Vintage Stihl FS80 Brush Cutter Carburetor Replacement Mod

[bird dogger]

I’ve been brushing and cleaning up windbreaks and woods lately. With the recent threads on chain/pole saws it dawned on me while using my 40 yr old brush cutter to post this mod I made years ago. Some will find it interesting and someone might even say, “Hey, I can do that to my idled equipment!”

Purchased new in late ‘70s to clear some recreational property that was so thick with brush you couldn’t see past 10 feet in any direction. With a saw blade on the end, the brush and small trees couldn’t fall out of its way fast enough! The old style FS80 cleared anything in its path for a great many years. But after a few carb rebuilds and 20+ yrs of service…..carb parts were no longer available and it sat unused because it wouldn’t run right and you couldn’t tune the carb. Here's my vintage FS80 brush cutter.

And here's the new carb with my mods:

I believe it was first made in Japan and rebadged for Stihl. Or at least the carb was Japanese and parts and carbs became obsolete and Stihl didn’t support that model anymore….at least for carburetors. After it had been sitting unused for a number of years, I needed to get it running again to help with maintenance around the farmstead we now live on. Somewhere along the road of life I had heard of making an adapter block to match a new carburetor to an old piece of equipment. An internet search turned up the closest carb match for under $20 and the project was started.

The new carb closely matched the old for the throat size, mixture adjustments, and for throttle hookup. But the mounting hole positions were off, along with another small port’s location on the carb. The original air filter housing also had to be scrapped. In the pics below you can see the mounting issues along with the misaligned small port just underneath or along side the new carb's throat.



More to follow....

 

[bird dogger]

I had some epoxy resin impregnated laminated maple wood (doesn’t everyone?) to use for a trial run at the adapter block. Holes were drilled for the original mounting screws and threaded holes made for the new carb mounting studs. That took care of the new carb to old FS80 mounting issues.


I drilled a round hole to match the new carbs throat size and reshaped it to closely match the oblong intake port on the motor’s port side. Then measured, located and drilled a small hole at the proper angle to match the other small port on the new carb to the existing port on the motor.


A little extra grinding on the adapter block and repurposing some generic gaskets made it look like it might just work. Fingers crossed!!



Next up: Mounting the new carburetor and making an air filter and throttle hookup.

 

[bird dogger]

With the new adapter block “setup” the carb could now be fitted to the motor. (Sharp eyes might notice that when I took this apart to take these pics that I reinserted the gasket on upside down…..as in the new hole for the small port alignment is in the wrong place. I noticed that before I put it back together but wasn’t going to take things apart again for another pic.)


My junk pile of gadgets provided an aluminum round block that with a little machining turned into a nice filter housing. An aluminum adapter plate served to securely mount both the carb to the motor and the new air filter holder to the fabbed up adapter plate.


The air filter holder is a tight slip fit over the round mounting adapter and is held in place with two screws. You can see the two tapped holes in the round adapter plate in the above pics.

For now, I just put my hand against the air filter to simulate a “choke” plate. Will have to fab one up one day. An easy job to have one flip over the outer opening. But when not in use it would be out in the open by itself and vulnerable to catching and bending. So far the palm of my hand works just fine for now.

Onwards to solving how to reattach the throttle cable and its hookup...... stay tuned.

David

 

[JimmyJazz]

Some might call your efforts hillbilly. I call it genius. Where craft and creativity meet to save a buck. An almost extinct notion today. You have restored my faith in humanity. Thanks.

 

[bird dogger]

Some might call your efforts hillbilly. I call it genius. Where craft and creativity meet to save a buck. An almost extinct notion today. You have restored my faith in humanity. Thanks.

Ha! The "hillbilly" part I agree with! It being "genius" might be a bit of a stretch! But thanks for the compliment and faith, JimmyJazz! We'll soon see if the "end result justifies the means". LOL
best regards,
david

 

[Matt Ellerbee]

Have you sourced a filter yet? UNI makes some that will clamp around that aluminum but on the carb. I have one on my play saw.

 

[bird dogger]

Have you sourced a filter yet? UNI makes some that will clamp around that aluminum but on the carb. I have one on my play saw.
View attachment 61657

Hey Matt! That almost looks like a beer coozie! But yes, my air filter was already done. I had planned to do something like your example. But then I found that piece of aluminum in my junk box that just screamed to be made into a filter housing. I could probably find a little bit finer material to cut into shape for a replacement filter element, though.

What's that gadget clamped to your main handle bar on the saw? I swear we share the same workbench!
David

 

[bird dogger]

Here's the final filter housing/element in place and fastened.


A small aluminum bracket screwed to the adapter block’s top provided a simple way to reattach the throttle cable….complete with the adjustment screw.


