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Yamaha RD250F Restoration Part 6

15th June 2016

Yamaha RD250 Exhaust

Welcome back to Alex's Blog which has been taken over by Adrian from our Oakmere store for the past few weeks... and will continue to be until his bike is finished. But firstly we have a brand new store opening in Uckfield - the second this is opening we will let you know but in the meantime here is another Blog about our new Uckfield store!

Now over to Adrian but first a  short recap:

Last week with Adrian's Restoration project we were up to the tank being painted – and who he chose and if they could do it....

“I looked in to lots of different respraying places and finally plumped for Alex of Axcel Airbrushing. His work is absolutely amazing as he does intricate designs on helmets etc, so this to him would be a walk in the park. My tank, tail unit, side panels and seat trims are away being sprayed as I type and I am eagerly awaiting their return. 


No time to rest on my laurels though, as the exhaust I had chosen had arrived so the fitting could begin!

Yamaha RD250 Exhaust

This was an exciting time, because once the exhausts were fitted it meant I could start setting up the engine. This is really important in a 2 stroke twin cylinder motorcycle because both barrels have to be working at the same rate. If one barrel is working harder than the other then this not only affects performance but also prematurely puts excessive wear on one side of the engine. It is crucial to get the engine working properly for the longevity and overall performance of the bike. 

With a handful of nuts, bolts, springs and pipes I set about mounting the exhausts to the frame and engine. This was a simple and pleasurable job as it can't really go wrong. I think they look pretty good and I cannot wait to hear what they sound like! 

Yamaha RD250 Exhaust on the frame

Yamaha RD250 Exhaust and back wheel

So on with the tuning of the engine! This is basically done via the carburettors. This is how I did it (I am by no way saying this is the best way, it was the best for me and one that I was taught many years ago by some classic 2 stroke experts).

The first job is to make sure the throttle cable is fitted correctly and that when you twist the throttle both carburettors open when they should. To see this happening you need to remove the air filters from the back of the carburettors. The  carburettor slides should both start to open at exactly the same time when the throttle is slightly twisted. If they don't, then turn the adjuster screws on the tops of the carburettors to get them in sync with the "fast idle screws" turned out.

Obviously before you do any of this you need to have checked that the main jets and pilot jets are the same in each carburettor and that the throttle slide needle is set the same ,again, on both slides. Next you remove the spark plug from one cylinder, place it in to the HT lead and leave it lying on top of the cylinder head. Then concentrate on the cylinder with the spark plug in.

Also you need to make sure the air mixture screws are both set to one and a half turns out. This is a good starting setting for the mixture screws. Now back to the setting up! With the cylinder that has the spark plug and HT lead in place, try to start the bike. It should fire, if not, turn the "fast idle screws" in, a quarter turn at a time, until it fires. When it does fire try to adjust the "fast idle screw" until you are happy with the tick over.

Once happy, move on to the other cylinder. Put the spark plug back in, the spark plug cap back on and remove the spark plug from the side you have just done. Repeat the above process for this side now. Then you can put both spark plugs back in with the plug caps on and attempt to start the bike again. Just before you do start the bike, twist the fast idle screw on each carburettor half a turn out. The engine will probably race if you don't do this.

Then do your fine tuning one carburettor at a time but replicate exactly what you do on one side to the other to get a perfect result. From here on work at a quarter turn maximum at a time until you have got the bike sounding like it did when it came out of the factory. The carburettors are now perfectly synced and maximum performance will be achieved! I did all this with an auxillary fuel tank (as mine was away being painted) but I just couldn't wait! "

Until next time,

Stay safe!

Alex