Tecumseh ???

phil25

Member
Morning: I got the quick pleasure of looking at why a MTD lawn tractor with the Tecumseh engine, backfired and died...
I found the exhaust valve push rod was off the rocker, and no compression, had the valves reseated/new head gasket, and valves operating proper.
Still no compression, and it pushes air out around the pushrods[through the guide tubes] on the compression stroke,slight sucking of air through the oil fill stand pipe, same when the exhaust is covered with your palm.
When the valve cover is held in place enough air comes out the two securing holes to make a hissing noise.
I suspect broken ring? Anyone able to confirm for sure or some other reason,better to have the owner go for a replacement engine?
Tks for reading.
 
First thing to do is check crankcase breather valve,same may be full of oil or valve stuck,you shouldn't have that much pressure buildup in crankcase when cranking eng,although in wouldn't cause a loss of compression.

Next double check valve clearances,most/all tecumseh ohv eng set at .004" on intake/exhaust,if clearances correct do a leakdown test on eng to confirm possible cause of compression loss.

Possible causes of compression loss,scored cyl wall/stuck rings as Hal said,broken ring(s) or excessive ring/cyl wear,valves ground/seated incorrectly,warped valve(s)/valve seat(s),valve seat loose in head,worn valve guides,weak/broken valve spring(s),worn cam,head torqued incorrectly,head warped.
 
Evening: I was reading your reply"s and am thinking them through, and can add this info to the no compression part.
i slacked off the rockers and removed the push rods so that both valves are closed full time, and with my hand on the flywheel i can turn it over easily.
There is this hissing of air as i mentioned before up through the push rods tubes.
So where is this breather valve located[ first time with this machine], and when i cleaned off the piston top[ no holes] i didn"t notice any scoring of the cylinder walls, but wasn"t looking for it either.
Another feature is that it doesn"t suck in gas from the carb either, and when given a direct drink, it will backfire but not run, as when it stopped[ backfired and died]
I will get back at it soon and see what turns up
Tks
 
When it won't suck gas is an indication of low intake manifold vacuum. Hal
PS: The rings on your piston may be stuck in their lans (ring grooves). Put the push rods back in and adjust the valves. Then see if both valves are ever closed when on the compression stroke. Your valve timing may be off.
 
Afternoon: You just gave me a clue about it being possible to have the valve timing wrong?
i wonder when it was running,if the exhaust valve/pushrod fell apart first, messed up the timing, and hence the backfire and a dead engine?
Then i rememebered that i have both pushrods out now,and both valves are closed, and still no compression, guess its time to check those rings.
tks
 
In order to have compression, the engine must first bring in a charge of air/fuel mixture to compress. With both valves closed, the engine CANNOT bring in air or fuel. Also, there CANNOT be manifold vacuum with intake valve closed.

Regarding possible problem with valve timing, NOT LIKELY!! In order to have a p[roblem with valve timing, the engine would have to have been disassembled to quite a bit more than you have described.

An engine is basically an air pump. Intake valve opens, and piston travels downward to draw in a fuel/air charge. Valve closes, and charge is compressed. At some point near top center, spark is provided by ignition system, and mixture is burned causing piston to travel downwards on power stroke.

NOW, using deductive reasoning, does it not follow that if pushrods are removed, and valves remain closed, the cycle is disrupted? You need to have the pushrods installed, valves adjusted, and correct engine rotation to evaluate possible problem with compression.
 
Look close at those valve springs to make sure they're not broken. I had that problem on 1957 Chevy. Hal
 
Yep that"s right its a basic "suck,squeeze,bang ,blow" sequence, and with the push rods out nothing should be right in that sequence.
I expected to have either a resistance to squeezing what air was in the cylinder or resistance of trying to suck in air, i just get a nice easy roll over[by hand].But the valves are working in sequence and at the right times.
I should of cleared the air that its back together, valves set and still the same no compression,intake of air, when i wrote of the last test, my bad?
End result will depend on the owners decision of what to do with it, obvious more surgery, and time required.
Many thanks for the info and time/effort spent
play safe
Tks
Phil
 
All things considered, engines do not lose compression suddenly without involving a major mechanical failure - like a swallowed valve, dislocated seat, or a hole in a piston. Rings fail over a longer period of time, and generally are accompanied with some smoking and oil consumption.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top