LCGB Forums

Hi out there,
Last year I replaced the 26mm Jetex because it was leaking badly with a new Jetex of the same type from Rayspeed that was jetted to suit the 175 cylinder kit and 42mm clubman exhaust I have on my set up which is on a series 3 Li 150. The problem is that the Spark plug is showing lean running conditions and increasing the main jet size has no effect on the colour of the spark plug (still showing lean running) even though when I ride the scooter it feels like its running rich (sluggish with rpm increase). The jetting is as follows Pilot=45 Choke=50 Main=132. I tried going up to 135 main which as I mentioned earlier still showed lean conditions with Spark plug(B7ES) but felt like it was running too rich whilst riding the scooter. Any help appreciated. Thanks.

try a B8 plug.............and ditch the jetex

Agree with the B8 plug but don't ditch the jetex.I don't think thst is your problem although imo the main is way too big. I suspect your plug problem is an air leak somewhere, inlet, head or exhaust.

Rich'

I personally would have jetted 50 pilot and 128 main and try swapping out plug to a B8. But if plug is still not brown you will have to look for an air leak.

Have you modified the air box from standard, inner elbow cut out back to the top and drilled the sides of the air box ? Lifting the air scoop about 10mm with a packer also flows more air.

Done some of these mods myself in the past but find it can be very hard to jet the 26mm jettex or dellorto.

Speed and power were not much down keeping the standard box unmodified just lifting the air scoop and removing half of the elbow but it was much easier to jet the 26mm carb, seems they work a little easier if they can create a slight vacume in the air box.

If the air box was heavily modified I found I had to run mains up to 138 and also modify the atomisers to flow more fuel for less depression.

Good luck, Mark.

50 pilot is a good idea because the lean condition could be occurring on deacceleration when your carb is only flowing an idle mixture through the pilot jet and the engine is still at high revs. Classic seize scenario.
What atomiser is fitted ? what fuel oil mix ratio are you using ?

I would use some carb cleaner or WD40 etc and check for a air leak around the inlet manifold first to eliminate a possible air leak weakening the mixture.

It would be a useful diagnostic to get the engine up to temperature then put a clean plug in and do a high speed run shutting the engine off with a wide open throttle pulling the clutch and coasting to a stop .if you check the spark plug it should give you a clear indication of whether the lean condition is only occuring when using the main jet ie at WOT.

I second that. Plug colour is only good for THE moment. Going out for a run and then parking up the plug shows the last moments of running.
I have had a lot of Jeter cars and if set up they are fine but I always found they needed a 50 pilot.

Hi Guys,
Thanks for all the information. One thing I didn't mention with this new jetex is the float stuck on and petrol came out of the overflow on the carb all over the floor and got into the cylinder fouled the spark plug which I had to ditch. I had to fettle the float where it was catching on the carb where it is held in place by the metal pin. This lean condition seems to of been worse since then . I will do some more investigating.

thats probably the most common problem you'll come across on the Jetex carb . its due to poor quality control and cheap materials .
Once you have made sure that the float can swivel freely on its pivot pin check that you have the correct length float needle valve fitted as it governs the correct float height and hence fuel delivery at high rpm

Let us know which atomiser is fitted as this can have a pronounced influence on air and fuel delivery ,more than your main jet at idle to 3/4 throttle

johnnyXS wrote:thats probably the most common problem you'll come across on the Jetex carb . its due to poor quality control and cheap materials .
Once you have made sure that the float can swivel freely on its pivot pin check that you have the correct length float needle valve fitted as it governs the correct float height and hence fuel delivery at high rpm

Let us know which atomiser is fitted as this can have a pronounced influence on air and fuel delivery ,more than your main jet at idle to 3/4 throttle

All the 26mm Jetex carbs I have had, I have changed the atomiser for an Allstyles drilled and modified 5899-2, and all have worked very well.

Rich'

Hi again,
I am running on 4 % Fuel/Oil ratio. As regards the atomiser I am not sure what it is as I can't find any identification on it. Its the one supplied with carb after AF Rayspeed jetted it to suit 175 Cylinder and 42mm clubman exhaust, if that gives any clues? Thanks.

if there is nothing marked on the atomiser then it is almost certainly either the rubbish atomiser supplied with the Jetex carb from India or a generic copy .Either way its not suitable . You have absolutely no way of telling which atomiser is fitted to your carb unless you use jet sizing tools.

