no compression on Craftsman Mulcher, Briggs & Stratton flat head lawnmower.

[ggrigo33]

Hey guys, I am not experienced at fixing small engines, in fact I bought this lawnmower which is about 12 years old just to play around with.

It was working before I got to it, now... not so much now...So here is the issue. I completely disassembled it to clean off carbon, change piston rings and gaskets. I watched enough videos on youtube to learn about how to disassemble and reassemble it. So after I cleaned it off and reassemble it I couldn't get to start. I ruled other possible problems, so the only thing that is wrong with it is no compression. I connected my compressor through the spark-plug to test where is the airleak and it turned out that the exhaust valve does not hold the seal, so as the piston rings. So I took the piston out to make sure that the rings are placed in a correct order and in a correct way and I made sure that ring openings are evenly spaced apart. The exhaust valve is the original valve I just cleaned it off carbon.

I am not sure what went wrong and why there is now compression. Is it because I used carburetor dip to clean off valves and rest of the engine from the carbon build up? Please help.

 

[ThaxtexGM]

Did you reseat the valves?
Or are the valves right, have you got the in let in the out let??
So ease to do this

 

[Michael72]

Ok....first thing i would do is have a look at your valves...turn over so piston is top dead center and rotate flywheel 1/4" and not 1/4 turn...(both valves now closed)...at this point if you can turn valve easily or they they are noticeably loose then they need attention...exhaust vale not seated correctly will cause no compression check online for your valve clearance specs and adjust..

Please check your head gasket!!!... sometimes they can look fine but the weak point is close to exhaust valve (the smaller valve) ..if im doing a top end change i will always replace head gasket.. the amount of time folk change replace the top end because of compression drop when often it is just the gasket or valve seating issues...you will need a feeler gauge to adjust vale clearance..:thumbsup:

 

[ThaxtexGM]

Ok....first thing i would do is have a look at your valves...turn over so piston is top dead center and rotate flywheel 1/4" and not 1/4 turn...(both valves now closed)...at this point if you can turn valve easily or they they are noticeably loose then they need attention...exhaust vale not seated correctly will cause no compression check online for your valve clearance specs and adjust.. Please check your head gasket!!!... sometimes they can look fine but the weak point is close to exhaust valve (the smaller valve) ..if im doing a top end change i will always replace head gasket.. the amount of time folk change replace the top end because of compression drop when often it is just the gasket or valve seating issues...you will need a feeler gauge to adjust vale clearance..:thumbsup:

Also timing is your crank and cam lined up ??

 

[Michael72]

Also timing is your crank and cam lined up ??

Ah don't mind that small detail :laughing:

 

[ggrigo33]

Did you reseat the valves?
Or are the valves right, have you got the in let in the out let??
So ease to do this

hmm I did not do that. I guess it will be my next thing to do.

In terms of the head gasket the origional one seemed to be in a good shape but I did replace it with a new one anyway.

Timing of the crank cam shouldn't be a problem because I alligned the mark on the cam's gear with mark on the crank shaft's gear just like I saw on youtube.

would clearence change if you dissassemble and reassemble the engine? I thought if all of the parts are the same then clearences shouldn't change.



What about the piston rings and cilinder is there a way I can mess them up buy putting new rings on the piston?


Thanks for all your reply guys!

 

[Michael72]

if rings are the wrong size then yes....just check valves first tho IMO...if valve clearance is ok and valves are seating correctly i would try a leak down test if you have the equip

 

[ggrigo33]

if rings are the wrong size then yes....just check valves first tho IMO...if valve clearance is ok and valves are seating correctly i would try a leak down test if you have the equip

Alright, last couple of days I tried to lapp both valves. I don't know for how long to grind them, but they both look uniform withought any spots and nicks and I did spend quite a bit of time grinding them both.

However despite that no compression. I hear that the air escapes. When I changed the rings I made sure I bought the right part and prior the exchange I measured width of old rings and new rings with calipers to make sure they are the same size and they were. I checked the cilinder and didn't see any scraches. Do I need to hone the cilinder? Can it help?

 

[ThaxtexGM]

If the valves are settled right and you have new rings, then it's down to the head and gasket.
1. Is the head cracked ?
2. Get a new head gadget ?

I'am in the UK happy to Skype don't know if time different's will get in the way????

 

[Michael72]

Some trickery afoot here :biggrin:.....Loss of compression = head gasket, valves, valve cover gasket, rings, crank seals etc... when you lapped the valves did you adjust clearance from valve stem to push rod?

If you add compressed air and remove oil dip stick are you hearing lots of air and no air loss by valves and head?.....if so there is a piston and rings issue here...you will hear some air loss from dipstick here but if its gushing out then thats a no no... check muffler gasket too..