Small engines: is the priming bulb supposed to be full of gas?

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.


Gotta get outta here...
Sep 20, 2003
tackling yet another chainsaw finetuning.

This one was working fine last time I tried it. Just now, with nothing changed except gas put in, it will start but won't keep running unless I keep the choke on half way and even then it's iffy if I play with the throttle.

Tellingly, perhaps, I don't see much gas at all in the little flex priming bulb, whereas I would assume it should be mostly full. Am in correct on that? If not, that would suggest that the fuel intake is clogged, I take it.
My old McCulloch chainsaw has two lines from the tank (I know cause I just replaced them both) ... one feeds directly to the carb .. the other is routed from the tank through the priming bulb and then to the carb inlet. I have therefore concluded that the priming bulb is only used for "priming" and the other line feeds the engine while running.

(to add - no I don't believe the priming bulb needs to be full for the engine to run ... and in fact since the feed line to the primer bulb is not connected to a filter - in my case anyway - it should not be feeding the engine while operating fwiw)

This is only conjecture because despite this presumed knowledge I "think" I have - I still can't get that McCulloch to run properly!!
Last edited:
this particular one is a small Homelite Super 2 from the mid 80s I think. Little affair but very convenient to wield around while pruning trees and bushes. Much lighter than my 025 Stihl.

Good point about the 2 lines, didn't think about that, had assumed that was the one.

I do vaguely remember seeing it full of gas before though, which is what made me wonder. Could be wrong and confusing with something else, though.
the primer bulb should be mostly fuel,

check the lines they get old and start cracking. replace with tygon. there is a fuel pump in the carb operated by pulses from the crank case, the diaphragm in it can can go bad also.
At least half full on the ones I have. If not there is a line collapse,stopped up filter or line that is cracked. Mike
usually running a partial choke means the carb is gummed up, or so I've found. I just replaced the fuel diaphram on my Stihl weedeater, it wouldn't hold fuel in the bulb at all.
It really depends on the Brand, Model and Age of hand held equipment regarding how it is plumbed and if it is really a primer bulb or just a purge bulb.
...:bang: D'Animal once again has provided invaluable insight. I had presumed that if it had a "bulb" it was for priming (my Honda trimmer calls the bulb a "priming bulb") and assumed that meant "pumping" gas into the carb venturi ... when it apparently means it is pulling gas through the filter and into (and through) the carb - in that way "purging" air from the carb in order for fuel to brought in :doh:

thanks again! That certainly helps splain a lot! And now I will have to reverse the primer/purge thingy I installed on the McCulloch ... as it has a one way valve -- and I am pretty sure I smugly put it on the wrong way!

(once again I am going through life ... pushing on doors marked "Pull")
The older (up to late 1980's/ early 1990's) equipment had actual Primer bulbs exactly as AJP described. You push the bulb and fuel squirts into the venturi. It is easy to flood the units. If you have this system you may or may not have fuel in the bulb.

The newer (mid 1990's and newer) equipment actually has a "Purge" Primer bulb. It works exactly as AJP described. You push the bulb and it sucks the air out or purges it out of the system and replenishes it with fresh charge of fuel. The discharge has nothing connected to it inside the fuel tank. Nothing goes into the venture so you cannot flood the unit. You should have fuel in the bulb when running. If you do not, the check valve in the primer is bad. It is essentially not seating and drawing air in through the discharge port.

Then there is the other style that is a little more complex but works on the same principle as the Purge Primer.
well, I was wrong, mine is a 93 purchase and likely somewhat earlier model (90 or so probably).

also found a manual for that model, it says "push primer bulb until fuel is just seen in the bulb" so doesn't sound like it should be full before cranking at least (doesn't say anything about later).

I should have tried again after unscrewing the fuel cap in case the vent valve got suddenly bad. Didn't think about it.
doesn't make any difference if the cap is unscrewed. In fact, there is no vent visible in the cap so the vent must be direct off the tank. (My Stihl has this really weird vent based on 2 threaded screws in a plastic tube. What's with that odd design?)

cleaned the fuel pick up filter, no difference. Will try without it altogether but don't expect that's the issue.

Cute, the oil pump sits on top of the carb body.

Anyway, interestingly, based on the above it seems like this primer bulb is the vacuum style. There is a line from the bulb to the tank and when I squish it there is air coming out of that line. So it seems like it's just sucking fuel into the carb somehow. Which suggest it should not be full of gas I guess.

well, learned something new -yet again from D- didn't know there are 2 kinds of primer bulb, I had just assumed that thing *pushed* gas into the carb. Goes to show ya how much I know... :)
Last edited:
If you have replaced the fuel lines feeding the carb, try a carb kit and replacing the primer bubble (they are usually sold seperate).

Users who are viewing this thread

Top Bottom