lightened flywheel question

[mickey c]

Hi all
As briefly as I can, may I pick your brains for some advice please?
I’ve been steadily improving my GP, it’s had a mugello for a while and I’ve recently put a sporty pipe. The good news is it runs really well and feels a lot quicker but does start to shake when I open her up. I know this is par for the course to some extent.
I do have a lightened flywheel (small one, looks like it’s off a Vega) knocking around my garage. I’ve never used one before. My question is would / could replacing the flywheel for this may be reduce the shaking a bit? It’s got a SIL crank but would this suddenly having a lot less load cause any problems? And would I reduce the cooling ability of the scoot? This is my work horse that I use for work and generally thrash about but equally sit in commuting traffic a lot!
It’s got a pretty new BGM 12v electronic stator in case you experts need to know that too.
Thanks as always for any thoughts.
Mike

[missing lynx]

I wouldn't have thought it would fit and you would need an electronic one with a gp tapper

[mickey c]

That's what I think / hope I've got. It's what I bought anyway. But good point, I'll check it with a spare crank that is lying around in the same cupboard

[Storkfoot]

The AF light flywheel looks like a Vega flywheel. It could be one of those.

As to whether your crank is the cause of your vibrations, it's certainly possible. It could also be numerous things such as engine mounts worn out or the nuts too tight, front hub, forks, bent frame, four stroking, carb manifold touching frame, the list goes on sadly.

Have you discounted forks, hub, ill jetted carb causing 4 stroking and engine mounts?

[Warkton Tornado No.1]

Your vibration needs sorting & could be many things from a twisted crank, failing engine mounts et cetera.

However, a Vega/J flywheel can be made to fit with the correct centre, preferably GP, utilising Series III stators with points.

It is a very practicable conversion as there remains ample inertia for, say, a 200 engine. My SX 241.5 ticks over nicely with one fitted.

I have not had over-heating issues though I will be modifying the flywheel cover as a precaution to stop any risk of cavitation as there is a significant difference in fan size & gap from fin to flywheel cover.

Loads of road & race riders ran the same fan with AF electronic ignitions without modifying their cowlings.

[mickey c]

thanks for the replies guys - it is an AF flywheel so will give it a try.

I have seen there are probably a hundred reasons for a scooter vibrating, although I didn't know about the engine bar nuts being too tight! I will check few others bits out but think its pretty solid (touch wood).

I've not been on here for a while as I'm just finishing a Vega which I believe "shake your teeth out" as my local scooter shop owner laughingly told me so been trying every trick / good build technique to minimise this.

but regarding my GP, it was not very shaky but obviously now it goes faster, it vibrates more reaching a speed its not been up to previously - so much so that I get scared and slow down! just looking for a potential quick fix I have lying around otherwise I spent lots of money to do a speed that is not comfortable.

[Warkton Tornado No.1]

It’s very difficult to analyse where the vibration comes from, but, nonetheless, I am sure we are all concerned.

Purely in an attempt to help, perhaps you could confirm whether you feel the vibration is related to the speed of the engine or the speed of the bike?

If its engine related, then most probable of the causes would be your SIL crankshaft unless it has been rebuilt by a competent engineer.

IMO, @ the very least, it should have had a decent big end bearing fitted & checked for true, preferably a complete good quality connecting rod used too. SIL crankshafts are only good for their webs & should be rebuilt even for use in a standard engine, let alone a kitted one as yours is…

[mickey c]

Thanks loads for your input & advice. A very good question too which I should've considered myself!
Now I think about it, I think it's the speed of the bike, not the speed of the engine. I've not red lined / had it screaming, it's more it gets to 60 ish with gas / throttle to spare but is shaking so much I'm scared & ease off! It could be me shaking at that speed.
Prompting these thoughts does lead me to think something in my build quality, forks, engine mounts etc. Only done about 3k miles with all new, quality bearings, springs, shocks etc! Think I'll change the mounts anyway as I think I read BGM 1's are good.
I am also thinking your thoughts on my crank are valid and worrying! I'll look at this when something (inevitably for me) major happens & I need to strip the engine again.

[rossclark]

Worth just going round and checking everything is tight. Axel nuts, shock, engine mount nuts, hubs. Spin the wheels and check they're true, no rubbing or rattles. Chock the stand up and get a grip of the fork legs and feel for any lift or back and forward movement.

Take the flywheel cover off and grab the flywheel see if there's any in out or up down movement which might indicate bad bearings. Pop a fixed wire pointer on one of the cowling screws and place it very close to the flywheel edge, take the plug out and turn the engine over by and to see if there's any sign of run out on the flywheel. While you're doing that rock the fly wheel around and see if you feel any knocking which might indicate worn big or small end bearings.

I h ave heard reports that the BGM mounts are very 'firm' which tightens up the handlin but can transmit more vibration

[mickey c]

thanks again
I am changing the rear hub for a series 2 one and 1 SIP rim I picked up recently (although its a right squeeze) and i'll swap the front rim when I can afford another SIP one. I did get paranoid reading about failing rear hubs. Whilst doing this I was going to check everything is tight.
But Ross that flywheel check is gold dust - I've not heard / read of that before and obviously now makes perfect sense!
maybe I wont get the BGM mounts then and just get some other new ones to see if that smooths things out a bit!

[mickey c]

Just to up date; I managed to get in the garage last night & put the lightened / little flywheel on. It went on easily, taper appeared correct as it spun, engaged the crank & piston etc but as soon as I tightened up, it seized solid! I tried a couple of tones with & without a washer but the same wax time! I've gone back to the original flywheel & will check other potential causes. I will also book it on my local dyno to check if my set up can be improved.

[holty]

i would guess the the flywheel is hitting to bolts that hold the stator in place, af supply shortened bolts to overcome this, some people have the inside of the mag housing machined back 1 or 2 mm to give clearance, i skimmed a bit of the rear of my flywheel so it wouldnt touch.

[mickey c]

I guessed it could be the bolts into mag flange & possibly / probably the stator itself. Someone mentioned they came with slightly smaller specialist stators. I'll consign this to the next project pile or wait for an engine to blow up! Meanwhile I'm just checking other bits to reduce the rattle.