difficulty jetting in

[olliewtf]

hi all,
running a ally barrel tuned by myself with suzuki piston and 60mm x 116 rod (or 117 either way) making it 195.
jl3, phbh 28mm carb running with mb panel filter, indian ignition with vespa flywheel, reedspeed cdi with advance of around 23 degrees back to 17 degrees at around 4k rpm.
current jetting as of today, trying to cure a mid range lean spot, is as follows: 50 pilot, av266, x13 2nd clip and main jet at 120. main jet is probably too lean, only because i was running x13 on 1st clip with 128 main, which was a tad rich but cleared once engine was warm. however, on 1st clip, although the engine picked up really well, it was definitely lean in the mid range once the porting and pipe came into the power.
so before an 80 mile jaunt today with some fast a roads, i put it back on 2nd clip, and dropped main jet to what i had lying around, hence 120, thinking the 128 was compensating for too lean mid range. it is still a bit of a pig in the mid range, even once it was fully warmed up, holding it at mid throttle once in the power it was 4 stroking just a little bit, and wasnt picking up as well as it should.
i briefly tried the 128 again, and this made anything from 1/8 up way too rich. so there is something to be said about main jetting compensating for lean mid range.
However, i am a tad unhappy that between the two clips im facing either way too lean, or too rich. i know that you can get a spacer to make the needle sit between the two, but im not keen on this as the change seems too drastic, and what if the spacer makes it slightly on the lean side?
just to add a bit more info, before this i ran a x7 needle on 2nd clip, which responded exactly the same, too lean once in the power, and too rich on 3rd clip.

another odd problem im facing is that when i throttle off, particularly if throttling off when in the power, the revs make a very odd sound, they kind of thin out, similar to when my needle position is too lean, so im convinced that when throttling off im too lean. after thinking about this problem, my next step is to up the main jet, and up the pilot until that problem goes away, although i couldnt get anything over 50 to tickover nicely. however, im going to try a higher tickover to see if that helps (only reason for not doing so was because the slide adjuster was bottomed out already.)

in short, as i have a bit of a peaky set up, although the advance certainly gives me quite a bit of torque back, i struggling to find a happy medium, and am starting to think i should re check my top end for leaks.

whilst out today, i changed my plug from 8 to 9 as i was concerned about these lean situations on the faster roads, and this seemed to help a bit.

starting to become a bit of a pain this one!


cheers all

[tavspeed]

had the same problem on my cast 225 with a dellorto phbh30,with x13 needle and various mains upto 135 was way too rich even with circlip on leanest setting,changed to a x7 was still rich on 2nd clip from top,and too lean on top clip,there are various needles and atomiser in between the x7 x13 range,have a look on wasp performance,ended up putting my trusty oso pwk on and have it running sweet,went to cleethorpes and back and never missed a beat'
dellortos have that many variables they are very difficult to set up in my opinion,probaly best taking it for a dyno session.

[olliewtf]

it does seem that way. i hear what youre saying about other needles, might be worth a punt. could try an x10 and see how that fairs.
has anyone else tried these inbetween needles?
it seems to me the ones recommended for lambretta use are at quite large intervals to each other, and would clearly not be useful in every application. the clip settings imo dont cover the gaps, so is there is something im missing here? why do we not see more x8, x9, x14 etc in lambretta carb set ups?
i will try a washer first, but it seems to be that this is not a perfect solution as the jump between both clip positions is drastic.

i still would like confirmation on whether my thoughts about the pilot jet are correct. if you set up a rich tickover the traditional way, is it common to then find, on a peaky set up, that when throttling off it isnt supplying enough fuel as the revs drop?

