i read on another post that says "If the connecting rod (or anything else was bent) then the engine either would not run, or would vibrate so badly you'd know something was wrong",made by DBCherry.
I have been driven crazy about some engine viberation,it really viberates at certain rpm's and just above a high idle.This last weekend i spent aboout half a day workiing on it,looking for something that was loose,the only thing i could find was a screw kinda loose from the engine mount to the firewall,it seemed to be better but the vib is still there,i'm not afraid to take the engine apart to check out a bent rod or anything else BUT should i take it somewhere that works on these ?,or a competent LHS ?.Engine is .61 Evolution.
Thanks

You haven't changed props or spinners, have you? Engine vibration could also be magnified by a loose fuel tank, something cracked or broken in the airframe, or even loose landing gear. If it really seems like it's vibrating more now than before, and you haven't done anything to the engine to cause you to think your con rod was bent, chances are something, somewhere on the plane is loose. Check your whole plane over carefully, especially firewall to fuselage joint and the wing box joints. Also check that your empenage is still secure. Some vibration is normal with a small single cyl. engine, just the nature of the beast. The larger the engine, the more intense the vibration will seem.

Andy

I have changed two props,i'm on my third one now,the second prop was way too sloppy,like it was reamed out too big.I noticed the sides of the plane,forward of the firewall really shakes,i'm beginning to think it's gonna shake the plane apart.
Also had a small crash all by myself in a open field that was recently planted but the ground was soft.It was viberating before the crash.[]

My SS 40's cowl cheeks shake at certain rpm's, but they've never come apart. Engines have harmonics, certain rpm's where the vibrations are worse. Sometimes the type of prop will magnify them, plastic props are probablly the noisest, where wood props seem to have some natural dampning characterists (sp?). Just keep an eye on the firewall to fuse joint (you did reinforce it with epoxy and back it up with a piece of stock while building, right?) and your plane will be fine.

Andy

I didn't build it,it was a RTF

It's still a good idea to go over teh high stress joints with either epoxy or thick CA, just to make sure whoever was building it didn't have a bad day. Especially the firewall joints, and the landing gear blocks.

Search around and you'll find lots of posts on the subject of reinforcing glue joints on RFT or ARF aircraft.....


Andy

Thanks for the info ,now i guess everybody knows i'm not perfect at my flying.It just rained cats and dogs here so the flying field will be too wet to fly tomorrow,that will give me a chance to really look it over and do some reinforcing with some epoxy.I did take the fuel tank out and looked it over real good with a flashlight,looking into the corners and stress joints as best i could.

Just got a idea,if the rod was bent,wouldn't i hear some metal noises ?,maybe i'll put a two blade good balanced prop on it and see if it's the same.
Thanks

Bent rods do different things based on how they bent... The end result is almost always a snapped rod (occasionally they bend enough before breaking that the engine won't run) but the symptoms can be VERY mild before it snaps.

A bent crankshaft can be the result of a crash. this is more likely than the bent rod, which is typically from hydraulic locking or sticking something in the exhaust port to allow cranking down on the prop nut. (Or using the infamous devices sold to stick in the glow plug hole... which I consider to be con-rod breakers.)

Yuo can CAREFULLY watch a prop as it rotates with the ctrankshaft and see if its badly bent. minorly bent cranks can only be verified with a "dial indicator."

Continuing to run an engine exhibiting problems of this nature with multiple props... you'll wipe out the engine.

The loosened engine mount bolt is sometimes a symptom of the vibration... It being loosened allowed the engine more freedom to move due to some mbalance... and thats why tightening helped. But with the symptoms you describe... the engine needs help before further damage is done (if its not too late already).

This won't solve the root source of the vibration, but this might be one possibility:

[link=http://www.sullivanproducts.com/MotorMountsMainFrame.htm]Sullivan Products[/link]

Those look like some good mounts,i have seen some other types but i want to get to the bottom of the source,there has to be a reason why it does this,seems it started not long ago.
Thanks

I had the same problem yesterday, excessive engine vibration with O.S. 46Fx and I lost 1 carb screw 2 times up in the air. Prop was balanced and tried different prop also. Mount, firewall was okay.

The problem seems to be the spinner backplate, it is the GP nylon and the back plates looks like it is bent. It was not in a crash or something, but somehow it is bended.

I will try a prob nut and to see whether it makes any difference, if not then connecting rod might be a problem for me.

