Hi I’ve just purchased a 1959 125 sport I’m struggling to get it to idle it’s ok if I keep on the throttle at around 3500 4000rpm I’m unsure if it’s had a cam etc the air screw is all the way in and it’s unhappy if I turn it out a turn and a half any idea would be appreciated
If nailed on originality isn’t a key aim, it may be worth considering a carb conversion to a brand new Mikuni. A pal of mine runs his ‘48 Indian Chief on new Mikunis and the trade off between originality leading to breakdowns, missed rallies, and not getting out of the garage, versus reliability, ease of starting, first kick every time is one he has been very happy to have.
I have give this some thought but I’m trying to keep it pretty original also it’s difficult to know what other carbs I can use size wise and fitting etc
Motocarb (Steve Doran 0151 924 5383) may be able to look into that Mikuni side of things. All too often, the original carb is simply worn out, a 63 year old precision instrument is unlikely to meter and be within calibration to work as originally specified. So it may be worth trying to find a Dellorto NOS replacement. Eurocarb is a Dellorto specialist and without spending much more time on their site, it looks as if they’ll provide you with a modern alternative, this not preserving the originality, either. https://www.dellorto.co.uk/shop/del...rts/phbh-26-to-30mm/r4047-phbh-30as-4-stroke/ If you don’t make any headway with either of those, or via the excellent help that will come your way from others on the Forum, please let me know via pm and I will make enquires to a small group I know in Spain that specialise in restoring the singles (not confined to Mototrans!) Good luck
Good to see you found your way around Nigel Welcome into the mad house and plenty of good advice to be had with our lovely forum family
Hi Nigel, welcome to the forum. What sort of carb are we talking about? A UB series DellOrto by any chance? I've had a fair bit of experience with tuning and troubleshooting these. Do you have any information on the stock settings for carb, ignition etc? I've used Ducati Paddy for vintage DellOrto spares. He can work out a little cheaper than Eurocarb. https://www.ducatipaddy.com/
Yes Dellorto UB22BS, I’ve just bought the bike. It starts fine so I’d think the timing isn’t far out, it’s got a Lucas conversion I think on the ignition side . I’ve had the carb off and checked pilot jet etc all seams fine just don’t want to go out on it and it stopping at junctions etc. Thanks for the response
Good. Now we are getting somewhere. A 22mm carb is a pretty hefty carb for a 125. I'd expect something like a 18 or a 20mm maybe. Is it the one it should have? I have a UB22BS on my 200cc F B Mondial. I know that the UB22BF is used on things like the MV 175 CSS Disco Volante as well as a lot of the sporty 175s from a variety of manufacturers, so my guess is that its over carbed. There are lots about, and spares are comparitively easy to come by, but not cheap. I had a number of problems to solve with it. I found in the owner's manual the original specifications which were as follows: Main Jet 110 Idle Jet 45 Slide #70 Needle E2 Emulsion tube 260B These work OK, but I have moved down to a 108 Main Jet, as I could see it was a touch rich at the plug. As bought however pretty much everything was wrong. The idle screw wasn't long enough to adjust the slide stop, the slide cutaway was wrong #60 I think, as were the jets. The float needle clip was rusted through, and the float needle was stuck in the float. As you have probably worked out, the mixture adjustment screw is upstream of the slide, and is therefore an air bleed screw, I.E. it weakens the mixture when you screw it out. It sounds a little like it might already be weak at idle, and gets worse when you unscrew the mix screw, so you might want to try a larger idle jet? Fault diagnosis especially of the carb sort is tricky at one remove from the machine, so some, all or none of the above might be useful!
Oh, just one more thing. I don't ever use the choke. I just give it a tickle, and a kick and off it goes. What happens when you operate the choke?
No choke on my carb, I just tickle it and away it goes second or third kick, your right about the air screw as soon as I turn it out it’s unhappy, I’ve also noticed i small amount of fuel coming back in the air filter very slight
Nice bike! I think I'd pop the carb off, and measure it to see if it really does measure 22mm diameter inside, and that the inlet tract on the head matches it. Maybe someone has bolted a bigger carb to your head? Check that the slide stop is working properly. I never knew that you chould get different lengths of screws till I found that mine was too short. Next have a look at all the jet sizes, particularly the idle jet. If you have a box of small drills, you can size the hole in the jet. I never trust the sizes stamped on jets unless I have bought them from new, as you can never tell if someone has been in there with a jet drill before you! Always give the slide a wiggle in the bore and see if it feels baggy, if it is, then the carb needs rebored or the slide is worn or both. My carb body was in pretty good condition, so with the proper new slide, jets and needle, my Mondial seems to idle nicely and pull well, so, if in good condition, they are a fairly good carb. Let us know what you find.
It’s 22mm carb and block so it’s obviously had a bigger carb fitted I have another UB22 coming so I’ll look at that once cleaned etc and rebuilt that I’ll have to see if anyone has some base setting jet wise and go from there it’s all fun this Ducati lark
