Compression issue after rebuild

[gregjo1948]

I'd guess you aren't setting the valve lash correctly. Most small engines have a compression relief built into the cam. It opens the exhaust valve slightly and you may be adjusting the lash wrong.

 

[Mowerstevo]

Need to check to make sure all your replacement parts are correct you used to rebuild it. Compare with the old ones…As long as the lash is set and the dots on the cam and crank are lined up, no sheared key, it’ll start. I have been down this road of sent wrong parts and key slots were off by a few degrees or timing dots were off because I was sent the wrong parts. This issue sounds like timing. If timing isn’t good then it won’t run at all or like crap.

 

[StarTech]

I'd guess you aren't setting the valve lash correctly. Most small engines have a compression relief built into the cam. It opens the exhaust valve slightly and you may be adjusting the lash wrong.

First the intake valve is the one furthermost away from the flywheel, closest to the PTO end, as this engine uses a cross flow head.

And second the ACR does affects the intake valve. Plus Briggs says to adjust clearances at 1/4" pass TDC compression stroke. This with the piston moving down 1/4" at normal engine rotation direction. I find it is just easier to adjust each valve independently when the opposite valve is fully closed.

 

[Hammermechanicman]

 

[grumpyunk]

Did the new cam have the movable part that is spring loaded? That is the compression release that bumps the tappet/lifter when the rpms are low, and then swings out to stop bumping when rpms increase. It must be movable and also must return to rest when the engine comes to a stop. They are known to break readily and are the highest cause of cam replacement. Junk design that works for a while, and breaks on a whim. No fix except cam replacement at >$100 a pop. Extra cash and bonus for the designer...
If you read the shop instructions, the clearance is set with the piston 1/4" down from TDC(after). Or very similar. Get the book, PDF online I think.
If your compression at low speeds, with the valve clearance set at .003-.005", is more than the starter can handle, the likely the CR is not working. And it works as writ above. And means you will likely have to take it apart and inspect the cam/CR for sticky/bent/broken/?? parts.
tom

 

[StarTech]

Yes Hammer this one is a drama...45 posts and still not fixed.

 

[Hammermechanicman]

The valves are not set correctly.
See post #37 and #38.

 

[Gord Baker]

I suspected valve/cam gear timing. Since you have solved that, check the Flywheel Key to be sure it is not partially sheared.
A worn cam lobe would close valve(s) quicker so I doubt your new cam is at fault. Re set the valves per spec and check coil positioning. Perhaps you are running it too lean? Good Luck.

 

[casual observer]

Help everyone…. Ok, this is going to be lengthy. I have completely rebuilt my Briggs and Stratton model 31C707-0026-G. It all started with a broken piston Rod arm. I figured while I was rebuilding and the engine was old I replaced multiple parts. (Cam shaft, cam shaft timing gear, tappet valves, all gaskets) I just put everything back together, started it up with out the spark plug, turned the ignition key and the flywheel spins freely. Put the spark plug in, tried to turn it over and the fly wheel doesn’t want to turn. There is a lot of resistance. The flywheel just doesn’t want to turn. I did a little research and found that it could be a valve issue. I have a Briggs and Stratton service manual and adjusted the rocker valves to the required specs. Put the valve cover back on, cranked it over and had the same results. I now have back firing out of the carb and exhaust. It’s also very hard to turn over to the point where my starter was smoking, no issues with the battery and the starter is fine. After adjusting the valves, the flywheel will spin much better when I have the throttle cable to the lowest setting. When it’s at max, flywheel barely turns. I am suspecting that something is messed up with the timing in the cam shaft even though I lined the marks up. Does anyone have any suggestions?

 

[mtodd101]

I had similar problems with my motor. Came down to the flywheel key had sheared off and the flywheel was out of position. Fixed my motor. Good luck.