Join Date: Nov 2009
Location: georgia's toe-nails of the foot hills
Re: Bolens 1203H engine trouble
Have you looked at the Predator V-twin from Harbor Freight? It would be higher than an opposed. As far as replacing a 12.5 with an 18, you should have no problems as far as power for the machine will only use what it needs, and that depends on how much the operator loads it. For example, you might slow down when in tall grass due to engine limitations, but with a more powerful engine, you could go faster without laboring the engine. It is still doing the same amount of 'work', cutting the stems, but at a faster pace. If you slow down to the previous rate of cutting, with the 12, you would be loading the equipment the same, no matter whether you had a 12, 18 or 24hp engine installed. Operator decides...
If the Briggs is a flathead, you may have a valve seat that got loose as I understand that can happen. I would try rotating the crankshaft manually, with the spark plugs removed. Using an available thumb, attempt to hold compression on both cylinders. If you get no compression on one or both, remove the cylinder heads or use a tiny camera to inspect for valve/seat problems. A sticky valve guide is also possible, which could make the valve less likely to seat.
In some cases, you can substitute a single in place of a twin. Some have used the small vertical Predator on riding mowers to replace NLA Tecumseh and others. The available hp seems to be enough. The smaller horizontals seem to work well also if you don't want to or can't fit a twin in place.
There is at least one youtube (backyard machinist?) who has installed a horizontal V-twin Predator on a zero turn, making a mount and modifying the PTO as needed. He was very pleased, FWIW.
simplificate & add lightness
It's not how hard you work, it's how much you get done.