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Carburators

Ady00

Website User
Non-VOC Member
Hi I have just treated my Rapide to a pair of new pre monobloc carbs to replace the mkI's, can anyone advise me on setting them up? I'm use to setting up mkI's also are the throttle and air slide cables common to both type's of carb's..thanks
 

Tom Gaynor

Well Known and Active Website User
VOC Member
Carburetors

The settings are in the literature. I assume you mean 289's or the like, i.e. original carburetors. About 200 main jet, 3 1/2 slide, and needle in the middle position is a good start - and often a good finish. Weak makes more power, but rich doesn't burn holes in pistons.......
 

andygbsmith

Website User
Non-VOC Member
Every bike will be different, and it depends on so many factors.
I have bought a real time wide band fuel air meter to take the guess work out of it.
With the standard 276s on my Rap I am down to 140 main jets and the mixture is still rich.
I have checked several Vins and they are all way to rich.
And yes I do ride the bike hard and have done approx 1500 miles with those main jets in, that included thrashing around the IOM for a week.

The only way to do the job is either buy a meter or pay for dyno time.
Don't be tempted to buy other peoples set ups (I have heard of some people selling there results on to try an cover some of the cost) because as above every bike will be slightly different.
Remember most of the advice in the books is old now and the octane rating has changed, this will have an effect.
 

Tom Gaynor

Well Known and Active Website User
VOC Member
Carburation

A friend races a 350 Manx I am familiar with. This year he couldn't get the carburation right, and kept leaning it out. At the last meeting of the year, with 30% smaller jets than standard, it seized. What the dyno had apparently not shown was that the problem was the 2MTT magneto, which was producing a big enough spark to start the bike, and run it, but not enough to burn the fuel completely, hence the sooty plug.
I had a similar problem with my own 500 at the same meeting, but I knew that the timing was out, so left the jet (on a Gardner) where it was although the plug was dark brown - usually it is "dark white". My new PVL ignition system needs an approximate static setting to get the motor to run, but needs a dyno for a final setting, and i hadn't done that yet.
140 main jets in a Rapide is 5 sizes down on standard. That's a lot. 2 sizes is quite a lot. I suggest therefore that you make certain that carburation is the problem before you suffer ventilated piston crowns.

Every bike will be different, and it depends on so many factors.
I have bought a real time wide band fuel air meter to take the guess work out of it.
With the standard 276s on my Rap I am down to 140 main jets and the mixture is still rich.
I have checked several Vins and they are all way to rich.
And yes I do ride the bike hard and have done approx 1500 miles with those main jets in, that included thrashing around the IOM for a week.

The only way to do the job is either buy a meter or pay for dyno time.
Don't be tempted to buy other peoples set ups (I have heard of some people selling there results on to try an cover some of the cost) because as above every bike will be slightly different.
Remember most of the advice in the books is old now and the octane rating has changed, this will have an effect.
 
Last edited by a moderator:

andygbsmith

Website User
Non-VOC Member
Of coures what you are saying is correct Tom.
Thats why I have tried the equipment on several Vin's as well as more modern bikes.
I am running a modern ignition system so the spark should be good and I hope the timing is correct it would be nice to strobe it but static is spot on.
One other thing to take account of is that Amal jet's are sized differently from most others, they size them on flow rate ,Mikuni and most others are a drill size.
I got some figures from Berlin on actual hole size and if you compare it to Mikuni you find that going from 170 to 140 in Amal jets is only the same as going from 170 to 160 in a Mikuni.
Food for thought.
 

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