Dell Orto carb mixture screw problem

[78jotadave]

On my 78 Jota, the # 1 cylinder carb mixture screw has me scratching my head. No difference in if I screw it all the way in or all the way out, the revs on the engine do not change. The spark plug is as black as black, and sooty. I removed the mixture screw completely and the float bowl and fired a couple of shots of compressed air through the mixture screw hole and I could feel a strong stream of air from the carb bowl area. This did not change the condition of the mixture screw issue. The bike runs OK but #1 remains rich, and the throttle response is "fluffy" right off idle. What's going on?

 

[Paul Marx]

Float bowl level?
Your bike is running too rich, but not via the idle circuit, if the idle circuit checks out.

Paul

Paul

 

[1200ts]

Also check the choke plungers. If it's not seating fully do to not enough slack in the cable or the rubber seal has hardened it will do this.

 

[piranha-bro2]

Definitely sounds like a choke (enrichment circuit) issue to me. Assuming the inlet-side port that feeds the idle circuit isn't blocked.

 

[Jugs_S]

The air for the idle circuit is supplied from the small port at the bottom of the in let face L or R depending of carb model. this has a restrictor jet in it about 25mm down the port. From memory the size is bout 0.8mm. This port and jet easily get blocked or restricted. If it happens you get no change on your idle screw. The best Ive found so far is Three bond Carby cleaner (green can) spray it in soak the run bike at idle and spray in port. its not 100% but I've has good success.
Be very carful if you stick any thing down there to clear jet. if you enlarge jet there is no hope of getting the idle screw to work.

Also the idle screw only has any affect at idle slides fully closed. as soon as you crack the slides its all Idle jet and cut away on slide. Most IJ are set to large for modern fuel. in a 1200 motor with standard 60/2 slides Ive gone down to 45 IJ with a O2 meter in the exhaust and the first 1/3 throttle where you mostly ride is only just getting above 10.5 air fuel ratio with target of 12.5 (I would not recommend this with out a O2 meter to check)

 

[Paul Marx]

The air for the idle circuit is supplied from the small port at the bottom of the in let face L or R depending of carb model. this has a restrictor jet in it about 25mm down the port. From memory the size is bout 0.8mm. This port and jet easily get blocked or restricted. If it happens you get no change on your idle screw. The best Ive found so far is Three bond Carby cleaner (green can) spray it in soak the run bike at idle and spray in port. its not 100% but I've has good success.
Be very carful if you stick any thing down there to clear jet. if you enlarge jet there is no hope of getting the idle screw to work.

Also the idle screw only has any affect at idle slides fully closed. as soon as you crack the slides its all Idle jet and cut away on slide. Most IJ are set to large for modern fuel. in a 1200 motor with standard 60/2 slides Ive gone down to 45 IJ with a O2 meter in the exhaust and the first 1/3 throttle where you mostly ride is only just getting above 10.5 air fuel ratio with target of 12.5 (I would not recommend this with out a O2 meter to check)

Nice to hear that modern fuel implies going down on idle jets.
I've found the same thing. Down 10 points to 45s on my 750 S and progressively going the same way on my SF2.

Will look into the 3C later.

Paul

 

[CLEMTOG]

you say that you removed the screw completely
was there, and is there still a spring a brass washer and an O ring in place, if not you are flogging a dead horse.
CLEM

 

[Paul Marx]

you say that you removed the screw completely
was there, and is there still a spring a brass washer and an O ring in place, if not you are flogging a dead horse.
CLEM

If the O ring isn't there Clem, the screw will be sucking in air, making it too weak.

Paul

 

[78jotadave]

Thank you for the comments and advice received it is appreciated.
I understand the operation phases of the DellOrto carb as used on Laverda and that the idle mixture screw only affects the mixture at idle.
I have a copy of a DellOrto tuning manual which provides me a good understanding of the phases of operation.
I did check the choke plunger and that all seems to check out fine. The idle fuel screw is assembled with spring then washer then O ring which is in good condition. I have tried a few shots of carb cleaner and compressed air with little to no change in the engine revs when adjusting the screw.
I will take a look at the float bowl level too

 

[Ventodue]

I did check the choke plunger and that all seems to check out fine.

Best way? Stick a bit of plastic pipe on the end of the uptake jet and try blowing up it. Any leakage is immediately obvious. (Altho', to be honest, I replace them systematically anyways ...)

I will take a look at the float bowl level too

Absolutely critical. Follow the Dell'Orto book. The biggest difficulty is judging exactly when the valve closes. Trick I learnt from Stevio years ago is:
1. (Once again ) Stick a piece of tube on to the inlet banjo.
2. Blow lightly down the tube while using a gauge - it is the best way - to measure just where the valve closes.

Piccie of home-made gauge attached.