Briggs 3.5 HP: very low compression; 4 different observations...
I am currently trying to fix the Briggs 3.5 horse power motor of an old finger-bar mower.
The initial problem was that the motor, although starting well, did not develop power. When clutching in wheel drive and mower bar, it tended to stall. Also, the motor tended to stop by itself after, say, 20-30 seconds of operation.
I serviced the carburetor, giving it a new membrane. I think the carburetor as such is okay now. Air filter is new and oiled, too.
I noticed that on turning the motor (with the start cord), there is a cough of air blowing out of the carburetor (when air filter is removed). This made me think that there might be a problem with the intake valve. So, I removed the cylinder head cover, cleaned everything (quite some carbon residue there...) and checked the valves (removing them, etc.). Valves seem good, especially the valve seat. However, in the process I discovered three things where I am not sure if they indicate a problem or not.
1) I observe a strange behavior of the intake valve: On its way back into its seat it stops for about 1/8 turn of the main axle (drive shaft, also starter cord shaft), staying about 1/3 millimeter above its true seat. Just after the additional 1/8 turn (45 degrees) of the main shaft, the valve does the last 1/3 millimeters of its way. - I am not sure if this is a feature (?) or a bug. Shouldn't the intake valve sink back into its seat smoothly, without such delay?
2) I observe that I do not get good compression. I tried to measure compression with a standard compression measurement manometer (screwed into the spark plug place), and I do not get any visible measurement. Okay, there is of course some compression anyway; I feel it when turning the motor. Also, as mentioned above, the motor in principle starts. It just seems that compression is very, very low. - However, attention: My measuring the compression was done with the motor oil emptied, so the crank shaft case practically empty. I do not know: Would I need to measure compression with motor oil filled in? (I did not do this so far.) - The manometer as such is okay; tested on other devices.
3) I found the cover of the breather assembly damaged. It has a crack, caused by a drive belt cover pressing unfortunately against it during operation over years. So the cover is not air tight. - The breather assembly cover is also the cover of the valve spring compartment, so to reach the valve springs (and remove the valves) you first remove the breather; that's how I noticed the damage. - I do not know: Is the crack in the breather cover a problem? Needs this to be air tight? Or is it not that relevant for the motor to function?
4) With no motor oil in the crank shaft case, when turning the motor, I observe a cough of air blowing out of the motor oil neck (when its cover is removed). Okay, the piston would of course create pressure in the crank shaft case (and that's why there is a breather, finally), but is it okay that it is so strong? - Or is it again because I did this with no motor oil in the engine sump, and the effect disappears when oil is filled in?
Thank you for your help and your expertise on these four topics (or individual ones of them)!
Kind regards,
Christian
post edited by chgeiselmann - 2024/07/15 06:09:10