coming from dipstick tube?



## Msbrockman (Mar 28, 2020)

Hi, I acquired a Snapper 5HP i5222 snowblower with a Tecumseh 5HP motor. Overall, it’s in good condition but when started it will run smoothly but at a lower rpm than the full throttle that the throttle control is set at. After about 10 seconds, the motor will slowly reduce rpm then die. 
The last couple times I tried diagnosing it, I had the dipstick removed and there was almost as much air coming from the dipstick tube as there was from the muffler. 
I gotta believe there can only be one cause and that’s a problem with the piston, piston rings or both. 
Are there any other possible causes?
Thanks,
Mike


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## enigma-2 (Feb 11, 2014)

Not being an expert by any means, but have you checked the crankcase breather to see if it's clogged?


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## CO Snow (Dec 8, 2011)

Take off the gas cap and see if it will run. If so, the gas cap is not venting properly.


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## Msbrockman (Mar 28, 2020)

With the gas cap removed and the dipstick removed, no change in how it runs. (Low rpm till is slowly dies in about 10 seconds). There is definitely a lot of pressure relieving from the crankcase through the dipstick tube.


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## badbmwbrad (Jul 30, 2019)

Gas blow-by past the piston rings will enter the crankcase and exit from the oil filler tube if the oil fill cap is removed while the engine is running.

In addition, your engine's valve tappet clearances may be too small. As the engine warms up, the valve stems grow longer from thermal expansion. 
The clearance between the tappet and lifter is reduced to zero. The valve(s) do not fully close during the piston's compression stroke. The engine stalls due to insufficient compression. The most likely cause of loss of valve tappet clearance is exhaust valve seat recession. 

When the hot engine stalls out. Pull the recoil starter rope and try to estimate if there is any compression. If no compression then it's probably the exhaust valve.

Remove the carburetor to access the valve cover. Remove the valve cover. With the piston at top-dead-center on compression stroke, insert a feeler gage between valve tappet and lifter to check clearance. 

Increasing the clearance requires valve removal and grinding material off the tappet. You can use electrical tie-wraps to constrict the valve springs which then enables removing the keepers. Remove the cylinder head. I re-use the head gaskets; especially of a machine that has come to me at no cost (like from the junk yard).


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## ST1100A (Feb 7, 2015)

Msbrockman said:


> Hi, I acquired a Snapper 5HP i5222 snowblower with a Tecumseh 5HP motor. Overall, it’s in good condition but when started it will run smoothly but at a lower rpm than the full throttle that the throttle control is set at. After about 10 seconds, the motor will slowly reduce rpm then die.
> The last couple times I tried diagnosing it, I had the dipstick removed and there was almost as much air coming from the dipstick tube as there was from the muffler.
> I gotta believe there can only be one cause and that’s a problem with the piston, piston rings or both.
> Are there any other possible causes?
> ...


Were you giving the engine a "Leak-down Test" when you heard all the air coming out? 
Did you pressurize the combustion chamber with air when you heard it leaking air out?
Or where you just turning the engine over or had it running when you had all the air coming out of the dipstick?
Otherwise that is normal if the engine is turning over and the air is coming out. When the piston moves down in the cylinder it will push air out the tube, when the piston travels up, it will suck air into the tube by means of the change in displacement of the crankcase when the piston moves up and down, and blow back out, that is perfectly normal.
Your engine speed change could be from worn throttle linkage parts not holding the throttle linkage in position with the force of the governor acting against it, causing the slow down in speed, which is how it is supposed to work to keep the engine from over-revving. But the throttle should stay in place from built in tension of the control lever so the speed will remain the same under whatever setting it is at and when a load is placed on the engine.


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## ST1100A (Feb 7, 2015)

badbmwbrad said:


> Gas blow-by past the piston rings will enter the crankcase and exit from the oil filler tube if the oil fill cap is removed while the engine is running.
> 
> In addition, your engine's valve tappet clearances may be too small. As the engine warms up, the valve stems grow longer from thermal expansion.
> The clearance between the tappet and lifter is reduced to zero. The valve(s) do not fully close during the piston's compression stroke. The engine stalls due to insufficient compression. The most likely cause of loss of valve tappet clearance is exhaust valve seat recession.
> ...


