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Isleblue65

Valve tappet clearance

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Isleblue65
I think I may have found part of, or the whole problem with my Briggs that runs rough and stalls once warmed up. For what I've checked and what's been suggested so far, you can see the whole story here (3 pages of info) I didn't want this to be lost at the end of that post. http://www.simpletractors.com/club2/topic.asp?TOPIC_ID=85458&whichpage=3 I checked my tappet clearances and here's what I got: Intake: .0075 (should be .007 - .009) OK Exhaust: .0215 (should be .017 - .019) I am assuming that somehow once the motor gets hot, the exhaust valve clearance is increasing, preventing the valve from opening up enough and killing the motor. I would have thought the opposite would happen (the valve would expand or stretch, tightening the gap)? My manual says that the clearance is adjusted by grinding the end of the valve stem, but this of course would make the gap even bigger. It says that if the clearance is too much, you cut the valve seat. Is it common for the clearance to increase? Most people who talk about valve tappet clearance adjustment talk about grinding the end of the valve stem, not cutting the valve seat.

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D-17_Dave
The clearance decreases causing the valve stem to "ride" the tappet. This prevents the valves from seating and maintaining compression and a strong, clean power stroke. Such a loss of clearance spread over so many years would result in a slow loss of power and would not be noticed until the engine could no longer do what was required of it. It may serve you to remove the head and inspect the valve seats for deformation. You'll likely find the valves are worn heavily allowing them to fall deeper into the seat. This would account for the loss of clearance on the tappet adjustment. You will not be able to correct this condition without the necessary specialised tools but you can "lap" the valves in and return a consistent seal. You should notice an immediate improvement in engine power and performance.

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Isleblue65
Dave, Mine is doing just the opposite of what you are saying should happen. My valve stem to tappet gap is much bigger than it should be on exhaust valve only. Could my valves not be seating all the way into the valve seats due to something being stuck between them and the seats, OR could something on the bottom end be worn, such as the cam follower? There's no mention in my manual of cam follower wear being the cause of an excessive valve stem to tappet gap. The valve stems are smooth, not mushroomed. I've run Seafoam through the carb, so I don't think there would be that much carbon to hold the valve up off of the seat. Any ideas?

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tec2484
The only way you can have a larger gap is if the end of the stem is wearing, or the valve is not closing all the way. Pull the valve and look for build up on the stem and/or carbon on the face or seat. While you have it out take the time to lap it.

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UCD
Increased gap = valve not opening as it should. Resulting in no intake of fuel on intake valve or no exhaust of gases on exhaust valve. Increased gap can be caused by a worn cam lobe, worn valve stem, sticking valve stem, bent valve, foreign debris between valve & valve seat. Less gap = valve not closing completely. Resulting in blow back on intake valve and on exhaust valve loss of intake vacuum and compression. Less gap can be caused by worn valve seat, worn or mushroomed valve head.

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Isleblue65
I'll pull the head to check for obvious foriegn debris between the valve and seat. Other than that, I'd assume special tools are needed for repairing the other possibilities. For instance, I checked the price of a valve seat reamer and the kit was about $240. There are probably more special tools needed for removing/ replacing a cam, follower, etc.? Maybe then it would be worth considering a decent used motor, or biting the bullet and having a shop do the work? For repairs that I will probably only do once during the time I own this tractor, I don't think dropping several hundred dollars into special Briggs tools makes much sense, unless I could find them used for $50 - $60. What do you think?

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PeppyDan
You could bring the engine to a engine shop and have them grind the valves. I had a Kohler 12hp done a few monthes ago at a local NAPA. They charged $20 to put a multi angle grind on both valves and clean the block. I did have the engine apart so they did not have any time in tear down and I reinstalled the valves after they were done. They would have instaalled the valves for an additional $10. Dan

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MrSteele
Not knowing how to grind valve seats could cost you much more than the cost of the tools to do it with. Check around for a machine shop willing to do the work for you if they need to be ground. Otherwise, lapping the valves is relatively easy, can be done on your bench, and costs little. Be certain, though, if you use Clover compound to lap them, that you clean thoroughly, then clean a couple more times to get all the lapping compound off, as the compound is stupid, and any left in the engine will continue to cut metal while the engine is running. The trouble is, the compound will get into the rings and wear the rest of the engine out and you will have a fine problem.

