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Lapping Valves?

#1

J

jeruss

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.


#2

G

gregjo1948

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


#3

I

ILENGINE

gregjo1948 said:
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
Click to expand...

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.


#4

J

jeruss

What grit is considered to be fine enough? 240? Also, what is best to use for removing the carbon deposits on the valves and head? Thanks.


#5

Fish

Fish

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.


#6

G

gregjo1948

ILENGINE said:
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.
Click to expand...

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


#7

Fish

Fish

gregjo1948 said:
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
Click to expand...

Wow! I agree with you totally!!!! Good post!!


#8

pugaltitude

pugaltitude

jeruss said:
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.
Click to expand...


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.


#9

M

motoman

Lapping has great appeal to the home mechanic , but (IMO) is not a good practice to rely on. Proper valve and seat grinding and blueing check is far superior.


#10

J

jeruss

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.


#11

S

SeniorCitizen

motoman said:
Lapping has great appeal to the home mechanic , but (IMO) is not a good practice to rely on. Proper valve and seat grinding and blueing check is far superior.
Click to expand...

Finally a fellow that agrees with me. I've seen too many horror stories about it and wouldn't put that stuff on my worst enemies valves.

It's decent for sharpening clipper blades on a lap wheel and other similar work but that's about it.


#12

M

motoman

Senior, Well, give fish some credit . He really doesn't like lapping either!! I believe total removal of the grit can be a problem and there is always the human fraility of... "if a little is good, more is better," which is not true and overzelous lapping can degrade that nice ground surface. motoman


#13

T

turboscrew

I would check all the adjustments (including the carb), spark plug and diaphragm first.

When it comes to lapping, I think a good idea might be a ckildren's bow with suction cup arrows and a screwdriver/drill with care and low speed. It's hard to find small enough lapping tools.


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