Half throttle/idle issues

[fields_mj]

One of my 036 saws recently developed a starting issue. During a recent cutting session, I stopped to refuel, and the saw wouldn't restart. I was short on time, so I just walked in my shop and picked up another saw. Over the course of several days I tried to start the saw, tweaked the L jet every way I could think of, and nothing. The saw fires one time with the choke on then nothing after. It was a cheap aftermarket carb, so I replaced it with a tillotson that I had picked up. Now the saw fires like it always did. After removing the choke, but leaving the throttle locked, it starts well but then runs erratically with the RPM increasing (a lot) and then decreasing (a lot) and then increasing again.. If I unlock the throttle and try to let it idle, it dies. I can't get it to stay running at idle to see if the RPM is fluctuating at idle or not. If I stay on the throttle, I can run it wide open, but the tachometer says that the RPM is bouncing around a little more than I would normally expect.

Am I correct in saying that the variations in rpm indicate a vacuum leak, and the fact that the variation is extreme at 1/2 throttle means it's a pretty big leak? If this is the case, since this came about rather suddenly, I'm hopeful that its the intake boot and not a crank case seal but wanted to get some opinions from the "experts" before I start taking it apart since I've never dealt with a vacuum leak before.

Thanks
Mark

 

[grizz55chev]

One of my 036 saws recently developed a starting issue. During a recent cutting session, I stopped to refuel, and the saw wouldn't restart. I was short on time, so I just walked in my shop and picked up another saw. Over the course of several days I tried to start the saw, tweaked the L jet every way I could think of, and nothing. The saw fires one time with the choke on then nothing after. It was a cheap aftermarket carb, so I replaced it with a tillotson that I had picked up. Now the saw fires like it always did. After removing the choke, but leaving the throttle locked, it starts well but then runs erratically with the RPM increasing (a lot) and then decreasing (a lot) and then increasing again.. If I unlock the throttle and try to let it idle, it dies. I can't get it to stay running at idle to see if the RPM is fluctuating at idle or not. If I stay on the throttle, I can run it wide open, but the tachometer says that the RPM is bouncing around a little more than I would normally expect.

Am I correct in saying that the variations in rpm indicate a vacuum leak, and the fact that the variation is extreme at 1/2 throttle means it's a pretty big leak? If this is the case, since this came about rather suddenly, I'm hopeful that its the intake boot and not a crank case seal but wanted to get some opinions from the "experts" before I start taking it apart since I've never dealt with a vacuum leak before.

Thanks
Mark

Take a min and pull the muffler to inspect the piston and cylinder, then proceed to look for the problems mentioned. Low comp will cause the issues you are describing, so you have some work to do, it's a rabbit hole sometimes, so you gotta start somewhere.

 

[9050LX]

Check the fuel line and filter.Sounds leaner than it has a right to.

 

[Old2stroke]

That kind of erratic running is often due to an ignition module going bad. Only valid test is to replace it.