Valve Lash Oddity


#1

While not related to cars, thought I would post this oddity just in case someone else suffers the same problem with a lawn tractor. The Briggs 17.5 Horse in mine had quickly gotten balky about starting. The symptoms were similar to:
Dead battery.
Bad starter motor.
Hydrolocked engine. (Engine very balky about rotating through the compression stroke even by hand.)

Battery tested fine and it took about 3 minutes to remove and disassemble the starter motor. Nothing wrong there with any of it.
A local starter/alternator shop owner told me that several people had been in with similar complaints and it was caused by the valves but he wasn’t sure on that aspect of it.

So removal of the valve cover and a check showed both intake and exhaust valves had about .010 clearance. I readjusted both to about .005 and problem solved. Turns over like a new one and fires right up every time.
I can understand the premise of cam lobe duration/overlap in an effort to create a compression release but the measly .005 of an inch making all of the difference in the world kind of surprises me.

Something to keep in mind if mower motor decides to get cranky; or non-cranky as the case may be. :slight_smile:


#2

Sincere thanks. I love this forum. I learn constantly.

I was having trouble with my 12-year-old lawnmower a few years back. (Tecumseh engine… they’re out of business now). A typical fuel metering problem, cycling rpm after the engine warmed up. I took the carb apart, cleaned everything up (no ethanol deterioration of the plastic parts) and it went back to running great. Then last winter I left the gas in it. Back to running crappy. I swore at it and kicked it and that didn’t fix it, so I treated myself to a new one. This one has a Briggs. Hopefully it’ll run better… assuming, of course, that come winter I store it properly…

I must be getting old.


#3

@ok4450 … I had a similar problem with my Briggs OHV engine. My brother-in-law runs a large mower shop and he told me the trick to setting the valve adjustment and it works great. Turn the engine to TDC and then use a chopstick to measure 1/2 inch of travel (stop at TDC and mark the stick in the sparkplug hole) then add another mark 1/2 inch above the TDC mark. Make sure to rotate the engine clockwise to reach this extra 1/2 inches at the new mark. Set the gap at .002 (loose) for exhaust and .004 (loose) for intake. Rotate the engine several times then recheck the clearances. I don’t really know what that extra 1/2 inch of travel after TDC does but it works.


#4

The camshaft has a spring loaded actuator that momentarily opens the intake valve at near TDC to significantly reduce compression while cranking. If the valve clearance is too tight the compression will be high enough to damage the electric starter or the recoil starter and both are expensive. I have broken both.


#5

One thing I have noted on recoil start 9 cubic inch B/S vertical shaft engines is that the exhaust valve gets lifted a few thousandths about half way along the compression stroke. This compression release is enough to ease the pull over the compression while still allowing enough charge to remain to start the engine.

I suspect that when the engine is running at working speed, the slight cracking of the valve does not affect the charge volume significantly or cause added exhaust valve heat. I once ground the exhaust valve tip to get rid of that lift and that really increased the starting crank pull.


#6

So that’s how the compression release works:


#7

Well, I’ll add another “just in case it helps” with my Briggs 18hp. A couple of years ago it started dying in the middle of mowing. Always seemed to be long after it heated up. It would start to sputter and would just stall out unless I killed the blades and throttled down. Even that didn’t help sometimes. I ran through the gamut. Checking spark when hot, compression, fuel filter, blew out fuel lines, pulled & cleaned the carb/fuel tank. This is also when I learned that you’re actually supposed to adjust the valves periodically, so I did that. (No! I didn’t read my “owner’s manual”!)

Eventually I arrived at the fact that it had to be fuel starvation, but I couldn’t figure out why. The filter was clean. The fuel flowed fine. The carb was nice and clean. Except - every time I pulled the bowl it had one or a couple of tiny little pieces of black crud. And every time I did this it was fine again for a while. But I couldn’t figure out how that crud was getting there. Anyway - the anti-backfire shut off solenoid at the bottom of the bowl had a bunch of corrosion inside of it. Every once in a while, at random moments, it would puke one out and clog up the main jet.

This is easy to check. I found a bolt of the right size and appropriate sized washer & o-ring and just plugged it. That was the end of that. I still haven’t decided whether to buy a new one. So far I live with the occasional backfire.


#8
  1. Lawn tractor with Briggs side-valve engine would stall 10-15 minutes into mowing. Turned out some insect packed the gas tank cap vent hole with mud.

  2. Same engine started running rich and getting worse over 2 years. Replaced carb inlet needle and added shut-off valve in fuel line. The puzzle was that the needle valve wasn’t leaking enough to ever drip gas outside of the carb.


#9

Obviously my memory is fading. The actuator undoubtedly operates the exhaust valve. Next thing you know my good looks will be fading also.


#10

Insightful;
Good thing it was only the vent hole.

Years back, I parked my mower next to the house to finish up the mowing later…after an appointment that I had to get to. I came home and it started, but ran only a minute before acting like it was out of gas. I got the gas can and filled the tank, but it took little gas to fill and I still couldn’t get the thing to run at all. After checking spark I checked the fuel flow and it had good flow. I finally had a hunch to peer into the open gas tank and I thought that I saw something. Turns out that the neighbor kid…about seven, was playing gas station and filled the tank from the hose and added a few sticks and leaves to top it off.
I pulled the tank and got all the debris out but never got the thing to run for long after that. Turns out the previous owner must have…for some reason poked a hole thru the screen at the bottom of the tank and it had no fuel filter.
I must have sucked in a little piece of leaf that plugged a carb passage and I never did get it cleared so it would run for more than a minute or two. I even removed the carb and disassembled it and baked the carb in the oven for a few hours to try to turn the leaf into ash.
Don’t ask what degree…it didn’t work anyway!!!

Finally bought a new mower.


#11

I think on my Corolla the valve clearance specs are in mm, and the spec range is something like 0.2 mm to 0.3 mm. So that’s like … hmmm … ok, 0.008 to 0.012 inches. So I guess something on the order of 0.005 inches off can indeed make a difference even in a car engine.


#12

@ok4450, thank you very much for posting this. I had a similar problem with a 10.5 HP OHV B&S engine. Battery and start both checked out, but was always hard to start, and seemed to jam up on the compression stroke. At first, hooking up the battery booster managed to overcome this, but that got harder and harder to do. I just did the valve lash adjustment, for which both intake and exhaust were way out of spec. Now, it turns over like new and runs great! I was a little upset that the owners manual has no specs for the valve lash. I had to google it. I found a you-tube video that shows the best way to set the valve lash is by pulling the spark plug first, rotating the engine to TDC, then using a stick to mark the depth of the piston. Make a mark 1/2" up on the stick, and rotate the engine clockwise until the piston depth is at the new mark. Then set the valve lash.


#13

Glad that helped you out. It’s kind of a strange phenomenom and my owners manual also makes no mention of valve lash. My lawn tractor has been fine ever since the adjustment also.

It’s still odd to think that both valves being a measly .005 or .006 loose would cause an engine to appear to hydrolock.


#14

I have a vague memory that on the larger Briggs engines there is a compression release that is dependent on proper valve lash for its operation…


#15

Watch the video on Page 1…