Brad, I see more tight intake valves than I do see tight exhaust valves.
The exhaust valves usually are built differently to seem to last longer with more hardened valve faces that don't wear as quick as the intake valves do, plus the stems are different to combat against the heat they are subject to, the intake valves do not have that and wear faster.
I do the same as you with a lot of head gaskets and re-use them, just spray a little bit of copper gasket coating on them and they seal up good. I have engines I re-used the head gaskets on now running for over ten years without any problems.
Like you mentioned, check the valve clearances and adjust if necessary, that is common on the older engines like you stated. And while you have a valve out, check the guide wear to see if thee is a lot of play in them. There is usually an oil consumption problem or exhaust smoking when the stems and guides are worn, plus excess carbon buildup in the combustion chamber.


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## Msbrockman (Mar 28, 2020)

Thanks for the advise gents. I pulled the motor apart today and was surprised to see that it was so normal. Very little carbon anywhere, no ridge in the cylinder, the head gasket was intact, the rings were not seized or broken or scored and the gaps were 180 degrees apart, the cylinder has no scoring marks and still has some crosshatch. Just before I tore it apart, I did a compression check on the cold engine with three different testers (because I couldn’t believe what I was seeing) and the needle on all the gages didn’t move off of zero. Nada!! Zip. 

I suppose I will hone the cylinder and replace the rings and hope for the best. 

The discussion about valve stem clearance going to zero after it warms up, makes sense. I will reassemble it and check clearances. And will reduce the length of the valve stems till I get to about 0.008”. 

Thanks again.


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## badbmwbrad (Jul 30, 2019)

Can you check the valve clearances without putting it all back together? If the cylinder head is already removed, now is the time to check clearance between the valve stem and the tappet (lifter). Some people even lap the valves against the valve seat. That's a pretty easy thing to do if the valve is already removed to grind the valve stem (gains clearance).


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## ST1100A (Feb 7, 2015)

To grind the Tappet/Lifter, he would have to take the engine apart and remove the camshaft to get the Tappets out.
He doesn't want to grind the Tappets, he wants to grind the end of the valve stems to set the clearance.
I know what you mean about grinding them,"valve stems" to set his clearance while he has part of the engine opened up and has the head off, now would be a good time to do that, plus re-lapping the valves if the faces are not worn too much, and check his valve seats to make sure they are in place, and didn't come loose and are not pitted, that way they will seal properly.
Once in a while I have seen a valve seat come loose and pop out of the block where it is seated into, and that will cause a loss of compression. He would hear the leak out of either the intake or the exhaust when doing a "Leak-down" test.
You can usually "Peen" the seat back into place to keep them in tight if they ever come loose. He would "Peen" it around the edges of the seat and the block, peen the block material over to keep the seat in place, don't try to "peen" the seat or you could crack/ruin it. The seat is a very much "Hardened" material.


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## orangputeh (Nov 24, 2016)

did you check breather tube and oil level before tearing everything apart.

simple things first


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## Msbrockman (Mar 28, 2020)

Update and last post: To the guys that suggested it was the valve lash, you were right. My feeler gages started at .006” and the intake valve was very tight and the exhaust valve was very very tight. I ground the intake till it was .008” and the exhaust valve till it was .012”. It runs like a raped ape now. 
Thanks for the advice gents!


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## paulm12 (May 22, 2015)

Glad you got it running. But a word of caution, the specs on those valves may have been 0.008 min and 0.012" max, not 8 for intake and 12 for exhaust. It will still run well for quite a while, but you could be at the min and max of the range. I don't know which engine you have, but I know the specs in several places (for Tecumseh flat-heads) are misleading on this. Info for the next engine you work on.

tx


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