Small parts challenge

[cathead]

On my last excursion to the scrap yard I brought home a Stihl FS90 AVE brush cutter. After looking it over, I could see that it was missing a few carb parts
and the blade end didn't turn. After loosening up the cutter head it was on to the carburetor. I could see two ways of repairing this thing to functionality.
One way would be to affix a different carburetor to the engine or make the missing parts for the existing carb.

Before making parts, I decided to see if it had spark and it did. Then I instilled a little gas mixture into the carburetor and gave it a pull. It fired right off!

The parts are metric as well as the threads on the carburetor so somewhat challenging. I machined up some rough approximations and threaded the
inside of the top cover using a 12x1mm tap. So with that accomplished, I made a 12mm cylinder and grooved it so it would fit in the hole. After finishing that,
I threaded a short piece of brass and tapered it to fit the metering hole. This is all trial and error at this point as I have no dimensions to work with, just
using common sense and trial and error. The 12mm cylinder could be fashioned in brass but the aluminum one will be good enough to see if it will run.
I'm at the point where it is assembled and ready to see if it will run at idle with a little experimenting likely. Then I need to come up with throttle cable
parts which I should be able to come up with. It's probably 30 years old or better so likely hard to find parts.














This is the power head with the parts installed ready to see what happens next.



Below are the small parts ready to install. I left the brass pin a little fat in case I need to adjust the mixture.










This style of carb does not use the new style units with the bulb to press on. They don't hold up well anyway so if I can make this work, it will be a better
setup. The button that says push on it activates the diaphragm and also opens a little valve for easy starting.



It's another snow day and I had to shovel my way to the shop. So, later today when it quits snowing, I will have to move some snow with the skid steer,
an old 12C Hydra-Mac with a 2 cylinder 2ab1 Isuzu engine. I it has 9500 hours on it but still does every job I ask it to.

Y'all have a good day!

 

[markba633csi]

Hobby Machinist to the rescue! Looks great
Needle type carbs often have some way of raising or lowering the needle for different altitudes
I'm guessing you dispensed with that and are just going to take material off until it runs ok?

 

[markba633csi]

Did you get it going Cat?

 

[cathead]

Thanks markba633csi !

It has an idle speed and a full throttle mixture so will just have to adjust it until it purrs(hopefully).

Did you get it going Cat?

I have it all together but at present it won't start. I think it is time to disassemble the carburetor and
see exactly what is going on. It fires right off with a little gas, then quits so it is definitely not
getting fuel. I will post more when I have something to report.

 

[matthewsx]

Just a couple of things. First, many small engines you can get replacement carbs pretty cheap so check that if you like.
Also, with two cycles it’s not uncommon to have a leaky crank seal. This can cause all sorts of strange issues and run you around in circles. Not too many good ways of diagnosing.

Good Luck,

john

 

[cathead]

Just a couple of things. First, many small engines you can get replacement carbs pretty cheap so check that if you like.
Also, with two cycles it’s not uncommon to have a leaky crank seal. This can cause all sorts of strange issues and run you around in circles. Not too many good ways of diagnosing.

Good Luck,

john

I have looked at those aftermarket carbs on E-Bay and the design has changed significantly since my unit was built.
The pressure/vacuum inlet to the carb is in a different place for one thing. The other thing is that all those new carbs
use the plastic bulb thing that requires another inlet to the fuel tank for fuel. I do have an old Walbro carb that I could
possibly use and that has no bulb thing but the throttle access would be problematic. I'm aware of the crank seal
problem associated with small two stroke engines and actually there is a good way to diagnose crank case leaks.
One needs a low pressure gauge and a way to pressurize the crankcase to determine if it leaks off pressure. I like the
older Stihl chain saws and equipment and have quite a collection of them. My favorite of all is the 028 Super Stihl
and for bigger wood I have four Stihl 038 saws plus a bunch of other Stihl saws including a 041 Farm boss that still
gets used occasionally. This morning I will be looking into the carburetor innards and see what the problem might be.

I'm not a small engine guy, just a guy with a lot of small engines.....Oh Yeah, and some BIG engines too........

 

[cathead]

Yeeeee Haw!!!! It runs! After an hour of tinkering(something I am good at) it runs pretty well. There
were several things that I found that made the difference. In taking apart the carburetor this morning,
I found the diaphragm was pretty much stiff as a board and as luck would have it I had several spares
from some chain saw repair kits I had laying around. The other thing was that there actually is supposed
to be one of those plastic primer things on there, something I hadn't noticed before. Gas was dripping
out the tiny hole beneath the bracket that is supposed to hold the bulb. I plugged the hole with a small
piece of rubber as I didn't have a primer bulb that small. After that it fired right up and even idled somewhat.
It was running rich so I turned in the main jet screw and it straightened right out and ran great. It was getting
blue in the shop so I decided to finish up the project later today. All that is left would be to rig up a
throttle control of some sort and add a kill switch, both of which will be relatively easy. I'll be gloating
all day over this but one needs a few successes once in a while to keep one going in these crazy times.
Thanks for reading along everyone.