GT186 kit vibration

[srx600]

Hi, I have just been out for the first run on my rebuilt Series 2 and it isn't running quite as expected/hoped. At lows revs there is a lot of vibration. I dont have a rev counter but I would guess anything below say 3000-4000 rpm is quite unpleasant to ride at. Above that rpm it is quite smooth and pulls well. The vibration is enough for the top of the legshields to be vibrating back and forth about 15mm.
On the basis it goes quite well at the top end I was guessing carburation, that said if I close the throttle going down hill it still vibrates.
Any thoughts on where I should start, or would I be better taking it to a dyno to get it right ?


The spec is
- GT186 kit with GT head, squish is 1.3mm
- New GT 60/110 crank
- New Mains (FAG)
- New Mid weight flywheel SIL based (I think)
- New Read speed magneto
- 19/47 gearing
- GT tensioner
- Li150 gearbox
- New chain, layshaft, output bearing ....
- Casa Remote air filter
- Avanti Clubman
- 25mm Dellorto jetted as per GT instructions
- Needle D22
- Slide 40
- Needle Jet AQ266
- Pilot 55
- Choke 60
- Main 102 (upped from 97 because of remote filter)
When I built it I did a leakdown check and it was all fine.

[Toddy]

On personal experience I would say crank is worth checking for trueness I had the same symptoms

[srx600]

Ok cheers, will check it out.

[nsaints]

My own personal experience and nothing todo with the kit, it was the newly fitted BGM silent blocks
Which turned out to be the last thing I checked

Swapped for some SIL ones from SR and the vibration vanished.

[srx600]

My own personal experience and nothing todo with the kit, it was the newly fitted BGM silent blocks
Which turned out to be the last thing I checked

Swapped for some SIL ones from SR and the vibration vanished.

Oddly enough I have also fitted a pair of those.

[srx600]

My own personal experience and nothing todo with the kit, it was the newly fitted BGM silent blocks
Which turned out to be the last thing I checked

Swapped for some SIL ones from SR and the vibration vanished.

Hi, when fitting the SIL mounts are the holes in the same orientation as those in the BGM mounts ?

[nsaints]

Yes..
Page 72 of Sticky's latest manual covers this topic - I recommend you buy a copy, if you don't already have
you must have

'3-hole engine mount with the bottom hole at 6'oclock, the other holes facing forward'

[srx600]

Yes..
Page 72 of Sticky's latest manual covers this topic - I recommend you buy a copy, if you don't already have
you must have

Thanks for pointing me in right direction. I do have it but couldnt see it for looking.

[nsaints]

No worries and good luck
for my vibration issues I tried everything including the twisted crank - if it wasn't the mounts, the last thing on my list, I was beyond grasping at straws

Hope you have equal success

[srx600]

Crank was spot on and I couldnt find anything else which obviously a problem. I have removed the BGM mounts and fitted SIL ones, hopefully that will sort it.
The only thing I did notice was that the drive sleeve that fits on the crank and holds the front sprocket was rather a sloppy fit on the crank. It was quite a good fit on the original crank. My guess is the load in the cush drive spring creates enough pressure on sleeve to main bearing contact point to make this irrelevant. does anyone have any thoughts on this ?

[roli150lam]

it is not advisable to starve the engine of fuel when going downhill.best practice is regularly pull clutch in and give it a blip of throttle.i.e.keep the cylinder charged with cool fuel.

[ULC Soulagent]

it is not advisable to starve the engine of fuel when going downhill.best practice is regularly pull clutch in and give it a blip of throttle.i.e.keep the cylinder charged with cool fuel.

+1

[GP Kevo]

Does anyone still stock the 2 hole SIL engine mounts? Those are the best I have used.

[srx600]

Hi, I have just been out for the first run on my rebuilt Series 2 and it isn't running quite as expected/hoped. At lows revs there is a lot of vibration. I dont have a rev counter but I would guess anything below say 3000-4000 rpm is quite unpleasant to ride at. Above that rpm it is quite smooth and pulls well. The vibration is enough for the top of the legshields to be vibrating back and forth about 15mm.
On the basis it goes quite well at the top end I was guessing carburation, that said if I close the throttle going down hill it still vibrates.
Any thoughts on where I should start, or would I be better taking it to a dyno to get it right ?


The spec is
- GT186 kit with GT head, squish is 1.3mm
- New GT 60/110 crank
- New Mains (FAG)
- New Mid weight flywheel SIL based (I think)
- New Read speed magneto
- 19/47 gearing
- GT tensioner
- Li150 gearbox
- New chain, layshaft, output bearing ....
- Casa Remote air filter
- Avanti Clubman
- 25mm Dellorto jetted as per GT instructions
- Needle D22
- Slide 40
- Needle Jet AQ266
- Pilot 55
- Choke 60
- Main 102 (upped from 97 because of remote filter)
When I built it I did a leakdown check and it was all fine.

