tecumseh cam timing

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  #1  
Old 07-20-04, 10:03 PM
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tecumseh timing

tore down the 1979 10hp tecumseh again today. i found that that pts were breaking at tdc on what i believe to compression stroke. when i moved piston down to .09in as puey said the points were 1/2in. form opening and there was no clearance on exhaust valve. it was still open a smidgin. intake at .09in. is still at .008. exhaust valve i ground at .008 at tdc when i worked on it before. i think i could rotate armature to have pts open at .09in. but i think someone else had set it at tdc. remember i mowed yard 3 times before replacing pts, cioil, and condenser. are the valves to be ground at tdc or 09. in.? also should i go ahead and retime engine with piston going up on compression stroke at.09 in making points break at this pt. also i am having trouble following the 4 strokes on this egine when i rotate the crank. the only pt where both valves close with .008 clearance is what i believe to be the compression stroke at tdc. am i on the right track? please reply. thank you, l.mitchell
 
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Old 07-21-04, 12:50 AM
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http://www.cpdonline.com/692509.pdf

Technicians hand book for 3 to 11 HP
 
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Old 07-21-04, 10:06 PM
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tecumseh cam timing

went to library and xeroed pictures of 4 stokes of engine and i think i may have found the reason why my 1979 tecumseh 10 hp will not start after new coil,pts, and condenser were put in. intake- book saysintake valve should be closed at bottom dead center. mine closes at 1/4in form bottom dead center. compression- matches with book, power stroke- book says exhaust opens at bottom dead center mine opens 1/4 in. from bottom, exhaust stroke- book says intake closed as exhaust opens mine -intake partially open as exhaust open. which might explain why it backfires thru carb and pops thru exhaust when trying to start. is the cam shaft or cam gear off time. how can i tell if there is cam damage without tearing it to pieces. or should i reset the magneto to .09 in. and try and restart it, magneto timing was at tdc during the testing. please reply thank you, l.mitchell
 
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Old 07-22-04, 12:28 AM
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I merged this thread with the previous one you started about this engine to keep continuity of the discussion.

Have you checked the valve clearances? They can and will affect the operation of the valves. Was the engine taken apart since it last ran? Any chance someone mis-timed it? Some engines do not operate exactly like the info you described, but 1/4 away from TDC and BDC is a bit much. Is it before tdc, or after, that the intake valve opens (when turning the engine the correct direction)?
 
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Old 07-22-04, 10:49 PM
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this am i took the diagrams to town and showed them to the most knowlegeable person i knew. he said on some engines valves will over lap. on this engine at the end of the end of the exhaust stroke with piston near tdc going up the intake opens before the exhaust closes. he told me that it is not possible that the cam gear jumped a tooth. i have not opened the crankcase. i lapped and ground the valves at.008 at tdc. the exhaust is a tight .008. with this in mind and pueys advice i set the magneto at .09 in. from tdc and the points at .20 using an ohmeter. i could not believe how touchy this adjustment was. one would throw the other off. it took 6 tries before i thought it was near perfect. the magneto ended up about a millimeter from where it was before and the points were probably wider originally. anyway ibolted the engine up and noticed a blue spark shooting about an in. away. i put the carb on and the engine took right off. i mowed in 95 degree heat at hi rpm for about 40 mins. engine mowed with great power then got hot and started cutting out. let it cool 10 mins, changed plugs noticed it was carbon fouled, leaned the carb about a turn, and mowed again for about 20 mins. it ran ok but i thought it was progessivley losing power . let it sit for 10 mins. started right up and drove it into garage. yes the engine had been worked on before because i found jb weld marks behind the starter where it had thrown a rod and had been put back together. when i got it from the previous owner he said he thought it ran when he parked it in 1989 under a pine tree and left it but he thought it had an electrical problem. is the valve overlapping causing the overheating and loss of power? the engine only acts up under load at higher speed. when you pull plug out engine will turn easily. when you leave plug in it is hard to turn but does have recoil. if the cam gear was put back in years ago off a tooth would it cause valve over lapping? what would cause missing under load after several mins. of operation. thank you, l.mitchell
 
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Old 07-23-04, 12:20 AM
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Sounds like the cam is timed correctly...no doubt. 2 things that come to mind that would cause poor running after several minutes of running well are: the fuel cap vent is plugged, causing a negative pressure to develop in the tank (vapor-lock), or...a bad coil breaking down from the heat.
 
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Old 07-23-04, 07:27 AM
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Forget about the valve overlap being your problem. Most four cycle engines are designed to have both the intake and exhaust valves open for a brief time during the cycle. Briggs & Stratton engines do as does my Chevy pickup truck engine.

Air cooled engines are very particular about being properly cooled. Make darn sure that your cooling system is in proper order before running the engine. The main source of cooling air is from the fins on the fly wheel. That's your fan. Make sure that all obstructions from the suction side of that fan are clear of debris. There will also be air baffles to direct the fan air discharge past the cylinder fins. Make sure the fins themselves are clear of dirt & grass. My riding mower has an engine air discharge on the right side where I can feel a very strong discharge of hot air when the engine is running.

I could see that if the cooling air system for the engine is faulty on either the intake side or the discharge side that you could have the engine loose power and slow down after running for a while. At least it's something to check before you start replacing coil's & other ignition parts.
 
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Old 07-23-04, 10:11 AM
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Sounds like a thorough carburetor reconditioning is in order here...or maybe possibly, simply getting the adjustments on the money.
 
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Old 07-26-04, 08:01 PM
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engine passed leak down test today with flying colors. is it possible that i was sold a bad coil or condenser? or is magneto timing so precise on these old timers that even being off a minute amount would cause over heating and missing? the last time i mowed the engine ran well for 10mins. started missing and died at the 45 min. mark. 10 mins. it started and ran perfectly. any replies appreciated. thank you, l. mitchell
 
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Old 07-26-04, 10:45 PM
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I'm still standing by my previous post as a possibility.
 
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Old 07-28-04, 10:43 PM
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got engine back today. they said they tested coil on a machine called a mercaton or something. they said the points were bad even though they were new causing intermittent spark. now its down to the carb but i wont find out till i bolt everything back up and try it. all adjustments were fine. will post results. l.mitchell.
 
  #12  
Old 08-03-04, 09:20 PM
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this engine runs good now. it appears the getting hot and having to adjust carb hi speed from 1.5 turns out to .75 turn out when hot and sometimes continued missing was due to a partially closed choke -linkage problem and a seconday coil wire that was being squished by the blower housing. i adjusted the the choke to only close past hi throttle and rerouted coil wire so it would not come in contact with the center partition in blower housing. i also insulated it. the engine runs cooler and does not miss. the carb hi speed stays perty well adjusted at 1.25-1.5 turns out. engine starts cold or hot. i was told before that sometimes a hot engine when flooded will not compress gas when hot. before this engine would get hot and i couldnt even turn it over with the truck battery due to increased compression. when it would cool the gas would evaporate and it would turn right over. in summary the engine appears to have been choking and /or grounding itself out. thank you for your input. l.mitchell
 
  #13  
Old 08-04-04, 07:24 PM
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Good job! I'm glad you just had to move the coil wire instead of replacing the whole coil.
 
  #14  
Old 08-07-04, 08:12 PM
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the engine well today. mowed entire yard in record time running circles around neighbors brand new snapper!
 
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