Engine Questions


Old 10-04-02, 05:39 PM
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Question 4hp head on 3.5hp B&S

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?
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Old 10-04-02, 08:43 PM
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Hello dave4.3, actually what it will do is make the 3.5 a 3.5 with weaker compression.....Mike
Old 10-04-02, 08:56 PM
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Then the hp must come from a longer stroke then? The carbs seem the same.
Old 10-04-02, 09:07 PM
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Question Checking rpm's on a single cylinder

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?
Old 10-05-02, 04:18 AM
J A Boggan
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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.
Old 10-05-02, 05:34 AM
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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
Old 10-06-02, 12:59 AM
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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.

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