I am still having issue with this, I have done the following but still no improvement
- New BGM flywheel
- Replaced BGM engine mounts with SIL three hole ones
- Checked crank for trueness, it was spot on
- Refitted original SIL stator
- Refitted the AF Clubman copy
- Dropped needle one notch (back to 2nd notch now)
- Reduced main jet size to 97 (back to 102 now)

The main issue is when I change up from 3rd to 4th the engine note sounds flat and it vibrates a lot. It runs fine outside of this rev range, above 4000 or so it sings along and at a low throttle around town is fine.
It feels like carburation to me, but changing the needle position didnt effect it, would it be worth playing with idle jets ?

Does anyone have any thoughts on what to check next ?

[Toddy]

Is the woodruff key sat proud on the crank making flywheel sit off a bit ? That’s one thing worth checking as this can cause vibration

[Storkfoot]

Your gearing. Have you quoted it correctly ?

You say you have an Li150 gearbox. Assuming this is the Italian Series 3 box, that has a fourth gear of 34 teeth on the gear itself and 20 teeth on the cluster. If you have a 47 rear sprocket with a 19 front sprocket, I’d suggest your final drive ratio of 4.20 is at least a large part of your issue. Wonderful kit as the GT186 is, in my opinion, it won’t pull 4.2 irrespective of what exhaust you fit.

My GT186 did many thousands of miles with a MB/BGM Clubman on the same gearbox but with 46/16 sprockets. That is a 4.9 final drive ratio.

[srx600]

Your gearing. Have you quoted it correctly ?

You say you have an Li150 gearbox. Assuming this is the Italian Series 3 box, that has a fourth gear of 34 teeth on the gear itself and 20 teeth on the cluster. If you have a 47 rear sprocket with a 19 front sprocket, I’d suggest your final drive ratio of 4.20 is at least a large part of your issue. Wonderful kit as the GT186 is, in my opinion, it won’t pull 4.2 irrespective of what exhaust you fit.

My GT186 did many thousands of miles with a MB/BGM Clubman on the same gearbox but with 46/16 sprockets. That is a 4.9 final drive ratio.

Hi, hadn't spotted that it looks like a typo on my part. I just checked and I have 17/47 which gives me 4.7 final drive

[srx600]

Your gearing. Have you quoted it correctly ?

You say you have an Li150 gearbox. Assuming this is the Italian Series 3 box, that has a fourth gear of 34 teeth on the gear itself and 20 teeth on the cluster. If you have a 47 rear sprocket with a 19 front sprocket, I’d suggest your final drive ratio of 4.20 is at least a large part of your issue. Wonderful kit as the GT186 is, in my opinion, it won’t pull 4.2 irrespective of what exhaust you fit.

My GT186 did many thousands of miles with a MB/BGM Clubman on the same gearbox but with 46/16 sprockets. That is a 4.9 final drive ratio.

Hi, hadn't spotted that it looks like a typo on my part. I just checked and I have 17/47 which gives me 4.7 final drive.
I also checked tht gearbox ratios when it was apart and it is an Li150 box.

[Storkfoot]

A final drive ratio of 4.9/5.0 is more suited to a standard GT186, in my opinion. I ran an Avanti Clubman too for a short time although, in hindsight, the MB/BGM was better suited to the kit.

Anyway, whilst your gearing may well show itself in a lack of power when you go from third to fourth, especially when going up hills, I can’t see it being responsible for such bad vibrations unless the jetting is totally wrong. I can see that you got your jetting from Richard Taylor’s guide so I wouldn’t have thought it is far out, if at all.

That said, in my fairly limited experience of the PHBL25. I have found that a leaner slide and richer atomiser/ needle set up helps so, for example, I have a Casa 185 with standard exhaust. I have a 45 slide with Aq270/d29 on that. It hasn’t been on a dyno but I know it runs much better than it did with the usual 40 slide and leaner atomiser/needle.

As regards the vibrations, have you checked that the forks haven’t come loose in the headset, the front hub halves aren’t coming apart, the nuts on the engine bar and shock absorber aren’t too tight, or that anything, such as the inlet manifold isn’t touching the frame when you are riding?

[srx600]

After the comments about gearing took it out last night and using a GPS speedo thingy and rolling the throttle on in top I found that the vibration occurs between 36 and 44 mph which equates to around 3,400-4,200 rpm. When changing up from 3rd to 4th I am dropping into the middle of this window, so changing the gearing may help to lessen the effect of this.
As for the chassis etc, I can remove the rear shock and drop it down to the bump stop without anything touching, as for forks etc nothing has changed from when I had the standard engine which didnt vibrate this way.
With this

the nuts on the engine bar and shock absorber aren’t too tight

I didnt quite get why being too tight would be an issue.