[Trevor D]

Got a Briggs motor that compression locks when you try start it. Thought battery was no good so bought a new one with even more CCA abd a starter solenoid thinking that would fix it but didn’t. Pull the plugs and she spins no problem. No rocker arms to set on valves they are just direct to cam. Found on the internet that a lot of those size motors have a centrifugal valve lifter to release compression on start up attached to the cam and they like to blow up. So pulled engine thinking that must be it but this engine doesn’t seem to have one.

Any thoughts?

Mtr 422707-1236-01

 

[Walt 2002]

One of the more common causes of this in this engine is carbon buildup in heads. Another is poor battery cable connections especially the ground to frame. Third s starter its' self. The top bushing wears oblong letting the armature rub on field winding OR he starter was simply held on start too long overheating the winding causing it to loose torque. Do not hold starter on start longer than 5 seconds followed by twice as long a rest/cooling period.

Your engine does not have the ACR mechanism you reference, it uses the "Easy spin" cam grind where the Intake cam lobe is ground to hold the Intake valve just slightly open well into the compression stroke relieving some compression.

I can send you a Service Manual for your engine IF you like, address below, put in proper format and remind me engine model number and what you want.
Walt Conner
wconner5 at frontier dot com

 

[ericforster]

I would first look at the starter. It is most likely the issue.

 

[38racing]

I had one flathead briggs once with compression release problems because somehow the valve clearance on the intake was too much. I had opened the engine to replace a rod. I had to buy a new valve to get the clearance back down and it solved the problem.

 

[tomw0]

If the clearance on an intake valve was too big, the valve would open late... taking up the clearance before starting to open on each intake stroke - and would close early ... thus reducing the volume in the cylinder. That would effectively lead to lowered compression all the time. OTOH, if there was a CR of any sort, and it used the intake valve to function, it would be defeated if the clearance was greater than spec.
FWIW, I have heard of welding a dab of material onto the end of a valve stem in the case where the clearance was larger than spec on a flathead. The tip would then be ground flat and clearance adjusted as normal. I kinda think the exact flatness of the tip would not make a huge difference as long as the tappet made reasonable contact. The 'push' is straight on, so if there was adequate contact area, it should work. How long would it last? Notaclue.
tom

 

[Walt 2002]

The valve stem needs to contact the cam follower, tappet, squarely to prevent side thrust wearing the valve guide. The proper way to cure slight over clearance to re-cut the valve seat.
Walt Conner

 

[Vigo]

If the clearance on an intake valve was too big, the valve would open late... taking up the clearance before starting to open on each intake stroke - and would close early ... thus reducing the volume in the cylinder. That would effectively lead to lowered compression all the time.

Most compression release mechanisms lift the valve very little. Therefore, excessive valve lash 'eats up' your compression release travel, making it effectively not have a compression release. If the valve rides directly on cam lobe and clearance is excessive, all you can really do is replace the valve or cam or both, or try to sink the valve deeper to shrink the clearance by machining the valve face, valve seat, or both.

 

[Forest Gump]

I see per your info that is a Briggs twin L-head.
When I suspect or want to test if excessive valve lash is the seizing problem on L-head or even the OHV adjustable valve lash engines that are hanging on the compression stroke so as to bring the valve lash to minimum specs without wasting a lot of time is take feeler gauge and measure the existing lash, subtract that reading from the minimum spec'ed lash insert feeler gauge (two gauges, one on each intake valve stem if it's a twin and see if the engine then turns over ok. I've even used feeler gauges on some to take the valve lash to less than minimum specs and the engine would hang on compression stroke indicating that the problem is something other than valve lash. (such as weak crank electrically (such as weak starter/batttery,/connections/flywheelkey) or some of the really old Briggs single cylinder engines (old 8 hp is one) had a special grind on the camshaft and the cam lobe gone bad)

Try this feeler gauge shim test and let us know the results?

I've got some surprises several times that the cranking seizing issue WAS NOT VALVE lash.
Doing the above tests using feeler gauges will save a lot of time and effort playing around with trying to adjust valve lash when the major issue is actually elsewhere.
Also a hint about using feeler gauges: I've seen some of the Briggs single OHV 31 and 33 series engines that have the wimpy camshaft compression release that is know to fly apart that would still hang on compression stroke at the spec of .003 but operate ok at .002. (probably due to some wear on the internals???)I've found that I can more easily determine such by using feeler gauges than by hit and miss adjusting valve lash on these model engines.

 

[Ariens93GT20]

Have you checked the flywheel key?

 

[tomw0]

Most compression release mechanisms lift the valve very little. Therefore, excessive valve lash 'eats up' your compression release travel, making it effectively not have a compression release. If the valve rides directly on cam lobe and clearance is excessive, all you can really do is replace the valve or cam or both, or try to sink the valve deeper to shrink the clearance by machining the valve face, valve seat, or both.

Understood about excessive lash defeating the CR. My comment re lowered compression was 'overall' when the engine was running there would be a slight change in the valve timing, and thus a reduction in the intake flow. I expect less change than the CR was intended to make. Having clearance that was too large is effectively similar to having a worn cam lobe.
OHV clearance can be adjusted in either direction without a lot of effort. Flathead is definitely a lot more difficult, and most I know of decrease the clearance with age & wear. To increase clearance I think would need wear on the tappet or cam lobe, which are less likely that wear on the valve & seat. I think.
tom

 

[Forest Gump]

Have you checked the flywheel key?

Try cranking the engine with both spark plug wires off the spark plugs but with the spark plugs installed. If it cranks over easier it's a hint that
If ignition firing too much BTDC (before top dead center) the engine will will be harder to turn over and a hint that flywheel key is bad.

 

[tomw0]

The valve stem needs to contact the cam follower, tappet, squarely to prevent side thrust wearing the valve guide. The proper way to cure slight over clearance to re-cut the valve seat.
Walt Conner

Walt,wouldn't replacing the valve be a bette way to cure over-clearance than cutting the seat? Once the seat is cut, that material is gone forever from the engine block. Adding a dab or weld to the tip could be done, and then the surface ground properly to gain a 'square to the guide' surface which would meet the constraint you mention, just as grinding the original valve stem tip is done. I realize the factory suggested manner to get proper adjustment do not include that option, but given the lack of available parts for older machines, it seems a reasonable method to adjust the clearance when other options are limited. If there are other options or considerations I would love to read them.
tom

 

[Vigo]

Welding on the valve stem would be a bad idea in an OHV engine where the rocker 'sweeps' across the valve stem, requiring it to be a uniform, nicely finished surface to not quickly self-destruct. But on a side-valve/flathead engine where the entire valvetrain is in-line/in-axis, i think that would be fine. It still might not be the easiest solution, but i need visuals of the 'cam follower' or tappet or whatever the thing between the valve and the cam lobe is called in this instance to come up with a better idea.

 

[dave_r]

I would try making some kind of thin shim that can be inserted in the valve train, rather than welding on the end of the valve stem. But it likely is non-trivial to make, as it needs to be very flat on one side, maybe an indentation on the other side so it doesn't move on the valve, and should be harder than just plain mild steel.

 

[38racing]

if you haven't already check for play in the upper bushing of the starter. I did all sorts of stuff to fix a slow crank and it was the starter binding due to the excess play.