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DeltaBravo
The exhaust valve is only 2 thousanths more than the high end of the spec. The valve is closing and sealing perfectly, but .002" lift has been lost when opened by the tappet. I don't see this as root of the problem. On mechanical tappet engines like this, typically you might hear a little more tappet noise when the engine is cold. I'm sure on a one lunger this amount is hard to hear over the exhaust noise. I've followed the thread but can't remember everything written, so I make these suggestions. You could have excess carbon build up. Briggs even recommended pulling the head after so many hours to scrape off the carbon. I think pulling the head after all these years is a good PM. The cast iron block head surface and valves will clean up nicely with a single edge razor blade. A new one can be used on the aluminum head if you are careful and go slow, using only one side of the blade. I first use Castrol super kleen and clean up stubborn spots with the razor blade. After cleaning the block surface, rotate the crank to take a look at the back side of the valves. A lot of build up will absorb fuel and possibly cause problems.

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DarrylF
If there's nothing obvious holding the valve open, then consider just buying a new valve, lapping it in and grinding the stem to spec. That's the cheapest and simplest repair. I'm sure the perfectionists won't like it, but it should get you up and running. Darryl

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Al
Hi, If your valve clearance is only .0015 over I doubt if this will affect your engine at all. If the clearance is too small, the expansion may cause the valves not to seat when hot. If clearances are out of spec they should always be on the loose side. I say this and then say if you have one of the New OHV Briggs singles, the valves NEED to be right on the minimum spec, or the starter don't have the torque to turn them over because of the loss of the compression relief action. On the older engines years ago some mechanics actually set the valves loose to reduce the compression relief in the belief they ran better this way. I owuld suspect that someone ground the valves and took a little too much off the stem. They probably didn't want to take any more of the seat out ant just let it go, knowing no one would probably ever know. Might have happened the factory even. As to power loss, lets just pick a lift # like .375" .0015 would be less than 0.4% or an insignificant amount. This extra clearance might cause a slightly increased amount of valve noise, but that would be the only effect. Just remember every bit you take out of the seat is gone forever, so when you are grinding the stems be very very careful. On my valve grinder taking off .001 you can't hear the cut and there are no sparks. One thing I would look at is the stems, do they have carbon on them? I would wire wheel them off and take a wire brush and spray carb cleaner and clean out the insides of the guides. Then after I had checked the valve face contact I would if necessary lap or grind them. Redo the clearance and put some of the Briggs valve stem lube in the guides and put it together. One should check the valve and seat with "bluing". Put a thin film of bluing on the valve face and put the valve in the engine. Push the valve the seat and pull it out. DO NOT TURN IT! You should see a contact ring in the bluing all the way around. If you turn the valve you will get false readings as one high spot will create a contact ring all the way around the valve. When lapping valves DO NOT turn the valves, just rock them about 1/4 th turn back and forth. This will allow you to check the seat contact. If you go around and there is a high spot on the seat it will make a ring on the valve and the valve will look like it was seated all the way around. This would be a false indication. Al Eden

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JimDk
I would like to share a tip about valve seating told to me by an old time engine re-builder. After lapping a valve, with the head of the valve clean and level, place a few drops of gasoline between the head and seat. If the gas disappears quickly, you do not have a good seal. If it holds gasoline, it will hold compression. Jim

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Isleblue65
Great information here! My clearance is actually .0025" over high spec, not .0015" - but this might be splitting hairs. I have noticed that the last 2 times I've mowed, the exhaust has been a lot more choppy and loud. It almost sounds like the rotors of a helicoptor "Chop chop chop chop". It is audibly clear and easy to hear over the motor and deck noise. This is what made me think the valve clearance was most probably the problem. If it is not, I really at am a loss as to what I'm going to check next. I'm in the middle of insulating my garage, so there hasn't been space or time to dig into the motor yet, but this weekened I'll finish up the garage and get the work bench back into place. I'll probably just pull the motor and put it on the bench for a winter once over. I will follow the advice given on lapping, bluing, checking stems, carbon, etc. Question: does the head gasket need to be replaced every time the head is removed? I would say 'yes' if it were my car, but maybe not necessary on an old Briggs? Thank you.

