View Full Version : Vibrating belt

profqwerty - n/a
2-Jan-08, 04:53 PM
I've (finally) managed to resurrect the 447 (went out and bought some brand new petrol, and started it on a very weak oil:petrol mix - is this bad?). however the belt vibrates sideways a LOT.

Different rpms show the difference resonating frequencies http://hovercraft.org.uk/images/icons/smiley_icons/icon_cool.gif

But I'm sure this isn't good for it. Is the solution to simply tighten it up - shim it a bit more? or do I need an idler pulley?

Ta http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif


Iceman - n/a
3-Jan-08, 08:17 AM
The oil mix ratio should be 50:1. I would not deviate from this.

Belt vibration is always there but in different degrees. I get the feeling your engine is not running very well which will lead to extra belt slapping. You should do two things, check and adjust belt tension and investigate your engines rough running.

hovmart - n/a
3-Jan-08, 09:06 AM
if its an old system it may be the pulleys are worn this will mean they do not engage corectly and this will give you problems

kevinf - n/a
3-Jan-08, 10:02 AM
Are you running old nylon pulleys? If so, they are prone to swelling over time - apparently they absorb moisture - this will lead to a mis-match between your belt and the pulley causing the belt to jump and cause excessive vibration. Check for this by wrapping the belt around the pulley and checking to see that the belt stays in mesh all the way round. If it starts to ride up onto the teeth, the pulley either needs re-cutting to size or replacing with a decent aluminium one. I had this problem many years ago - it cost me a lot of busted blades before I found out the reason.


profqwerty - n/a
3-Jan-08, 01:04 PM
It's a brand new system from Ken...

I do agree with Iceman - the engine did seem to be rougher than I remember - any ideas?????

I'll try tightening it a little bit this weekend see if that works.

Iceman - n/a
3-Jan-08, 01:07 PM
Hi Toby,

how old is the engine? is it points or electronic ignition?

profqwerty - n/a
3-Jan-08, 02:54 PM
I would say pretty old to very old. I have no idea - I got it from a bloke off his microlight. It looks and feels old (eg wires look worn).

It's points with bosch coils. the bolts tapped into the crankcase holding the coils have mostly stripped the thread from inside the holes!

I re-did the ignition connections though so they are ok (i hope!)

Iceman - n/a
3-Jan-08, 03:08 PM
Its hard to say what the problem might be without seeing the engine but you could check the usual and do a little bit of maintenance to improve matters.

Things to check are:

fuel system - clean

air filter - clean and oiled

plugs/caps/HT leads - good condition

compression - good and balanced

timing - within tolerance

points - clean

max rpm - 6500-6800 (with carb slide fully open)

Mart366 - n/a
4-Jan-08, 12:47 PM
Its not the problem i possibly had? which David correctly guessed.

Are you running a flexi coupling? i had a problem where the flexi coupling was causing bounce, and preventing the second cylinder from firing correctly.

A quick further pull on the starter usually resolves the issue.It did for me http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

With regards to vibration.

I had a belt which had some vibration / slack and decided to fix it http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif (after having a very good weekends racing)

I re tensioned the belt, by adding washers under the bottom plate, and it was disastrous, http://hovercraft.org.uk/images/icons/smiley_icons/icon_sad.gif the belt wandered and came off the back of the pulley. Wrecked the pulley, and the belt. Turns out my re tensioning had affected the alignment of the pulleys/ shafts

I ordered a new belt, repaired the pulley, and reset the belt making sure both drive shafts were parallel and aligned, and not over tensioned.

I now have some vibration at tickover revs, but under load its fine.

I used a straight edge clamped onto the back face of the upper pulley to set the vertical plane for the back edge of the lower pulley. i then inserted a tapered shim in my case to alight the pulleys in the horizontal plane. this stopped the belt from wandering excessively


profqwerty - n/a
4-Jan-08, 12:58 PM
Yes it's flexi coupling.

I'm using K&M's 'lower pulley/bearing block', on a K&M frame so I'm confident it's aligned http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

I will try putting another washer on the bolts first, and the pulling the starter again. I'm pretty sure both cylinders are firing - it sounds ok in that sense.

under load it's worse. I'm assuming the belt is stretching slightly and so has more slack in it.

I'm going to try it out tomorrow morning - bring on the weekend!