170 gas run on!

Interesting post. My 170 has been doing a little of that too. Looking forward to the intel provided.

Very slow low idle (400-450 RPM the slower the better) with 30 to 60 seconds of idle time before key shut off. Late/incorrect ignition timing can make things hotter in the combustion chamber, which can cause run-on. Use cooler spark plugs like AC43-XL instead of 45-XL.  Better octane gasoline. Some competitors had enough problems with run-on they had an electric solenoid screwed into the main jet passage that would cut-off all fuel when the key was shut off. Last ditch effort would be just dump the clutch when shutting off the key.

Edited by DrAllis - 21 Oct 2023 at 9:34pm

Try setting carb idle screw to almost an “off” position. At cold it will shut off, Maybe after warmups the engine will barely “lope”?
Then if your throttle cable allows, you could have a small amount of low idle control via its throttle lever? So during operation, you could back throttle completely off, but then by moving throttle just a bit, catch engine before it dies. When you want it to shut off put throttle all the way back. If engine dies, then turn off key. Like DrAllis stated, if throttle is completely backed off & engine still lopes. Let it lope for 30-60 sec before turning off its key.

It's pretty much all related to the higher (8.0 to 1 ) compression ratio. Seldom would you ever get a D-17 to do this unless you were plowing and just rolled up to the end of the field and turned off the key. I've seen it before on a brand new 170/175 OH with new pistons (clean) and sleeves (not enough slow idle time), not that carbon build up may not aggravate it. One might also try leaving the transmission in any gear with the Power Director clutch in neutral/foot clutch pedal up. This would put a little drag on the engine and take away the rotating mass of the P.D. clutch housing itself.

Edited by DrAllis - 22 Oct 2023 at 1:16pm

Check Mechanical Advance weights and springs inside distributor. Remove distributor cap and turn rotor clockwise with your fingers and release. Clockwise is full advance and spring return is full retard. It should spring return to the full retard position on its own. If it doesn't, remove the rotor and oil the felt pad in the center of the distributor shaft with ATF. 6 drops is all you need. Wipe up excess with a rag. Put rotor back on and cycle the advance mechanism until it frees up and returns on its own to full retard position. If it doesn't free up or return to full retard, remove the points base plate and check 2 weights and 2 springs under base plate. Rust and missing springs or broken springs will prevent return to full retard and cause run on. 

Edited by Phil48ACWC - 23 Oct 2023 at 6:17am

I find it hard to believe timing or a stuck distributor advance has any effect on dieseling or run-on, since the distributor is not supplying any spark after the switch is turned off.

Edited by WF owner - 23 Oct 2023 at 6:13am

If the mechanical advance does not come to full retard at slow idle, it will idle faster and contribute to run on when shutting off the engine. In any event the mechanical advance must work properly to get the combination of performance at higher RPM and return to full retard at low idle to prevent dieseling or run on. Not to mention the ignition must return to full retard to allow ease of starting the engine especially when the engine is hot. All other parameters such as mixture, coolant temperature, spark plug heat range, valve condition especially exhaust valve, lack of intake manifold leaks, lack of throttle shaft leaks, good fitting carb butterfly, properly operating throttle linkage and governor must be in proper order. We used to fight all this on cars prior to fuel injection and computer engine management back in the '70's. Thank God for EFI.

Edited by Phil48ACWC - 23 Oct 2023 at 7:13am

My WD-45 has 175 pistons in it. I've found that if the gas gets old it was to do that. It does finally shut off, it just wants to run a few extra seconds