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Need to rebuild your lawnmower engine ? I purchased a 2nd hand Year 200 Briggs & Stratton 16” cut lawnmower, which worked okay but did smoke rather while running. It also ‘hunted’ while running, so I decided on a full rebuild.
After all it is only a simple little 150cc single cylinder side valve engine, isn’t it ??

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During the last 2 decades I’ve rebuilt 2-stroke triples, four stroke singles, four stroke DOHC motorcycle and three car engines, so this one should be easy right — WRONG !! This lawnmower engine is the most awkward engine I’ve ever seen. If you have engine building experience have a go otherwise, leave it alone, get someone else to do it or chuck it !

I had decided I would grind the valves in and put new rings in; the rings gaskets and sparkplugs were about 30.
Stripping the outside covers, removing the card, air filter and ignition system is relatively easy. The rotor declares a special pulley is needed, but if you’re careful you can gently tap all around it and it will loosen. Don’t loose the woodruff key. It is a good idea to drain the engine oil out a day before so that the engine is as dry as possible inside. At this point the cylinder head and valve cover would be removed as well as all the other stuff.

Now for the really great bit!!. The crankcase splits in half vertically (relative to the cylinder) but the bottom half cannot be taken off because of the blade support, and the conrod is holding the crankshaft in the other half! The next bit requires tenacity and patience. Clean the bottom crankshaft part really well.

 

When that is really clean and smooth undo the crankcase bolts and the
bottom half will be able to slide down the crank reasonably well; as below.
CARE is needed at this point. The bottom crankcase half can be rotated but you must try to keep the beige camshaft wheel engaged with the wheel on the crankshaft. The valve timing NEEDS to be checked and marked if possible. The small wheel on the crankshaft has a small pin that should!! line up with the tab on the camshaft wheel. If it doesn’t you need to mark it, see the picture below.

NOT AN EASY TASK!! There is a gap of only about 2 inches to do it through. The best method is to keep the camshaft and oil splash wheel in the bottom half — below.

Now for the big end cap !! Unless you have 9 inch figures with molegrip strength, it is easiest if you have a flexible or jointed 3/8 drive to the big end cap bolts out — pic on right.
A gentle tap and the big end cap will spring off. You will need to push the piston and conrod up to nearly TDC to get the crank clear of the conrod big end. Then make sure neither valve is being lifted, and it should be possible to withdraw the crankshaft and other wheels away from the top half.
Note: there are 2 valve pushers loose fit into the crankcase. Take care of the decompression mechanism on the camshaft wheel. Hopefully you will have determined the valve timing is correct or you have marked it.

At this point you will replace the rings and grind the valves — which I don’t need to describe.

If the valve timing hasn’t been determined; set the crankshaft to your best estimate of TDC (pic. below) and set the camshaft to midway between activation of either (ie where both lobes are furthest away from activating the valve stems. The adjust the camshaft one tooth backwards (engines always fire before TDC). Now ‘dummy’ assemble the 2 halves connect up the conrod and see if the timing looks like it will work. If this looks okay mark the camshaft. This is the best estimate you can do.

Now I recommend to dry assemble, with gaskets, and liberally oil all moving parts — but don’t put oil in at the moment. Assemble the rest and if it starts, super ! — stop it immediately and put the engine oil in. If it doesn’t start disassemble and check the camshaft timing and move it back or forward one tooth and mark. It will be best guess at this point — but you should get it right 2nd or 3rd go.

GOOD LUCK.

e&oe

I hope my advice can stop some B&S engines being smashed to bits with a club hammer - this one came close to it.

 

Dealer quote "oh, it depends what day it was made as to whether the timing mark is accurate" !!!

Contact me:
mark @ linesi.co.uk

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