OT briggs and stratton valve problem

[bill b va]

aquired a 5 hp L head tiller with very low hours with low compression . found intake valve not fully closing because there is no clearance between the valve stem and lifter . i know to shorten the valve stem but can't figure out why this happened . any ideas ? i can understand how the clearence would be more but not less .

 

[Hendrik]

Not familiar with B&S engines. Is the lifter restricted in its travel? In other words, does the lifter follow the full cam profile from lowest to highest and vice versa?
HTH, Hendrik

 

[Bob Bancroft]

I'm going to guess it was assembled with very little clearance, and as the valve seated in a little, any clearance was lost. This is probably why the tiller wasn't used much. It didn't work well or very long after new, and was given up on.
I have a newer, bigger overhead valve v-twin. I just discovered the intake valves were set way wrong at the factory. Clearance is supposed to be .004-.006". It was .010-.011" ! In my case this had disabled the compression release, making the engine hard to crank.

 

[Geo-TH,In]

valve seat recession happens on flat head engines with aluminum block. Usually the opposite happens on overhead valve engines. The exhause valve clearance is very ciritcal on overhead valve.

 

[Indiana Ken]

You might check the valve seat to verify it is/was seated and tight. The seats are a press (shrink ?) fit in the block. If the seat was not installed correctly it could have loosened in service and opened up the clearance you are now seeing.

 

[Tramway Guy]

Are you sure you aren't seeing what is actually the compression release St work. It holds the valve slightly open till a certain speed...

 

[cwtech]

New valves have to be clearanced when installed.

Someone may have installed a new valve without setting the clearance.

Are you checking the intake valve clearance when the exhaust valve is fully raised?
...If not, the intake valve may be on the part of the cam lobe which is designed to raise the intake valve off its seat slightly as a "compression release."

This feature relieves compression while cranking the engine, but once the engine is running, this bleed-off of compression is negligible.

On most small B&S engines, you check compression by rotating the engine backwards to negate the effect of the compression release ground into the cam profile.

 

[Bryan inIowa]

As George said.valve seat recession. Seat wears or gets loose .

 

[Southern Ray]

it has happened on mine, but after many hours.
take a small flat file and run it back and forth between the lifter and valve stem.
you might chain the tiller down real tight.
running the file back and forth in that tight quarters is tough with the engine jumping around like you are extracting a tooth.

 

[NCWayne]

Take a look at the seating face on the valve. Usually you'll see a groove in the valve where it has been sealing against the valve seat, even if it's only a few thousands of an inch deep. Then you've also got wear to, or displacement of, the seat itself caused by erosion from exhaust gasses and/or mating with the valve, and also the chance of it getting pushed deeper into the block by the constant hammering of the valve, etc, etc. Because of the wear, etc to both the valve and the seat, it basically allows the valve to seat deeper into the taper of the seat. This, in effect, drops the valve closer to the lifter and reduces the clearance required for it to close and seal.

I had one on an old Briggs last year that a customer brought to me after taking to another shop. The other guys told him the engine was worn out and he needed a new one. I bought a new valve for like $7 and ground the end to get the correct clearance, did a few other minor repairs to the carb, and had him out the door for around $50 total. Alot less than the cost of a new engine....

That all said, just insure that wether you choose to regrind the old valve ((((any goove in the face needs to be removed if you are to reuse it, and the old ones are usually worn pretty bad, so a new valve is usually your best bet))), or buy a new valve, that when you grind the end that it is ground perfectly square. Too insure you grind a very little bit at a time because if you go too far there's no turning back. Good luck.

 

[Jason S.]

The clearances for the old 3-5hp engines was .005-.007 intake and .009-.011 exhaust.

 

[bill b va]

thanks for the help . i used a dial indicator to check the valve action .i was able to see compression release hump on the cam using the indicator . i had the valve out and i could see no problem other than no clearance . i filed off the end to get clearance put it back together and it started right up and runs fine . as pointed out the seat must have moved .

 

[Loosehandle]

I think thats usually the exhaust valve for compression release

 

[Geo-TH,In]

It is possible you are running it a little fast. Back in the day, leaded gas helped cushion the valve seat. Today valve seat recession is very common on flat heads. If it happens again, grind off more valve stem and run it slower. When it starts to happen, you will notice it runs rich and you can even see the air going backwards through the carb.

George

 

[bill b va]

i saw gas vapor comming back out of the car . never saw that before . had me puzzled .