A slight twist in the bracket end to line up the "pull/push" stroke and the throttle works perfectly.


I think it's time for the end result. That's coming up next!
David

 

[Matt Ellerbee]

What's that gadget clamped to your main handle bar on the saw? I swear we share the same workbench!
David

That is a tachometer and a 3D printed holder. I was tuning after adding a bigger carb and filter stack.
As for the work bench, I’m sure you and others are the same, I know where everything is!

 

[Creature Meadow]

Have you sourced a filter yet? UNI makes some that will clamp around that aluminum but on the carb. I have one on my play saw.
View attachment 61657

I have nearly the same saw but mine is a 026 PRO, I bought it new around 2000.

 

[Matt Ellerbee]

I have nearly the same saw but mine is a 026 PRO, I bought it new around 2000.

They are great saws. I have another that has been ported, it hates wood.

 

[bird dogger]

Time to give the new carb on the 40 yr. old brush cutter a go. With throttle now hooked up and everything back together it’s time for a trial run. Prime the bulb. Two pulls with the “palm” choke. Put the plug wire back on.

Not sure what to expect, whether it’s nothing…..fire….explosion…..or success. Turned the switch to “run” and gave it a pull. It fired right up! A quick warm up and carb tune had that old machine running like new! WooHoo!! It’s been running great ever since. If the new carb can’t be rebuilt and finally dies there’ll be another replacement carb and adapter block made. The carb is by far the weakest link on this rugged machine. I found some new circular blades online that fit and it zips through heavy brush just like new again.

I thought I’d have to re make the trial adapter block out of aluminum. But the laminated maple block is still holding up fine. Until it starts to fail it’ll stay in place. If anyone has trouble finding carb parts for your old equipment, refitting a new carb via an adapter block is actually a viable solution. It would’ve been a shame to scrap that brush cutter just for want of a new cheap carb. The old style FS80 machines are almost legendary! And a new comparable Stihl is well over $600.

The other thing I’ve noticed: The new carb has the old FS80 running as strong as new. But it seems to be much more of a workout now than when I was in my late twenties. Might need to re calculate the weight/balance and make some more changes. Yes, that must be it!

 

[bird dogger]

Here's a couple of pics of the brush cutter blades. One pic shows the original with the new. And a close up of just the new blade. Those new thin kerf carbide tipped blades made the old blade seem like a butter knife...even when it was new!



and here's a pic of my chainsaw lineup: In the background is the Stihl 026. Up front is my 1970's Husqvarna 61. The Husqvarna is more of a beast compared to the Stihl and just will not die. It'll probably do me in one day instead!

 

[Matt Ellerbee]

Here's a couple of pics of the brush cutter blades. One pic shows the original with the new. And a close up of just the new blade. Those new thin kerf carbide tipped blades made the old blade seem like a butter knife...even when it was new!

View attachment 61715 View attachment 61716

and here's a pic of my chainsaw lineup: In the background is the Stihl 026. Up front is my 1970's Husqvarna 61. The Husqvarna is more of a beast compared to the Stihl and just will not die. It'll probably do me in one day instead!
View attachment 61717

I have that same blade on my weedeater, it does a fine job!

49cc vs. 60cc on the saws.

 

[RCW]

‘dogger - -

I have an old FS86 cutter I bought new1991. The thing has done a lot of of trimming and brush cutting in it’s day.

Several years ago, got “hit” with ethanol gas problems. My own doing. Have tinkered off-and-on for a few years to get it resurrected.

I put a knock-off carburetor on it couple years ago. Better, but still a shadow of what it used to be.

I replaced it with a 4-cycle FS130 a few years ago, but still like the 2-cycle power better.

The old cutter just hangs there. Your thread has inspired me to to re-visit the old timer again.

 

[bird dogger]

‘dogger - -

I have an old FS86 cutter I bought new1991. The thing has done a lot of of trimming and brush cutting in it’s day.

Several years ago, got “hit” with ethanol gas problems. My own doing. Have tinkered off-and-on for a few years to get it resurrected.

I put a knock-off carburetor on it couple years ago. Better, but still a shadow of what it used to be.

I replaced it with a 4-cycle FS130 a few years ago, but still like the 2-cycle power better.

The old cutter just hangs there. Your thread has inspired me to to re-visit the old timer again.

View attachment 61749

RCW, That would be great to get your FS86 running like a top dog again! Either machine would be a nice backup for the other as well. Put it on your to do list as a nice rainy day project or even a winter project. They're too nice of a tool to be a wall hanger when a cheap new carb can have them back in service with a little ingenuity thrown in. Keep me posted! (And glad my post played a small part in your renewed plans!!)
David