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hello - another possibly idiot question;

my LI3 currently has a 175 Gori Kit with a 200cc jetex (with a 108 main), an AF and MB fast flow filter through a drilled late S3 box. gearing is 125 box with 46/17. fast flow tap now fitted but struggles to hit 60 kllcks...


I'm taking it to be be strobed next week. Whilst I am very confident that it'll be well looked after, what degree am i looking at? i've followed a few threads but I don't think I understand it quite right yet.


[youtube-share]https://www.youtube.com/watch?v=o0yGOyyzcfg[/youtube-share]

Thank you!

what engine size was it originally ? when you say Your taking it to be strobbed is it being done by someone that knows Lambretta's and how was it set when fitted ?

just looked at the spec for the gori 175 kit and it looks as if it is just a mildly ported std 175 so I would imagine if all the ports are blended and matched and the carb and timing are spot on with the right exhaust you will only see about 65-70 if your lucky also looking at the scooter help gear selector at 6000 rpm they expect 61 mph

Just reading with interest as I have a similar set up to yours -Li 125 gearbox 46 16, 175 indian barrel, 42 mm kba clubman ,cheap steel bird air filter ,gp air scoop ,22mm jetex but my main jet is way bigger at 126 it will run on 125 but plug looked a bit lean.
I've not bothered measuring the top speed for a while (gps on my phone) but I had 58 mph on the flat which I thought was plenty fast enough .I can keep up with everybody else in my club and this year have done 3500 miles at home and Europe .
Not long after I got the scooter on the road I rode It as a bog standard 125 and Im not joking it was shocking some guys on push bike's came past laughing and taking the p155.
.
I think if you can get between 55/60 mph your doing great on 10" wheels with drum brakes and brick like handling

as Phil says your Main jet is looking awfully small at 108.

I run a stock LiS 150 with 22mm Jetex 5889-2 Atomiser and 106 Main jet with standard exhaust and mine can cruise at 50Mph on 1/4 throttle with lots in hand.

An easy way to check if your Main jet is adequate is to find a steady hill somewhere you can maintain a wide open throttle to put the engine under load..... then slowly apply a little choke.
If the engine picks up then your Main jet is too small. (If it bogs then its too big but that is extremely unlikely in your case)
(Note its not the test speed that is important here but your throttle position. You could do this test in 3rd gear just as well as 4th

Between 3/4 and WOT ....only the main jet is contributing to the fuel delivery so nothing else matters. if you have some spare jets then start with the largest and work down until there is no appreciable improvement in speed and then I would advise going up one size from there. Better to be slightly rich at WOT than slightly lean

Good advise Johnny, though this task is not made easy by the stupid choke position on the Li Series models.

The alternative test for undersize main jets is to hold the scooter under load on WOT, and then throttle back a bit. If the scooter speed picks up, this indicate that the main jet is too small. This is because the balance between the amount of air and the amount of fuel is optimised as the throttle is closed.

In an ideal set up, the ratio of fuel to air should be constant from closed throttle to wide open throttle.

Knowledge wrote:Good advise Johnny, though this task is not made easy by the stupid choke position on the Li Series models.

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I know ... I did think about putting a warning because there is a chance whilst doing this test that the scoot could suddenly bog and you could lose control with just one hand on the bars.
I'll try your suggestion next time I'm out and see how it compares.

Knowledge wrote:The alternative test for undersize main jets is to hold the scooter under load on WOT, and then throttle back a bit. If the scooter speed picks up, this indicate that the main jet is too small. This is because the balance between the amount of air and the amount of fuel is optimised as the throttle is closed.

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Would an oversize main jet also give the same symptom ?

When in WOT you may be "flooding" the engine but when backing off you would optimise the air/fuel and the speed would pick up................just a thought and I may be wrong

MickYork wrote:
Would an oversize main jet also give the same symptom ?

When in WOT you may be "flooding" the engine but when backing off you would optimise the air/fuel and the speed would pick up................just a thought and I may be wrong

an interesting question Mick. I have not tried this myself yet because the choke method is more reliable.

By coming off WOT you are introducing 2x different changing factors simultaneously.
You are reducing the size of the venturi which has the effect of increasing the vacuum (Bernoouli) which would draw more fuel through the main jet at the same time as decreasing the air flow. Naturally you would have to be careful not to reduce the throttle to anything close to 3/4 or you'll then be involving the Atomiser and dewpending on the car type , the needle too.

We'll have to test this to see if it gives a reliable result compared to the choke method . Applying choke maintains WOT and introduces only fuel which will definitely enrich the mixture so you know exactly what you are changing to the main jet supply

Fair point about trying the choke with the engine under load at WOT and I am interested to see what happens with my set up so I am going to try it even though I was'nt the person who posted the original question .You can operate the choke if your careful with the heel of your foot its slightly easier than trying to reach down especially when you've got stumps for arms like moi.

Ok apologies for the last 2 posts . The first answer was made without really engaging my brain and the second answer was made whilst cooking dinner and struggling with a stupid 10" netbook ...god how I hate laptops and netbooks

Now ...what we are trying to achieve here is to test if the Main jet is too large (rich) too small (lean) or about right.

The only way to determine this reliably on the road is to introduce either more / less air .....or more / less fuel.
As we cannot introduce any more air than is available at WOT we are left with fuel enrichment .

