Spark Plug Specs for PT 425 w/ Robin EH72

[cqaigy]

After reading on TBN, that the robins engine should Not be idled down before shutting off, i haven't had the diesling problem. My Father has a sears lawn tractor w/briggs engine and it's just the opposite, in that if you don't idle down it will backfire.

I've also noted on other engine on trenchers and such that they say not to idle down either.

 

[Barryh]

Hmmm .. well maybe there's a nice compromise here. Shutting down from full throttle feels a little harsh to me, of course there is no basis for that just my opinion.

Maybe half throttle will accomplish the same thing. I will have to try that tonight and see if that helps. For those with a heated garage a cooler plug may be the answer /forums/images/graemlins/crazy.gif

 

[JackRobin]

I've passed the questions to our engineers, as I'm kind of scratching my head here. I'll get back to you guys when I'll get some answers.

Regards,

Jack Gawecki
Robin America, Inc.
Technical Support Dept.

 

[JackRobin]

Alright, to all of you with the "bang" /forums/images/graemlins/grin.gif problem, would you please state the amount of hours on your machines?
Also, I was instructed to ask you to change your plugs from BPR4EY to BP5ES.

Please let us know asap what the results are.

Regards,

Jack Gawecki
Robin America, Inc.
Technical Support Dept.

 

[MuncyBob]

Jack...I have 148 hours on mine...but I had had the bang/run on problem from day one.

 

[KentT]

</font><font color="blue" class="small">( After reading on TBN, that the robins engine should Not be idled down before shutting off, i haven't had the diesling problem. My Father has a sears lawn tractor w/briggs engine and it's just the opposite, in that if you don't idle down it will backfire.

I've also noted on other engine on trenchers and such that they say not to idle down either. )</font>

In general, if the engine doesn't have a fuel shutoff solenoid, you DO want to idle it down before shutoff.

Without a solenoid, every engine revolution is sucking more fuel/air mix into a system where there's no active sparkplug to ignite it -- and pushing it right on out to a hot exhaust system. As long as the engine is turning over, it's pumping fuel/air mix out to the hot exhaust. Shutting one of these engines down at full throttle literally risks a backfire strong enough to blow the muffler off....

With a fuel cutoff solenoid, the flow of fuel is shut off as soon as you turn the key off -- so it only pumps air without fuel after that... at high RPMs, the next pulse of fuel-less air comes so soon that it blows any fuel/air mix right on out the exhaust before it has time to ignite...

My experience and opinions only -- your mileage may vary...

 

[MossRoad]

So is the cuttoff solenoid part of the carb? Just curious, as I like to learn stuff. /forums/images/graemlins/laugh.gif

 

[KentT]

Yes -- they're called "anti-diesel solenoids" by some -- they typically close the main jet of the carb when power is removed from them by turning off the switch.... When operating correctly, you literally have to turn the carb "on" so it will flow fuel. This makes them "fail-safe" if you know what I mean...

VWs had to put them on their engines around 1970 as the smog laws started to impact the operating temperatures of air-cooled engines. They also had to use sodium-filled exhaust valve stems, to deal with these increased temps. As the smog laws tightened further, they had to move to fuel injection, and then the smog laws finally killed the whole use of air-cooled engines...

It's starting to happen now in the OPE industry. Kohler already makes a fuel-injected version of your basic CH-25....

Hopefully we won't be required to put catalytic converters on them -- or that'll be really dangerous.

 

[Phils]

Hey, MR! My solenoid is mounted on the lower right rear side of the carb.

For Jack: 98 hours on mine, but it's done it the entire time since new.

KentT -- your explanation makes perfect sense and will be the next thing I try. I've tried shutting down at medium speed but, like you, had a mental block about shutting down at full throttle.

tracdoc... since this apparently isn't a problem with this engine in other applications, maybe you are correct in mentioning the PT exhaust manifold?

And something else: I always thought spark plug heat range ONLY dealt with how hot the tip stayed during operation. I've seen cutaway diagrams and the longer the porcelain traveled up the inside of the plug body before contact meant the plug was a "hotter" heat range. That difference was only significant concerning dieseling and spark knock. I don't really see where that would make any difference during the start revolutions, whether the weather was warm or cold. Am I misunderstanding something here (as often happens)?

Phil

 

[KentT]

Phil,

Some types of mufflers will run hotter than others, based upon how restrictive they are, in addition to the materials they're made of, their location in any airflow, etc. If the exhaust system used on the PTs is unique to the PT -- this could be something that Robin hasn't experienced elsewhere....

Or, something is keeping that muffler hotter than in normal Robin applications -- like that blanket around the muffler that I've seen pictures of... or less airflow around the muffler than in other applications, etc.

I have no dog in this hunt, but it wouldn't surprise me that Robin isn't used to hearing about these problems because they're not having these problems in other applications.