HELP! RPM's cycling up and down...

[coveforest]

When at full throttle my Stihl 028 will suddenly drop in rpm almost stalling....if I keep the throttle full on, it will recover, max out then drop again. The time it takes to go through this cycle is about 5 seconds. I just replaced the fuel line and fuel filter, but that didn't help.

Any advice would greatly appreciated!

Chris

 

[Metals406]

Sounds like maybe the diaphragm in the carb... Carb kit's 10 bucks or so. Rebuild and see if it fixs the problem.

 

[Metals406]

Also, you could have knocked something loose while replacing line, check that all lines aren't rigid, cracked, and are fit up properly.

 

[Jacob J.]

When at full throttle my Stihl 028 will suddenly drop in rpm almost stalling....if I keep the throttle full on, it will recover, max out then drop again. The time it takes to go through this cycle is about 5 seconds. I just replaced the fuel line and fuel filter, but that didn't help.

Any advice would greatly appreciated!

Chris

Is yours old enough to have the separate ignition "trigger" unit? Sometimes they act like that when they're on the way out...

 

[coveforest]

Thanks for the ideas. I did notice that the vent tube was rotten at the base, so I removed it and rammed a piece of thin wire down into the tank to make sure it wasn't plugged. Will order a new tube tomorrow, but I started it and still the same problem.

It wasn't doing this before I replaced the fuel line, so I'm thinking I might've bumped something... I replaced the line because it was cutting out suddenly like it was out of gas even though it had half a tank. Someone said that my fuel line probably had a hole in it. For what it's worth...

 

[Jacob J.]

Sounds like maybe the diaphragm in the carb... Carb kit's 10 bucks or so. Rebuild and see if it fixs the problem.

So did you rebuild your carb or no?

 

[coveforest]

No, I did not rebuild the carb. Sounds like I may have to.

 

[Lakeside53]

check you inlet screen as part of the process.

 

[coveforest]

New fuel line and fuel filter (pickup unit). Is that what you mean by inlet screen?

 

[Jacob J.]

I think what he means is the inlet area on the fuel pump side of your carburetor...After you remove the fuel pump cover and diaphragm, you'll see an opening with a fine mesh screen, those would get clogged on the older saws with the foam insert style fuel filters as the foam would deteriorate.

 

[coveforest]

Problem solved!

It was the vent tube. Must've bumped it when installing the new fuel line and pickup body...it was pretty rotten. $5 part. Saw is good as new!!!

Thanks for everyone that chimed in with suggestions!

 

[PaulDaisy]

You mean the fuel tank vent?

 

[JONSEREDFAN6069]

check the crank seal's, had to replace em on my 026, same exact symptom's as your saw. i went through all the carb and fuel suply line checked vent to see if it was plugged took off carb and cleaned and retested same thing. sprayed some brake clean when idling on recoil side and would rev up by itself then go back down, asked and was told crank seals. worth a look.

SORRY SHOULDA READ ALL THE WAY THROUGH

 

[coveforest]

PaulDaisy - Yes, fuel tank vent. There is a very small tube running from the tank to the outside. There are 2 brass inserts inside this tube - I guess they serve to prevent junk from plugging the tube.