Dellorto PHBH Atomiser question

[alanscottj]

If I am using a AV266 atomiser in my Delly carb (Mugello 198) and change it for a AV268 which I believe is richer, where would I see the difference?

[drunkmunkey6969]

From tick over upwards, right to the top end.....but most of all in the midrange.

[alanscottj]

From tick over upwards, right to the top end.....but most of all in the midrange.

So if mid range is already bordering on too rich, I would be best to leave it as is?

[drunkmunkey6969]

Depends what slide, pilot, needle, clip, and main you have. And what problem you have which you are trying to cure.?

[alanscottj]

Depends what slide, pilot, needle, clip, and main you have. And what problem you have which you are trying to cure.?

I am just trying to make mid range a bit safer as that's where I think I went wrong last time I had it set up. I had a reed valve fitted and was told to upjet my main which I did (significantly), then went on to hole my piston. It has been suggested that I might want to go up on the atomiser too, to be on the safe side

At the moment I have:

130 main
X4 needle (2nd clip down)
AV266
55

Runs ok, maybe a little rich 1/4 throttle to full throttle

[alanscottj]

Forgot to mention the pipe - BGM clubman

[olliewtf]

Your jetting seems fairly typical, but i am questioning the needle... x4? according to this: http://bristollambretta.com/dellortojettingneedles.htm, youll see that the common lambretta needles have the same dimensions bar the length of the tapered part. Im not saying that therefore they are the only ones you can use, but it is a trend!
It does show however that your needle is pretty much the same dimensions as an x7 needle, apart from the outer circumference of the top part of the needle. I would imagine that this therefore is leaner particularly at lower throttle openings, and could potentially be a factor in your problems, as when you set your cruise position on the throttle, it is generally below half, depending on how your engine is set up or how powerful it is etc etc.

Personally, I would change it out for x7, 2nd clip. I would then also try an x13, see how it fares. if you have any lean spots still, up the atomiser.

I went through this, could get up to 2nd clip x13 with av 266 and it was too rich, but when in the powerband it still had a significant lean spot under load. I then upped the atomiser and it helped alot. im still finalising my jetting, but that was a key step in the right direction.

Also dont ignore the massive impact the main jet can have on your mid range. for example, i have been running a 122 main with x13 top clip. the mid was a bit too rich, the main a bit too lean, but still the lean spot lower down too. today i figured up the main, lower the mid a tad. went for 130 plus x7 2nd clip (my working theory here is that the higher the needle is out of the atomiser at low throttle the richer i can make my low down lean spot) and it was silly silly rich. thats with a leaner mid range. the main jet drowned it above 1/4 throttle. so now i will steadily come down on the main to make sure im not leaning off the mid range to compensate for a too large main jet!

read that last sentence again. Its very important.

right. waffled a bit but i hope that info helps in some way, ive been jetting mine in for nearly 6 bluddy months as its so complex sometimes!

[drunkmunkey6969]

...........It does show however that your needle is pretty much the same dimensions as an x7 needle, apart from the outer circumference of the top part of the needle. I would imagine that this therefore is leaner particularly at lower throttle openings, and could potentially be a factor in your problems, as when you set your cruise position on the throttle, it is generally below half, depending on how your engine is set up or how powerful it is etc etc.

Personally, I would change it out for x7, 2nd clip. I would then also try an x13, see how it fares. if you have any lean spots still, up the atomiser.
!

X7 is leaner than an X4.

[drunkmunkey6969]

Depends what slide, pilot, needle, clip, and main you have. And what problem you have which you are trying to cure.?

I am just trying to make mid range a bit safer as that's where I think I went wrong last time I had it set up. I had a reed valve fitted and was told to upjet my main which I did (significantly), then went on to hole my piston. It has been suggested that I might want to go up on the atomiser too, to be on the safe side

At the moment I have:

130 main
X4 needle (2nd clip down)
AV266
55

Runs ok, maybe a little rich 1/4 throttle to full throttle

I would suggest that 'having a reed valve fitted' would require bigger main jet than a 130......if it was fitted properly with boost port and piston modifications.

As for your mid-range issues, a dare say needle taper and/or clip position is the issue.

My advice.......stop messing about if you don't know what you are doing, go get it dynoed.

[alanscottj]

Depends what slide, pilot, needle, clip, and main you have. And what problem you have which you are trying to cure.?

I am just trying to make mid range a bit safer as that's where I think I went wrong last time I had it set up. I had a reed valve fitted and was told to upjet my main which I did (significantly), then went on to hole my piston. It has been suggested that I might want to go up on the atomiser too, to be on the safe side

At the moment I have:

130 main
X4 needle (2nd clip down)
AV266
55

Runs ok, maybe a little rich 1/4 throttle to full throttle

I would suggest that 'having a reed valve fitted' would require bigger main jet than a 130......if it was fitted properly with boost port and piston modifications.

As for your mid-range issues, a dare say needle taper and/or clip position is the issue.

My advice.......stop messing about if you don't know what you are doing, go get it dynoed.

Thanks for the advice, I will be getting it dynoed at some point, I am just trying to get it running ok until I have the time to get to one. I might not know exactly what I'm doing but I am enjoying the learning in the meantime I'll stick to experiment and plug chops......