8.5 hp tecumseh issues

[corvairbob]

my brother dragged his poulan pro 8.5 hp flathead engine snowblower over the other day with what the repair shop said was low compression. i put a gage on it and it has 30 psi using the rope. he was complaining of it not having the power it used to have and he said it sounds like the engine is slowing down when it gets into a snow load.
so i first hooked up the tester and did a dry test 30psi then i pumped 5 pumps of oil and pulled the rope a few times to get the oil around the cylinder and i had 35psi then i put oil on the valves and the same 35psi
so i went to my 9hp tecumseh flathead and i got about 40psi and no change with oil
so i went to my ohv tecumseh and did the same and got about 50 psi
so i'm thinking this is due the the decompression causing the low compression for cranking with the rope and the electric start.
he thinks he wants me to rebuild the engine which is rings and i may grind and lap the valves, but i wonder if i should be looking at the decompression on the cam? i looked thru the plug hole at the valves and the exhaust looks a bit dark around the seat and on my 9 hp flathead it had a bit of a ring that tells me that valve is seating.

today i will tear it apart and take a closer look but i'm leaning on valves and not rings. but because i have it apart i will do rings also being they are cheap and it is apart

but dos this sound more like the valves and maybe the cam decompression?

thanks

 

[ILENGINE]

Snow throwers are not used a lot in most cases, but that engine is probably old enough to have issues with the valves clearances closing up and could be floating the valves. Take the head off and then push down on the valves and see if you can turn them with thumb pressure with them in the closed position.

 

[sgkent]

the head is loose or the valves need adjustment, or both. I went thru the same thing on a flat head pressure washer about six months ago. Compression went from 30 PSi to 70 PSI cold. Factory spec on my old pressure washer new is 70 - 80 PSI cold. Even with worn valve guides it runs much better now. If it has a choke, make sure it is off when you test again and the throttle on full open if it has multiple positions.

 

[corvairbob]

no the head was not lose. the gasket was sealing perfect all around and i had to scrape it off and there is no valve adjustment on this engine. it is just a horizontal lawn mower type engine. although the exhaust valve guide is wobbly but in the parts listing i do not see valve guides. the cam exhaust decomp looks to be intact and the little pin that holds the valve open during cranking is loose enough for the lifter to push it back down out of the way

i'm leaning on the exhaust valve being wobbly to not be seating real good. and i checked the carb and it is clean on the inside, he said he drains the tank and runs it out of gas every spring.

the rings have top ring .020 center ring .024 oil ring .035 piston is 3.09 dia bore is 3.12 so that all looks good. i did notice the coil to be rusted a bit so it is possible it may have prr spark. anyway he wants new rings and if i can figure out how to get the valves out so i can grind them and lap them i will do that. so far i can't do the exhaust guide because i ahve not found the part. if someone know of these being replaceable let me know the exhaust valve wobbles maybe 1/16" or a bit more

 

[Rivets]

You are wrong, you adjust the valve to lifter clearance by grinding the valve stem. Valve clearance should be .008“-.012”. This manual may be of some help. https://www.mymowerparts.com/pdf/Te...L840-HXL850-TVS840-TVXL840-ENGINES-694988.pdf. I hope you didn’t remove the magneto assembly, because if you did, you will have to retime the engine. Procedure is in the manual and I hate to say it, not easy unless you are an experienced mechanic.

 

[corvairbob]

Snow throwers are not used a lot in most cases, but that engine is probably old enough to have issues with the valves clearances closing up and could be floating the valves. Take the head off and then push down on the valves and see if you can turn them with thumb pressure with them in the closed position.

yes i can turn the valves by hand, the exhaust has maybe 1/16" side wobble the piston is 3.009 dia the bore is 3.12 dia the top ring has the end gap at .020 thecenter at .024 the oil ring is at .034 i have not measure the valve stem dia yet but they do not look worn or scratched up the intake wobbles a bit but that most likely is normal for when the engine heats up. but the exhaust i believe is just to much. if it were my corvair with that much i would be replacing it. both air cooled.

i see the intake has a 1/16" ring around the head of the valve where it has been seating but that is at the bottom of the seat. and the exhaust has something that looks like a tiny shiny ring around the most bottom of the seat. if it turns out i have to leave the guide alone and just regrind the valves i will be putting the seal at the top of the valve for a better interference. the ring gap looks to met to be serviceable. but i will put in new rings because my brother wants new rings.

