LawnMowers and SnowBlowers

(From: Brian (Aquakidd13@aol.com).)

On old 3.5 and 4 HP Briggs and stratton engines the cam acts weirdly and opens the exhaust valve slightly on the compression stroke thus loosing air and fuel mixture and compression. This is probably due to wear of the valve or valve seat resulting in the cam pushing the valve up when it's really not supposed to be touching the tappet. You need to pull the head off and crank it slowly and watch the exhaust valve, if both valves close and then the exhaust valve pops up, an adjustment is required. To fix this grind the exhaust valve tappet side down 0.5 MM put the valve back in and check it out if it still does grind a little more off. the engine will run smoother than before and a little better

(From: Sam.)

Note that many small Briggs and Stratton (and other) engines include a compression release feature to make starting easier. This has a similar effect in that it opens the exhaust valve during part of the compression stroke reducing the effort needed to pull the starter rope. This should not be confused with what Brian is describing, above. At normal operating rpm, the governor moves a secondary piece on the exhaust valve cam out of the way so that the behavior returns to normal. Unfortunately, unless the crankcase is open, there is no easy way to determine whether the compression release is present on your engine without a service manual.

Oil sump/crankcase cover

• Check the crankshaft for burrs at the blade lock key or other key and/or where any front wheel drive pulley setscrew was tightened. It is essential that these be carefully removed with a fine file before you attempt to remove the cover to avoid damage to the main bearing. In addition, any rust and/or dirt buildup must be removed with steel wool, sandpaper, or emery cloth to allow the shaft to pass through the main bearing without damaging the bearing or oil seal. Remove all traces of rust and grime but don't go overboard - it doesn't need to shine. Wipe with a very slightly damp cloth to remove **all** abrasive residue.

• Set the engine flywheel-side down on wooden blocks so that the flywheel mounting shaft is clear of the table.

• Once again, wipe down the underside of the engine, especially around the crankcase/oil sump mounting bolts and the seam where the cover will separate from the engine block.

• Remove the 6 to 10 hex head crankcase/oil sump mounting bolts and set them aside.

• Use a soft rubber mallet if necessary to help free the cover. If mild persuasion doesn't work, check for any bolts you may have missed.

• As the cover comes free, gently lift and turn at the same time. It should slide right off the crankshaft. If there is any resistance, you probably didn't find all the burrs or rust. Go back, identify, and correct the problem. Don't force it as you will end up with scratches on the bearing surface and/or damage to the oil seal.

• The gasket will likely tear in the process of removing the cover and will need to be replaced. It is not worth trying to repair it. You will have to scrape the remnants of the old gasket off of both mating surfaces before installing the new one (later).

• Check for any washers that may come free with the cover. There will be at least one on the crankshaft. It may be sticking to the bearing surface on the cover.

• Usually, the flyweight governor is just left in place unless parts need to be replaced. Inspect it for damage to the gears, flyweights, or cup. Individual parts can be replaced if needed (and if you can get them!). The post is a press fit and should not be disturbed unless damaged.

Camshaft/camgear

• Remove the camshaft driven plunger type oil pump. This is in two sections which have a definite relationship (the flat faces out). Inspect for damage and set aside.

• Carefully rotate the crankshaft until the timing marks align. These will be a line, dot, or hole on the camgear and crankshaft gear. For most engines, these should line up perfectly with each other at one position of the crankshaft. However, on some Craftsman engines, they are offset by one tooth. Check and note this before removing the camshaft/camgear!

• With the timing marks aligned, valve pressure should be released (if you removed the valves, this won't matter) on the camshaft and it should slip out easily.

• Inspect the camgear for chipped or broken teeth and wear. There should be no chipped teeth and no detectable wear on any of the gear teeth. Chipped teeth or significant wear will require replacement.

• Inspect the cam lobes for wear or wear. There should be no detectable wear and no damage.

• Check the compression release mechanism on the camgear for damage and free operation. The weight should snap back to the shaft when released. The little lift pin should move smoothly.

• Remove each of the valve lifters and inspect for wear. There should be no significant wear. Note which went where so that they can be replaced in the same location. Most are of the same length but once a wear pattern is established, replacement in the same location is desirable. Sometimes, they are of different lengths and then this is more critical.

Connecting rod

• Rotate the crankshaft so that the rod bolts or nuts are accessible.

