Top Speed

[Duccout]

What top speed do 250 owners get out of their bikes when riding normally? I ask this because my 250 has always refused to exceed 60 MPH when seated normally. The bike has normal footrests and high bars, and I am 6ft tall, so make quite a sail, and the engine has small valves, 8:1 piston and a 27mm carb and the camshaft is a white code version. I should add that 60 is plenty for the riding that I do, and it runs straight up to this speed, but seems to hit a natural barrier at this speed, irrespective of gearing - I originally had it on lower gearing but 60 was still the top speed, which makes me think that it is down to power, ie there is not enough of it to push me along any faster!

Incidentally, the engine runs flawlessly with the new Amal Concentric, so if anyone is contemplating a new carb, I have to recommend Amal over Mikuni simply for the fact that the correct jetting is readily available.



Cheers,


Colin

[IanHood]

I think you may have a breathing issue. It's a long time since I have ridden a 250 but I can certainly remember doing 70mph on a scrambler (I'm 6'2") and I believe other 250 models were capable of considerably more than that.

Ian

[Bevel bob]

The higher compression piston ,nominaly 10 to1 but actually more like 9 to 1 would make a difference , as would the 27mm inlet valve and a sharper cam. Beyond that I would not go. I would expect to get 80 mph sitting up. I have gone much faster on a 250 but the trade off in traffic manners is not worth it.

[ranton_rambler]

That sounds a bit disappointing. I can (eventually) get to an indicated 70mph (actually ~65mph) on my 175. It has a 175 Sport piston and 24mm carb but standard small valve head.

[themoudie]

Aye Colin,

May I ask if you have an air filter fitted to the AMAL, or concertina bellows type hose between the carb and a remote air filter.

What sort of exhaust silencer do you have fitted?

I have never run an air filter on the 450 and only a short dumpy standard AMAL bellmouth, not the long polished version that "look" very nice. According to Nigel Lacey, the long bellmouths do not improve engine breathing when tested on a dynomometer.

I know that with an Armours, Gold Star type silencer fitted, my 450 would not go above 55mph - 60mph and a blued ring appeared around the body of the silencer on the exhaust pipe side of an internal plate/baffle. Inside the silencer. The central hole in this "baffle" appeared very restrictive and I enlarged it with a sharpened fencing puch of 1¼" OD and whilst this allowed better gasflow, I think the flat plate was causing too much reflected gas waves, back up the exhaust pipe and interferred with the removal of exhaust gases by scavenging. I have not butchered the silencer to see exactly what is the culprit, but even with my Motad oil filter design baffle that disrupts the exhaust pressure waves, but uses a rounded surface to do this, the 450 breaths/revs very well up to 6,500rpm, which is my self imposed limit.

I would remove any air filtering, go up a main jet size by one size, and fit a less restrictive silencer, if the machine were mine.

If you would like to view dimensions for a Motad type silencer, drop me a PM with your email and I will try to sort something out for you. Alternatively, put "Motad" into the "Ducati Singles Main Discussions" forum search window (upper left of screen), not the Motoscrubs forum search window (upper right of screen) and you should get 5 posts to read.

Good health, Bill

[ecurbruce]

I went 74 mph on my monza 250 on flat ground, really tucked in to the bars. That was with the phone taped onto the gas tank to see the GPS MPH.
I was running mikuni 26, and a hand built megaphone shorty.

Bruce.

[Duccout]

Hi all,

Thanks for the thoughts. I am running a K&N air filter, but I have run the bike without, and with a 28mm Mikuni, but it made no difference to the top speed. The exhaust is the standard Mototrans silencer as fitted to Strada and Forza models, and by the amount of decibels emanating from it, I think that it is pretty de-restricted!


Cheers,


Colin

[Dave354]

Hi, can I ask what gearing you are running on the final drive chain?
Regards

Dave

[Duccout]

I started with 16/47, but that was too low so I am now running 18/47, but both gave the same top speed.



Cheers,


Colin

[themoudie]

Aye Colin,

So what is the specific model of your 250 engine?

The usual ratio of the primary gears for the 175, 200, 239, 250 and 350cc engines is 24 x 60 or 2.5:1
The usual ratio of the primary gears for the 100, 125 and 160cc engines is 21 x 63 or 3.0:1
The usual ratio of the primary gears for the 350 and 450cc engines is 27 x 57 or 2.1:1

Which of these primary gear combinations do you have fitted?

Insofar as I can see all of 250 engines, either 4 or 5 speed are fitted with gear clusters, with a final gear ratio of 0.97:1

Your final drive sprocket combination of 18 x 47, has a ratio of 2.61:1

So, if we assume that your engine revs are at 7,000rpm, the usual 250 primary drive (2.5:1) will reduce that to 2,800rpm (7,000/2.5) of the gearbox input shaft.

The final gear ratio of 0.97:1 will increase that to 2,887rpm (2,800/0.97) for the gearbox output shaft.

In turn this means that your rear wheel will be turning at 1,106rpm (2,887/2.61).

If you have a 160 primary gear combination fitted, the primary drive would be (3.0:1) and assuming the same engine rpm as in the previous calculation (7,000rpm), then the gearbox input shaft turns at 2,333rpm, the gearbox output shaft at 2,405rpm and the rear wheel at 921rpm.

Depending upon the overall diameter of your tyre, you should be able to calculate the distance covered per minute and by multiplication, miles per hour.

If these calculations are incorrect, please accept my apologies, even "simple" arithmetic is not my forte!

Good health, Bill