stihl 025 wont run

[W33kendWarrior2017]

I recently was given an old stihl 025 from a friend. When I got it, it had a piston that was in bad shape, and the crank bearings and seals were shot. After replacing the piston, seals, bearings, cylinder, impulse line, and gas line, the saw was able to start, but not idle. I thought maybe it could be an air leak, because I thought it sounded like it was running lean. But when I hooked up a compression tester on the spark plug, I was able to get it up to 145psi after a few pulls. So I decided to look at the carb. I took it apart and sprayed down with carb cleaner, although it didn't appear to be too dirty. After reassembling, the chainsaw will no longer start. I can spray starting fluid into the carb and it will crank a few times, but won't stay running. I tried resetting the adjustment screws by turning them all the way in and then backed them out 1 turn, but that made no difference. By the way this is my first time doing any kind of work on a chainsaw, besides changing out an air filter. Does anyone have any ideas on what my next move should be? I don't know if I should have even touched the carburetor.

 

[cuinrearview]

Buy a carburetor re-build kit, and when you tear into it again make sure that before you re-assemble all of the carb cleaner has dried. If the diaphragms weren't stiff before they likely became that way from the carb cleaner.

You could also buy a cheap replacement carb on eBay, but those are hit and miss and re-building is so easy.

Oh! And also, don't use starting fluid on a two stroke. It is a fast way to ruin all of the new parts you just installed. There's no oil in starting fluid.

 

[SteveSr]

You may still have an air leak but won't know for sure unless you test it with the proper equipment. What sealant did you use on the clamshell when you put it back together? These can have a significant gap that requires a curing sealant like Dirko. There is a reason that Stihl uses this type of sealant.

 

[SoopermanLuva]

Have you tested for spark? Unscrew the plug, connect it, put it against the cylinder. Crank a few times with the ignition off just to get excess fuel out of the cylinder. Since the saw doesn't run, there's probably some in there. Switch on the ignition and crank the saw with the plug against the cylinder. Should give a frequent and consictent spark. Otherwise there may be a problem with the fuel supply. Check fuel line and impulse lines. The latter is very important and if there is a big leak there the saw will not start. Also, what do the membranes in the carb look like? Stiff, deformed? The compensator membrane, the one on the bottom with a metal plate in it needs to be flexible and the one on the other side acts as part of the fuel pump system and as a valve so it needs to be flat, no bent reeds. If the spark plug has fuel on it after you try starting the saw, that will tell you that fuel is at least getting into the cylinder. The compression measurement doesn't help as far as telling if you have leaks or not. The metering occurs on the top of the cylinder and there is literally nothing that can leak. Also check the intake boot, it has to be on the cylinder really tight, as far as it can go. And it's an easy one to miss after a full disassembly. And yeah, the cylinder on these needs a fair amount of sealant to keep it from leaking, but that's easy enough on a clamshell.
The saw needs basically just 4 things to run:
1. Fuel
2. Air
3. Airtight seal in the crankcase
4. Spark for combustion to happen
It would help to have a quick rundown of the stuff you did NOT replace.
Oh yeah and does the choke acually work? Does the lever actually close off the airflow into the carb?

 

[W33kendWarrior2017]

Yes it does seem to have good spark. The sealant I used on the crank case was Permatex red rtv, which is what my Stihl dealer suggested. I did not replace the intake boot on the engine because I thought it seemed like it was in good shape, and it looks like it is seated all the way against the cylinder. I had replaced the impulse line, and it is connected. I believe it to be a fuel issue. The diaphragm in the carb does seem a little stiff, I think I will start there. Thank you for all of the good advise. I will get the parts either tomorrow or Tuesday.

 

[HarleyT]

Get a new OEM carb from your Stihl dealer, they are only @ $22


1123 120 0603 CARBURETOR (MS250) 21.99


1123 120 0604 CARBURETOR (MS210) 21.99

1123 120 0605 CARBURETOR 21.99

 

[SoopermanLuva]

Yes it does seem to have good spark. The sealant I used on the crank case was Permatex red rtv, which is what my Stihl dealer suggested. I did not replace the intake boot on the engine because I thought it seemed like it was in good shape, and it looks like it is seated all the way against the cylinder. I had replaced the impulse line, and it is connected. I believe it to be a fuel issue. The diaphragm in the carb does seem a little stiff, I think I will start there. Thank you for all of the good advise. I will get the parts either tomorrow or Tuesday.

Does the choke work though? Is the lever connected properly? Even with a bad carb the saw should at least show some signs of life. You can try unscrewing the H and L screws and blast some carb cleaner into the holes. I'd say there's a fair chance the problem is carb related then. The question is whether or not it can be fixed. Looking at the price of a new OEM one, I wouldn't bother rebuilding. I would just try to troubleshoot and clean first. The question is also: does the saw starve or flood? What does the spark plug look like after a starting attempt? Any fuel in the cylinder?