Intek engines M# 31H777 bad valve seats

13 years ago

I am wondering if anyone out there is having the same problem with the briggs and stratton intek aluminum head in the 31h777 model motor. This motor is in a sears riding mower. I have replaced the head twice in 3 years because the valve seat on the exhaust valve keeps coming out of the head. I was told by a local dealer that these motors are junk because of this problem. The dealer said that when the head gets hot the aluminum expands and allows the seat to fall out of its pocket. I have kept the motor clean and even tried to cut 1 gear slower so there was no extra strain on the motor, but this didn't work. Now I have been waiting 5 days to hear back from briggs stratton and have heard nothing. Anybody out there know anything, I have over an acre of grass to cut and a 20" walk behind is really a pain to use but I cannot afford another mower. Thanks for reading

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Comments (9)

walt2002
13 years ago
I have not heard of very many valve seats coming out on this series of engine and I am fielding hundreds of queries on this series. Have you tried staking the seat back in place per the instructions furnished by B&S for their "L" head engines? My first thought would be overheating but you seem to have eliminated that. Perhaps that engine compartment is not providing enough ventilation.

Walt Conner
mownie
13 years ago
It might be a good idea to go through the carburetor to make sure nothing is restricting the volume of gasoline the carb can deliver. A thin flat piece of debris having become lodged in the main jet would cut the volume enough to create a lean fuel/air mix that burns very hot.
Also check that the fuel solenoid works properly. If the fuel solenoid plunger movement is restricted by grit or rust in the barrel of the solenoid body, the plunger might not be opening enough to flow all the fuel it is supposed to.
Another possibility (though a long shot) is that the carburetor has the wrong jet in it. If your carb somehow was built with a "High altitude main jet", it will not flow enough fuel for operation down in the "flat lands". You would need to refer to the Briggs service manual to find the specs on jet orifice diameter and then examine your main jet to determine if yours has the correct jet for your locale.

You need to "take a reading" on the color of your spark plug to get an idea of how lean (hot) your combustion actually is. The ideal color of the plug should be tan or very light brown. If the spark plug color is an ashy white color, the combustion is too hot, meaning "lean burn" (not enough fuel).
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When pulling the valve cover, you will also want to know if the studs for the rocker arms came loose from the head- thus increasing the clearance. A visual inspection and a quick check with a 10mm wrench will tell. Sometimes they loosen to the degree the cap comes off from between the valve and rocker. If the push rod is laying loose- let us know if it is bent and what else you find. In 2007 builds they discontinued the rocker gasket and used sealant. RTV or Right Stuff will replace the gasket in either case if you desire. Any problems reaching Walt -just let us know.
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Hmmm, no comment from Walt, guess he is mad at me. Anyway, I did take time this past weekend to remove the cover, muffler pipe assembly and air intake hose from the engine in order to remove the cylinder head. Once the head was removed, I noticed that the piston was about three quaters toward the front of the cylinder. I then turned the crank to see if the piston would move, but this time, I was only able to turn the crank about one quarter turn, seems for some reason I have lost most of the travel in the crank area. The piston did not move. I was able to push the piston back into the cylinder a little more than half way, the piston moved freely without any binding when I did this. I then turned the crank again several times back and forth, but did not see any movement in the piston. Sort of sums up the fact that the piston is no longer connected to the crank. Not sure if it is just the crank collar or a broken piston rod at this point, but on Monday I did speak with a gentleman from a mower repair shop who was familiar with this type of engine, he said he actually had one in the shop for repair, about 7 years old that apparently has a broken rod. He indicated to me that this model engine (31H777-0202) seemed to have this problem after a few years of usage, and that the newer model (31P777-0202) should have better parts to last longer. When I asked for an estimate of the cost of repair, which I assume would require a total breakdown and rebuild of the engine, he was not able to give me an exact figure, but indicated that there would not be much difference in the repair cost and the cost of a new engine. I have not done any more research to verify if this model engine does have this type of breakdown history, but if this is the case with this model B&S engine, then regular maintenance of this engine may not have made any difference in this outcome. At this point I need to put more of my time going in the direction that will get this tractor back on-line, so the plan will be to acquire a new engine for the best price. Guess I will shelve the old engine until I can either find someone who is willing to buy it from me, or use it for parts, or rebuild it myself. If I do a rebuild of this engine myself, I guess the suggestion of taking pictures will be beneficial for future hobbyist. Thanks for all of the input guys, I will stay in touch. Howard
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bushleague
13 years ago
Mownie is right, fuel cools. There are many variants to the 31 series, the 31H is around 15.5 HP? I should have this head, I'll take a look tomorrow.
walt2002
13 years ago
"Hi Walt,, Thanks for the advise. Your fix worked just fine. Running as strong as ever."

Just got this message back from someone I advised on this problem.

By the way, those 31 series heads are interchangeable even with 28 series.

Walt Conner
wconner5 at verizon dot net
tomplum
13 years ago
That's odd. I never thought about the 28 and 31" engines having the same head. I always keep gaskets around for each and I do recall seeing a part # change on the 31's gasket last summer also. Could maybe the excess heat be from a partially sheared flywheel key or the wrong coil? I never noticed either that the 31's could take a different coil until about 2 weeks ago. One would think they fire same same. What about increasing the exhaust valve lash a couple thousandths? Maybe verify the lift between the 2 valves. I've replaced 2 heads ever on Briggs for seats btw. It would be interesting to know if the OP had other service done, is the engine seemingly running strong and what fuel is being pumped into this thing. If the local dealer is really seeing more than his share- what else could be a common denominator?
walt2002
13 years ago
"I never noticed either that the 31's could take a different coil until about 2 weeks ago. One would think they fire same same. "

Sears Tech friend says coils with small square hole punched thru laminates of coil indicates presence of an advance.(?)

Walt Conner
tomplum
13 years ago
Interesting on the advance. Must be one of things a guy hears and never think you would use again. I had a guy w/ a Scotts and no engine #s w/ an F series. (31F707-0112-E1) Here's what his parts list said. There were also different flywheel #s -with slightly different breakpoints. Emissions? The 341 coil I stock has the square holes stamped above in the lamination. Unsure on the 315.
Coil 795315 +07100300
Coil 492341 -07100300
walt2002
13 years ago
Don't know, the 31 series that have a letter in the model number as above, are almost a completely different engine than the 31s with just numbers in model number.

Walt Conner
bushleague
13 years ago
I remember the flywheel update, didn't this coincide with a camshaft update on early 31's?

Intek engines M# 31H777 bad valve seats

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mownie

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