Spark Plug Specs for PT 425 w/ Robin EH72

tracdoc · Oct 20, 2005

( If it affects only PTs, and not Robin EH72's in other applications, perhaps the "funky" PT exhaust manifold/muffler design may be at fault, developing hot spots causing detonation in that location. Just a thought as to a possible cause. )
Jack,

That's what I meant with my earlier comment & query /forums/images/graemlins/grin.gif

J_J · Oct 20, 2005

I would like to add something, that I read. Even though you have a fuel shut off solenoid, there is fuel left in the bowl. If the fuel solenoid seat is not in perfect condition, it may allow some fuel to be sucked up the carb even after shut down. I believe the fuel shut off solenoid was developed for safety reasons, in case of a roll over, the seat safety switch would kill the engine and deactivate the fuel solenoid. A similar solenoid is used in many different engines to prevent dieseling. The Kohler on my Zero turn mower has a Kohler engine, with the fuel solenoid, and if some trash gets in the bowl, the engine would act up, surging and trying to shut down. After cleaning out the solenoid, it ran like it is supposed to.

There is a simple test that you could do to check and see if the solenoid will shut off the fuel, with out turning off the ignition switch. With the engine running at half throttle, locate the fuel solenoid wire, there may be two wires, one will be connected to the start terminal, and the other will be connected to the run terminal. and unplug or disconnect the wire from the fuel solenoid to the run terminal. and see if the engine will shut down. Check your electrical diagram to make sure that you know which wire is correct. The wiring for the Robin will be a little different. Jack Robin should be able to help there.

According to Kohler's web page, they say to shut down from half throttle.

Phils · Oct 26, 2005

Just an update, I tried shutting off at full throttle and it made no difference -- a single "pop" occurred after shutdown.

Phil

tracdoc · Oct 26, 2005

Another update:
Last weekend, I tried the NGK BPR5ES plugs (0.030 gap) that Jack Robin suggested. After mowing for an hour, I shut down from half throttle, and BANG! a single backfire. This scenario was repeated throughout the afternoon. I even tried shutting down from 2/3 throttle, with same result.

Undaunted, this week I tried the next range colder plug, the NGK BPR6ES which was the one "recommended" in the printed and on-line manuals. This made no difference in the shutdown backfire. This plug was indistinguishable from the _5ES.

When I read the plugs, neither the 5 nor the 6 showed any evidence of fouling (a potential problem as plug heat range gets colder). Of note, I used the _6ES this morning, and the PT started right up with ambient at 39 degrees. I left it outside just to see how it would fare. Maybe as the plug ages, cold starts could be a problem, but not right now.

I am going to stick with the colder plug for now . After seeing the damage that a too hot plug can do to an engine, I choose to deal with the minor annoyance of possible balky starts and maybe fouling which can be cured for $2.50 X 2.

Back to the problem of the shutdown backfire.... I seem to still be at square one. Will be interested to see what the Robin engineers come up with. /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

JackRobin · Oct 27, 2005

Alright, after talking to the engineers we figured that the engine on this application possibly could have a carburetor with two shut down solenoids instead of one. Currently on this carb, there is a acceleration pump instead of a second shut down solenoid, just like we have on our generators.
We use a carb with two shut down solenoids because the muffler of the generator is slightly enclosed and runs a little bit hotter than any other application.
This could be a cause in PT tractors, not enough air flow to cool the muffler down quickly after the shut off causing an afterburn effect and a big boom.

You guys might want to talk to PT about this, they probably could get together with our engineers to discuss possible changes.

Regards,

Jack Gawecki
Robin America, Inc.
Technical Support Dept.

toolz_not_toyz · Oct 27, 2005

( According to Kohler's web page, they say to shut down from half throttle. )

I had the same backfire problem with my JD long tractor and 25hp Kohler twin engine. If I didn't shutdown at 3/4 or full idle the engine would backfire. It was just an old habit to always go to idle for a bit to let the engine "relax" before shutting down. That strategy makes sense for an air-cooled engine that has no fan or shutdown solenoid. But that's basically wrong for the Kohler and possibly for the Robin engine as well (if it is the same basic design with a solenoid and fan).

Think about it. If the engine has a fan, then cutting engine speed will slow the fan down and reduce cooling ability. That's not what you want. It is far more efficient (from a cooling standpoint) to stop work and continue running the engine at 3/4 or full before shutdown. Idling will just work against you.

Similarly, if the engine has a shutdown solenoid you should let it do its job by running at full throttle and letting the solenoid shut off the fuel supply.

You could also try a higher octane fuel as an experiment. But I don't think that helped any on the Kohler and with today's fuel prices it's probably not the best fix.

Phils · Oct 27, 2005

I've used hi-octane fuel exclusively except for one tank and it made no difference on the "PT Pop". And from Jack's post earlier it appears that there's going to be no easy fix.

But today premium gas is the best deal it's ever been. It's still 20 cents more than regular, just as it was when gas was 50 cents a gallon, which is a pretty small increase percentage-wise when dealing with 3 dollar per gallon gas.

Phil

J_J · Oct 27, 2005

All I can do is to refer you to the kohler web site.

Kohlerengines.com

tracdoc · Oct 27, 2005

( And from Jack's post earlier it appears that there's going to be no easy fix. )

/forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif I agree. I don't know enough about the setup of the carb to know where the second solenoid would go, or how it would be wired.

Jack,

Some of us have engines that are still under warranty. Would this presumed retrofit fall under that umbrella? I don't think that Power Trac would be particularly receptive to inquiries from customers regarding engine difficulties, especially since they specifically state that any and all engine issues during the warranty period should be addressed to the engine manufacturer.

How do you suggest we proceed with this in a manner that is fair to all?

toy · Oct 28, 2005

My pt 425 with the kohler engine has backfired since day one. It may mot be what is recommended but I throttle back some before I shut down, but it still backfires. I wonder if when you cut the machine off if somehow the fuell pump would cut off first and still have a delayed power cutoff to the spark plugs to allow it to burn the excess fuel off while it is in the combustion chamber wouldn't solve the problem. That would fire the remaining gas mixture inside the engine and not letting it get to the exhaust manifold.