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Engine question

P

Palm Bay Airboater

Member
I just bought an airboat with a lycoming engine. I took it out today and it ran great but when I shut it down it ran on for a couple of seconds and then ran backwards for a couple of seconds. I am running premium gas in it and it ran fine. Did I shut it down wrong or what. Also when I would run it on the trailer for a couple of minutes it would shut off fine. This just happened after running it for a couple of hours. The oil temp was good.

Also I just found out I need to put mufflers on. Any one already put them on? Any tips?
 
this issue has been asked several times, all with pretty much the same answer. the term used here is "dieseling".....when the motor chokes and gags and spins backwards and makes all kinds of racket. don't worry its normal. if your engine is an angle valve, i don't care what anybody says, you should be running avgas. forget all the lead additives and octane boosting crapp. if you wanna play with the power you gotta pay at the pump. don't ruin a $12,000 engine cause you wanna save $1 per gallon. if it is a straight valve, then the pump gas is fine. with all that said, here goes.

i've found the best way for me to kill mine (0540 straight valve) is to idle it down for a few minutes, cut off the straight mag, let it idle down for about 10 seconds and then lean it all the way out. it'll run for about 3 seconds and then cut off absolutely like a dream. works every time. kinda tricky coming into a dock because its hard to adjust the cutoff time to wind, drift and other docking factors. what i do there is the same thing except i'll go ahead and cut off the impulse mag immediately after i've leaned it out. that should fix your problem. ALWAYS idle your motor down for a minute or two. if you skip that step, it will pretty much always diesel on you. hope this helps........
 
Duckluv, that also allows time for the temperatures in the engine to stabilize which leads to longer engine life. It's a win - win. Good post.

feather
 
Good advise for ya there Palm Bay,
Is your engine rigged with a lean to cut-off cable?

I it is not, and has an Aircraft carb on it, I would rigg it that way.
They do best when shut off by cutting off the fuel, via the provisions provided in an aircraft carb.

Warm that baby up really good when before you run it, Especially if your gonna run it really hard right off the bat. It will last you a whole lot longer.

Let her idle for a little while before you shut it off, especially if you've been running it real hard. This will let you run some oil through the rocker covers, and let the cylinders cool down some.

Anyway, those are some of the things I do, and the advise on fuels above, is good advise.

Larry has posted some muffler pictures in his gallery. There are smaller ones that he saw on a 4cyl A/C engine. He said the were very effective, and did not seem to cause any issues for the operating temps on the engines.

Your engine is bigger, so you may want to "search" the forum with key words such as "Aircraft Mufflers" or other terms applicable.

Theres allot of info in here on that subject.

Go get it.
 
Thanks for the info. I have a straight valve 0540 and I do hav e a lean to cut off cable. I guess what I did wrong is I didn't cool it down long enough. I had just got done loading it on the trailer so it was probably a little bit warm. Also how do I tell the difference between the straight mag and the impulse. I bought this boat from my mother in law after my father in law passed away, so I don't have anyone around that I can ask about how he set it up. I do know he ran pump gas in it with no problems.
 
I see alot of Aircraft boats shut down right away after running. I believe that is one of the worst things you can do to a motor. Let that baby idle for awhile, then shut er down.
 
the impulse mag is the one you should be starting the motor off of. it'll be the the shorter mag usually on the left side looking at the motor from the bow. btw, its probably best to turn the prop over a few times and then turn on your impulse mag when starting. that takes some of the load off the mag. when it cranks, then turn your straight mag on.
 
I have had the same problem on my 0470 GPU and posted about it here not too long ago. What I did was follow some recommendations given here as follows, and for the past two times out, no more dieseling.

I adjusted my idle down to 550-600
I added some Octaine boost to the fuel
After running I made sure I sat at idle for at least 3 to 4 minutes

Then I kill my battery and fuel pump, wait a few seconds and then kill the mag.

Although it might not "hurt" the motor to diesel, it just don't sound right!!
 
