How to tell if there is a bad intake Valve.

"With the air filter off and I hold my hand over the intake I get a blow out and then a good suction as the piston rotates. Does this indicate a bad intake valve?"

That may be natural, hard to tell from here. The Compression Release bumps the Intake Valve open a few thousandths to relieve compression for starting on the compression stroke hence some blow back thru the carb. That should be followed by the power stroke, then the exhaust stroke, then intake suction. IF the suction comes immediately after compression stroke then perhaps the Intake valve is sticking OR the valves need adjusting.

I have detailed instructions for adjusting the valves IF you would like them. Address below, put in proper format.

Walt Conner
wconner5 at verizon dot net

Thanks Walter
That was me that you sent the email to about the valves adjustment.
Thanks. will get into it in the morning.
The engine is a 31P777:thanku:

Ok Here it is in a nutshell I have a B&S 91P777 18.5 HP OHV motor.
The engine was shut off to dump the grass containers and it has not started sense. This is what I have done:
Change the plug, fuel pump,fuel filter, cleaned the carb, Checked the Valves for adjustments .005 on the intake, .006 on the exhaust. Checked compression and it read 110 psi, pulled the fly wheel and checked the key way. Removed the armature and cleaned and reinstall with .010 clearance.
When trying to start with in full choke it will try to start with no power then will back fire and pop. Will not even burp when not in choke. I loaded it on the trailer to take to the shop but the shop closed early . So I still have the mower but on the trailer. Any suggestion before I totally give up and the mechanic hit what I been over looking and charge me a bank load?

Valve seat came out...

"Valve seat came out..."

Could be - you need to remove the valve cover, the spark plug and turn the engine by the flywheel cooling screen and see what is going on with the valves, rocker arms and push rods. You adjusted the valves so you would have seen a bent or broken push rod.

By the way, .005" is too wide for Intake Valve even IF you measurement was really accurate. I am betting it is even wider but that would not likely cause your problem.

Might try unplugging the "Kill wire" where it connects to the Mag Coil and see if that makes any difference.

Walt Conner

Walt 2002 said:

"Valve seat came out..."

Could be - you need to remove the valve cover, the spark plug and turn the engine by the flywheel cooling screen and see what is going on with the valves, rocker arms and push rods. You adjusted the valves so you would have seen a bent or broken push rod.

By the way, .005" is too wide for Intake Valve even IF you measurement was really accurate. I am betting it is even wider but that would not likely cause your problem.

Might try unplugging the "Kill wire" where it connects to the Mag Coil and see if that makes any difference.

Walt Conner

Click to expand...

Briggs SPEC was at one point 0.005"-0.007" intake, and 0.005"-0.007"Exh, as I recall...but we always set em both to 0.004" cold...

They really dont seem to effect the starting till they get wider than 0.015" or so...Ive had em falling off the valves they are so loose and still start...go figure.

"B&S 31P777 "

"They really dont seem to effect the starting till they get wider than 0.015" or so...Ive had em falling off the valves they are so loose and still start...go figure."

"Briggs SPEC was at one point 0.005"-0.007" intake, and 0.005"-0.007"Exh, as I recall...but we always set em both to 0.004" cold.."

First off, I have never seen a 31P777 or any 31 series that would crank with an Intake Valve anywhere near .015", maybe a completely worn out one.

B&S Service Manual I have calls for .003" - .005" on Intake, .005" - .007", always figured B&S knew more about what to set the clearance to than I did. Most people do not get the correct clearance without 2 - 3 tries, moving the rocker arms, etc. generally wind up too loose.

Walt Conner

I meant to put 0.003-0.005" intake, I mistyped that.

I see em quite often though, with the rockers so loose that you can wiggle em off the pushrod..true, they werent starting "then" but were starting immediatly prior to that point.

I think Briggs had more subtle cam wear issues than they let on...

And some of em will barely crank if they are more than 0,010"

Well this project was all self inflicted. When I pulled the fly wheel for inspection I didn't relies you had to use a impact to reinstall the fly wheel to keep the key from shearing. All better now. Timing Timing Timing. I did see where somebody said timing. Thanks to all and hope this adventure will help somebody save some time.

Can I get more info on the impact reinstall of the flywhel? I never heard that before. Might explain why my kawasaki 17 single sheers the key often. Of course now the crank is worn so the key can twist. DUH!

I have used J-B Weld to "weld" a new key into a worn keyway on one engine I had,and I also coated the worn tapered areas on the crank with it too,then sanded it smooth back to the original size..hasn't loosened up or sheared yet..

I only tried using an air impact a few times on flywheel nuts,both times on Tecumsehs,the crank snapped off flush with the flywheel,and thats the LAST tinme I tried using one on them!..I didn't "whale" on the nut either,one broke when I went to REMOVE it,not put it on,too!..

I use the impact to take OFF the flywheel, but before I do, I mark the position of the nut relative to the flywheel. I like to put the nut back on by hand until it is just seated and use the gun to bring the marks back to lining up. Easy.