carburettor wrong one

[maxifields]

Hi yer,
        Trying to sort out my  running issue (died on me after a mile ) I think its a carb issue which I'm  going to strip down .I've noticed it a Dellorto 20-20 D and read for a Sprint 69 it should be a 20-17D whats the difference? 

[vespasco]

ah! the sprint carb debate! im sure we had this not too long back...which carb is best and with which jets..try a search or someone else here may have a better memory and tell you where to look

[shipscat]

Jul 31, 2016 10:54:11 GMT maxifields said:

Hi yer,
        Trying to sort out my  running issue (died on me after a mile ) I think its a carb issue which I'm  going to strip down .I've noticed it a Dellorto 20-20 D and read for a Sprint 69 it should be a 20-17D whats the difference? 

a 20/20 carb is 20mm all the way through 20mm top 20mm bottom
a 20/17 carb is 20mm at the top and 17 mm at the bottom
i think the d reference is something to do with the choke mechanism , some of the older type carbs had a flap in the air filter that was a choke(or at least my vnb1 was like this)

some one has tried to improve the engines performance by adding a bigger carb

[henri]

the flap in filter ,a "strangler" choke, thats its real name i'm not joking, thats on earlier piston ported vespas .ss180's/gs's/vna's . apart from that its as shipscat says , 17=17mm venturi , a 20/20 will work on that engine but will need a larger main jet than standard . H

[maxifields]

Thanks for your replies pretty interesting . found a 20-17 which i'm going to restore.
Thanks again

[maxifields]

I've  now changed my carb with 20 17. 102 main jet  42 slow running jet but when I start it up it ticks over is really fast like its on speed.I've tried adjusting throttle stop screw  and pilot jet screw but little effect .any ideas?? Cheers 

[bryno]

Sounds like the slide is not closing properly, is the cable too tight?

Try backing off the cable adjuster until you have a tiny bit of play on the cable

[maxifields]

Thanks will check this ,was just reading it could be an air leak ,If the scoot wasn't so pretty I think I'd go back to fishing. bad day

[sime66]

Cheer up mate; a carb leak isn’t such a biggy.
If you've checked the slide is moving freely and throttle cable isn’t too tight, so it’s closing properly to the Idle screw stop, then a leak around the carb might well be the cause of your engine revving high.
Causes are uneven tightening of carb bolts, or uneven mating surface - probably on underside of carb, or maybe if you’ve used an old carb gasket and/or got some muck between the surfaces when you put it back. There’s also another mating surface under carb box onto engine, but that’s a bit more of a job to sort, though still easy enough*.
If you take your carb back off, check the underside, on ideally plate glass, but I use an old mirror, to see if it’s flat or rocking. Then flatten it off on a flat sheet of wet and dry on the glass – I squirt it with WD40 too and go in figure-8 motions. Replace gasket with a new and/or thicker one, and make sure there's no muck in carb box getting between the surfaces, and torque bolts down correctly and evenly – not too much! (about 15NM I think) Then make sure the slide is still moving freely afterwards, ie that you haven’t torqued it down too much. That should sort you.

*I imagine it's same for Sprint; when you take carb off there's still a screw holding carb box onto engine, as long as this is secure and carb box isn't loose, then the mating surface and gasket under the carb box is probably undisturbed. If you think it might be leaking between carb box and engine, you need to lift carb box and deal with that seal in the same way - flat, clean, new gasket.

[maxifields]

Yeah sorry was on a bit of a downer yesterday..I had cleaned the carb and renewed the gasket and fibre washers and seal in float chamber.going to have another look tomorrow and do what you suggested,
Thanks for the advise

[sime66]

The leak that gives you high revving like that is not within the carb itself, it's between carb and carb box, or carb box and engine.
Air gets sucked in, diluting the fuel/air mix after the carb, and leans it for combustion - so it races.

[maxifields]

Carb rebuilt 2nd time and everything seems good.Had help.(cheers john) but neither of us could see what I'd done wrong the first attempt.Maybe the wet and dry with wd40 did the trick.Rods packed away.

[sime66]

we do like a happy ending!  

[johnnymarr]

thanks for the tip with the mirror, wd40 and figure 8, one to keep in mind
cheers