22mm Jetex

[sunrisemac]

before you start opening up the atomiser , try fitting a 5899-2 Atomiser . The atomiser you have fitted has a 2mm bore which is probably too rich for a 150cc The correct Atomiser 5899-2 has a 1.75mm bore and should improve things from 1/2 to WOT.
You'll probably need to adjust the main jet size though .

An easy test to see if your mixture is too rich at WOT is to run the scoot up a steep hill at maximum throttle then apply a little choke and see what happens.The engine should immediately bog if your mixture is too rich . If it stays about the same then your WOT mixture should be about right in which case you might need to look at your timing .

Your choke applies a completely independant fuel supply direct to the engine and does not alter your normal running mixture so its a good test.

Thanks for the advice, I am pretty sure the timing will be OK butcould be retarded a couple if necessary, it is 19 at present (static measured from flywheel)

[EddieStone]

Running a sh2 on a 150, make sure you have the right slide. It makes an immense amount of difference. I had the wrong slide in mine and it was always too rich no matter what I did with the jetting. Fitted the right slide and it cleaned up a treat.

[Chadley]

I like that - I'd be interested in one for my chop.

As for atomisers/emulsion tubes, I would fit a different one rather than drill the one you have - here's a guide I used to jet mine properly:

Atomiser.jpg

Thanks for the chart, I have been trying to find that info as reference to help me set up the carb.

I don't know where that chart originates from but the measurements don't match some original Dellorto atomisers I have previously measured. For example, two 5899-4's from different sources, one of which is my GP150 that hadn't been touched since the 70s, has the upper 'low rpm' hole at 1.25mm not the 0.98 shown. I also measured a Scootopia remade 5899-4 at 1.20mm.

[sunrisemac]

The atomiser I have fitted is a scootopia one and I have a couple of others that are unmarked which I intend to use for testing so will they be in line for modification, any info regarding hole sizes gratefully received thanks.

[sunrisemac]

Running a sh2 on a 150, make sure you have the right slide. It makes an immense amount of difference. I had the wrong slide in mine and it was always too rich no matter what I did with the jetting. Fitted the right slide and it cleaned up a treat.

Thanks for that, I think as I have bored the carb out to 24mm I may have to try different slides before getting it right

[johnnyXS]

yes I was going to post a warning that the atomiser hole sizes were way off but didn't want to kick anything off
Last year I bought a set of Atomisers and measured all the Atomisers with sets of Jet guages and double checked them with Vernier calipers I'll post my results tomorrow.
Commonsense should confirm that such non standard irregular bore diameters are unlikely


As sunrisemac says its important to make sure that you have the right slide fitted .
For 150cc engines fitted with a Jetex 22mm carb you should have a 7895-2 Slide ( has scooped out back and cutouts ), fitted as per GP150 but many Jetex carbs have been supplied as suitable for 150cc with slides jets and atomisers fitted that are meant for GP200cc engines .

In my limited experience most Lambretta parts suppliers haven't a clue about which parts should be fitted.
For GP200 and other 200cc engines using a 22mm carb a 7895-1 slide is standard fitment .(plain back with no cutouts) .Also 175 and 185cc kits
However that said... the poster has modified his carb considerably so its a question of trial and error testing of all parts.

There is an excellent table of base carb settings on the Cambridge site including modified engines
http://www.lambretta.co.uk/downloads/carbjetting.html

[sunrisemac]

yes I was going to post a warning that the atomiser hole sizes were way off but didn't want to kick anything off
Last year I bought a set of Atomisers and measured all the Atomisers with sets of Jet guages and double checked them with Vernier calipers I'll post my results tomorrow.
Commonsense should confirm that such non standard irregular bore diameters are unlikely


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I look forward to seeing the results of your measurements

[johnnyXS]

yes I was going to post a warning that the atomiser hole sizes were way off but didn't want to kick anything off
Last year I bought a set of Atomisers and measured all the Atomisers with sets of Jet guages and double checked them with Vernier calipers I'll post my results tomorrow.
Commonsense should confirm that such non standard irregular bore diameters are unlikely


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I look forward to seeing the results of your measurements

mmm so do I

Can't find them at the mo. Must be on a backup drive somewhere.

[sunrisemac]

yes I was going to post a warning that the atomiser hole sizes were way off but didn't want to kick anything off
Last year I bought a set of Atomisers and measured all the Atomisers with sets of Jet guages and double checked them with Vernier calipers I'll post my results tomorrow.
Commonsense should confirm that such non standard irregular bore diameters are unlikely


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I look forward to seeing the results of your measurements

mmm so do I

Can't find them at the mo. Must be on a backup drive somewhere.

doh........

[P D: SC Chester]

Any results and sizes of main jet etc.
I posted in another section about my GP150 and me fitting a Sterling.
Can you help a novice on boring out my 22mm jetex? And what jets, pipe you use.

Cheers

[johnnyXS]

be happy to help if we can Chester but you're going to need to tell us a little about your engine displacement , state of tune , exhaust, ignition system and timing advance
Right now I can tell you do not bore out your 22mm carb before you try and tune it properly as it is. A 22mm jetex has a perfectly sufficient venturi size for your current engine tune provided its tuned properly

Ok I've done a little checking and it seems you are running a GP150 with a 150c engine in standard tune and a 22mm Jetex carb and clubman exhaust yes ?

This is probably the best combination given your current engine displacement and tune. Done right you should see an appreciable improvement in power and top speed over standard spec but bearing in mind the size of the engine and its state of tune you need to be realistic about what you can hope to achieve .

Its too late now but I'll add to this post tomorrow about selecting slides jets atomisers and tuning the carb

[P D: SC Chester]

Thanks for the reply XS.
Its a totally standard Indian GP with 12v electrics and stator.
Sterling pipe and 28mm delly.

Cheers

[johnnyXS]

sorry I must have misunderstood .?
I thought you were asking about jetting and tuning a 22mm Jetex carb.

A sterling pipe and 28mm Delli on a standard 150cc untuned engine would be a complete waste of time to my mind.
I'd be happy to help you get the best out of a more realistic setup like a clubman and 22mm Jetex .

[P D: SC Chester]

Thanks XS

[johnnyXS]

sorry chester I don't have any tips for your proposed setup.

Setting up and tuning a scooter isnot just a question of bolting on a big carb and changing the Main jet ...its a complete package thing which includes timing & ignition

have a read of the Cambridge LAmbretta tuneing guide it will give you a very good overview and an excellent tuning guide
http://www.lambretta.co.uk/downloads/carbsetup.html

[P D: SC Chester]

The Jetex is back on, poppin away, I will keep the delly, for when I can afford to stage 4.

Cheers

[johnnyXS]

Chester here is an excellent tuning guide .
there is no point in my attempting to rewrite this in a few lines of text .
Work your way through the guide and let us know if you have any specific issues.
A video with sound track would be very useful to help us diagnose any problems you may have
http://scootrs.com/tech.cfm?tip=tunecarb

[P D: SC Chester]

Thanks XS, thought id posted before, but something with the Interwebtrickery went a bit wonky.