Just put an older 8hp Briggs together. Mainly just needed cleaned up, honed and a set of rings and lapped and set clearance on the valves. Valve seats appear good. Starts and runs great but has a real hard place in the cycle just as the piston passes TDC on the downward stroke (power stroke). It rips the starter cord out of my hand. Checked the valve clearance twice and ruled out anything else I can think of. I can feel the tight spot in the same spot of the cycle when it's pulled slow. Don't think it's a bent crank since it's fine on the other cycles. I did do a hillbilly plane on the head and block deck with a piece of granite and wet/dry paper but can't imagine removing enough surface to mess up the valve clearance.? Stumped
Just put an older 8hp Briggs together. Mainly just needed cleaned up, honed and a set of rings and lapped and set clearance on the valves. Valve seats appear good. Starts and runs great but has a real hard place in the cycle just as the piston passes TDC on the downward stroke (power stroke). It rips the starter cord out of my hand. Checked the valve clearance twice and ruled out anything else I can think of. I can feel the tight spot in the same spot of the cycle when it's pulled slow. Don't think it's a bent crank since it's fine on the other cycles. I did do a hillbilly plane on the head and block deck with a piece of granite and wet/dry paper but can't imagine removing enough surface to mess up the valve clearance.? Stumped
Yep. Horizontal. All been checked and rechecked. Starts right off and runs and accelerates smoothly. Just has that really tight spot I can't figure out. Pulled the rope till it got hard to pull and pulled the head off to see where it was sticking. Rolls over smooth with no compression
Yep. Horizontal. All been checked and rechecked. Starts right off and runs and accelerates smoothly. Just has that really tight spot I can't figure out. Pulled the rope till it got hard to pull and pulled the head off to see where it was sticking. Rolls over smooth with no compression
You can remove the spark plug and turn enging slowly by hand to check for any nechanical interference. If no i interference and valve lash is good probably a decompression issue.
#11
Fish
How much did you shave off? It doesn't take much to throw your compression ratio way up.
Not sure how to measure that. Only sanded till surfaces were evenly shiny. Hard to imagine that I could've taken that much off with sandpaper and block but.....?
Really can't figure why it gets so hard to roll over only on the downward stroke of the power cycle. Would think it would be on the compression stroke.
I turned the motor manually until it got to the tight spot and removed the head to check the position of the piston. It was just past TDC. Good to know that about the decompression on the cam. Didn't know about that. I have another one I'm going to replace it with. Only thing that makes sense since it's the only part that rotates twice to the four times of the crank.
#17
StarTech
Yes the ratio is correct but it is 1 full camshaft rotation to two full crankshaft rotations, just to clarify. These engines are four stroke not eight stroke. Each stroke is half of a complete crankshaft rotation.
Intake stroke (piston moves down), Compression stroke (piston moves up), Power stroke (piston moves down), and Exhaust stroke (piston moves up). Then everything repeats.
I turned the motor manually until it got to the tight spot and removed the head to check the position of the piston. It was just past TDC. Good to know that about the decompression on the cam. Didn't know about that. I have another one I'm going to replace it with. Only thing that makes sense since it's the only part that rotates twice to the four times of the crank.
To expand on a previous thought, the engine work along with the light resurfacing may have increased compression by enough to cause (pre)detonation. One thing that may help without modification is increasing the octane of the fuel you are using. Drain the tank and carb, refill with 93 octane and see if the problem persists...the questions about blades and other rotating weight is also important. If you are using new blades that are lighter than the original or something else that is supposed to rotate is now lighter, you may be just noticing the change in the forces that are usually dampened by those weights. I have tried to start a push mower with the blade removed and had the cord snatched painfully from my hand...The blade does more than just cut. It acts as a weighted flywheel.
#19
Hammermechanicman
It is very difficult to get the compression ratio of a flathead briggs up to the point you need 93 octane fuel. I have chainsaws running 200psi of compression with about a 10:1 compression ratio and they run fine on regular gas. 8hp briggs will have a cast iron flywheel. No need for a blade.
It is simple to diagnose. Remove head and slowly turn engine over. If it binds up something wrong internally.
Put head on without sparkplug. Slowly turn engine over
Does it bind up? If so valve hitting head.
Put in spark plug and slowly turn over. Does it bind up? If so compression issue.
#20
VRR.DYNDNS>BIZ
If you still feel the resistance with the plug out, and you reused the head gasket and stoned the head/block as you said, this engine has very tight clearances to the head and may be making contact. Loosen all the head bolts and pull slow and feel the head and if problem disappears.
The recoil has bearings on the flywheel pull start assembly. they are probably rusted or just need some oil. when you pull start it you should be able to hold the pull cord as long as you want. and slowly let it back in. if it snaps back in then there is something grabbing with the 6 ball bearings
