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I promised to do an update on my 250D rebuild in the very helpful 'Rich Running' thread back in June but the plague and a massive list of "while you are locked down" home maintenance projects, helpfully provided by madam, have intervened, sorry.
We are having some glorious autumn weather here in Italy so I've been thrashing the nuts off the 250D on our local winding roads roads, purely for testing purposes, officer...(or Dottore, as the carabinieri around here like to be addressed).
As reported previously the bike is now running strongly with only a bit of carb stumbling at part throttle. I haven't yet embarked on a carb swap but the Sachse ignition, Dyna coil, careful ignition timing and new 'short Conti' (Scrambler) silencer seem to have all worked wonders. The LED bulb swap has also gone well, much easier than a 12v conversion. I haven't ridden it at night but the beam seems strong with a good dip cutoff but the light is disturbingly white after the previous yellow glow, I'm sure it will be marked down by the rivet counters. I will give details of what I used and suppliers in the next post, promise...
Finally a couple of dumb questions;
I couldn't help remembering that the blue and gold 350 mk3 I owned back in the '70s had great power characteristics and suited the chassis better than either 250 or 450 versions so I wondered what might be involved in converting the 250D to a 350D? It's a wide case of '74 vintage, I think the stroke is the same but what about cylinder stud spacing and the head? Anybody tried chucking on a 350 barrel and piston? A bit of squish can be a (very) good thing!
Finally the bike came fitted with a brand new ( in 1985!)set of 90/90 Dunlops on Borrani WM2 rims and they still had some nibs on, I haven't yet replaced them and they seem o.k. in the dry. I would normally have binned them immediately , we sometimes used a complete set each hour in the old 'Team Bike' endurance racing days so I have no emotional attachment to tyres, but I'm not sure what I should replace them with. Anybody have any recommendations?
Cheers
Zed

Hi Zed

I use Avon Roadriders on my 250D and Velo Thruxton, and have no issues at all.

HTH

I think that RoadRiders are the favourite.

Zed, I think you have got a bit mixed up with your 350 conversion- the 350 has a longer stroke and it would be quite a bit of work to convert a 250 to a 350. Nice to hear of your progress!

Zed, I am using Bridgestone bt45 battleaxe front and rear on my 250 desmo and on my 350mk3 ......I have used these for years on my Yamaha 350 ypvs and put a set on the 250D as an experiment, they were so good I then put a set on the 350 as well. Converting a 250 to a 350 is pretty straight forward, however you need to change more than you think......barrel and piston, crankshaft, and the primary gears, frankly its a lot easier to get a widecase 350 engine and fit your 250D head to it though as far as I know (I have never measured it)the squish profile is supposed to be slightly different between a 250 and a 350 desmo head. I have a 250D head stashed away to fit to my 350mk3 someday.....just havn't got to it yet.
Cheers,
George

I have Roadriders on my Guzzi 850GT and BT45 on the 1000. Happy with both. I only bought the BT45 because they were actually cheaper!
Ducati 175 had Pirelli City Demon, which come in smaller sizes.
Ian

Hi,

I made tis conversion, when I built my 350M3D from scrap in 1980. I had bought a ratty 1970 350SCR engine together with the Desmo. It's cases were painted black and heavily corroded, because the engine was stored partially under water ... so I decided to use the 250 case instead. The cases bore for the cylinder liner had to be opened with a facing and boring head on a mill, by a few mm. Otherwise the liner of the 350 cylinder could not be slipped in.
Please note:
250 cyl. - 8 fins
350 cyl. - 9 fins
450 cyl. - 10 fins
So all have different lengths, and bores as well.

As there are no different heads for 250 and 350 Desmo (see parts manual), the re-use was not an issue. From the 350 I took also the upper bevel pair and the protection tube.

The 350 crankshaft was in an unusable state (worn taper for the alternator rotor, worn big end bearing), so I bought a new one with the 32mm big end pin. Crankshafts for 250 and 350 are different in stroke, conrod length and shape of the crank webs!

Also different are the primary gears:
250 - 24/60
350 - 27/54

The 350 alternator rotor is made from bronze, hence has more inertial mass, while the 250 rotor uses aluminum. I stayed intenionally with the 250 rotor.

You need the 350 cylinder head bolts because of greater length.

The lower flange for the bevel shaft protection tube also differs for 250 and 350.

There might some more minor differences which I do not remember well

My advice: carefully examine the parts catalog for different part numbers assigned to the same parts.

cheers Hans

Big thanks for responses, I don't know where I got the idea that 250 and 350 strokes were the same, so thanks for putting me right. The conversion would be a heroic project as very well described by Hans, so I'll put the idea on hold unless I come across a 350 engine (some hope...) and re-learn how to ride a 250 properly. It actually pulls quite well but my advancing years and even more advancing beer gut give it a lot of work to do.
Thanks also for the tyre recommendations, I use AM 26 s on my twins so will probably go down the Roadrider route although I have heard good things about the BT45s as well.
Cheers
Zed

Anybody use the shorter stroke 250 bottom end with a 350 barrel and piston?

This makes no sense unless you shorten the 350 cylinder by the appropriate amount, to achieve the correct compression ratio.
And as well, you have to oen the bore in the case in order to fit the outer dia. of 350 cylinder liner.
If you finally succed, you only gain max. 2-3 mm of extra bore diameter.

Hans

Hans, Ref your earlier post.....are the 350/250 widecase conrods not dimensionally identical? I thought they were the same, ......obviously the crankshafts are different.
Cheers,
George