[tavspeed]

do you know what your port timings are,have you a 1mm packer for the 60mm crank,i think the suzuki piston makes the timings quite racey and then you will have raised them another 1mm,whereas the inlet port floor might want lowering,
they might not be now compatible with the jl3
might be worth measuring them and posting back,quite a few on here who know what timings etc will suit the jl3

[olliewtf]

i tuned the barrel around the piston and crank combo, timings are roughly, off the top of my head, 186 ex 129 tr and 165 inlet. no need for a packer in the end, wouldve pushed transfer timing quite high. barrel wasnt shortened as the rod evened out the shorter piston, plus extra mm from crank, so as said barrel tuned around this. those timings should work with a jl3 imo, and it does have a strong powerband. it seems to me that when the power comes on it wants much richer jetting than i can manage to make low down running easier, even with ignition advance. it seems really that im nearly there, but its kinda on the knife's edge if you will!
head profiled and chamber matched by harry barlow, so im not convinced compression is a factor.
will try and find a suitable washer to lower needle a bit, and richen up the pilot to sort out the throttling off problem. its only noticeable when throttling off at high revs, at lower revs all sounds normal.
bit of a b@5t@rd on a fast road, hold it open and it wants to pull and pull, so drop throttle to attain cruising speed and it seems to lean out. with the richer mid range its better, but if i release the throttle its really not happy, which means for now i cant do any fast miles as it scares the bejeezus out of me. at some point i will build a handlebar choke, but really want to get the jetting nailed without having to dyno it! nearest dyno (im in london) is southend, and i dont really want to go. i mean, im sure its a lovely place to visit, but i wanna be able to suss it!

[tavspeed]

the inlet looks very high timings and your blowdown is 28 deg,have a look at mb,s tuning pages,your inlet timing is suited to 10,000+ rpm,link below
28 deg blowdown make it difficult to jet in he says
http://www.lambrettaspares.com/info/mb- ... -+140.html

[olliewtf]

I hear what you are saying, but I've heard so many differing opinions on tuning that I decided to follow a median of all the various bits of information I have heard. My inlet is high, but I dont suffer enormous amounts of spit back. High blowdown was something I wanted after a few discussions with people.
It certainly is a difficult one, and I have read MBs pages again and again, but I did start wondering how all the fast kits, imola, rb, ts1 were completely different timing wise to what MB was suggesting. Now I don't have the luxury of trying these things scientifically, and I may have overdone it, but it is a fast engine which is tractable low down too.
But as you say, maybe thats why im struggling. having said that, the phbh carb works on many other 2 strokes that are peakier than mine and create more power, although maybe that functionality is down to gearing.
Alas, there's not much I can do about the tune, but Im sure there is something I can do about the jetting.
we shall see!

[Jazzy]

Have you thought of altering a needle to suit your set up,
something I'm contemplating at the moment.

[eden]

28 degrees is fine.
Have you tried a power jet?
My cast 240 used to suffer in the same way. I couldn't get the jetting bang on through the full rpm range due to it needing to be much richer once into the powerband. Fitting a power jet and setting it up right did the trick for mine.

[olliewtf]

i think i may have to go down that route. not wholly confident on getting the position right following adams post on it recently!
tbh the mid range is probably where it wants to be for fast use as once its really in the power the spluttering is nearly gone. its just i drive mainly round town for work and it makes it a bit of a pig, and im concerned that richening up the main, which needs doing, will exasperate this problem.
just to clarify, when you throttle off at high revs, whats feeding the engine? is it just the pilot and air circuit or is it also the slide cutout due to the vacuum created by the falling but higher revs? if so, im tempted to assume my slide, currently 40, may be lean. i will first try upping the pilot to get rid of this odd lean sensation when throttling off, but im starting to think the slide may play a factor.

also, this may sound stupid, but would dropping the atomiser back to 264 help to bridge the gap or is that too much of a change? as it seems to be so finite between clip positions im thinking maybe the 2nd clip, proving a bit too rich, is actually ideal in terms of taper and length, but the atomiser could be supplying too much fuel overall? i may just try this soon anyhow, but im not convinced by my own logic here....

powerjet sounds good, how exactly does it work though? if for example i lean off the midrange for better low down response, does it richen this up at higher revs regardless of throttle position due to the greater vacuum created by the faster rpms? who supplies a good kit? cam lam ive seen but thats 60 quid, not cheap for me, is that kind of the going rate? wouldnt want to fit it myself either! sounds like a good shout though.
eden, do you recommend using non conventional needles, ie x10, or am i going in circles and is this just a case of trying to jet for two very idfferent applications in one go?

thanks for everyones input!