I have gotten a lot of old engines going even with poor compression. With a Briggs if you have a good spark, good plug, decent compression and the carb is pumping gas, it will run even if it smokes like hell. If it smokes excessively or has a knock, you can then decide if it’s worth a trip to Northern Tool or Tulsa Engine Warehouse for a replacement engine.
Let’s start from the beginning.
Compression
Check the valves and head gasket at the same time by removing the head. Inspect the head, head gasket and block for signs of leakage and burning. A bad head gasket or burnt head will drop compression in a heartbeat. Glue a piece of 80 grit sandpaper to a solid very flat surface. Grasp the head and place it flat on the sandpaper. Using a circular motion and even pressure lap the head, turning it 45 degrees every couple of strokes. Check for flatness by coloring the entire lapped surface with a magic marker and then rubbing it on a very flat dry surface like a table saw deck. Look for high spots. Continue until you decide it is flat enough. Clean off carbon. Either get a new head gasket or reuse old one if it looks OK and is not burned.
Rotate engine to open and close valves. If valves are burned they can be ground if they are not to bad. Some engines burn valves early because the factory doesn’t leave enough gap at the tappet end. If burned, pull the valves out and using a bench grinder remove about 1/32 off the tappet end. This will ensure good seating after they are lapped. Lap with a good valve grinding compound until you get a good sealing surface all the way around both the seat and the lip of the valve. If the valve seat appears loose the engine may still give you good service by taking a punch and peaning the aluminum all the way around the seat to resecure it. Then lap it to mate it to the valve.
Keep the valve-grinding compound the hell out of the engine. If you didn’t have problems before try a teaspoon of carborundum and see how long it takes to throw a rod.
When everything is spotlessly clean and no grit inside, install the valves. With the springs installed, and the valves closed you should have at least .010 valve clearance at the tappets as long as you are not in the compression release area of the cam.
Put the head back on with Coppercoat sealant. Torque the head bolts to about 20 ft. lbs. but do not strip out the aluminum threads in the block. A lot of times this is all it takes to get her going again. Or not.
Fuel System
We always blame the magneto but quite often it’s the fuel system. If I am thinking about your tiller right it has a 2-5 HP engine. Check to see whether your engine has the gas tank separate from the carb. There are two different types of fuel systems and the steps are different.
If the tank is separate you will have a gas line and perhaps an in tank filter or a shutoff valve sediment bowl. Whether the tank is attached to the carb or not look inside the tank and if it’s rusty or gunked up, you must remove it to clean it. Remove the in-tank filter, if it has one, because it will be ruined by the next step.
Clean the tank out with lacquer thinner and a handful of nuts and bolts. Shake it up real good using the impacts of the fasteners to dislodge the debris stuck to the walls of the tank. When you think it’s ready, dump out the corrosion soup and rinse it out with hot water and blow the inside dry. Reattach the fittings and gas line to the tank after cleaning filters and valves with solvent. Put a little clean fuel in the tank, disconnect the fuel line from the carb, if no fuel comes through, check upstream for problem. If you are getting adequate flow, the problem is downstream.
A lot of those engines carb design put the fuel pump diaphragm a half-inch from the muffler. The diaphragm doesn’t last long when it gets that hot. Remove the muffler and replace the fuel pump diaphragm. It should cost under $2 dollars. Screw a ½ inch street el back into the engine block when you replace the muffler. It will direct heat away from the carb.
Still won’t run?
You’ll need to pull the carb.
Clean your fuel pickup tube, if it has one, with carb cleaner. If you can’t blow through the pickup tube, gas can’t get through either.
If you have a float type carb, disassemble and clean the float as well as the rest of the carb with lacquer thinner or carb cleaner. Do not forget to carefully ream out the tube the float valve slides in with the proper size drill bit. I have found corrosion there on several occasions which puts the valve in a bind and keeps it from sliding freely. Be careful not to nick the rubber seat down in the bottom of the slide tube. Hold the carb with a rag over the float area so that when you blow air thru the gas line into the carb you don’t loose the rubber seat if it gets blown out. Make sure that the venturi tube is clear. Remove all your needle valves and small parts and clean with carb cleaner or lacquer thinner and dry. Don’t soak rubber parts in lacquer thinner or they will swell up.
