Whirley Bird

My Feedback: (1)

My friend decided that after 4 years It was time to start flying his TH trainer MK II with a 60 OS engine on it.
The engine was stuck tight so I used oil and took my time and rocked it loose.
Now it seems to flip over easy with my finger and all feels normal.
However when turning the prop slow,I feel the piston come up on the compression stroke, then it seems to fall into a notch right after TDC.
So that leaves it in a condition where I need some extra finger pressure to turn the prop back either way.
As I said above when flipping the prop in a normal manner it seems fine but I made no attempt to try to start it.
Is this normal?
Vegas/

Dorsal

My Feedback: (17)

Sounds normal to me - it's the area where the pushrod bushings are going from compression to tension and there's probably a ten-thousandth or two of slack in there. There has to be a little clearance or the crankshaft wouldn't turn.
Put some fuel in the tank, hook up the glowplug and fire that baby up! Make sure to run it rich and NOT at full throttle to let it break in.

Whirley Bird

My Feedback: (1)

Were going out now.
I'l run it but it's the 1st time I ever experienced anything like this.
I pulled the muffler and I see this happens just when the piston wants to start going down and then it just seems to fall into a spot when the prop feels locked
Well run it and i'll post the results here later
Vegas.

Jetdesign

My Feedback: (8)

Every one of my engines does the same thing. Totally normal

bdtsr

My Feedback: (5)

This is normal. There is a point at TDC when the piston is stopped but the crank swings across from one side of the stroke to the other. From compresion to power. Nothing to worry about.

BantamTJ

If you were to pull the backplate off the engine, & observe the same piston position you'll notice that at this point is where the connecting rod travels it's extent, then "breaks over" from TDC to continue on it's downward stroke, rock it back & forth and observe the rod, you'll see, it'll all make sense then.
Good luck just break in in CORRECTLY & it'll be fine.

Whirley Bird

My Feedback: (1)

[quote]ORIGINAL: bdtsr

This is normal. There is a point at TDC when the piston is stopped but the crank swings across from one side of the stroke to the other. From compresion to power. Nothing to worry about.
[/quote
I know what you mean and I know what that rod reaches TDC,the crank is sort of in a fulcrum position but what I don't understand is when I move that prop just a tad, it seems locked.
Since it already did pass TDC I assume it would be smooth from that point on.
This happens when the piston starts going back down just a few thousands of an inch from TDC
Well flipping the prop it feels normal.
I ran a full tank and made adjustments as per instructions.
It flew like a raped ape so tomorrow I'll be flying mine.
I have no choice but to fly RTF plane due to a part paralyzed hand so these RTF planes allow me to do some mods and enjoy the hobby[8D]

pimmnz

The condition is totally normal, the reason the piston doesn't want to come back down is because, having squeezed all the air out, the piston/cylinder seal isn't letting any air back above the piston. The partial vacuum created by try to pull the piston back down is trying to suck it back up. All the signs of a good initial set up. Run in carefully and never run it lean and it will look after you for a long time.
Evan.

Whirley Bird

My Feedback: (1)

Thanks pimmnz
That does make sense but for the piston to hold a vacuum all night long seem normal?
I think not but it runs smooth.
I ran it again with 5% nitro and ran it very rich
I have a new not run yet EV 61 and I can just almost feel that
Oh I forgot to say that It does the same with the glow plug removed so then the vacuum would not be an issue.
It's a bugger and I can't figure out what is causing the notch where the prop seems to fall is 1 spot and almost seems locked as if I was forcing the piston against the glow plug but this happens when the piston starts coming down and no plug installed.
But it runs well so why fix what isn't broken.
Vegas/[8D]

MikeL

My Feedback: (3)

It's because of it being an ABC-type engine. Anything with a taper-fit piston and liner is going to have that pinch, or it wouldn't work properly when running.

Whirley Bird

My Feedback: (1)

Yes,
I understand that and I have been a mechanic most of my life.
However,even with the glow plug removed,i turn the prop slow then it just hits TDC then a turn it a fraction more and it feels like it's between TDC on both sides and it seems to take a good amount of pressure to move it past that point plus it makes a noise as if it were dry and had no oil.
After a break in and 3 full flights it still does it.
I decided to tear it down and I will let all of you know what I found.
Thanks to all who responded.
Vegas/

BantamTJ

Let me say this once more, & if you insist on disassembling a good engine, take note of what was mentioned in the above post, possibly if you start there, you'll need to go no further, also note that when the conn-rod is remove,... so is the condition,... & you'll notice also the the piston no longer exhibts the condition as well, (except for the "pinch" of ABC engines), link those two back together & you get what's mentioned in the post here.

Jetdesign

My Feedback: (8)

Yeah I had to take two of my engines apart for cleaning and this phenomenon doesn't happen when the engine is apart, only once the back plate and head are screwed back on. Maybe it's worth the piece of mind to just remove the head and back plate to see it stop and hopefully stop there.

OP have you taken an RC engine apart before? This video is priceless (2 parts):

http://www.youtube.com/watch?v=TuzEco3EDUM

I know you're a mechanical guy but we all can use some tips whenever doing something for the first time - marking the back side of the con rod, etc.

ElectRick

My Feedback: (2)

If this is indeed an ABC engine, you do NOT want to run it slobbery rich. Not even for break-in. You WILL ruin the engine otherwise. Slobbery rich (four-cycling) break-in is for ringed and lapped piston engines only.

ABC engines need heat from combustion to expand the cylinder slightly at the top (cylinder is tapered) to establish proper running clearances. You cannot achieve this by running it slobbery rich, as the extra fuel and oil keeps the combustion chamber too cool. If you do run it excessively rich, you'll be breaking in and wearing out the piston/sleeve fit all at the same time.

A slightly rich two-cycle needle setting is fine for break-in runs, maybe four or five clicks to the rich side of peak, depending on how sensitive the needle is just so you see a somewhat smoky exhaust. After the fit loosens up a bit, run the needle in until RPM's are peaked out and then back it off two or three clicks for flight.

Rick