OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines rebuild problemo

hey its meeeee again.ok i pulled this 12 hp brigs 281707 4029 01 91052211 apart and gave it a new set rings and gaskets i put her back together with a new coil and get good spark and plenty of compression when i spin it on the starter but she wont fire.i lined up the timing marks etc even gave it a shot of start ya bastard.it backfied thru the exhaust once and that was it . any ideas????regards mike the 53 year old aprentice

couple of quick questions for ya, have you checked the exhaust and inlet valve tappet clearances and also have you checked the flywheel key?


joe.

flywheel key is new and in position valve clearances i dunno

Mike, are you sure you have the camshaft correctly timed? It is essential to line up the dots on the timing gears precisely, when you put it back in. If you are certain about the cam timing, you need to check the tappets as Joe said. Once you have those points correct, you might try squirting a spoonful of fuel in through the spark plug hole and checking whether it starts and runs at least briefly.

If you can report back on the results of those items, we can run a diagnostic process if necessary.

im certain the timing dots were aligned and the coil is set with the buisness card ill try the fuel in the piston trick .ill report back tomorow after i have another go

You should check the tappet clearances Mike, it only takes a couple of minutes and may save you a lot of messing about if that is the problem. You might also try starting it with the muffler removed, in case it is blocked. Once those simple things are checked we can work in an orderly way through more subtle problems.

Once we know that air is moving freely through the intake and exhaust systems, the ignition system is OK, the valve timing is right, and it has compression, we need to know if it will start with fuel in the cylinder. If it will, we have a fuel system problem. If it won't, we are into relatively serious headscratching and can all have some fun with this.

how do i check the tappet clearances

If you look below the carburetor on the side of the engine you will find a sheet metal cover held on by a couple of small screws. That is the cover of the valve chest (it is also the breather valve, but that is irrelevant for this purpose.) Remove the screws and the cover. Behind it you will see the valve springs and the lower part of the valves. Underneath the valves are the tappets: the camshaft followers, which lift and lower the valves. Rotate the engine until the flywheel has rotated an inch or two past top dead center on the compression stroke (i.e, just after it has fired and the piston has moved down the cylinder say, a quarter of an inch). Now, there should be a very small amount of space between the lower end of the valve stem, and the top of the tappet. That space is the tappet clearance. You need a set of feeler gauges to measure it. You select a feeler gauge that is 0.006" and see if it will fit into the clearance space under the intake valve stem. Then you select a 0.010" feeler gauge and see if it will fit into the clearance space under the exhaust valve stem. If it will fit in both cases, without pushing it in so hard that you are lifting the valve against its spring, the tappet clearance is OK. If you were doing an overhaul you would also check that it didn't have too much clearance, but that doesn't matter for our current purpose (so long as it doesn't have a lot too much clearance, which would indicate a disaster to the camshaft or tappet).

If both valves have a reasonable clearance, that isn't your problem.

hi grumpy,
hey i certainly enjoy tour replies to quite often complex questions , always very well answered with precision & background knowledge , love it , i am also an indentured tradesman. now im sure you will not take this to heart , this is just a little "dig" , but im sure that i was led to believe that it was the valve spring that was responsible for the valve closure , not the rocker or the tappet,
love you grumpy, johnno.

johnno, the valve spring provides the force that tends to close the valve, we are on the same page there. When you are checking tappet clearance, if you try hard to push the feeler gauge in on these small low-speed engines with very weak valve springs, you are likely to succeed in getting it in even if there is not enough clearance, because the valve is fairly easily lifted by compressing its tiddling little spring. It won't happen with a car engine, which has vastly stronger valve springs.

The Briggs and Stratton small engines until not very long ago, were all side valve. Mike's engine was made in 1991, and is one of the side valve range. With those engines the cam follower, or tappet, rides on the camshaft as usual, but the top of the tappet acts directly on the valve stem: there is no pushrod or rocker in the system. That is why you have to check the tappet clearance by removing the crankcase ventilation valve from the valve chest low on the cylinder/crankcase casting, so you can get access to the end of the valve stem.

I'm not clear on where we were disagreeing to start with, but that is my hopefully more detailed explanation of what I was getting at. I don't want Mike to use vigour to get the feeler gauge in there, by compressing the valve spring to do it. If your point was in connection with me saying the tappet "lifts and lowers" the valve, I agree it needs the help of the valve spring to lower it, but the spring has little hope of doing this without the cooperation of the tappet. Which one lowers the valve? Both of them do. Given enough time, gravity might even do the job of the valve spring, I suppose.

i pulled the head off it and the bore is scored now.guess i stuffed up id fitten new rings as directed .oh well shelve it for a while till i get over the poos

Now that you have achieved that result, I'll say something I wanted to say earlier but didn't want to be negative, since you'd already made the mistake: never use an ether based starting fluid, it may cause the bore to score, particularly if you didn't oil the rings and ring grooves thoroughly before you put it together.