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Why do you need to prime vertical shaft engines and not horizontal shaft engines? Answered

I am just curious. I know some vertical shaft engines that don't need to be primed but most do.


It is just the engine manufacturer way of building it. I know that some of the older B&S and Tecumseh engines have the priming bulb and they are 4 stroke. There are some older snow blowers that are horizontal shaft and they have a primer bulb (check out the older toro 2 stroke snow blowers). Lots of the stihl power equipment have bulbs on them but they only bring fresh fuel to the carburator. So you can press the bulb 50 times and all it will do is keep on cycling gas though the fuel system. Now i'm not sure about this next comment but I believe that EPA standards may of made manufacturers get away from the whole primer bulb squirting raw fuel into the intake due to the amount of unburnt hydrocarbons that come out the muffler (the puff of black) I could blab about this stuff for a little while but then it's getting to be to much like work.

Your question is vague, but like Re' says: it's in the design. Priming may not be necessary, it's like a choke adjustment in that it generally works better for a cold-start.


Lemonie gets the Grand Prize for correctly identifying the specific difference and Re gets first prize for identifying the general difference.

"Primer" carbs do not have a choke. "Non-primer" carbs do. The choke cuts off most of the air from mixing with the gas, thereby creating the very rich fuel mixture needed to start a "cold" engine. Primers do the same thing in a different way, and keep an engine from being stuck on choke. It is as Re said: a design difference. My personal choice is a choke, but that is what I grew up with.

BTW, many newer 4 strokes come with primer bulbs. Chokes are harder to find on the smaller engines.


In my experience a two stroke needs to be primed and a 4 stroke doesn't. It's not the orientation but the type that matters.

I have 2 4-stroke engines that have primer bulbs. The fuel just naturally gets pumped when the engine is running. When you're trying to start, the tubez aren't full yet, and thats the time you need just a touch more oomph, so you prime.

I don't think I've ever seen one on a 4 stroke.

I guess it has more to do with the carb. design.