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tube frame hunter
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242 Posts
Discussion Starter · #1 ·
Hello,I have a question on the tecumseh OH engines used on tube frames,I know they have a internal compression release,I know what a compression release does, but I can't understand what/how does that spring loaded "weight" on the camshaft do anything? the cam seems solid,it doesn't pivot on the gear to advance the cam and I don't see anything going to the tappet or pushrod of either valve to lift it,the factory manual just says new camshafts with mechanisims are available.My pop has one that I'm sure its not working but the"weight" moves on the cam and the valves are adjusted right.Anyone able to enlighten me on theory mechanicals of this particual engine Thanks alot in advance Steve!! :thanku:
 

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There is a pin that installs into the cam lobe. The fly weight pushes it out to provide a small amount of lift when starting. When the rpm comes up at start up the flyweight swings out letting the decompresiom pin to retract back into the cam lobe and no longer provide valve lift on compression. Sometimes the pin will break off and disable decompression. The pin and flyweight can also become worn decreasing lift. Even a few extra thousandths of valve clearance can affect its function.
 

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tube frame hunter
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242 Posts
Discussion Starter · #3 ·
Hey Bushwacker I am looking a a spare cam from a scrap engine and I see a 1/8 inch hole parrallel to the cam lobe.there is nothing in it!! I assume that is where the "pin" goes.I know the cam in question in my pops engine is the same [nothing in mentioned hole].I assume the pin has a "head" on the end closest to the weight to keep it from flying out?I wonder where that pin went,sure didn't see any debris in the crankcase when it was apart.I can actually see a wear pattern on the "shoe" of the weight on this junk cam so it was there on this cam at one time.I now know what to look for when I tear one of these down or look for a spare cam.BTW did these cams have a tendency to flatten lobes? The lobe farthest from the gear is flat on top,nothing like the lobe closest to the gear which is nice and perky.P.S.If I may ask where in Wi are you I'm just west of Milwaukee in town of Waukesha .Thaks again Steve!!!
:thanku:
 

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The pin has an E clip on it to retain it. I am not sure about the profile on the lobe but I am pretty sure that the spec for the exhaust lobe and intake lobe are the same measurment so if they measure the same I would say it is ok. I dont have the book in the house but if you need the spec to measure for wear let me know and I will dig it up. As for wear you may want to look at the lifter for wear also on the side with the flater lobe. I live in the Kenosha area about 30 miles south of Milwaukee. If you cant find a cam let me know I may have one with a good pin in it. I just had my dads engine apart this summer thinking that that pin broke again because it was starting hard and it turned out to be the CDI unit. It was fixed and it ran up around 14 hrs and then started to quit intermitently and then finaly quit running due to no spark. Had to install another CDI unit and now it is running again.
Roy 262 945 7693
 

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I have a problem with a Tecumseh OH140 compression release. Does anyone have any specs on this 4 piece mechanism. Mine is all there but worn, cam OK. Does anyone know the exact length of pin. Can I lengthen the pin to compensate the ware where the pin rubs on the counterweight.
Thanks, Lou
 

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The longest pin I have measured was .960" long. It provided .042" of lift at the push rod and .032" lift at the valve with .010" valve lash. I believe that the .042" measurement would be the same amount of pin sticking out from the cam. I have another engine I measured .032" lift @ the push rod and .012" lift @ the valve ( did not measure the valve lash but it must be around .020") and it seems to function fine with only .012" decompression valve lift. The surface that holds the pin out does tend to wear and decrease pin length. I tried welding one up and ground it back smooth and I must have recontured it wrong as it had more pin sticking out but the valve spring pressure was unwinding the fly weight resulting in next to no decompresion valve lift. I took apart several cams and used the best parts to obtain the .032" and .042" lift mentioned above. I also did some compresion tests with valve lash set @ .010" and then opened it up till no decompression lift was present. If I remember correctly compresion went from around 80 psi to 120psi.
 
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