Stihl 028 has air leak

[glock_556]

Local saw shop says my saw has an air leak and since it's an old saw, doesn't want to invest a whole lot of time looking for the leak.

Is there a common place I could start to try to find the leak? Right now, the carb has to be set at a high idle for it to stay running at high RPMs/during cut (per mechanic).

I took this thing in for a carb cleaning and it ran fine before that. The saw shop but in a carb kit and now I have this problem.

Thanks in advance.

 

[Johnsobri]

Impulse line or intake manifold would be the easiest to check. If it is not there you will need to perform a pressure and vacuum test to find the leak. What are the symptoms?

 

[glock_556]

From the way the mechanic talked, he was having trouble getting the carb adjusted after putting in a carb kit. In order for it to run while in the cut/wide open, he said he had set it so it would idle fast.

 

[spudulike]

The symptoms of an air leak are
1) screaming on WOT
2) A fast idle that the idle adjustment screw has little effect on and may need the L screw to be unscrewed to bring the idle down
3) Holding on to revs after WOT and not falling to a steady idle straight away after the throttle is closed.
4) fidgety idle that you cant dial in a steady idle.

Normal areas of failure - Inlet manifold, impulse line & crank seals.
Pressure checking this saw shouldn't cost too much as it is a pretty standard construction and would be better to get it done rather than guessing as to what has failed.

It could be a worn carb issue. Sometimes the throttle valve rod wears in the alloy carb body and when the throttle is closed, air is dragged in from the outside of the carb past the worn rod in to the engine and because the amount of air changes and isn't controllable, the idle becomes VERY fidgety. You can get the saw on idle and squirt WD40 or oil on the ends of the throttle rod and see if it changes the idle speed or kills the engine.

 

[tomdcoker]

I check for air leaks with carb cleaner. Use the plastic tube and with the saw at a medium high speed squirt some cleaner around the base of the carb. and behind the carb. manifold to check the carb. boot and impulse line. Remove the clutch and check the PTO side. Crank the saw then remove the rewind and try to squirt some behind the flywheel. This is a quick fix that will probably need a v & C test to pinpoint the actual leak, but it will tell you if there is a leak. If there is a large leak the saw will die. Tom

 

[rd35]

I took this thing in for a carb cleaning and it ran fine before that. The saw shop but in a carb kit and now I have this problem.

Thanks in advance.

Based on your last opening statement (quoted above) I'd be taking a real close look at that carb. If it ran fine before the carb work, then that's where I'd start. I'd check to make sure the carb is sealing to the boot correctly, check that impulse line connection and hose, and then I'd pull that carb apart to make sure it was re-assembled correctly.