Help with a Stihl 4-Mix engine

[NewToStihl]

Okay, not a chainsaw, but with the knowledge in this section of the site I figured it was the place to be.

I have a BR550 blower that's acting funny. It starts easily, idles like a dream, and runs perfectly to about 3/4 throttle. At that point it sounds to me (based on my BR600) like it's running right at full throttle and purring like a pussycat. BUT, if I pull that last 1/4 or so it just falls on it's ass and goes back down to very near an idle state. It's doesn't die out, but gets very close. It's honestly nearly like an idle condition, but just a hair rougher.

The specs I found only show an idle RPM, so I haven't yet bothered putting a tach on it at the max RPM or compared actual numbers to my BR600.

Anyway, here's where I'm at; I disassembled & cleaned the carb, adjusted the metering lever (needed a slight raise), and put the H&L screws back in at factory settings which is where they were at when I started. The air filter is clean. The fuel filter is new. The spark plug is good & has a correct gap. None of the above made even the slightest change. The only thing left that I know to check is the valve gap, but it's been my experience that an out-of-adjustment valve would result in a poorly running machine in general, not just at the last leg of of high throttle.

Anybody else have experience with this? Any opinions on what I should try/check next?

 

[Modifiedmark]

I have a BR550 that just out of the blue started acting all messed up. Long story short after going through the carb a couple times to no avail, I just bought a new carb for it and its been great every since.

 

[NewToStihl]

Yeah, I figured that was a possibility. I was hoping to avoid a new carb...

 

[cutforfun]

My 550 started running funny last week, it would idle and I could slowly give it throttle and gain rpm. Then it would fall on its face back down to an idle, it was a crack in the fuel line going from the tank to the carb, stihl does not sell the line it is just bulk fuel line.

 

[KG441c]

Possible carb rebuild or a new one. Weak ignition coil?

 

[roundhead]

if it is a 4 mix it may need to have the valves adjusted. sounds more like a carb or ignition as others have suggested

 

[Rookie1]

Ive heard if the cam shaft starts wearing it will idle fine but run rough at higher rpm. I say carb or fuel lines. Curious to see what problem is.

 

[KG441c]

If the valves are too loose it would loose compression and cause it to wanna die. May be carbon buildup under the valve seats or they may just have became too loose. Adjust both valves to .010 but to do this u will need valve tool from Stihl or take a regular feeler gauge and grind some off both sides until it will fit into valve contact area. Be sure to take burrs off the edges after u grind though. Is it hard to start? Check compression and
adjust valves. If still no go try a carb kit

 

[KG441c]

Be careful adjusting the valves as if you accidentally loosen them its possible to loose the pushrod into the cam follower area and get stuck, (magnet may retrieve) , in the bottom end and cause damage. To get the pushrod out if dropped in u would need to pull harness plate and fan off to access cam area cover on back of blower

 

[NewToStihl]

Thank you for the suggestions all.

I've adjusted valves on many 4-Mix engines and do have the specially shaped Stihl feeler gauge. I'll eventually try to adjust them, it's just not acting like that's the problem; compression is where it should be, and it doesn't run rough at all up through a high RPM - smoothe as can be - it just instantly falls on it's face with the last bit of trigger.

I suppose the coil could be suspect, but would a bad coil allow it to run well up through a pretty high RPM and hold steady, but reduce down to an idle-like speed immediately at full throttle? I have an old coil off of a BR600 - since they use the same one I would swap that out to see.

I'll check the fuel lines more closely this evening and perhaps do a pressure check.

Thanks.

 

[KG441c]

resistance of coil could be bad but swapping out would sure be a good start at process of elimination

 

[KG441c]

Im also with Rookie 1 that the cam follower could be worn and causing irratic opening or closing of valves?Sounds like its running out of fuel. Checked the tank vent??

 

[KG441c]

 

[blsnelling]

Adjust the valves and eliminate that variable. Otherwise, you're guessing. It only takes a few minutes.

Remove the limiters from the carb and see if it's a tuning issue.

 

[beermeatguns]

If it's a choppy running it's usually I've had several that seemed like the rev limiter was kicking in at lower rpm
than it was suppose to and that was a coil issue.

If it's a bogging down issue replace richen the high up to 2-3 turns and see if that works had some that needed a ton of fuel on the
high side. If that doesn't work replace the carb. they're around $40

I only had to adjust valves usually on high hour commercially used, it's a low hour machine that's unlikely
it needs to be done.

The BR550/600 have been really reliable and only usually see these couple things wrong with them.

 

[lone wolf]

Thank you for the suggestions all.

I've adjusted valves on many 4-Mix engines and do have the specially shaped Stihl feeler gauge. I'll eventually try to adjust them, it's just not acting like that's the problem; compression is where it should be, and it doesn't run rough at all up through a high RPM - smoothe as can be - it just instantly falls on it's face with the last bit of trigger.

I suppose the coil could be suspect, but would a bad coil allow it to run well up through a pretty high RPM and hold steady, but reduce down to an idle-like speed immediately at full throttle? I have an old coil off of a BR600 - since they use the same one I would swap that out to see.

I'll check the fuel lines more closely this evening and perhaps do a pressure check.

Thanks.

One thing you could look at is rust on the coil pickup. I had that happen on time.

 

[swatbwana]

Adjust the valves.
My br600 mag acted the same way put a cap or two of seafoam in the tank and adjust valves to spec should also get teh decarbonizer from the stihl dealer and follow the directions.


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[NewToStihl]

Well, I finally had a chance to work on the BR550 this evening. I was hoping to be able to report back on what the cause of the issue was. The good news is that it's running like new again . . . the bad news is that I have no idea what was causing the issue. I hate it when this happens, especially when it's a piece of equipment I'm planning to sell...

The carb on this blower has no limiters and I first verified that I had properly set them at factory specs after the cleaning - they were spot on. Ran it again, no improvement. Second I swapped the coil out with a known good one. Ran it again, no improvement. Then I checked the fuel line - not compromised. Then I checked/adjusted the valves. Both were just a hair tight - arguable whether they needed adjustment at all, but the way I was taught was so that the feeler gauge has a similar resistance to if you were to push a pin through a piece of paper. Ran it again, no improvement. Just for the hell of it I checked the gap on the plug. It was a tad on the heavy side, so I closed it down to the .020. Ran it again, no change.

Then, due to the suggestion from beermeatguns above I decided to play with the carb settings. I leaned the H about 1/2 turn and YAHTZEE! Revved right up through the full trigger pull and held solid. Then I held the trigger and and slowly richened the H back up to see if I could re-create the problem. Nope. I backed it out another full turn with virtually no change in operation. WTF?!?! So, I returned it to the factory H spec where it had been - 3.5 turns out - and it proceeded to run just like it should. I'm only grasping at straws here, but all I can figure is that there was perhaps a slight blockage in the H jet that I didn't get when cleaning the carb and messing around with it just happened to have cleared it out? Thoughts?

Thanks for all of the suggestions.