.........especially when your guest has never been in an airboat before....,"is that normal?" lol.....whatdaya say to that. hehehe
 
okay I went and looked at my mags. They are both the same size but the one on the left (looking at the engine) has a ground wire attached so I am guessing that is the impulse mag.
 
On a lycoming the impulse mag will have a spacer inbetween the mag and the accessory cover, about 1" thick. If both of your mags have this spacer on them then they are both impulse mags.

Larry
 
good post Larry.....both of your mags in one way or the other are grounded. some ground to the mags and some ground to something else. just depends how you have them wired. VERY IMPORTANT!!!! this is the most important information you'll ever know about a/c engines: mags are naturally hot. to kill the engine you are grounding out the mags. if you ever send current to your mags, YOU WILL fry them like an egg. or if a hot wire ever touches what grounds your mags......SIZZLE SIZZLE.
 
after letting her idle a few min. and if your using a electric fuel pump just cut off the pump.let it run out of gas. if your using a mechanical pump i don't advise shutting off the fuel to it as when restarting it dry if can pop the diaphram and then u gotta buy a new one. same if your using both electric and manual. i use only an electric pump and always shut off the fuel pump letting the carb run out of gas.
by the way dieseling is very destructive to the engine. if it runs backwards it can break the impulse spring on the mags even turning the propeller by hand the wrong direction can damage the impulse spring on the magnetos.
 
And to any car motor guys experiencing the same problem...It's due to the increased idle air (throttle blade opening) required from the load of the prop. Of course this applies to A/C guys too, but they have much easier/practical fixes.

For both parties wanting to figure out the physics of it...That's all it is....Too much air at idle. (Or more to the point...The throttle blades are too open)...When it has fuel in the mix, and a heated combustion chamber, especially combined with a fuel that ignites easy....It will "run-on".

Use that thinking for either application, and there are many ways to get rid of the problem....But chopping the air off to the engine will be the most effective. (and arguably so...But definitely cutting the head off of the snake perse' since that's what really causes it) I.E. Dashpots, fast idle solenoids, A/C solenoids etc....

Now if you use this base thinking....You can start to appreciate where carburetor stumble, and idle quality are weak in airboat apps. Airboats and aircraft are never truly given a chance to "idle"...They are under load at all times and very few carbs are designed/metered for that.....

Food for thought
Felber
 
You can hear the impulse mag click when you turn her over. Just shut off the mags and crank her over and listen......
 
To tell the impluse coupling mag- hold had on mag when the prop is turning over with the starter you should be able to feel it click. Both mags can be impluse. if one is not then it must be turned of when starting. You can remove the plug in the side of the mag. rotate the pro by hand with mags cut off & lookt through the hole . An Implused coupling mag will stop rotating for about 25 to 45 degrees of rotation of the prop before firing ( you hear the click). The mag will stop turning 20 to 30 degrees before top dead center of the pistion & then release 5 to 10 degree after top dead center. The amount of retating is a function of the type of impluse coupling installed.
To stop motor set idle as slow as you can helps 350 to 400 RPM when hot.
 
good info. i don't like to set my idle that low though. seems like the motor is just struggling. i have mine set at 600. not very fast but still allows the engine to turn the prop with no trouble. in other words, its a lot smoother idling at 600 than it is at 450. i feel like i have to nurse the throttle to keep it running. i say to set the idle at a smooth idle and use thr R & L adjustment to kill it. its too easy and beneficial for the motor.
 
And if you can put a simple solenoid (dashpot/fast idle solenoid) on the throttle to drop the blades completely closed on shutdown....You can idle it as high as you like/the motor likes, and the run-on is gone. All problems solved. That's just me, and I certainly understand if people don't wanna put any more gadgets on their stuff than nescessary....But that one in particular....I like.

Felber
 
Felber, do you agree that its better to idle your motor at a good, smooth idle; ie, 600rpms versus 450? i hear a lot of people talking about turning the idle down to 450, but to me i don't want my motor struggling to idle, especially on a morning where the temp is 25 degrees.
 
when you start to Iddle one above 500 RPM it will keep running when you shut it off. I have set my idle below 500RPM for over 25 years with no trouble. !5 years on the same motor.
 
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