Bogging fix

[dadstillsurfs]

Full disclaimer: I found out about this fix on YouTube, I didn't figure it out myself.
My Stihl FS-55 string trimmer had not been used much in the last six months, but we needed it to "brush in" our new waterfowl blind. I put all the Service Pack parts in it, plugs, air filter, gas filter.... still bogging like crazy when I pulled the trigger and wouldn't reach it's top end at all.
I tried a technique from Chickanic on YouTube and it lives and breathes fire again.
Go through the normal cold start, ignition on, choke on, pump the primer bulb, (fuel on if that's an option). Bump the pull cord a couple times to prime some fuel into the carburetor and then start like normal at Wide Open Throttle. Once it starts and runs, warm it up for about a minute. Then go back to WOT and actuate your choke from full choke to zero choke until the diaphragms get wetted well and operate normally. It may take a minute, it may take five minutes or more.
It worked for my little Stihl, I hope it works on your neglected small engines.
If this works for you, check this young lady's channel out. She seems knowledgeable and is attractive to boot.
Cheers

Chickanic

When all else fails and you are in need of help, ask a Chickanic! Bringing you insightful small engine tutorials, product reviews and little humor to brighten your day. (A little about me) My name is Bre. I took two years of small engine repair at the local college. When I left school, I fell...

youtube.com

 

[dadstillsurfs]

Oops I linked you to her home page, this is the video of Chickanic fixing a trimmer with dry carburetor diaphragm.

 

[EFSM]

There is no fix that I know of for stiff diaphragms other replacement, although the technique of choking can help the carburetor to pass dirt.

 

[hacskaroly]

Once it starts and runs, warm it up for about a minute.

Welcome to the forum!! I have watched a few of her videos and found helpful information in them.

As for the warming up...my opinion is that is a good idea, not only for helping the carb diaphragms, but also to give the piston and cylinder a chance to warm up together. I worked on a MS 261 the other day that a mechanic stated the unit had been straight gassed (scorring on piston and cylinder (at least through the muffler and spark plug holes). The customer stated that they used fresh 2-stroke fuel. Talking with the customer they stated that when they cut wood which is mostly cutting enough of a tree over a road to clear it, the saw is started, immediately starts cutting and then shut down and put in the back of the truck. This is the cycle of use. Once I pulled the cylinder found that the scorring was localized to the exhaust side of the piston and transfer on the exhaust side of the cylinder. From a cold start to immediate work, the exhaust side of the piston heated up faster than the cylinder causing it to expand and friction burn against the cylinder until the rings embedded, scorring occurred and saw lost compression.

I find that it works best to start a saw and let it idle for at least a minute before starting work and when you are done cutting, let the saw idle for about a minute before shutting it off to allow it to start cooling down (air passing over the cylinder from the flywheel and cooler fuel entering the cylinder) and then shut it off.

As stated above by @EFSM replacement of stiff diaphragms is the best bet, some people use ATF or WD-40 to try to eek out some life, it may get you by in a pinch, but best bet is to replace them. If you use pump gas, when the saw is not in use for an extended period of time, drain the fuel and run dry, though if you use can fuel (Stihl, Husqvarna, Harvest King, VP...etc.) then you should be fine leaving that in.

 

[dadstillsurfs]

Thanks for your voice of experience re the "diaphragm fix". Don't shoot me, I'm just the messenger, and it seemed to get my trimmer up and running. I'll update y'all next week how it functions.
Via Con Dios

 

[EFSM]

I wasn't trying to take any pot shots. Do what works for you, and welcome to the forum!

 

[hacskaroly]

Don't shoot me, I'm just the messenger, and it seemed to get my trimmer up and running.

There wasn't any intention of saying you did it wrong...this is all part of the learning process we go through, we see something and try something and sometimes it works and sometimes it doesn't. Hopefully your trimmer keeps going for a good while longer, but if it doesn't then at least you have a good idea that a carb kit should be able to get it back up and running relatively cheaply (as opposed to buying a carburetor or new weedeater!).

 

[EFSM]

Do a search on here about replacing the accelerator plunger o-ring as well. Those Zama carburetors need them replaced occasionally or they will bog and never receive full fuel at WOT.

 

[Woodwackr]

So, not a plugged muffler screen?

 

[Sloughfoot]

Thanks for your voice of experience re the "diaphragm fix". Don't shoot me, I'm just the messenger, and it seemed to get my trimmer up and running. I'll update y'all next week how it functions.
Via Con Dios

Every string trimmer I've owned, 2 stroke and 4 has to warm up a long time before it'll run right. Never knew why, just did it. The times I wasn't patient had to kill it and start over in a while. Makes sense that the diaphragms were dry and pulling the choke in and out would wet them. Learned something today. Maybe these little stringer motors need to be choked a little longer to get enough fuel passed to wet the diaphragms.

 

[hacskaroly]

Makes sense that the diaphragms were dry and pulling the choke in and out would wet them.

I don't understand how actuating the choke wets the diaphragms, doesn't it just restrict air flow when engaged and pass air when disengaged, hence pulling the choke on a running machine should kill it? This is a legitimate question from me, carbs are my weak point on this equipment and I am still learning about them.

 

[Sloughfoot]

I don't understand how actuating the choke wets the diaphragms, doesn't it just restrict air flow when engaged and pass air when disengaged, hence pulling the choke on a running machine should kill it? This is a legitimate question from me, carbs are my weak point on this equipment and I am still learning about them.

You've got a point. Maybe there's some build up in the carb in the split second the pulse weakens from RPMs slowing. I'm guessing.

 

[EFSM]

Every string trimmer I've owned, 2 stroke and 4 has to warm up a long time before it'll run right.

Most 2-strokes need warmed up for a few seconds, but 10-15 seconds is usually enough, unless the carburetor is adjusted too lean, which was probably your struggle.

Maybe these little stringer motors need to be choked a little longer to get enough fuel passed to wet the diaphragms.

The diaphragms can't get any wetter than they get when priming.

I don't understand how actuating the choke wets the diaphragms

It doesn't. However it can budge a stiff diaphragm and depress the metering lever enough to give more fuel flow.

 

[hacskaroly]

It doesn't. However it can budge a stiff diaphragm and depress the metering lever enough to give more fuel flow.

That makes sense, I guess it creates enough pressure when you choke-unchoke-choke-unchoke-choke-unchoke that it will help budge it.

 

[huskihl]

That makes sense, I guess it creates enough pressure when you choke-unchoke-choke-unchoke-choke-unchoke that it will help budge it.

When you close the choke, the engine is still pulsing vacuum every time the intake opens up. It can’t pull much air so it pulls fuel through the high-speed nozzle and draws down on the metering diaphragm