Two Stroke Engine Fuel Primer

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Not sure if this is the place to ask this question but I will give it a go.
I have several petrol driven devices in my garage, Lawn Mower, Strimmer, Pressure Washer, Chainsaw etc. Some are four Stroke engines and some are two stroke.
Only the two stroke devices have fuel primer device whilst none of the four stroke devices have.
Is this just co-incidence or is there a reason for the two stroke engines requiring a primer device.
It is not a problem but I am just puzzled why the primer is required on the two stroke engines.
 
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Primer bulbs occur on four stroke engines too - largely in the absence of a choke mechanism for a cold start.
The reason that two stroke engines have a primer is largely due to the fact that they use a carburettor without a float chamber, (reason below) so fuel can't be drawn into the cylinder by the usual Venturi method. Priming the carburettor floods the drillings and chambers inside the carb, hopefully to promote a good start.
You'll find that when the two stroke engine is phased out due to pollution regulations, the four stroke replacement engine will have the same priming bulb on the carb.
Carbs without float chambers are able to operate at any angle, including upside down......pretty important when using a chainsaw or strimmer for example.
John :)
 
Hi John,
Thanks so much for your reply. That certainly explains the reasons for the priming bulb on my portable devices. As you pointed out, my Lawn Mower and Pressure Washer are devices which remain horizontal (on Wheels) during operation and the other devices are hand held and as such operate in various positions.
Thanks again for your helpful reply.
Ian
 
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A pleasure, Ian (y)
Just as an addendum - as I've got the anorak on at the moment - some two stroke machines have no primer bulb, and in this case the engine relies on crankcase decompression to work the pumping arrangement of the carb to get the machine to go. Typically this is the system used in stone cutting saws and chainsaws where the carb is completely enclosed for its own protection against dust or whatever.
Popular carb makes are Zama, Walbro and Tillotson, and there are many copies of these designs out there.
John :)
 
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