[grouper12]

i was having a problem when sihfting from nuetral to forward the engine would die , i could pull the trottle to the side for high idle and rev it up ,then before it when back to idle speed shove it in gear ,[ had to do it to get back to the dock] i checked the thermosensor , knock sensor , cam pos sensor ,all good but the ,throttle pos sensor i cant get a propper reading ,maybe because i am not using a yamahe dig .meter ? on standerd dig meter on 20k setting i get 5.ohms . but when i adjust the sensor the engine will rev up or down . also i changed the filter was dirty added water remover ,octane booster ,seems to be running better [in a test can prop removed ], but will cough at high dle around 10-1300 rpm like it loses spark all at once then recovers , any ideas what i should check ?

[Ray Neudecker]

The throttle position sensor should read smoothly from minimum to maximum. Any digital meter should work for this. If there are dropouts it is defective. A common cause of the problem is the primer pump leaking back. This drains the fuel off the top carb. Blocking the hose off from the bottom of the bowl is the easiest way to test it.

[grouper12]

thanks for the replay ray i,ll check with anew primer bulb and see how that works

[rodbolt]

not a primer bulb.
that engine has 2 fuel pumps and a primer pump. actually a fuel enrichment pump is more precise.
it has an A and B solinoid.
either or both can open due to various sensor inputs and enrich the fuel mix by about 20%( memory may be a tad off)
the cough is a lean out, its a backfire into the crankcase but it cannot get past the reeds. it also destroys fuel and primer pump diaphrams, can be caused by a failed primer pump or dirty carbs or both in combination as sometimes one lean sneeze can take out the pump.
best way to check the TPS, and the rest of the sensors is with a DVM using the voltage setting. if you suspect a bad TPS due to an intermittant open then use an analog meter.
a bad TPS may cause a stumble but not a sneeze.
the TPS other than telling the micro procesor throttle angle and opening or closing speeds doesnt control fuel. the only fuel control is from rapid opening or closing of the throttle,knock detect and thermosensor inputs.
while very advanced for its time its still a carbed motor.