At wide open throttle (WOT) with the engine under load ,(max RPM) it is only the main jet that supplies all the fuel and the open venturi that supplies all the air .
If under WOT we have reached and maintained our max engine speed RPM (note: not max road speed ) what we need to do is to introduce either an increase or a decrease in fuel and observe the engine response.
This will test the main jet.


If we turn the choke on it will supply more fuel from the choke jet straight into the venturi and we can note if the engine bogs , picks up or stays the same. As it is only fuel that we have changed we can be pretty certain that we are testing just the Main jet fuel delivery (richness)

If on the other hand we were to roll off the throttle slightly we would be introdcing several variables at the same time some of which would be impossible to quantify.
Closing the throttle would decrease the venturi opening to the engine which would increase the air velocity and I would expect alter the amount of fuel drawn through the main jet from the fuel bowl. So theoretically it should have a similar effect as applying choke and enrich the mixture but it will also effect the air flow characteristics to an unknown extent.

I would chose to use the Choke method personally as I know precisely how I am effecting the fuel delivery mixture but without testing the two methods back to back I have no idea if you can achieve the same result using the throttle reduction method. What I do know is if you are not careful and reduce the throttle to near 3/4 you will then be introducing the needle and needle jet or the atomiser on the early carbs which would make the result worthless

If the weather is good tomorrow I am going to try both methods. I'm pretty sure I have the correct Main jet ( 106) as the the 108 definitely effects WOT and top speed adversely.

Phil D wrote:Fair point about trying the choke with the engine under load at WOT and I am interested to see what happens with my set up so I am going to try it even though I was'nt the person who posted the original question .You can operate the choke if your careful with the heel of your foot its slightly easier than trying to reach down especially when you've got stumps for arms like moi.

you could always tie a bit of string on the choke lever and run it up to the clutch lever housing and give it a yank whilst keeping both hands on the bars

I think the trick is to introduce a little choke gradually , noting any change . If you plonk full choke on I think you'd swamp even a lean Main jet with too much fuel and the engine would bog.

Doh
ok what about employing a midget to sit on ones legsheilds and operate the choke .

Phil D wrote:Doh
ok what about employing a midget to sit on ones legsheilds and operate the choke .

nahh... that would discriminatory and exploitation
I might be able to train my cat to do it though

Oh yes, you're right, I'm wrong.

Blame it on jet lag. I've just come back from Wales.

Thank you for the advice. The block was built elsewhere before I fitted it, which is why I thought it may need strobing, and the spark is not the brightest. I was curious to how many degrees it should be set to.

I tried the choke test (that is tricky!) today with no discernable effect. The engine bogs on the flat if I put the choke on. But since re-reading the posts I think I did it too quickly so will try again tomorrow. Cambridge L.W. has 106 for a TV which is why I tried the 108 but I appreciate that was on a 20mm carb...

It's off to the workshop for strobing next week.

Knowledge wrote:Oh yes, you're right, I'm wrong.

Blame it on jet lag. I've just come back from Wales.

Wales eh ! commiserations

Actually its our favourite UK touring destination .

You may well be right I don't know . I'm hoping to try both tomorrow if it isn't persisting.

I had a go at testing my main jet today using the choke enrichment method and was not surprised to find it tested ok
Has anyone else tried this yet ?

Andie wrote:
I tried the choke test (that is tricky!) today with no discernable effect. The engine bogs on the flat if I put the choke on. But since re-reading the posts I think I did it too quickly so will try again tomorrow. Cambridge L.W. has 106 for a TV which is why I tried the 108 but I appreciate that was on a 20mm carb...

Smaller carb so is drawing in less air hence it doesn't need as much fuel amd therefore a smaller main. If you are looking at the CamLam carb set up guide then you would be better off looking at the lines for budget 175/185 kits with 22mm carb, this suggests a 5899-2 atomiser, and either a 118 main with 45 pilot or 125 main with 48 pilot. 125 sounds a bit too big but no harm in starting there and working down.

burnside wrote:

Andie wrote:
I tried the choke test (that is tricky!) today with no discernable effect. The engine bogs on the flat if I put the choke on. But since re-reading the posts I think I did it too quickly so will try again tomorrow. Cambridge L.W. has 106 for a TV which is why I tried the 108 but I appreciate that was on a 20mm carb...

Smaller carb so is drawing in less air hence it doesn't need as much fuel amd therefore a smaller main. If you are looking at the CamLam carb set up guide then you would be better off looking at the lines for budget 175/185 kits with 22mm carb, this suggests a 5899-2 atomiser, and either a 118 main with 45 pilot or 125 main with 48 pilot. 125 sounds a bit too big but no harm in starting there and working down.

I'm getting a little confused (it doesn't take much these days ) . I thought Andie said that he was using a Jetex carb with a 200 slide (no cutouts) and a 108 Main jet I assumed that it was a 22mm Jetex ,although you can get a 26mm Jetex .
Where has the 118 Main jet come from ? I must have missed something somewhere. as you say thats way too rich for this application.

To ANDIE
To do the choke test properly your engine must be under maximum load which is impossible to achieve on the flat..
You have to do this test up a steepish hill or with a heavy passenger on the back. Wait until your speed has reached maximum with a WOT then apply the choke and hold it on for several seconds to allow the extra fuel to effect the engine.
If your engine then bogs your main jet is too rich .