Hi, Guys. I have a 14.5 HP Briggs OHV single cylinder on a riding mower. It currently runs somewhat rough as it spits through the intake and occasionally backfires. Is this most likely the valves not seating correctly? Should I lap the valves? If so, what grit should I use? I adjusted the valve clearances and no change. Thanks for your advice.
Lapping Valves?
- Thread starter jeruss
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gregjo1948
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Try a new spark plug first. Before taking the head off, recheck your valve gap. With the plug out check the gap in several spots on engine rotation. The exhaust valve , I believe is use as a compression release to allow easier starting. Start lapping with coase and finish with fine. I put a scratch on the valve head pointing to the center of the spark plug hole. When you lap the valve , turn it back and forth a small amount, using that scatch mark as a reference. Do not go around and around because it will give a false/bad lap. There should be a noticable uninterrupted line on the valve face. If there isn't, then the valve or seat may be bad. When reassembling, line the scatch on the valve up with the center of the spark plug hole. gregjo1948
You do realize that the valves rotate when the engine is running don't you. I was always told to lap back and forth with lapping compound then turn the valve several degrees and lap some more, making sure you have a complete circle around the valve face and well as the valve seat. I normally do three angle seat cutting, as well as, recut the valve face to remove any grooves and bad spots.
Fish
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I really do not use lapping compound other than to check things after re-cutting the seat and valve. I do a couple of little back and forth motions, then clean and inspect the valve to see where it is sealing and where on the valve face,
as well as evenness all around. Lapping beyond that destroys the seal that is given by the 1% interference angle.
as well as evenness all around. Lapping beyond that destroys the seal that is given by the 1% interference angle.
gregjo1948
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jeruss didn't say anything about grinding(cutting). He was just asking about lapping. If you move the valve around to several points to lap, it will not give you the trurh about the condition of the valve or seat. The valve or seat may be slightly burned or warped etc. The high spot is all that will be touching and will leave a line but the valve won't actually be seating. Lapping isn't ment to fix anything. It is for checking for a good seal. gregjo1948
Fish
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Wow! I agree with you totally!!!! Good post!!
pugaltitude
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I would start at the beginning and do the easy things first.
Whats your Model,Type & Code of the engine?
Have you used fresh clean fuel?
Have you replaced the spark plug?
Have you cleaned the carb?
Are the gaskets and o-ring ok?
what have you set your valve adjustments to?
Have you set it with the engine on the power stroke a 1/4 down past TDC?
The decomp is usually on the bottom valve which is your inlet?
It could be a head gasket blown between the valve chest and the cylinder.
Hi, Guys. I am sorry that it took me so long to respond. I took off the head and cleaned it along with the valves. The valves seemed to be fine. I put the head back together and put some brake cleaner fluid and no leaks through the valves. I put the head back on and adjusted the valve lash. The engine now runs fine initially but as it warms up it starts surging and sputtering until it dies. I took the carb apart and cleaned it and no change. I am thinking of replacing the carb.