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Author Topic: Upon starting. engines blows prop, prop nut and prop washer off????  (Read 1257 times)

Offline frank mccune

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       Hi All:

       What would cause the engine to fire so hard that everything connected to the crankshaft blows off for parts unknown?

       All things prior to starting have been checked and very tight.  Choke the engine about 5 times, flip forward about 5 times and connect the battery to the glow plug.  Turn prop to check for a bump and if a bump, flip the prop.  The next thing that happens is that the crank shaft is devoid of all the items that should be there. Back flipping and  an electric starter both provide the same results!  These are not expensive high compression engines but are a Fox .25 nd a Fox .29 circa 1950's.  Both have great piston cylinder fits and great compression Three of us gave up yesterday in total disgust!  They acted like they were flooded or the compression was excessive.  They were not flooded.

     Too much voltage to the glow plug? Not likely as I was using a 1.3v starter battery. 

      Any thoughts???

                                                                                                                                   Tia,

                                                                                                                                   Frank

                       

Offline Brian Hampton

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #1 on: June 02, 2019, 08:10:11 PM »
I had a similar problem once with my Stalker 61 when suddenly it went from being an easy hand starter to a vicious monster. I eventually pulled the plug and connected it to my glow driver and saw it was flaring like a supernova about every second or a bit less. Put in a new plug and all was well again. The problem of course was that the flaring advanced the ignition way beyond where the piston could get over TDC.

Offline BillP

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #2 on: June 03, 2019, 06:34:24 AM »
I don't know why but 2 engines doing the same thing must be sharing the same culprit. I'm thinking over compressed. Assuming both engines have the correct head shims or gaskets. The flaring glow plug is interesting but how likely is that going to happen with 2 different plugs at the same time? Rarely but could be the glow driver, plugs or fuel? My thoughts are leaning to the fuel. Did you try different plugs and fuel? Are you experiencing high humidity?
Bill P.

Offline frank mccune

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #3 on: June 03, 2019, 07:45:27 AM »
      Hi Bill and Brian:

      Thanks for the prompt responses:

       The engines were run at different times.  Only the symptoms and results were similar, difficult starting. The plug may be the culprit despite it being pulled from the engine and tested. 

        I will attempt to start the Fox .25 today as it is much cooler and less humid.  I can't see that this engine has high compression.  Oh yes, I will try a different fuel.

        Will report findings.

                                                                                                                                                                        All of the best,

                                                                                                                                                                        Frank

Tom Vieira

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #4 on: June 03, 2019, 08:35:33 AM »
make sure your prop washer/driver isn't smooth.  they need to "bite" the prop a bit, otherwise they loosen up.  use a dremel tool with a cut-off disc to cut grooves/roughen up the surface a bit, then try again.

Online Ken Culbertson

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #5 on: June 03, 2019, 08:49:08 AM »
I am curious if the common element between the two is a worn or smooth prop or drive hub?  Firing with abnormal compression can easily loosen the prop nut.  Happens a lot but if the hub and prop mesh nothing happens except maybe you grab the chicken stick or say something that Sparky will mask.

Well it looks like T.J. beat me to it!

Ken
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If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Fredvon4

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #6 on: June 03, 2019, 09:37:47 AM »
I do not think in 55 years I have ever had a glow engine that did not one time or another backfire

As I type this it is curious to me that I never have had any brand 2 stroke yard tool backfire....never  and this is a LOT of sizes and brands...so I guess gas oil and spark ignition is pretty immune to the phenomenon

Back to the glow engines.... I have a few ASP Magnums and one Fox 35 that were knuckle bangers and prop spitters..... with a nut or hub not a big deal...behind a spinner...a royal PITA

Internet taught me a cool hack that works very well
Cut a DRY WALL SANDING SHEET (about 60~80 grit) into a full sized fender washer for between the drive washer and back of the prop hub.... and tighten very well...the friction interference dramatically reduces* the un nutting un propping from a backfire....* not 100% of the time
"A good scare teaches more than good advice"

Fred von Gortler IV

Online Ken Culbertson

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #7 on: June 03, 2019, 09:59:58 AM »
Should have asked earlier - are these wooden props? I have used the sandpaper idea a long time ago with nylon props.  Put the gritty side toward the prop.  Used old sander belts.  Didn't seem to work as well on wood.  Too easily compressed and the prop quickly meshed into the drive hub.

