Belt tensioner ...

The 3 1/2 forum
Post Reply
123morini
Posts: 49
Joined: 30 Nov 2015 12:19
Location: Chenee Liege Belgium

Belt tensioner ...

Post by 123morini »

Hello ,

Years ago when I improved a 350 engine for races I made a new alternator plate to fit a belt tensioner to prevent belt slap . And I also changed it to fit an other type of alternator .
Than that was working really whell .

Photo ,

Here the back of the plate
Plateau prevu pour un tendeur vu de l'arrière.jpg
Plateau prevu pour un tendeur vu de l'arrière.jpg (111.5 KiB) Viewed 366 times
And here the belt tensioner .
plateau alternateur avec tendeur.jpg
plateau alternateur avec tendeur.jpg (122.84 KiB) Viewed 366 times
Chris .
simonnorthroad
Posts: 495
Joined: 16 May 2017 10:57
Location: Bath, UK

Re: Belt tensioner ...

Post by simonnorthroad »

Clever! But if you buy a properly sized belt, and it's not ancient, it wont sag or 'slap'.

I wonder how much power you lost for racing, dragging the belt over that unnecessary tensioner?
123morini
Posts: 49
Joined: 30 Nov 2015 12:19
Location: Chenee Liege Belgium

Re: Belt tensioner ...

Post by 123morini »

Hello simonnorthroad ,

Thank you for your answer

But I would say why I did this work . When I have a bike that I do not know when the last service was done I always change the belt .
And for a road bike it is ok if the belt has a small slap .

For the races when I tuned the engine I always used new belts . But sometimes the belts don't always have the right tightness and they slap .
On my tuned engine as the ignitions captors are on the camshaft when the belt slap the ignition timing is not perfect betwen 5000 - 9500 revs and this is why I made a tensioner .
And every times when I change something or when I adjust the belt tension I do it on the dynamometer .

Concerning about having an idee of the power here is a small video on a French track and it is between a honda 500 and my 350 . The action is more or less at 8"50 min . I hope you enjoy it .

https://www.youtube.com/watch?v=JUfLsO6KziY

Chris .
simonnorthroad
Posts: 495
Joined: 16 May 2017 10:57
Location: Bath, UK

Re: Belt tensioner ...

Post by simonnorthroad »

cool!
User avatar
72degrees
Posts: 1574
Joined: 31 Aug 2007 21:24
Location: West Midlands

Re: Belt tensioner ...

Post by 72degrees »

123morini wrote: 15 Feb 2026 22:02
Concerning about having an idee of the power here is a small video on a French track and it is between a honda 500 and my 350 . The action is more or less at 8"50 min . I hope you enjoy it .

https://www.youtube.com/watch?v=JUfLsO6KziY

Chris .
Great! That goes well. Which ignition system are you using?

Never did get to Croix en Ternois. Happy memories of riding my Voxan Roadster 1000 at Mas du Clos and Charade.
Morini: 1980 2c/375 The Old Gal plus all that remains of the Forgotten Error (the engine)
Gilera: 1957 175 Sport
Honda: 1984 CB250RS
Husqvarna: 2019 401 Svartpilen
Wishlist: 2026 Morini 350
123morini
Posts: 49
Joined: 30 Nov 2015 12:19
Location: Chenee Liege Belgium

Re: Belt tensioner ...

Post by 123morini »

Hello 72degrees ,

On my race bike at first I used genuine Kocusan ignition than after I fited Ignitech adjustable CDI with the Kocusan captors .
I also tried with the captors mounted on the camshaft .
And as I was looking for more power I changed the complete alternator Kocusan by small Chinese bike and I made my own system to fit the captors on the cranckase under the alternator .
In fact I changed the alternator because the Kocusan produced bad electric waves that were disturbing the CDI . After with the Chinese flywheel with an internal metal partition evrything was ok the bad waves did not appeared and the ignition was really better .

In fact, this is a summary of my ignition tests .


Chris .
noc668r
Posts: 22
Joined: 18 Jan 2026 11:48
Location: Northampton UK

Re: Belt tensioner ...

Post by noc668r »

Ternois en Croix is a great circuit, not far from Calais, brilliant classic racing.Definitely worth a visit.
Even the track day weekend is worth going for, so many different machines turning up,plenty of 3 1/2s.
When I went a couple of years ago, folk had come from all over Europe.
123morini
Posts: 49
Joined: 30 Nov 2015 12:19
Location: Chenee Liege Belgium

Re: Belt tensioner ...

Post by 123morini »

noc668r wrote: 23 Feb 2026 13:18 Ternois en Croix is a great circuit, not far from Calais, brilliant classic racing.Definitely worth a visit.
Even the track day weekend is worth going for, so many different machines turning up,plenty of 3 1/2s.
When I went a couple of years ago, folk had come from all over Europe.
Hello ,

Yes Croix en Ternois is a nice circuit .

The last time we were there , the rider Christophe won the race and had the best run at 1.06min on my orange 350 Morini .

Chris .
Post Reply