212 & built motor vibration

[noseoil]

I've seen threads about 212cc predator engines & vibration, just wanting to know about other people's rides & what they find is "normal" with a modded motor. The build is a 212 hemi, billet rod & flywheel, 11:1 Bullfrog piston, Black Mamba cam, mild porting, header & Mikuni VM22-133 carb with GPS intake. Nice combination & it seems to make good power once it's up on the can a little bit.

It runs well & is broken in now, but here's what I've found with this motor setup. This is my first modded motor build, so I don't have anything to compare it to. With the lighter flywheel & different internals, I have a vibration which runs from about 4,000 - 5,200 rpms. It's not really all that bad, but it's nasty until the speed builds up & it climbs out of this rpm range. Mostly it's just annoying, but it's bad enough to cause some discomfort when riding.

I know there's no real way to balance these single cylinder motors, unless it's for a given rpm range, but is this going to be a problem later on if I stay out of the "bad" range & try to run above & below the shakes? It's pretty reliable the way it's running & so far there have been no issues with the build (other than jetting for the first time on a Mikuni, PITA until I figured it out). It does kick once in a while, if I'm not careful with the starting rope & getting on compression correctly when I pull, I wear gloves now just in case.

Anyway, just wondering if anyone else has seen this to be a normal consequence of a build & harmonics at a given rpm range. Thanks.





P.S. Picture of final assembly is with the Chikuni VM22, not the VM22-133 which it's running now & tuned with.

 

[danford1]

I built up a mild Predator, billet flywheel, billet rod, CL2 cam, pipe, jets etc. It ran Great but the vibration was Much, Much more noticeable than stock. It had to be from the light weight ARC flywheel. It vibrated with the stock flywheel but that was mild compared to the ARC wheel.
Some guy came along and HAD to have it so of course I sold it, he didn't care about the vibes, he loved the bike so it went bye bye.

Here is something that helped. It didn't cure it, but it did help.

Danford1

Engine Vibration Isolation Mat.
https://www.oldminibikes.com/forum/mini-bike-parts/152090-engine-vibration-isolation-mat-15-free-shipping.html

 

[ole4]

Noseoil that sounds like a potent combo! I am going to assume you have checked for tight mounting bolts any engine mounting plate cracks etc. engine vibration forces increase with rpm. When you feel a vibration only within a rpm range it is usually because the engine vibration frequency is in resonance with the frame. Changing balance factor moves the vibration forces around the horizontal and vertical planes if the cylinder was upright. In the predator cal is at an angle. One thing you can try is to make a head steady which would connect the top of the head to the frame. They were used on the British twins back in the day as as well as some minibike.

 

[noseoil]

Ole, thanks for your input. As far as I can tell, it's all tight & there weren't any cracks I could see when I did the swap from the stock setup. I guess I could try damping out the vibration against the frame, just to see how it affects this rpm range & vibration. Hadn't thought of doing it, but it's not a bad idea.

I've been chalking it up to a greatly reduced damping mass in the flywheel, a different piston (I think it's heavier than the stock flat top, but I didn't weigh it to compare them) & the higher rpm range available with this combo. Guess I should think about a head support next, hmmm...

 

[Daniel Coop]

Noseoil, all mine do about the same thing in about the same RPM range too. The more seriously built, the more they seem to vibrate. Just keep it above 5200! Lol. In all seriousness though, that isolation mat and head steady that Danford1 and Ole4 suggested are both good suggestions I might have to try myself.:thumbsup:

 

[noseoil]

Thanks Daniel, Danford1 & Ole). My concerned with the vibration mat approach is that the mounting bolts will squash the mat ok, but it will create "pressure points" in the block as the fasteners crush the mat unevenly at the mounting points, due to the mat thickness & block webbing. This is putting the block in a bind & possibly making fractures at these points, when rpms are added over time & the block tries to flex. The block is designed & reinforced to take loads on a flat, hard mounting surface, so I'm not sure I want to do this on mine. Maybe a thicker plate would be better in this case to help smooth things out a bit?

Glad to hear the vibration is "normal" on a built motor & that it's just part of the trade-off for making more power. If it's just part of the build, I can accept it & move on. I'm going to look into some type of damper or strut to attach the block/head to the frame somewhere & see how it works. It would be nice to take some of the shake out of the bike, since 4,000+ rpm is a decent place to ride, just not really comfortable. I have 2 more frames, so I'm thinking of doing a different bike for making more power & designing it to be a little more stout in general.

Maybe a 1/8" or 3/16" mounting plate & a strut to the motor at the head would be the way to go, just to cut down on vibrations in general. I have to look at this for a while, to see what it takes to make it simple & stronger without getting carried away & making it into a lead-sled.

This combo makes good hp & pulls really well, it's just a hand-full at times, especially out in the desert at speed in sand, ruts & dirt. Soft soupy sand is really exciting, but mud is worse!

 

[ole4]

This is the stock head steady on my Atlas 5000TT

 

[noseoil]

Nice Ole, did it make a large difference in the vibrations, or a small one?

 

[danford1]

Noseoil, I can understand your thinking on the mat crush and web strength etc. I also understand a thicker mounting plate like 3/16" would be a good idea. How about doing both? Add the thick plate on top of the mat and tighten away. Would'nt hurt to try it.
Just thinking out loud...

Danford1

 

[TheDullCarbide]

I remember [MENTION=55159]BWL[/MENTION] had his Predator vibrate real bad towards the end of it's life. But that was because the jug split from the crankcase.

 

[ole4]

That is the way it came from the factory.

 

[BWL]

Yeah, these single cylinder motors are unbalanced by definition, but I had a bad clearance issues with the valves as I remember. I'm going with big blocks now.

 

[owend]

A few years ago, I used some of those small engine rubber engine mounts found on pressure washers to help with the vibration. It did work, but I had to use a tensioner with a stiff spring because they flex and my chain would fall off.