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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
After the carburetor adjustment, tamper-re-
sistant devices (limiter caps) must be installed.
The limiter caps are shipped with the new car-
buretor just for this purpose. Installing the lim-
iter caps ensures that the engine will continue
to operate within the standards set by the EPA
and engine manufacturer.
Final Running Evaluation
The engine must perform correctly after the
repair. Start the engine using the manufactur-
er’s starting procedure.
“Does the engine idle correctly?” The inex- ”
perienced repair technician has a tendency to
“nurse” the engine at idle by “blipping” the throt-
tle trigger. In reality, the engine should idle on
its own in the shop, just like it should idle on its
own for the customer. If the engine dies, adjust
the idle speed according to the engine manufac-
turer’s procedure.
Allow the engine to warm up correctly. The
correct process for starting and warming up the
engine is listed in the operator’s manual. Check
the operation of the throttle trigger, the throttle
interlock system, and any throttle control system.
“Does the engine accelerate to WOT cor-
rectly?” “Does the engine drop back to idle
correctly?” The engine should respond without ”
hesitation when moving from idle to WOT and
should drop back down to idle normally.
On a string trimmer, the engine should idle
without allowing the string head to rotate. With
the string length correct (as stated by the equip-
ment manufacturer), the engine should acceler-
ate promptly to WOT. Record the engine speed
at WOT. Make sure the engine speed is correct
according to the engine manufacturer’s spec-
ifications. When the throttle is released, the
engine speed should drop normally, and the
string head should stop rotating. Check to make
sure the line advances correctly.
“Does the engine run correctly under its
load?” In the case of a chain saw, the saw should ”
idle without the blade rotating. It is important
to not run the chain saw at WOT for more than
10 seconds. A chain saw is intended to oper-
ate under a load at WOT. Without the load, the
engine overspeeds and overheats. Check to see
that chain oiling system is working correctly.
Make a cut on an actual log at WOT to make
sure the engine works correctly.
It is important for the repair technician to doc-
ument the result of each running test. Record
engine speed at idle and WOT. “Operates cor-
rectly under a load” is an appropriate descrip-
tion that the engine is running correctly. Record
the results of engine acceleration and decelera-
tion. If the string trimmer string advances cor-
rectly, make a note of that.