What are the main reasons for engine vibration.



I just installed a used RCFG engine on my seagulls yak 54. Filled it with gas, put on a prop..... I will
try another prop first, just to make sure it is not that simple..... I got it to started right away. The idle trim
is not set right yet. It has a cock and load choke that I am trying to figure out how exactly to set the trim . I got it to go high speed it does smooth
out at high rpm. motor sounds good but it may need tuning. Can tuning cause motor vibration at low speeds (to rich or lean) ?? Motor is mounted tight.
Any other ideas ?
Thanks
David

No money for new engine !!

David, mechanical resonances occur at 2000 cycles per second just around the rpm of our engines at idle. Check you engine mount and structure for any weakness. Long standoffs will certainly contribute to the problem. Adjusting the low speed needle valve will have little effect except to change the rpm of the engine and avoid the resonance by luck. By the way, what is a cock and load choke? Any image of it? Dan.

Yes, tuning can cause an unsmooth engine, especially at idle. They come from the factory rich, but you say it is used so that should not be the issue. Check the prop for balance, tune it up a bit if need be and see how it goes. How used is used?

Being a used engine, if it was in a crash or had a prop strike at high throttle the crankshaft could be bent a little. You would need a dial indicator to check that and also check the prop hub as that could have been machined a little bit out of round.
The OP's reference to a "cock and load choke" I believe is a cock and lock choke. The spring loaded type that loads a spring when you set the choke on and after the engine is primed and fires you turn the choke off by advancing the throttle stick a bit and the choke snaps open.

RCGF has never had that type of choke. I have no idea what he means on that one.

If this is your first gasser, vibration, a LOT of it, might be the first thing you would notice the first time you fire it up? It's not just your engine, it's all of them. The bigger they are the worse they get too. It's why you see so many twins in the larger displacement classes. Those have pistons that fire at the same time to cancel the vibration of each other out.

On a single, there's just one piston going up and down, and only so much you can do from a design stand point to counter balance that piston while it's jumping up and down taking care of business. On idle, they can get an airframe dancing around pretty good - especially so if the wing isn't installed on it to help dampen some of that motion out. A lot of airframes require tail brace wires for just this reason as well.

Really rich settings can cause some extra vibration (almost a knock), but if you are tuning for max rpm at wide open throttle that shouldn't be an issue.

If everything is normal, that vibration will be routine. You don't even notice it after a while....

You aren't trying to get that engine to stay running with the choke closed are you? That choke shouldn't have anything to do with your idle trim.

They do have this choke it is a 20 cc rcgf rear induction motor. Maybe they don't make it any longer
Has a walbro carb. It has a spring loaded arm to hold the choke open . Then you lock the choke closed ...
I can show you the pics

Please do, as I have never seen such and would like to.

Thank you ahicks ! I think you are right. Some of this vibration is normal. and you may be
right again as I didn't have the wings on. And it is my first gasser !@ And my airframe has
a tail brace !!!! This choke setup is a pain I think. Maybe a lot of people have not seen this set up??

Here are the pics see the arm You push that arm to lock the choke closed

Sorry bud but that is not factory, that was added by a previous owner. You can remove that and hook up to a servo or any means you prefer.

Here’s the factory version on a WT-76-1. The arm on the left is the choke. The one on the right is the throttle. The first image shows the choke arm in the open position. Notice the notch in it. The choke shaft is not detented in position. The second image is with the choke closed. That notch has engaged the throttle arm forcing the throttle plate in to a fixed open position. That’s where the problem is. The throttle pushrod will either inhibit this action, get bent or put a tremendous load on the servo. When the throttle is opened further, the throttle arm releases the notch and the choke pops open. It is going to be terrible trying to live with this action unless someone comes up with a way to inhibit it. Dan.

Ah, I see. I have never seen one on an r/c engine before. Just replace the arms with standard arms such as we do when we need longer or different positioned arms. Or use a Dremel cut off wheel and cut off the dog ears on the choke and or the arm on the throttle. See pic, removing this arm would do the trick would it not?

I don't think that would work. The choke arm must have a spring on it to return it to the open position. If you just cut off the ear on the throttle arm, the spring would prevent the choke from staying closed when you needed it in that position.

So you might say just unhook the choke spring. Well, that might not work either. The chokes we are used to have a detent that holds them in either the fully open or closed position. If that model carb does not have those detents the choke would easily bounce out of the closed or open position just due to engine vibration.

I had one of those choke lock carbs on a Mintor 22. It did work but was a pain in the ass compared to what we usually have on most engines.

Mike

Yep my evolution 26 has the same thing. You have to have throttle down and trim down to set the choke. After it pops set trim and throttle to starting positions. The hook the previous owner added was probably to facilitate easier operation inside a cowl.

gbfan... My Tartan Twin has horrible vibration at idle if it is set too rich. I think it was ahicks who gave me advice on another thread to lean the idle adjustment, and that made a world of difference.

If this is a 26, there were a lot of guys pretty ticked off with that carb at one time. These 26's were one of the engines that introduced a lot of guys to gas. There was an issue with the pop off pressure set by that little spring, but not many people knew much about that yet. For that reason, a LOT of the OEM carbs were replaced when they would not pump enough fuel - even with a fresh kit installed (the kits don't include that spring!).

This carb, if you defeat that spring loaded mechanism, will not have anything to hold the choke fully open or closed. There's no spring loaded metal ball, or the detent in the choke shaft it engages to hold it in the open or closed position. Point being, if you want to scrap this type choke, it's best to plan on installing another carb.

Second issue is the engine is set up for an external pulse line. No big deal, you just need to be aware. Most carbs take care of this need with a port in the carb flange. This carb flange isn't going to have one of those. So, the new carb (if you go this route) could be adapted by transfering the diaphram cover from the old carb (the one held in place with the single screw?) to the new/replacement carb.

Easy to find/get carb would be from a DLE 20. That carb is 40.00 from Tower. Another might be the Walbro from a VV20, but not sure what that might cost. I'm sure there will be a lot of other ideas. I've never had to buy a whole carb....

Maybe this will work. Grind off the notch on the choke arm. Disengage the choke return spring. Remove the choke plate. Try to remove the choke shaft. Install a detent ball and spring. Do you guys think this would work? Dan.

Only the OP knows what his issue/problem (if it is a problem) with the choke is. We don't as he hasn't really explained it yet in any detail ...has he?

This comment, and my own experience with one just like it, are pretty much in agreement?

"This choke setup is a pain I think" (post # 9).