You are wrong, you adjust the valve to lifter clearance by grinding the valve stem. Valve clearance should be .008“-.012”. This manual may be of some help. https://www.mymowerparts.com/pdf/Te...L840-HXL850-TVS840-TVXL840-ENGINES-694988.pdf. I hope you didn’t remove the magneto assembly, because if you did, you will have to retime the engine. Procedure is in the manual and I hate to say it, not easy unless you are an experienced mechanic.

yes i know that what i'm telling you is i have to grind the valve faces and seats because they are not making good contact the intake has .309 and the exhaust has .306 for stem dia. and then i may have to adjust the clearance after i grind them.
. yes i did and i know how to time them thanks there not much to that as the mag fits one way and i have to set clearance. i have brass gages for that or may just use a business card as most are the correct thickness.

 

[Rivets]

Are you saying that you have an external coil assembly? If so then you are correct, but if your coil is mounted under the flywheel, then setting the point gap will not time the engine. If you look in the manual you will find a timing procedure, which involves using a dial indicator and either a buzz box or VOM. Most Tecumseh SnowKing engines used internal mounted coils. If you would supply us with the model and spec numbers we would know exactly which engine you have.

 

[sgkent]

sounds like the engine is already apart. Normally the head comes off, the valve spring gets compressed with your own tool or an inexpensive one you can buy, then the keeper comes off. The valve to tappet clearance gets measured, usually at a specific piston height past TDC then the tip end gets evenly ground on a fine cool grinder wheel until the clearance is in spec. If you gently rock the valve when it is about 1" high off the deck you can feel the guide play. These flat head engines are not worth the PITA to replace guides unless you already have the tool set to ream them out and press in new guides. It is not a car that you will drive 100.000 miles more. All you need is it to run well when blowing snow or whatever. If you want to rebuild it, do it because it is hobby, not to save money. You can buy a new engine for less than to properly re-manufacturer the old one. If you just do a new head gasket and valve adjustment your compression should be fine, unless the cylinder is scored.

 

[ILENGINE]

sounds like the engine is already apart. Normally the head comes off, the valve spring gets compressed with your own tool or an inexpensive one you can buy, then the keeper comes off. The valve to tappet clearance gets measured, usually at a specific piston height past TDC then the tip end gets evenly ground on a fine cool grinder wheel until the clearance is in spec. If you gently rock the valve when it is about 1" high off the deck you can feel the guide play. These flat head engines are not worth the PITA to replace guides unless you already have the tool set to ream them out and press in new guides. It is not a car that you will drive 100.000 miles more. All you need is it to run well when blowing snow or whatever. If you want to rebuild it, do it because it is hobby, not to save money. You can buy a new engine for less than to properly re-manufacturer the old one. If you just do a new head gasket and valve adjustment your compression should be fine, unless the cylinder is scored.

Some of those older Tecumseh engines didn't have replaceable valves guides. You reamed the guide out to a larger size and then replace them with oversize stem valves.

 

[corvairbob]

i have both but the one under the flywheel is for the generator. and it works the light the mag is on the outside.

tell me it seems that the loss of power he is having may be do to the valves.the intake is seating completely around the seat but the exhaust is only a hisr think shinny ring that is maybe half way around the seat. and the exhaust valve is all crusty looking like it has not seated in years and the guides are real sloppy as per some YTs so it seems tecumseh does not make new guides and i do not know if b&s has any that may work. so to fis this does tecumseh have over sized valve stems and then you ream the guides to fit the new oversize valve stems? and then i guess maybe new to spring guides that have to seal in them, but the oversized valve stem amy just work if these are made. the current valve stems are .309 intake and .306 exhaust. so if i can get over sized stems and ream the guides that would be about the same thanks

Some of those older Tecumseh engines didn't have replaceable valves guides. You reamed the guide out to a larger size and then replace them with oversize stem valves.

thanks i found out they are not replaceable. i wonder if the guides were put in the diecast mould before it closed for the shot so the guides were moulded into the casting? anyway i found out they make an oversized stem for this so if anyone has one i sure would like to get one. outside of that i'm now getting some car guys on a fourum to see if they have any car valves that have these numbers the i can make fit