Dellorto misbehaving.

[Nick from Kent]

I've put a Dellorto UB24BS on my Monza 160. Couldn't get hold of a 22 so I sleeved down the inlet and used the jets specified in the handbook. Trouble is, although she starts and ticks over on choke well enough, when warm and off choke the engine dies when the bike is leant over to the left, and races when leant to the right! I have stripped the carb and cleaned with compressed air and solvent, and checked the float moves freely in the bowl, and I tried the larger jets listed elsewhere on this site, but to no avail. Anybody come across this before? Maybe I should try another carb, a Mikuni perhaps? I can't find a rubber to join the carb to the air box, so I've fitted a short stack, but I don't think that should have an affect the angle problem.
Weird huh?

[Sooners]

When you say it dies and races when you lean it over...does this happen when the bike is sitting and idling and you lean it over, or just when riding, at part or full throttle?

[JimF]

I'm no physicist, but to my thinking when the carb and float are perpendicular to gravity the presentation of raw fuel at the mouth of the carb is optimal for atomization as the engine creates a small vacuum at the carb inlet. Leaning the bike to the right while the carb is idling would have the effect of lowering the carb inlet below the float's internal fuel level and an excessive amount of fuel would appear at the inlet, perhaps causing the extra fuel to increase the revs or flood the engine. Leaning to the right would place the carb inlet higher than the float's internal fuel level and starve the carb/engine for fuel causing it to die.


I guess this is dependent too on how far you are tipping the bike.


When you are at speed and cornering perhaps the forces of inertia, the different jets you are drawing fuel from or the higher vacuum at higher revs overcomes the non-perpendicularity (is that word?) of the carb/float combination and that is what makes carbs work at odd angles in practical applications.


Does the bike run OK otherwise? If you rev the engine up and tip the bike does it exhibit the same symptoms? Could it just be that on the idle jet you just shouldn't be leaning the bike over?


Jim

[Nick from Kent]

When you say it dies and races when you lean it over...does this happen when the bike is sitting and idling and you lean it over, or just when riding, at part or full throttle?

Just when sitting and idling. Seems ok when moving along, but sometimes dies when throttle off, approaching a junction for example. The odd thing is that the angle of lean is so slight - about 20 degrees either side.

[Nick from Kent]

I'm no physicist, but to my thinking when the carb and float are perpendicular to gravity the presentation of raw fuel at the mouth of the carb is optimal for atomization as the engine creates a small vacuum at the carb inlet. Leaning the bike to the right while the carb is idling would have the effect of lowering the carb inlet below the float's internal fuel level and an excessive amount of fuel would appear at the inlet, perhaps causing the extra fuel to increase the revs or flood the engine. Leaning to the right would place the carb inlet higher than the float's internal fuel level and starve the carb/engine for fuel causing it to die.


I guess this is dependent too on how far you are tipping the bike.


When you are at speed and cornering perhaps the forces of inertia, the different jets you are drawing fuel from or the higher vacuum at higher revs overcomes the non-perpendicularity (is that word?) of the carb/float combination and that is what makes carbs work at odd angles in practical applications.


Does the bike run OK otherwise? If you rev the engine up and tip the bike does it exhibit the same symptoms? Could it just be that on the idle jet you just shouldn't be leaning the bike over?


Jim

I can see your logic Jim, but it's a bit impractical to try and keep the moving bike upright with the throttle shut. I just wondered if anyone has come across this before.The carb has an unknown history, and although it looks to be fine, maybe there is some wear that isn't visible to the naked eye. Would a worn float needle contribute I wonder?

Nick.

[machten]

My Dellorto SS1 29 does the same thing, but I can't recall of the top of my head if the left/right aspect is the same or reversed. I'm going to ride the 250 today, so I'll confirm later today. I agree that the variation in lean required to make a difference is slight and I discovered this trait of my SS1 sitting at a red light and noticing the idle speead was affected by which foot I had grounded! For the SS1, I'd say the variation I can generate is about 1000-1500 RPM. The carb runs spectacularly well other than that and I now just accept it as part of the "charm". In fact, I find it kind of handy when the 250 (and the pilot!) is yet to reach full operating temperature at a set of lights. I can take my hands off the clip-ons to have a stretch and manage the throttle by leaning the bike.

Kev

[Nick from Kent]

Well I've got it working! Not the way I'd hoped though; replaced the DellOrto with an old Mikuni I had lying around and she started first kick and ran beautifully for a fifty mile potter around the lanes. I think I'll try and get a 22 choke DellOrto with an upright bowl and try again (anybody want to swap a BS24 for a BS22?) but for the time being the Mik' is fine and the little Ducati is great fun to ride.

[JimF]

I have a SSI29 on one 250 and a Mikuni 26 on the other 250.

There are many factors at work between the two bikes so what am I about to say is certainly not comparing apples to apples...

The SSI proves difficut to start and I have to blip the throttle at idle but everywhere else the carb is flawless. It is coarse if I can use that word to describe it; few adjustment, few jets, and tuning is finicky. But when dialed in it works well for riding hard. It won't tolerate an air filter no matter how hard I try.

The Mikuni seems more refined to me, more jets and needles seems to keep the carb happy at any throttle position, easy to start, easy to adjust it for all throttle openings. Air filter? No problem. Performance? It doesn't feel as strong as the SSI but then the SSI is on a Mach 1 and the Mikuni is a smaller carb and on a smaller valved engine.



Jim

[Winter]

Sorry to wake this thread up, but I have the exact same problem.
Looking at the idle jet, it sits on the right side of the emulsion tube / main jet. If the bike is leaned to the left, the idle jet is starved since it sits further up the chamber and is more sensitive to the fuel level.

What I beleive are stock jects in mine are 95 main, 35 idle. Going to a bigger idle may help, but if the issue is fuel delivery, it might stall it even faster as it depletes the available fuel reserve faster.

Therefore I think I will go the route of reeming the banjo on the bottom of the bowl to increase fuel flow to the whole circuit. Can't hurt, but might help.

Hope this might help someone.

Best,
Andrew

[Jordan]

Your analysis is correct, and don't worry as it's normal for a non-"concentric" (non-symmetrical) float system.
It's fine when riding (unless you have a sidecar, and even then the effect of side forces isn't usually a problem).

Jordan