Has anyone swapped heads from a 4hp to a 3.5hp? I notice the combustion chamber volume is bigger in the 4hp. Will this make the 3.5 a 4hp?

 

Hello dave4.3, actually what it will do is make the 3.5 a 3.5 with weaker compression.....Mike

 

Then the hp must come from a longer stroke then? The carbs seem the same.

 

How do you check the rpm's on a single cylinder? I have a dwell/teach tester but it only goes down to 4 cylinders. It is old and don't really remember how it works. Do I need a special tach to accuratly read a small engine at idle and at speed?

 

There is a single cylinder wireless (hand held) tachometer. This one has a spring sticking out of the top, which acts like it's antenna (or pick up ). You just hold it approximately 1" form the spark plug wire. Briggs and Stratton also have two kind: an electronic tachometer that clamps around your spark plug wire - and a mechanical one that works (very well) off of the vibration in the engine. I prefer the hand held unit because of it's convenience.

 

If I remember correctly the four is stroked a bit longer than the 3.5HP. Could also be that the piston size is larger. I'm sure the carbs are the same....Mike

 

If the unit you have uses an induction clamp on the plug wire, then you can use the reading you get for a 4 cylinder, and divide it by 8 if I am figuring correctly. The reason for this is a car engine sparks only on the beginning of the power stroke. A lawnmower engine sparks on both the power stroke, and the end of the ehxaust stroke. So you get 2 sparks per complete cycle on a small engine, where you only get one spark per complete cycle on an automotive engine.