Priming & Starting A Hot vs Cold Lawnmower Engine ?


Old 06-29-05, 04:34 AM
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Join Date: Apr 2005
Location: MA
Posts: 271
Priming & Starting A Hot vs Cold Lawnmower Engine ?


Have a new Toro self propelled lawnmower, and also a new Homelite string weed wacker. Have read the manuals, honest, but they really do a poor job in explaining these things. I imagine that my questions, and (hopefully) answers, probably applies equally to both:

When one pumps the bulb the few times that is required when starting the typical lawnmower, etc., exactly what is one doing ?

a. e.g., does the pumping action just draw gas into the bulb, and then the next push on the bulb squirts it into the carb, or... ?

b. If there is gas visible in the bulb before trying to start it, can one consider the engine already "primed" ?

c. If so, should one ever pump it more prior to starting
If there is gas visible in bulb:
-When cold ?
-When hot ?
Why ?

d. Same as (c), but no gas visible in bulb:

If someone might explain this bulb pumping routine a bit, and exactly what happens,would be appreciative.

Realize I really don't understand what is happening when I do it, and when I should, and should not, do it.

Much thanks, really appreciate it,
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Old 06-29-05, 09:53 AM
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Actually both primers in your situation perform slightly different tasks to achieve the same goal, easier cold starting.
Heat is part of the equation necessary for proper combustion of fuel/air mixture. If the engine is at operating temp, no priming "SHOULD" be needed

The string trimmer most likely purges the air from the fuel lines and carb. When u press this primer it pushes out of the primer to the tank via the return line. When you release the bulb it should draw fuel Thru the carb via the filtered line from the tank and up to the primer now ready for the push out the return and back to the tank. In this case you should also have a choke which will restrict air flow and enrichen the fuel/air mixture for cold starting. In this case no harm should come from priming a warm engine, and may be preferable if air bubbles are noticed in the primer bulb.

The lawn mower uses a different principle, and actually squirts a bit of fuel into the throat of the carb to enrichen the mixture and in theory, eliminating the need for a choke. In this case you should not need to prime if the engine is at operating temp. If primed when warm it most likely will only puff a bit of smoke but could foul plugs.
Old 06-29-05, 11:12 AM
Thread Starter
Join Date: Apr 2005
Location: MA
Posts: 271
To Azis:


Just the info i wanted.
Very clear explanation.

Much thanks,

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