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debshirl
to answer your question yes the head gasket needs to be replaced after you torqe cylinder head the gasket will crush and provide a good seal i always put antiseise on head bolts so that you will get proper torque on bolts and possibly use a straight edge to check cylinder head and block

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Isleblue65
I'll order the head gasket tomorrow. The muffler alignment is good. All bolts are tight and the exhaust pipe is pointing right through the hole in the hood. I know the bolts are tight because I had to remove the muffler bracket when I repositioned my coil to the top of the engine, and I went through and tightened everything, not just the bracket I'd loosened. The chop chop noise started before I moved the coil and continues since.

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Isleblue65
Pulled the motor today, removed the head and checked the valves. There was quite a bit of carbon inside the head, on the piston and block, on the valve faces and on the stem sides which I scraped off. The head gasket also had a leak, which I could see once the head was off. Once I removed the valve collets and springs and cleaned things up, I could get an accurate tappet clearance measurement. The Intake valve was dead on, but exhaust was actually .022" which is .003" over the high limit. Again, this is a small fraction of the total percentage of valve opening, so it may not mean much. I hope that the carbon removal and honing which I will do tomorrow to bring the exhaust valve clearance back into spec will make the motor run better, if not solve my long running problem of the motor cutting out when warmed up! The second question I have is: should I switch it back to a Magneto ignition now that it's all apart and accessible? I am running an automotive style Kohler coil and using points. I installed this when my Magneto failed in order to save me from having to pull the motor to replace the magneto. Well now the motor is on the work bench. My Kohler coil is new this year and I've been using it all summer, and have not determined for certain whether this is one of the causes of the poor running. I already have an electronic ignition module and would just require the magneto to use it and eliminate the points. On the other hand, if there's no major advantages to replacing the bad Magneto and bringing the stock ignition back plus an electronic ign module, why spend the money? I've dropped more than I paid for the tractor into parts and maintenace already.

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MrSteele
Again, if you are not comfortable with the innards of an engine, be careful. I have had to bore automobile engines for people that ran a hone through a cylinder to "clean them up" and cut too much material off the cylinders. This can be done to your Briggs, as well, and quickly! The hone slop can cause you problems, too, as can the lapping compound, if not cleaned completely from the engine. The 'slop' contains some of the abrasive from the hone shoes and will wear your rings/cylinder if not completely cleaned out. If you are using a short boring bar to remove the shoulder caused by ring wear, be EXTREMELY careful, as you can cut the bore off center easily. If your head gasket had a small leak, you may have already found your problem. Check the mating surface of head and block for flatness. If it is not flat, the gasket may not seal properly. It can be trued easily, then reassemble. Torque the head bolts while reassembling, and yuour problem might well go away. I am not completely familiar with your tractor. Are there any brackets bolted on with the head bolts? If so, be careful to properly tighten the bolts with brackets( Brackets for mufflers, starter/generator?) Many times, the brackets will vibrated the bolts loose and cause head gasket problems.

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Isleblue65
I meant 'Lapping' not honing. I'm not touching the cylinder, just lapping the valves to bring the tappet clearance down toward the low end of the spec. I can still see faint honing criss-cross marks on the cylinder walls, and there's no scoring from the rings. The followers and ends of the valve stems look smooth and square, so I think a bit of lapping in the valves will be all that's needed. I will make sure to remove all compound before reassembly. I've not checked the flywheel key, but that will be another item I'll check. I'm also going to pull off the bottom to check the oil splasher or pump (don't know what I have yet) and to replace the gasket which has been leaking for the past year. Once I know the condition of everything, I'll order all the parts at once from Jack's. Thanks. Good suggestions.

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