You need to use an original Dellorto atomiser which is stamped with the series number ie 5899-1, 5899-2 etc so that you know exactly the number, placement and diameter of the fuel and air holes on the atomiser body . This will tell you exactly the amount of air and fuel that is being emulsified and supplied to the venturi under all throttle conditions so you can test other atomisers and compare performance characteristics between them

As Rich says the best Atomiser for your particular setup is going to be the 5889-2 its the atomiser recommended for the LiS150 the series 3 Li150 the SX150 the TV175 the SX200 and the GP200.
The 5899-2 atomiser has a large 1.75mm bore diameter giving maximum full throttle fuel flow from the main jet and a 0.95mm idle fuel feed hole which is one of the largest . It also has the smallest idle air feed holes so potentially will give a richer mixture and snappier acceleration.

I suspect that your main jet is way too large at 118 but then I suspect that a 25mm carb venturi may be a bit of overkill for a standard 175cc engine. The larger the venturi the weaker the vacuum in the venturi created by the engine which is what sucks your fuel out of the fuel bowl into the inlet. . This means you need larger atomiser and pilot and main jet to flow more fuel to compensate for the reduced vacuum and fuel flow.

When you say the plug shows a lean mixture ... what colour is the electrode ? a picture would be very helpful.

Thanks again for all the info supplied. A lot of Helpful hints. I will attempt to add a photo of the spark plug current condition. Thanks again folks.

my 175 has all the filter box internals removed & its drilled & the air scoop modified c/w 42mm ancillotti, was sick of mine seizing every 15 miles, so upped the pilot from 45 to 48, changed the atomisor to a drilled out -2 (to 2mm) & steadily increased the main to a now 135!!!!!! as when changing gear (when warmed up) it used to race like joe buggery, but now its all calmed down with a nice tan coloured plug

Hi again,
I am struggling to get a picture of the sparkplug on here any ideas anyone? Thanks.

yes there is an image size restriction on most forums and I suspect that you are trying to upload an image at its original size ?

Right click on the image and check the file size in properties.

If its larger than 1024 pixels or 250kb you can reduce the size by opening the image in Windows Paint program and selecting 'resize' select the pixel width option and change it to 750 or 1024 pixel width then save it .
it should now upload to your post but don't forget to select the 'add the file' option before you submit the post

Hi Again,
Please see attached spark plug picture showing current condition.

thats an extremely small image to see anything in detail however it doesn't lookto be a particularly lean mixture to me .
It is clearly a new or cleaned plug so maybe there are not enough miles done to get a clear picture.

The ideal colour is somewhere between a light tan and a biscuit tan but it depends a lot on the type of plug used and the type of riding you do .

If the mixture was really lean the engine would have a tendancy for the revs to rise at idle and be slow to die to an idle when you closed the throttle on deacceleration .

Hi,
Maybe I need to do some more mileage and keep an eye on things. I have got hold of a 50 pilot and modified Atomiser from Allstyles plus a selection of main jets to try also. Thanks for everyones help.

did you check for an air leak on the inlet using a spray ?

Even an extremely small leak would have a profound effect on your mixture .!

To test the mixture is correct before and after changing jets and atomisers you need to run the engine up to temperature then apply a little choke at idle, 1/4 throttle 1/2 throttle 3/4 throttle and WOT .
Each of these checks will tell you what needs to be changed ie pilot jet, atomiser or main jet.
(The last 2x checks need to be done quickly to avoid stressing the engine too much.)

If you don't make these checks at each throttle opening you'll just be changing jets at random and learning nothing.

What should happen is the engine should immediately bog down ie too rich.
If the engine continues to run normally or it momentarily picks up revs a little then the mixture is too weak at that particualar throttle setting .so then you will know exactly what is working correcly and what needs to be changed.
Your mixture does not stay the same throughout the rev range,.... it is determined by a combination of things at each part of the throttle opening

Did Allstyles tell you which atomiser they were supplying ???ie 5899-2 etc ??
edit looks like Allstyles drill a 5899-2 out from 1.75mm to 2mm which is basically the same as a standard 5899-5 except the idle fuel and air holes are different .
If that is the only difference you can buy micro drills on ebay and drill your own 5889-2 atomisers and experiment with say 1.8mm 1.9mm and 2mm etc