The worst part of 1 carb screw loose in flght was to wait for the tank to empty. As the carb screw came loose there was an air leak from bottom and the engine kept running a little less than middle thorttle.

I'd say it's impossible to tell over the web if there is a real problem there or just the normal vibtration we get from our single cylinder engines. No matter how well balanced everything is, these engines will vibrate, and that vibration will vary with RPM. And at the right RPM it will resonate with the fuse or other parts of the plane and make it look like the whole plane is about to blow apart.

To tell if there is a real problem, you need to get with someone who's run a lot of engines, or better yet, find another engine of the same type and see if yours vibrates more than the other one.

Yes, it's possible you bent something, but I honestly don't think it's likely. Over the years, I've found an awful lot of creative ways to destroy an airplane, but I've never bent a conrod or even a crankshaft, it's actually pretty hard to do.

First thing I'd do in your case is replace the prop and spinner, and check all the bolts. Then run the engine at different RPMs and see what it does. If you find a spot where it really vibrates, increase or decrease the RPM just a hair, just enough to hear the difference. Does the vibration drop off noticeably? If it does, then I'd say you have nothing to worry about. If it remains pretty much the same, then you may have a larger issue.

Here's the virdict so far,after messing with it saturday i put it back,today i was gonna start it with the 3 blade prop to refresh my memory then change to the 2 blade,however after i ran the three blade i found it to be pretty smooth until 3/4 throttle then the cheeks did there flapping and you could hear it viberating..how do you spell viberating in the first place ?,so i took it up to full throttle and it was about the same,i slowly moved the throttle back and just above 1/2 throttle position it smoothed out,all the way down to idle,i pinched the fuel line and it ran for a few seconds before it almost quit.
HOWEVER,the highspeed screw is past the stop about 1 turn and it really runs,if i screw it back to the original place the engine don't run as fast but i'm thinking it would be to lean and not run as fast,maybe i'm running the engine too fast so that is where the vib is coming from ?.
Thanks

Ok,i cleaned the plane and brought it back in,looked at it and said what the heck,so i reamed my 2 blade spinner,put the 11X7.5 prop and took it outside,fired it up,runs fine except just before 1/2 to 1/2 throttle and vibs but at full throttle it runs smooth,didn't adjust the carb for the 2 blade.It's back in the house now.

If you are using the right prop you won't be running the engine too fast!

If you start with the needle rich (out) and then screw it in then the RPM will rise to max. At this max RPM most sport flyers will unscrew the needle a tiny bit to make the engine run rich. If you continue to screw in the needle at the max RPM point then the RPM will drop. This is the point at which the engine is running lean. It will overheat but will not be at max RPM.

If you want to check the rough needle setting without a tach then pinch the fuel line hard and then let go. If the RPM rises then the engine is rich and can be leaned out a little. If you pinched the fuel line and let go, and it nearly quit then it is too lean.

Is the prop you are trying balanced? Are you using a spinner? (if so, run it without the spinner and see if it is better). Are you using the same engine mount as before?

The pinch test shows it will run a few just seconds when i pinch it,both props are new,the trainer prop is pretty well balanced and the two blade is balanced,havnt tried without the spinners yet.
Thanks

I had a problem like yours and the back plate of the spinner was the problem When they cast or made the back plate they made one side thicker than the other I had to take it back to the LHS. Bob

When i first got this plane everything seemed ok,when i changed props tonight i changed the back plate as well but i'll look them over.
Thanks

Do you pinch the fuel line and let , or pinch it and hold it?

If you hold it then the engine will obviuosly stop. You need to let go to hear the RPM change. If you are letting go, and it is stopping afterwards, then I would richen your engine quite a bit and try again. If you don't then you run the risk of damaging the engine.

I feel like a jaskass,i checked the backplate that came on the plane,it's out of balance,not that much but still......I put the two blade backplate on without checking it,i checked it and it not only wobbles,it's out of round and really out of balance,i'll take it back this weekend,that will give me an excuse to go back to the LHS.

*I'm getting the feeling that it's best NOT to TRUST everything just because it's new and just came out of the package.* []

Give that man a cigar!

I've seen props come from the package with the hole drilled slightly off-center. Talk about a vibration generator.

The engine vib is at the same rpm with or without the Three blade spinner which is pretty close to balance.

* I think i'm worring too much lately,lots on my mind besides a plane right now *

I think i found my problem.