Part throttle jetting

[Parker3865]

I have done about 800 miles on my rebuilt 250 2c. I had noticed that it was very fluffy at part throttle (if it was a 2 stroke I'd say it was 4 stroking) and a stutter when opening tbe throttle. I have dropped the slide needle one notch and it is running much much better.
However! When adjusting the idle screw at tickover any more than 1/4 out makes no more difference. I believe I have the standard idle jets; 40. I have the shorter mixture screws and have read Mark's article in ATG 222 (thanks to Julian for that) and I assume it should be more like 1 1/2 turns out. F.Y.I. I have the blue scooter transducers and I think I have the timing correct. Currently using superunleaded.
FINALLY: the question I have read elsewhere on the forum that people have jetted down the idle jet; what size might be suitable?

[Steve Brown]

As yours is already a small-ish 40 I'd say not much less! 35 for instance would be a big % difference. Before you try that consider that the needle adjustment has been fruitfull and that maybe the slide cutaway is worth adjusting? A few of us have found raising the cutaway has helped a lot, as little as 0.5mm at a time. I'd also say check the fuel level first, the float position is easier to do but still doesn't tell you the actual fuel level. There are Dellorto docs somewhere that tell you how high it should be, something like 5mm below the float bowl joint face.

[BarryCambs]

Both the table on the archived Netherlands site and one sent to me by Euro Carbs say 40 jet size. Mine was fitted with 42 for some reason, before I got it. The problem is with anything this age is previous owners could have fiddled with jet/slide/needle sizes trying to correct ignition problems!

https://www.morini-riders-club.com/Nederland/carb.html

Half the older cars or bikes I come into contact with seem to have been 'fiddled' with, so I like to go back to standard and start from there. Bitter experience tells me, don't assume anything!. Just because it says Dellorto on the side, doesn't mean they started life on a Morini and not a Guzzi. On two occasions I've found carbs off completely different things. On one occasion a Dellorto on a Ford Zephyr which while the correct model (but wrong choke size) turned out to be off an Royal Navy generator, as the previous owner was told it would improve the fuel consumption!

Mine's fitted with SWF ignition and pulls well right through the range. I don't know anything about the blue transducers, but I have a pair of virtually new ones from Alesandro in Italy you are welcome to borrow and try.

dellorto_table.png (404.31 KiB) Viewed 4249 times

[Parker3865]

Thanks Barry. I put new jets and needles in the carbs when I rebuilt the bike and used the NL website as reference for what I needed. I will check the ignition timing again.
Maybe a trip to visit you needs to happen. Much apreciate the offer of the transducers and would like to try them. Are they for sale?

[BarryCambs]

Be great to meet up sometime given we're not so far apart. I'll sell the transducers if someone needs them, rather than wanting them for stock. They were only on the bike for 10 miles before I realised that while they corrected the original timing problem, the pickup was dodgy as well. knowing what I know now, I'd have gone straight to SWF or similar from the outset.

[julianharty]

I recently rode my 250 2C 2550 kms to Southern Ireland and back home. I had similar 8-stroking issues on it before I left. I fitted some brand new square throttle slides, i.e. 7447-50 with a 50 (5mm) cut away in place of the standard 7447-40 slides. I left the needle jets in the same middle position of the 3 available with the clip. I screwed in the mixture screws slightly but didn't re-balance the carbs as I was more focused on getting myself and the bike ready for the ride. The new slides reduced the 8-stroking but didn't cure it, particularly when I rode off the ferry at Rosslare at about 05:30 (i.e. in the early morning) where there was dew and close to 100% humidity.

I've just (5 mins ago) ordered replacement E51 needles for the bike as my needles are fairly tatty. I'll fit these when they arrive from EuroCarb in a couple of days.

My bike generally ticks over at roughly 1500 rpm indefinitely, partly as I've got the screw in air mixture jets sold for the 350 v-twins (a tip recommended decades ago in the club's ATG magazine which Richard's already mentioned in this topic). I think E5 fuel helps (in Ireland we were sold some super fuel that also seemed to boost performance slightly) however for most of the trip I think I ended up using bog-standard E10 fuel as that was often all that was available.

For me getting the carburation right is an iterative process. I'll probably take the bike with a collection of jets, etc. to a local dyno place (PDQ near Reading) if I don't manage to get it to run well myself.

I'll post updates here. I've videos of the engine running which I'm happy to share directly as they're not practical to upload to the forum.

[Parker3865]

I have done a few runs now and it seems to be running fine. Idle mixture screws about 1/4 turn out and needle dropped one notch. It happily ran to the Ace Cafe and back. So I think I am going to leave it alone now.