Trying to repair a Ryobi RY253SS 2-cycle trimmer. Starts up fine, runs for ~10 seconds and dies. It dies regardless of whether full/no-choke, full/no-throttle. Restarts right away, but exhibits same behavior (which leads me to think not an electrical issue). Here is what I've done:
- New 50:1 fuel in tank
- Dry compression: 80 psi
- Wet compression after dripping some oil into cylinder: 120 psi (so some leakage around rings? would this explain symptoms?)
- Cleaned passages in carb, gaskets/diaphragms seem in reasonable shape
- Intake and exhaust gaskets look fine
- Applied 10 psi pressure to carburetor fuel intake pipe and it holds steady
- Applied vacuum to fuel inlet tube going to tank, and it sucks up fuel, so no obvious obstructions in tube or fuel-filter
- Cleaned some carbon buildup from exhaust port
- Pressure tested crankcase: holds steady at 10 psi
- Vacuum tested crankcase: holds steady at 15 in Hg, returns to 15 when I rotate engine
Only adjustment on carb is throttle idle screw. Might be possible to adjust the needle on the barrel-valve of the carb, but looks like a special tool may be needed.
Would appreciate any ideas of what to try next.
Hi. As a specialist in small engine repair, everyone will be saying to check this and that. The first thing that you have to do with all 2 strokes is remove the exhaust and look at the piston. If you see bar codes (I call them) known as score marks, you are wasting your time, the engine is finished. If no score marks, make a hook out of a piece of wire and pull the fuel filter out of the tank, remove it from the line and you should be able to blow through it. Some filters have a water absorbing filter and it should be changed. If you can blow through it, then the problem is in the carb. There is a 10micron screen and it could be blocked with water, dirt or a micro film of old fuel across the screen.
As it runs, it’s not a spark issue at this point, unless the flywheel key has sheered.
Compression testers don’t work on 2 strokes, as you can have compression, but if you have score marks, the pressure collapses in the crankcase and the fuel sits on the bottom. The fuel and oil mix are no longer in an atomised state.
The thing that everyone has to understand is that from the fuel cap to the spark plug is that it’s a pressure vessel. A carb only controls the fuel going into the engine by movement of the piston.
Vacuum and pressure tests only confirm that there is no air leaks.
If all of your fuel related issues are confirmed not to be the problem, I would look at the flywheel next. A fella with a stihl some time ago had the same issue as you. He put a new carb on, fuel lines and filter and still the same thing. He contacted me and I told him “well I know it’s not a fuel issue and there were no score marks “
I told him to look at the flywheel and the key sheered and the flywheel rotated just enough for it to start, but quickly died. He put on a new flywheel and never looked back.