I am surprised they are doing that in practice/fun flying.  I thought the reason they let you leave your spinner off was recognition that the only time you threw a prop was on an official.

ken
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If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Online Brett Buck

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #8 on: June 03, 2019, 10:21:32 AM »
The most likely issue is that you are grossly flooding the engine, particularly, getting the crankcase full of fuel before enough fuel gets in the cylinder to make it fire. When it doesn't go, you choke some more, until finally enough gets in to fire - at which point it sucks the rest of it in. Usually this just makes it weakly run, then quit, but sometimes you get enough to hydraulic lock it, or temporarily raise the compression,  which will usually kick off the prop.

     This is sometimes difficult to avoid on upright engine installations and at least pretty common on profiles. I haven't had the prop kick loose a very long time, and the only times were when I was burping the engine, holding the airplane inverted with the cylinder up, and intentionally putting a lot of fuel in the venturi. David's 75 used to do it now and then, I know because my back still hurts from holding the damn thing up while he puts it all back together - when it would start backwards (too much fuel), start blowing the excess fuel out of the venturi, finally getting to acceptable conditions, and then switching direction.

    This is as least one reason that stunt engines are generally mounted in the inverted orientation, it's much harder to flood and generally easier to start.

     This will be particularly bad when using low-nitro fuel, it has to be much closer to right to fire at all. You might consider priming it with 15 or 25%, just to get it started without having to load it up with fuel.

     It's remotely possible that the compression is excessive, but that's almost certainly not true *as long as the engines have not been diddled with*. The Foxes did not come from the factory with excessive compression for 5% nitro, they would probably love 25%.

 While there is some general folklore along the lines of people becoming hysterically frightened of "overcompression, OMG!!!!" for some inexplicable reason, and curing every issue with 40 head gaskets, it's almost unheard of  in typical stunt engines and you will not have to wonder about the cause when you finally go over the top.

    Brett

Online Ken Culbertson

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #9 on: June 03, 2019, 11:08:43 AM »
I think Brett has nailed how it happens but I am having trouble getting the mental picture of what is happening.  I have had my share of backfires knocking the prop loose and I have had a couple where the engine tries to run backwards for a brief time then quit but I have never had one run away forward which would be necessary to throw the prop, washer and nut off.  Just out of curiosity, is the hole in your prop so tight that you have the thread the prop on?   This is the only way that I can envision that would result it the whole mess coming off if you got a compression lock(see Brett).  Otherwise it would just come lose and sit there mocking you instead of acting like an electric with a lose collet.   How much shaft is left after the nut is tightened.  Any chance you have one of those spinners with the big aluminum barrel nuts inside that replace the prop nut?  The threads on those would stretch with use, especially if you were using nylon props, and not hold too well.

That's it - it sees the future and wants to be an electric LL~

Ken
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If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline BillP

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #10 on: June 03, 2019, 01:19:24 PM »
Brett may have it. Frank said it wasn't flooded so I didn't go there but too much fuel in the case does make for problems. In all the yrs I've flown glow engines I can't remember ever shedding everything off the prop shaft. Even loosening a prop nut is rare and mostly on 60 size engines. Wish I could say flooding was that rare but it isn't. As of the last couple yrs on flooded engines i turn and close the exhaust port with the piston, leave the glow plug on and let it burn 10 or so seconds. This seems to vaporize the immediate area fuel and make a charge for the plug to stay lit. I think there are a few times I've turned the engine upside down to purge fuel too. I fly a 4 bolt back plate Fox 25 bushing on 5% or 10% nitro and it doesn't act like a high compression engine. Its a sweet runner with a couple of flips...1 or 2 if I pay attention. I'm interested to see what the outcome is.
Bill P.

Online Howard Rush

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Re: Upon starting. engines blows prop, prop nut and prop washer off????
« Reply #11 on: June 03, 2019, 01:21:26 PM »
What would cause the engine to fire so hard that everything connected to the crankshaft blows off for parts unknown?                     

The engine being a four-stroke.
The Jive Combat Team
Making combat and stunt great again


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