Carbs - bigger jet or raise needle? - V6

I put K&N’s on my V65 some years ago and increased the main jet from 105 to 115. The plugs have been a nice light brown colour ever since so I have assumed all is well.

The carbs are standard Dellorto PHBH30’s.

I recently fitted more free flowing silencers as part of a rebuild. I’ve only given the bike one shake-down run but it seems to run a bit lean (accelerating better with a bit of choke).

Do free flowing exhausts required a richer mixture (eg. bigger jets)?

Is it better to increase jet size or raise the needle?


It depends on whereabouts in the range it is running weak.

Here’s an extract from the Dellorto tuning guide.

I will probably get a lot of Flak now…
BUT, my experience with 5 different vintage bikes say: Do both!
10% larger mainjet (as you already have) - together with needle raised one step.
You will then experience a more lively motor, never running hot - but with lousy economy…
Naturally, these Things are always a question of trial & error - so you need to experiment.lasse2014-01-25 14:12:14

Have found that small block Guzzis are quite different to the big blocks for carburation,
All the best

I’m sure someone will correct me if I’m wrong and I would like to be corrected if I am. It is my understanding that the needle jet governs the mixture up to nearly full throttle and then the main jet does the job. If the main jet is too small then the needle jet is unable to supply enough fuel after a certain point depending upon how much too small the main jet is. Therefore I would go with what Lasse says.

Yes because now more exhaust gas can get out of the engine, so then more air and fuel can go in. Carbs need to provide this extra.    Problee know my opinion about needles by now, (leave as standard)  but I would at least suggest change only one thing at a time, if nothing happens or makes it worse put it back the way it was and try something else.