Visually inspect the throat of the carb for any tiny orifices which need reaming out. I use insulin needles to clean them out because they are much finer diameter than torch tip cleaners. A carb rebuild kit is always nice to have for the new gaskets and small parts but if you are careful taking everything apart you might not break anything which you can’t clean and reuse.
Reinstall the needle valves and after bottoming out, turn them all out 1 ½ turns. Reinstall the float and check that it moves the valve freely and that it is not so worn that it hits the carb casting. If it does you can bend the tiny tabs touching the valve so the valve closes before the float bottoms out. Otherwise the carb will flood. Same goes for the valve not opening before the float hits the bottom of the fuel bowl. If that happens, the carb will starve for gas. Make sure the pivot points that the float hinges on are not so worn that it cannot be set right. More than 1/32 wobble is not good. It will give you a variable fuel level which will cause the engine to surge.
If you have a cast float bowl, assemble the bowl to the carb, finger tight, without a gasket and check that somebody hasn’t bent the casting at some earlier time. If the gap is more than the thickness of a piece of notebook paper it will never seal. Over tightening the screws just makes the problem worse. Decide if you can lap the mating surfaces flat again or if you need a new carb body or cast bowl. If everything is mating properly, reassemble the parts and reattach the gas tank. Reattach the carb-tank assembly to the engine. Make sure that the gaskets are all good and everything is sealing properly. Reinstall the spark plug.
Put a little gas in the tank and cover the throat of the carb with your hand. Pull it through a couple of times and see if it’s picking up gas into the throat of the carb. If it’s still dry you missed something. Take it apart and see what’s wrong.
Does it live?
No?
Spark
Start with a new spark plug. Test the plug first in another engine which you are sure is running well. Suspect the ignition system if spark isn’t visible with the naked eye when using a new plug and grounded properly. Be careful that you don’t have a transistor magneto with a low speed cut out.
If you are sure it isn’t getting spark, Pull the shroud and make sure that the magneto and flywheel are gapped properly. Remove the magneto and make sure that there is no rust or corrosion on the flywheel, magneto or any mating surfaces where magneto mounts to the block. Clean everything with sandpaper. Reinstall the magneto. Gap the magneto with 2 pieces of notebook paper. Make sure that the kill wire isn’t shorted out to anything. Make sure the plug wire isn’t cut or shorted out somewhere.
Still not running?
To bad.
Buy a tune up kit for the point system or a new transistor magneto for $50 bucks. You can convert a point type system to a transistor magneto system by plugging the pushrod hole behind the flywheel to keep the oil from blowing out. Or just cut the wires running behind the flywheel.
If you buy a new transistor magneto once you set up the magneto to the flywheel gapped like we said above it should run.
If you have points and you want to go that route, you will need to pull the flywheel.
To pull the flywheel, remove the nut on the crankshaft. There is usually a drilled hole in the end of the crankshaft. Place the tip of a big fat center punch in the drilled hole. Place a big screwdriver behind the flywheel where you can brace against something solid on the block. Put about 30 pounds of force on the screwdriver to put pressure on the flywheel, pulling it away from the block. Give the center punch a good strong smack with a 16 oz. ballpeen hammer. If the flywheel doesn’t come loose with the first hit you weren’t hitting hard enough. Don’t loose the aluminum key when you pull the flywheel loose.
Open up the point cover and blow out any oil or dust which might have gotten in. If you bought a new ignition set, install as instructed. If you are fixing what you have got, take a piece of 120 sand paper or a nail file and file the points clean. If oil has gotten on the points it is just as bad as being burned.
Reset the point gap to .028 inches. Place the flywheel back on the crank with the key in place and the nut finger tight. Make sure the spark plug is removed. Set the magneto gap as described above. Briskly turn the flywheel so the magnet on the flywheel passes the magneto. There should be great spark. If not pull it apart and go buy a new ignition set which will include a new condenser. Install it like described above.
If you get good spark, you did good. Pull the flywheel back off and permanently reinstall the point cover with new sealant at the upper edge. Reinstall the flywheel and magneto as described above. Torque the flywheel nut to about 40 ft, lbs. If you strip it that’s not good.
If points are set right and clean, flywheel and magneto rust free and gapped correctly and all connections clean, ignition set to run, the carb is pumping gas, and there is gas in the tank, it should run.
There is that little thrill you get when that old junker pops off first pull after you jacked with it for a couple of hours.
Good luck.
