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My 250 has the air filter (Says Dell'orto on it, and I believe it is original), but I see many run a velocity stack or bellmouth on the carb. I know there is a performance reason for the stack, but I've always felt that the cleaner the air in, the longer the engine lasts. Is the velocity stack correct for a 250 Diana, or was this a popular mod?

" My 250 has the air filter (Says Dell'orto on it, and I believe it is original), "

____ That carb mounted air-filter is stock only to the Scrambler-models !


" Is the velocity stack correct for a 250 Diana "

____ Why are you asking about a 250 "Diana"? _ The 'Diana 250' was a separate (4-speed) model of it's own and not the n-c 250 'Mark 3' model, which you have !
Anyhow, I've never seen a factory-stock n-c Mk3 which came with anything else attached to the carb-intake. _ So if the Mark-3 is what you're actually asking about, then the answer is 'Yes' !


DUKE-Cheers,
-Bob

Black frame Dianas are always pictured with velocity stacks.

The dealer or importer may have swapped it out for local conditions or owner preference.

The main jet size quoted in the literature is for a stack and it would be interesting to hear what size main jet you have in the bike if it's different from the literature.

Put a stack on the carb and get a repro open mega and it will run like a totally different bike at the top end.

While we're on the subject of unsolicited advice, Oil is an important but touchy subject on these motors so expect many opinions, but here's my 2c.
Don't, whatever you do, put multigrade oil in that bike.
Run straight, high quality 40/50 weight dino oil and change it every 1000 miles.
And keep the revs over 4k whenever you're riding under load.

I'm going on personal experience with my bike in the 60's and a severe dressing down from Vic Camp after my big end failed (Castrol GTX being the culprit)
The reason has nothing to do with lubrication but everything to do with thermal heat transfer away from the bearings.

M.

I used to run with a oil-wetted sock-type filter (pod filter) over the stack, but the bike always ran too rich (could never get it jetted correctly for the filter) and eventually I went to the open unfiltered stack.

I only put a couple of hundred miles on a year, and change the oil often, and I also adopted the attitude of "who the h*ll am I trying to save it for?" so for all those lame reasons and more I just run with the open stack. I plug it when the bike is not in use so the garage dust stays out.

I don't know if that helps you or not.

My uncle bought this bike in 1967, and other than replacing the taillight and tank, it's as it was in 67. It currently has 3514 miles on the still working speedometer.

I know very little about these bikes, I'm more of a brit bike person.
Please excuse the noob questions.....