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1989 140 idle is killing me

L

loopie111

Member
my motor idles fine a 700 rpms but will stall (cough and sneeze) between 800 and 1100 rpms. it runs fine above 1100 rpms. I have re-built all of the carbs and replaced all of the fuel lines, primer bulbs and filters. I have done the link and sync procedure 3 times and cant nail it down. I have tested different idle timing settings to no avail. compression is good on all cylinders. How do you check for an intake leak? Any other Ideas? it makes it difficult to put it on the trailer!
 
Intake leaks can be found by spraying fogging oil or some substance of your choosing around various gaskets in the intake area. The substance being drawn in causes a rpm change. I've worked on many of those engines and frankly I've never incountered a intake leak per se.... the exception being perhaps a O Ring seal slipping out of place between a carburetor and the intake manifold/throttle plate.

With the carburetor face plate removed and the engine running at a rpm where the problem exists.... one carburetor at a time, stick two fingers into the carburetor throat acting as a manual choke. A normal functioning carburetor will run rich (load up) instantly causing the rpms to drop, BUT if the rpms increase and the problem vanishes, you've found an offending carburetor.

Let us know what you find.
 
Thanks Joe, I'll give it a shot later this week, I have to wait till I get it in the water to check under loaded conditions. It runs fine on the muff's.
 
Under loaded conditions.........

Keep in mind that with the carburetor face plate removed, the engine will (at full throttle or possibly some other high rpm setting) run lean resulting in having the engine actually run out of fuel eventually, slow down and stop rather quickly.

The thick rubber like gasket between the face plate and the carburetors acts as a air restrictor of sorts which results in more fuel being drawn into the crankcase for a proper mixture.

Strange that the problem doesn't exist when running on a flushette.
 
If the motor runs fine at 700 (in gear & in the water) but spits / quits between 800 & 1100 then you may be picking up throttle plate movement too soon and / or not enough timing advance. A common source of air leak on a V4 looper is the side plates on the carb body, a reed plate gasket out of place or a throttle plate opening too soon / not seating properly. However, those should show up at 700 rpm. If the motor is running correctly on the flushette but not in the water your issue is affected by exhaust backpressure.
 
Not sure if this help's but if I press the key in to activate the primer when the engine starts to stall it will immediately stall leading me to think it is running rich. Idle timing is per the manual 18 deg atdc. I have tweaked it +\- 5 deg with no improvement. Throttle plates are fully seated during the link and sync. I have also backed off the idle screw 1 turn so the cam pickup is not touching at idle.
 
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