• Bend out any lock plate that may be associated with the rod bolts or nuts. Many Tecumseh engines use 'Durlock' rod bolts with integral locking serrations and there is no lock plate and no lock washers. Note: Durlock bolts, lock plates, or lock washers should always be replaced with new ones if removed and not be reused. You really don't want the rod coming apart!

• Use the proper size socket to loosen the rod bolts or nuts counterclockwise. Start with small equal increments on each of them until loose to equalize stress.

• Remove the bolts or nuts and cap. Note the orientation of the cap and rod. If yours is the slant type, this is easy. Otherwise, look for match marks, casting numbers, or other identifying marks and make a diagram in any case.

• Where bolts protrude from the rod, immediately cover these with some bits of rubber tubing or tape to prevent them from hitting and dinging the crank pin journal or other precision surfaces.

Piston

• Remove any carbon ridge you find at the top of the cylinder. This will catch the rings and prevent you from removing the piston or if force is used, break the rings. Usually, it is a simple matter of scraping with a piece of soft metal like aluminum. On rare occasions with a really well worn engine, enough of a metal ridge will also be present to require the use of a ridge reamer tool.

• Push the piston up and out of the cylinder bore. The rings will expand but will not pop off unless they are actually broken.

• Replace the rod cap and finger tighten the nuts or bolts. This will help to protect the bearing surfaces from accidental damage.

• Inspect the piston for damage. There may still be significant carbon deposits but once these are carefully scraped off, the piston should be fairly smooth. There may be some vertical scoring but a modest amount of this is not serious.

• Inspect the rings and ring grooves for damage. It is usually not necessary to actually remove the rings from the piston to do this - which should be avoided if possible to minimize the chance of breakage. The outer surface of the rings should have an almost polished appearance with no significant pits, scratches, or corrosion. There should be no chips or other visible damage.

• If you must remove the rings, use a ring expander if possible and make sure you note the exact orientation - top/bottom and location - for each.

• If you are removing the piston pin, note the orientation of both the rod and pin as they must be returned in the same relationship. Pistons are not symmetric! Look closely and you will see that the pin is offset a fraction of an inch to one side. This is done to optimize the center of force on the rod bearing and rotating crank pin journal.

• Use a pair of needlenose pliers to remove the 'circlip' from one side. The piston (wrist) pin usually floats (moves easily) in between the two circlips but a slight lip of metal (probably resulting from the pin banging back and forth) may prevent it from being easily removed. Very slightly scraping around this lip will free it up or you can remove the other circlip and then use a drill or arbor press to push the pin out far enough to free the rod. There is no need to remove the pin entirely. Then, there will be no question as to the direction upon reassembly.

Crankshaft

• Lift while rotating the crankshaft out of the top bearing. There should be no resistance (unless you forgot to remove something).

• Inspect the crankshaft to determine if it is bent. Any deviation from perfection is cause for replacement. The proper way to do this is with some V-blocks and a run-out gauge. However, you won't have these tools so a visual inspection is the best you can do. However, unless your blade kissed a boulder, a bent crankshaft is not likely.

• Inspect the small gear for broken teeth and wear. There should be no chipped teeth or detectable wear of the gear teeth. If there are chipped teeth or significant wear, then this gear and the camgear will need replacing.

  Note: the small gear on the crankshaft may be a press-fit and may not be considered serviceable by itself without replacing the entire crankshaft. However, I have been able to remove it non-destructively by gently tapping on each side using a soft metal bar (e.g., brass) and a small hammer. (When I had to do this, the guy at the engine parts store was surprised that I was able to get it off without damage.) Heating the replacement gear will expand it and reduce the force needed to press-fit it onto the crankshaft. Similar gentle tapping will then work once the gear is aligned with the indexing pin.

• Check for any washers at the flywheel end of the crankshaft and set these aside. There are probably none.

Breather

• The breather allows the pressure inside the crankcase to vent to the outside and should result in negative pressure inside as this contains a (leaky) one-way valve.

• The breather cover is under where the flywheel is located.

• Remove the screw to remove the cover plate. There is a gasket but it will probably separate cleanly.

• Check the spring and valve disk for rust, dirt, and wear. They will likely be fine. Make sure the small vent hole is clear.

• Replace the cover as there is nothing else to do in there.

Oil seals

• Do not remove the oil seals unless you intend to replace them. If oil leakage has not been a problem and you don't think any damage resulted from removing the crankshaft, leave them alone. It should be possible to replace the oil seals after reassembly if oil leakage turns out to be a problem.

