Quick jetting question... Surprise surpirse!

[Hughieboy]

Hi All

With help from the good folks on here I've been setting my 30 PHBH carb up. I have a quick (?) question about lean/rich running beyond half throttle.

Scooter is Li150 with Harry Barlow 190/Suzuki Ts piston that I have matched and polished the ports running with a TSR Evo expansion and a Varitronic. Timing as per Cam Lam recommendation is 19 degrees at fast tickover and retards to about 15 for anyone who hasn't read my previous ramblings

45 slide (came with carb, I know most seem to have a 40)
55 pilot
AV264 with X13 needle set on second clip
70 Choke
120 main

Air screw 1.5 turns out.

As above it runs well but seemed to hold back slightly on full throttle and almost speed up a bit when let off slightly. Choke test made it initially splutter for a second and then run normally. ETG said the head temperature was 110 degrees centigrade, I wonder if this is a bit hot? Plug colour looked good to possibly lean.

Thinking this to be the case I changed the main jet to a 122, this made it splutter at three quarters to full throttle intermittently although it still seemed hot. I took this to mean the main was too big and went back to the 120 but dropped the clip on the needle to the third clip to richen up the mid range and now it's boggy and spluttery in the mid range.

I have limited experience but it feels like it's either too rich and too lean at the same time almost. The first set up (120 main, second clip) ran well but I felt it needed to be a bit richer for safety but 122 is too much. Third clip on the needle is also too much either way.

Anyone recognise this and could tell me where to go with it?

Thanks,

Hugh.

[Knowledge]

The "seems to speed up when you let off the throttle" is a classic symptom of an undersized main jet. However, the next size jet seems to make less responsive, and 120 seems big enough any way.

Some Dellorto jets, and especially pattern jets, are poorly sized. You might have a rouge jet size but I would also question the combination of atomiser and needle. A 266 and X7 might work better across the whole range inc WOT.

Let's see what others think before you buy more bits for your carb.....

[Storkfoot]

Carb jetting can be hard work but please just change one aspect at a time and make a note as to how the engine responds.

AV264/ x13 works well with a filter in my experience but may be a tad too lean if open. Also, get a 40 slide (I know they aren't cheap) but there is a significant difference between a 40 and 45 and the hanging up you refer to could easily just be the slide being too lean.

[Hughieboy]

Carb jetting can be hard work but please just change one aspect at a time and make a note as to how the engine responds.

AV264/ x13 works well with a filter in my experience but may be a tad too lean if open. Also, get a 40 slide (I know they aren't cheap) but there is a significant difference between a 40 and 45 and the hanging up you refer to could easily just be the slide being too lean.

Sorry, forgot to mention, I have a foam filter fitted.....

Just so I understand, what effect does the slide size have? I'm getting my head around it all but it's taking a while to sink in!

When I bought it I asked for it be supplied with a ball park set up, 'oddball' park set up more like, so far I've replaced everything in it apart from the choke jet! It's all a learning curve.....

[Storkfoot]

Bigger cutaway in the slide = more air coming in = leaner mix

A good dyno run to set up your carb can be way cheaper than buying loads of jets and wasting loads of petrol whilst trying to set it up

That said, if I was a betting man and adding all the usual riders re every engine is different, I'd personally try a new 40 slide before I did anything else.

Others may suggest otherwise though

Good luck and let us know how you get on.

[Hughieboy]

Thanks Storkfoot.....

In for a penny and all that. I think I'll buy a 40 slide, that seems to be the size most use. Real oddball this one, it came with 45 slide, x4 needle and a 100 main! seemed to run ok but very hot and got fuel starvation on the top end. Now I understand things a little better I'm not surprised so I've already dropped a few quid on changing everything, another £30 of it helps get things sorted will be money well spent.

I intend to get it on a dyno at some point but I'd rather do what I can first and going to someone with a dyno means a road trip, there's no one nearby.

I'll order one up and see how it goes,

Cheers.

[Nelly]

Must say I've learnt a hell of a lot about setting up the carb by trial and error, even though I'm ordering all sorts of different needles and jets to try out. Bear in mind I had mine set up on the dyno but still found it has a very lean spot between 1/2 and 3/4 throttle once under load out on the road. I had to ride home from Derby with the choke on to keep the engine cool on the motorway. The CHT sensor has proved it's worth...........
Dave at Wasp Performance has been very helpful when I emailed him with my questions, managing to let me narrow the needles down to those that have a 1.8mm tip. I actually need to try out an X4 Hughie, so that setup you had may well have worked on somebody else's engine. It's a minefield!
BTW I have found Wasp by far the cheapest for needles and jets, all genuine Dellorto stuff too. I originally bought my carb from them too.
Must add, I've found this page from Bristol Lambretta Club very useful to work out which needle is which,
http://www.bristollambretta.com/dellort ... eedles.htm
Thank you to any of the Bristol lads involved with this website, very useful.

[Warkton Tornado No.1]

Before you order a slide, check what float needle & fuel valve you have (the item that the float needle fits into)

The float valve is restrictive if it's too low a number. The float needle should be red tipped but that won't be the cause of your issues.

Otherwise, very good advice above from members.

Sorry about the next bit, but PM's often don't get noticed.

FAO Storkfoot

Please PM me about the toolbox door if you would be prepared to 'borrow' an Italian one, pending replacement with an identical replacement @ a later date. Unless you can 'dechrome' the only metal one I'd let go.......

[Hughieboy]

Thanks,

I already ordered a slide as I'm off on holiday tomorrow and I wanted it here for when I get back..... And thank you Nelly for the PM, that is helpful in getting towards a better understanding of how it all works. I have ordered most of the replacement bits from Wasp but unfortunately they don't have any 40 slides listed at present so I ordered one from Eurocarb, still better priced than most at £27 odd delivered.

Back to the needle and needle jet, having studied the charts a bit more I think I'm right in saying that the original x4 needle with AV266 is richer than the x13 with AV264 that I have changed it to, is that correct? I just swapped them both as I was told that it was a good combination however possibly the original was ok. It also originally had a 53 pilot but again, I changed it for a 55 as that seemed a more common choice.

I think the tune would have originally been a touring tune as when I got it, it was running a 22mm Jetex and a big bore pipe. I added the bigger carb and expansion plus matched the inlet and exhaust ports to give it a bit more oomph for blasting about on which is mostly what I do. It certainly has got a lot more go now and when I get the jetting right it should be great but I need to sort out this flat spot, it revs out to 7500-8000 rpm after 3/4 throttle according to the fairly random Stage 6 rev counter/temp unit I have fitted but I don't know how reliable that is and it still feels like there would be more there but I expect we're dealing with wind resistance and fat old S2 legshield a at that point!

I will check the float needle size etc, I haven't looked at that but it may have to wait until next week.

What else would you do with your time......!