Tuning carb

[Duccout]

Hi Colin,

Just a couple of thoughts: what about using a 32mm PHB jetted to fit an 860GT? Alternatively, what about an Amal? These were offered as an optional extra by Mick Walker motorcycles on the 450 (32mm) and were known to run well; I am sure that the jetting should be available on line. OR, what about a Mikuni? There is an Australian company which supplies them to fit Ducatis, including the 900SS, jetted ready to fit, so I am sure that they could help you.

Cheers,

Colin

[ranton_rambler]

I am far from expert, but when I set up the PHB24 carb for my 175 I found that the needle profiles are available on line, so I drew them out at a large scale to see what the differences at different slide openings would be.
There are knock-off Dellorto needles available - I got a kit of 10 assorted versions for the price of a single genuine one.
With the information on the profiles I simply worked through them in a logical sequence until they stopped getting better and started getting worse.

This was after I had sorted the idle by swapping pilot jets and the atomiser which seemed to influence how well it picked up from idle.
Main jet was the last thing to sort out after the needle. Again, there are lots of knock-off bits...

A really useful thing is to mark the twistgrip so that you can see how much the throttle is open and therefore work on the right element.

Having said all that, it's possible to hold a 175 flat-out without getting to silly speed. For a bigger motor you might be better paying for some dyno time.

Ian P

[IanHood]

Colin,

If you feel like playing around, I’m happy to send you my PHBH, along with an assortment of jets. If you get it to work well with yours you can send me a few quid (let’s say 50% of the new cost). If not you can send it back to me.

Ian

[NZ450Colin]

Hi Colin,

Thanks very much for your thoughts. For sure there are a number of options and yes, I have loosely considered most of these, but many come with complications to do with fitting into the available space, which is tight. I know its possible to make a PHF fit, possibly even "bolt on" with a GT manifold? and I'm sure with a bit of work I'd be able to make an AMAL MKII (rather than MKI)fit. Road and race do a Mukini kit but I've not looked closely at this yet.

Whilst I'd love to get a little bit more top end, ( mine tops out at 145kph @ 6000rpm) on my standard engine I think slapping a bigger carb on there is unlikely to magically translate into that, especially without it being at the expense of mid-range power. I'm really just looking for a better idle and anything else would be a bonus, so for now at least, given the potential complications of fitting a physically larger carb that's probably why I'm leaning towards the PHBH30 option.

But nothing is fixed in stone yet and comments from anyone who's been down this path are much appreciated, as I'm definitely open to different options.

The other Colin.

[NZ450Colin]

Hi Ian,

Many thanks for your very generous offer. Actually I think I may be able to temporarily get my hands on a friend's PHBH for a short trial, but that is yet to be discussed with him. If that comes up blank, I might take you up on your proposal. I will certainly let you know either way.

Cheers.

Colin

[Duccout]

Hi Colin,

My 450 Mk111 which was completely standard, topped out at 120 KPH, so you are doing better than me! The engines are obviously strangled and when it came to producing the touring 860GT Ducati upped the carbs to 32mm. Have a look at Mikunioz, who can supply Mikuni kits, but I would go for a Mk 1 Amal as these carbs are widely used, bolt straight on and are so simple to tune. Bike magazine tested a 450 Desmo with a 32mm Mk 1 Amal and a 13t gearbox sprocket and achieved 99.99 MPH. If you decide on a Mk 2 Amal, The Moudie, on this site has one on his 450 and could advise you on jetting, but obtaining a manifold could be difficult.

Cheers,

Colin