• It is very unlikely that the flywheel side oil seal would be defective or damaged.

• The PTO/blade oil seal can be damaged by neglecting to completely remove burrs from the crankshaft before removal of the crankcase/oil sum cover.

• To remove the oil seals, use a screwdriver to pry them out from the crankcase/oil sump cover and/or the flywheel side of the crankcase. Take care not to gouge the mounting surfaces.

Detailed inspection

Determining this requires a visual inspection and taking measurements of all critical dimensions of bearings, cylinder, piston, and rings. Some of the inspection is subjective - how badly scored a bearing surface is before it must be replaced or reground. A few score marks around the circumference of a bearing surface will not adversely affect operation or wear. How few is a few? Perhaps if less than 10% or so of the surface is affected. You are not going to spend as much to repair the mower as it cost in the first place in any case so don't lose sleep over it.

Measurements may come up marginal as well. For example, if the limit listed in your engine specifications is .0015" and you measure .002" will this be a serious problem requiring the replacement of expensive parts? Probably not. You may get less than optimal life out of the engine but it will probably still work fairly well and for a long time. So many other factors can affect life that this may have no effect at all.

The following items should be visually inspected. If any significant wear is indicated, precise measurements should be made:

• Crank pin journal and rod bearing. Inspect for wear, pitting, and scoring on both the crank pin journal and the inside bearing surface of the rod and cap. On a new engine, both of these surfaces are nearly mirror smooth. On a well worn engine, there may be significant scoring due to particles from the oil getting trapped. An engine that has failed due to a severe lack of lubrication may result in some pretty spectacular failures of these parts. Minimizing wear and the change of catastrophic failure is the primary reason for performing regular oil changes.
  • A few score marks around the entire circumference of the journal are unimportant as long as they represent a small percentage of the surface area.

  • Scratches, pitting, or score marks that run side ways are more serious. If slight, polishing with very fine emery or crocus cloth may be all that is needed. If they catch a fingernail, this may not be enough.

  • A serious out-of-round condition is unacceptable.

  Follow the instructions that came with the plastigauge to take measurements. Consult your engine specifications for acceptable limits. Use judgement in determining whether slight out-of-spec measurements will necessitate replacement or major rebuild.

  If you had a rod failure due to lack of oil (remember what we said about the importance of oil - see the section: Rod disasters - or why the oil and governor are kind of important) - then there could be a variety of types of damage that will make these measurements academic. The rod my have broken in half or the cap may have literally exploded into multiple pieces. In many cases, the crank pin journal will escape relatively unscathed but needless to say, you will need a new rod and cap - not cheap!

• Main bearing - PTO (blade) end. Inspect for severe scoring, corrosion, or other damage. It may no longer be mirror smooth but should not appear excessively worn.

• Main bearing - flywheel/magneto) end. Inspect for severe scoring, corrosion, or other damage. This will probably appear almost like new even on an old engine as there is a lot less load on this end and it is relatively well protected and well lubricated.

• Cylinder. Inspect inside the cylinder for excessive wear and scoring. If it appears fairly smooth without much scoring, it is probably ok but only exact inside measurements would confirm.

• Piston. Examine the sides for vertical scoring. There will probably be some but as long as the piston is not mostly score marks, it is probably fine. Only exact measurements would confirm. Check for damage to the lands - the surfaces between the ring grooves. If any are cracked or broken, the piston will need to be replaced.

• Piston (wrist) pin. This should be mirror smooth. There should be no detectable free play if you try to jiggle the rod.

• Rings. Inspect for damage, pitting, and scratches. The outside surfaces should be pretty much mirror smooth. Use an appropriate sized feeler gauge to check clearance between the rings and piston grooves.

• Oil passages. Inspect and use compressed air if necessary to clear the various oil passages in the crankcase/cylinder, camshaft, connecting rod, and crankshaft. The typical small Tecumseh engine has a hollow camshaft which is part of the oil pump and drilled passages in the crankcase. The oil path is from the plunger/barrel oil pump up through the center of the camshaft, over top via the passages in the crankcase to lubricate the main bearing (flywheel/magneto end) and also to drip on the connecting rod and crank pin journal. Some larger engines also have drilled passages in the crankshaft and connecting rod. There is even an oil pressure test port normally sealed by a small screw. A typical pressure measurement on an engine running at full speed is 7 psi but you won't measure this so just make sure everything is clean and clear.