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

#1

G

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.


#2

ThaxtexGM

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 :(


#3

Michael72

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:


#4

ThaxtexGM

ThaxtexGM

Michael72 said:
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:
Click to expand...

Also timing is your crank and cam lined up ??


#5

Michael72

Michael72

ThaxtexGM said:
Also timing is your crank and cam lined up ??
Click to expand...


Ah don't mind that small detail :laughing:


#6

G

ggrigo33

ThaxtexGM said:
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 :(
Click to expand...

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!


#7

Michael72

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


#8

G

ggrigo33

Michael72 said:
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
Click to expand...

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?


#9

ThaxtexGM

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????


#10

Michael72

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..


#11

G

ggrigo33

ThaxtexGM said:
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????
Click to expand...

I do have a new gasket installed and there are no cracks on the head or rest of the body.

I think skyping would be great. there is 5 hours difference between your and my time. how about over the weekend? my morning and your evening?


@Michael72

I did make sure the clearence is good after lapping. Infact if all it is probably a bit more than it should be. Last time I tested the compression I had the crankcase open. I mean does it matter? Piston rings should maintain the seal regardless if the crankcase is open or not, at least what I think.


I am going to try to add compressed air to make sure for sure that the valves are good.

Just in case if it is a ring issue, what should I do? I do stil have old ring set that I took off.

The funny part is that it was a working endinge and now...not so much. I mean the main reason why I decided to take it apart is to learn how things work and I am definatelly learning quite a lot of new things lol.


#12

Michael72

Michael72

Its all a learning curve....ok, ring/s wont seal off 100% with dip stick out you will prob hear a slight gurgle but thats all... any more and there is issues..have a quick look at valve springs ...just to make sure all is where it should be...if gasket and valves are ok and no muffler or manifold leaks which i doubt it must be a ring issue... I have no doubt you ordered the correct piston/rings....question is, did they give you the correct ones :smile:.... if you take head off then place your old ring into the cylinder, down about 1/4 inch (keep parallel to top of cylinder)with feeler gauge measure what gap is left between both ends if you follow... repeat process with new ring and if its more than your old ring then theres your answer....:thumbsup:


#13

G

ggrigo33

Michael72 said:
Its all a learning curve....ok, ring/s wont seal off 100% with dip stick out you will prob hear a slight gurgle but thats all... any more and there is issues..have a quick look at valve springs ...just to make sure all is where it should be...if gasket and valves are ok and no muffler or manifold leaks which i doubt it must be a ring issue... I have no doubt you ordered the correct piston/rings....question is, did they give you the correct ones :smile:.... if you take head off then place your old ring into the cylinder, down about 1/4 inch (keep parallel to top of cylinder)with feeler gauge measure what gap is left between both ends if you follow... repeat process with new ring and if its more than your old ring then theres your answer....:thumbsup:
Click to expand...

hmm, good idea i'll try that


#14

ThaxtexGM

ThaxtexGM

ggrigo33 said:
I do have a new gasket installed and there are no cracks on the head or rest of the body. I think skyping would be great. there is 5 hours difference between your and my time. how about over the weekend? my morning and your evening? @Michael72 I did make sure the clearence is good after lapping. Infact if all it is probably a bit more than it should be. Last time I tested the compression I had the crankcase open. I mean does it matter? Piston rings should maintain the seal regardless if the crankcase is open or not, at least what I think. I am going to try to add compressed air to make sure for sure that the valves are good. Just in case if it is a ring issue, what should I do? I do stil have old ring set that I took off. The funny part is that it was a working endinge and now...not so much. I mean the main reason why I decided to take it apart is to learn how things work and I am definatelly learning quite a lot of new things lol.
Click to expand...

Yes Skype over the weekend :)

Email me tony.thompson@thaxtedgardenmachinery.co.uk

Hopeful I spelt all that right!! No glasses on lol

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#15

G

ggrigo33

ThaxtexGM said:
Yes Skype over the weekend :)

Email me tony.thompson@thaxtedgardenmachinery.co.uk

Hopeful I spelt all that right!! No glasses on lol
Click to expand...

sounds good.

by the way so i put old rings back on the piston and lapped valves a bit more to make sure there is no leak. No change... I think last thing I have left to do is to hone the cilinder and see if it helps, maybe i messed it up on some micro level, not visivle to the naked eye. If that doesn't help i am giving up on this engine.

ps.

will oversized rings help? I know B&S have oversized rings too.


#16

Michael72

Michael72

ggrigo33 said:
sounds good.

by the way so i put old rings back on the piston and lapped valves a bit more to make sure there is no leak. No change... I think last thing I have left to do is to hone the cilinder and see if it helps, maybe i messed it up on some micro level, not visivle to the naked eye. If that doesn't help i am giving up on this engine.

ps.

will oversized rings help? I know B&S have oversized rings too.
Click to expand...


Hmmmmm :smile: Over size after a cylinder polish yes... but if mower was working fine before the op there should be no need....did you put old rings back on old piston?...are the rings in the correct order?.... no scoring to cylinder? :confused2: Cam re-installed correctly?....if you pop in your old piston and rings and check for compression then :confused3:


#17

ThaxtexGM

ThaxtexGM

ggrigo33 said:
sounds good. by the way so i put old rings back on the piston and lapped valves a bit more to make sure there is no leak. No change... I think last thing I have left to do is to hone the cilinder and see if it helps, maybe i messed it up on some micro level, not visivle to the naked eye. If that doesn't help i am giving up on this engine. ps. will oversized rings help? I know B&S have oversized rings too.
Click to expand...

Skipping you now :)


#18

Michael72

Michael72

You sure there's not a big gaping hole in the block somewhere (Oil on floor)....new ring n piston then no compression...put back the correct old gear...


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