Hot Topics: Lawn Mower Primer Bulb Not Filling

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Original Thread: Primer Bulb Doesn't Fill

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suobs - Member

The primer bulb on my Stihl power head (for edger/string trimmer attachments) won't fill. Of course, the engine won't start as a result. What to check first?

badeyeben - Member

Check to see if the bulb is cracked or the fuel lines. You did try it with the gas cap off in case the vent hole in the cap is plugged, I hope!

suobs - Thread Starter

The bulb's not cracked, and yes I tried with the cap off. With the cap on I get bubbles in the fuel when I pump the primer.

miketdjr - Member

Assuming you have a KM 55 or 90 (something along those lines), checking that the fuel lines and vent lines are in good shape and, of course the primer bulb isn't cracked, is a good start. Most newer stihl's use a vent line rather than a vented fuel cap. Next, is this the first time the machine has been run since last season? If so, my suspicion would be the check ball in the primer base. Primer bulb should be on the carb body.

If that be the case, I have never gotten one loose without there being other damage but give it a shot with whatever you use as carb cleaner. Also, if that is stuck the carb is probably plugged also. If that doesn't work, replace the primer base or the complete carb, some of the stihl carbs retail for less the 40 bucks, and that's a pretty easy repair.

suobs - Thread Starter

Yes, KM 55; yes, first start of the year. I can't find diagrams anywhere - where is the "check ball in the primer base"? In the gas tank? Are you saying to spray carb cleaner into the gas tank??

cheese - Forum Topic Moderator

I'd start by taking the carb off, open it up, and clean it out with carburetor cleaner. This may be all that is necessary.

suobs - Thread Starter

I guess that's the next step. I tried squirting carb cleaner into the carb. For awhile, the bulb sucked up a bit of cleaner, then it slurped up some black gunk and clogged and won't reinflate now. That can't be good.

I'm not real comfortable taking something apart without instructions or at least a diagram. For example, the 2 nuts in plastic depression holding the air cleaner assembly to the carb - how to find a socket that thin? I must be missing something.

cheese - Forum Topic Moderator

a 1/4" drive 8mm socket should fit right in.

suobs - Thread Starter

Thanks, Cheese, I'll stop making excuses and get on it tomorrow PM (ya right)!

suobs - Thread Starter

OK, I have the carb apart and soaking. The bulb is in good shape. I still don't know what and where the " check ball in the primer base" is. Or when I should decide to buy a new carb.

suobs - Thread Starter

I said the bulb is OK, but when I seat it on top and compress, it doesn't reinflate. So it seems like one of the holes in the top of the carb that the bulb sucks through (the one with the semi-transparent diaphragm on it, maybe?) is clogged or something.

suobs - Thread Starter

OK, I found a "repair manual" online, but it's kind of vague. When I remove the bulb by unscrewing the "square end cover" that holds it on, there's a plastic "flange" under it between the bulb and the carb body. I assume the bulb has to suck fuel through one of the two tiny holes in the flange and that one of them connects to the brass fuel tube sticking out of the flange.

When I seat the fuel bulb in the flange and squeeze, it does not reinflate, so this seems to explain my original problem. When I remove it again, it does inflate. So I assume that the hole it is sucking through (or the brass fuel tube) is clogged. I've tried spraying carb cleaner directly into the hole that has a metal ring around it, and into the brass tube, but no luck.

I guess the question now is: if I can't get it unclogged, whether I should try to buy the flange alone and how I find the one I need - since Stihl seems to make finding parts as difficult as possible - can't find a parts diagram.

suobs - Thread Starter

Final report is now the bulb won't even compress when it's seated on the primer flange. So the fuel line in the flange is seriously clogged. Any ideas? If not, does anyone know how to find a part for Stihl? Because I'm not even finding the whole carb for KM 55 (Zama carb with brass fuel lines, no part number that I can find).

cheese - Forum Topic Moderator

You might want to just check on the price of a new carb. Some of these come so cheap that it's not even worth trying to fix them.

suobs - Thread Starter

Well the problem is finding a carb since there's no number on it. I have the bulb reinflating and there's nothing plugged in the 2 fuel lines (fuel flows out when I turn the fuel tank upside down). But I am still unable to suck fuel into the bulb although it reinflates now. Just a squeaking sound.

Is it possible the problem is the fuel pickup body, which has never been replaced (about 3 yrs old)?

cheese - Forum Topic Moderator

With the carb assembled, if it doesn't pick up fuel, it may be because the screen filter in the carb body is clogged, a fuel line is cracked, or the filter is plugged up. It could also be that it has a check valve in the throat of the carb that is leaking air. There is (should be anyway, I don't think I ever saw one that was unidentified) a number stamped into the side of the carb. It's small.

geogrubb - Member

I think the "black gunk" it slurped up is the check valve that has been destroyed by whatever, save some time and money just get a new carb, they are cheaper than they used to be. Have a good one. Geo

geogrubb - Member

If it has a C1Q-S71 or S66, which it should, the flange should be, 4228-120-2200. Have a good one. Geo

suobs - Thread Starter

Other than the word Zama, some tiny numbers off by themselves, and some illegible ink markings, "C1Q" is the only marking on the carb. I'll see if the dealer can ID it.

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