Lambretta jetting help please! 175 Li

[EddieStone]

Can anyone help with jetting my carb? Engine details are: Li150 with 175 Stage 4 cast iron barrel, using a Sh1/20 carb through the airbox. It has a GP style air scoop with no baffle in it. The exhaust is a KBR clubman style. Carb jetting is 45 pilot, 105 main, 50 choke, a 5899-6 atomiser and a 5914-2 slide.

When it's cold, I use the choke and it usually starts by the second kick. When it runs, it's a bit 4 stroky around quarter throttle, but past there it runs fine, plug is a nice colour and it pulls OK. I changed the atomiser from a 5899-2 to the 5899-6 and it cured a lot of the low rev 4 stroke it had. It 4 strokes all the time if I put a 108 main jet in.

When it's warm after being left for a bit it can be a right bu99er to start. If it doesn't fire up first or second kick, it will take about 20 or 30 kicks to get it going. By then there's a smell of fuel, there's some dribbling out of the carb rubber. When it starts, it sort of chugs to start with and I have to tease it to get it to run properly. If I open the throttle just a little at the wrong time, it will stall. Sometimes it will fire just once when I kick it but it will not run. It seems to me that it's far too rich and flooding. When it eventually does fire, it doesn't smoke at all - in fact it's pretty not smokey in general, unless the choke's on. I tried a 42 pilot, and it fired up easily when cold, but I took for a 5 mile ride and it was even harder to start. I checked the plug and it was dry. So I'm puzzled if it's too lean, or too rich.

Anyone any idea why it's so hard to start when it's warm? I've had this scoot as a standard 150 since 1990 and it was never a poor starter. I like the performance of the 175 as it's much more fun/safer to ride, but kicking it over 20 times to get it started is annoying me now!

I'm starting to wonder if there any chance the timing might be the problem? Squish is 1mm and the timing I can't remember exactly, but I've checked it many times with a strobe gun and it was OK. It's a 6V points system, but when I when I was setting it up I remember having to turn the stator plate to the limits of the slots to get it to time up statically. But it timed up with a strobe so I've assumed it's OK.

Cheers for any help on this.

[johnduke]

Hi, my guess would be petrol spitback in the inlet hose . Do you turn off petrol before stopping engine ? . Maybe your 175s porting causing more spitback than your old 150 .

[Doom Patrol]

A Clubman type exhaust running on a 20mm carb never worked for me. Personally I don't think the 20mm is big enough. It seemed to struggle to fill the volume of the larger exhaust, and if you haven't got the pressure there it's going to make it hard to run, maybe even start. I've got no scientific basis for this, it's just a feeling, but it might be worth trying a standard exhaust.

[Daggs]

You need to treat the carb. as having three sections;
a) Starting
b) Moving through the gears
c) Cruising in top

a) Sounds too rich. Put the 42 pilot in, set the airscrew to optimum and make sure the choke is closing off properly.

b) When going through the gears, if it burbles and chokes. The atomiser is too rich. If it goes but seems to missfire at times, the atomiser is too lean.

c) When the previous two areas are o/k have a cruise in top, then check the plug colour. Adjust main to suit. (keep it a tad rich for running-in)

[EddieStone]

I think it's definitely spitting back, then this mixture sits in the carb/air hose making it too rich. How can I reduce the spit back? Is that just a tuning issue? Turning the fuel tap off seems to back things a bit more successful.

I have a SH2/22 off an Indian GP150 I could use as I now have the right air hose, but changing the manifold and the cable is something I'm not very enthusiastic about doing unless it's going to significantly improve things.

[scooterslag]

one thing which I think people over look when building a 175 engine and then try to jet the bugger in is what type of airbox and air scoop they use. I'm currently building a 175 lump (nothing special just a bored out SIL barrel, minor porting, Goetze piston matched with a standard pipe and 20mm) and I've got both a SX/LI airbox and scoop and the more free flowing GP airbox and scoop available. Problem is which would be best/easier to set up? the SX/LI airbox is the same spec as the TV175 set but there are differences with my barrel to the TV item so would the GP type be more suitable or would the extra intake of air available be too much and effect the jetting and run lean? would the SX/LI type not provide enough air? combinations of different airboxs and scoops? questions, questions...

[LI150]

I had a similar problem and it was spit back. To help this I pinched the idea of someone on here.
Firstly if you can get a series 2 air hose to fit with the drain on use that. If not copy it.
I took a cable adjuster and ground the top down so it was almost flat then put a small hole in the bottom of the hose, put the adjuster through and put a washer and nut on the other side to hold it in place. Place a length of pipe on the end of the adjuster. Job Done

[Chris in Margate]

You certainly do NOT need a bigger carb. 20mm is a lovely little carb and works perfectly with the Clubman.
Check for worn out float needle first and make sure it's a red tipped one. Keep off the throttle when restarting.
The chug chug chug after 30 kicks is the excess fuel drawn into the crankcase struggling to burn and escape.
Turning fuel off before stopping is indicative of a float fault and only masks the problem.

[EddieStone]

Right, just to close this, in case it helps anyone else with a cast iron Stage 4 175 kit...

The starting issue was solved by changing the slide from a 5914-2 to a 5914-1, which is leaner at ticker and up to 1/4 throttle.
For anyone else opting to use an SH1/20 the variables are:

Main jet: Controls jetting from around half to wide open. Mine is a 105, which seems small compared to others, but I'm running through a standard airbox and filter, but with a GP airscoop.
Pilot jet: Controls tickover, and (presumably) starting. If your mixture screw won't go lean enough fit a smaller pilot, if it won't go rich enough fit a bigger one. The pilot jet is used through the whole rev range, so if you fit a smaller one, you may need to increase the size of your main jet to compensate.

Choke jet is 50. That's that.

Atomiser is not mentioned much, but makes a difference from 1/4 to half throttle, when the main jet takes over. The number after the 5899 indicates how lean it runs at lower revs. The 5899-2 is richer than a 5899-6. So I'm running a 5899-6 to make it leaner at lower revs as it four strokes more with the 5899-2.

This is from my experience so far - between the slide, the atomiser, the main jet and the pilot jet there are a lot of variable to set up the carb properly. In theory!

I'm planning on de-restricting the airbox as I suspect there is more power in the top end than I am using, but for the moment I'm glad just to get it running OK.

[Scooterdude]

Interesting thread, I have a 175 barrel fitted to my LI 125, done by Harry Barlow, he's opened up all the ports, cleaned up the inlet manifold and advised me to run it without a head gasket, I'm using the standard 18mm delorto with slightly bigger main jet and atomiser + a Scootopia long cone clubman, 16 tooth front sprocket, I'm getting about 125 psi compression pressure and it pulls like a train! But like above I have noticed petrol spit back in th carb rubber but just put that down to the fact that the barrel has been ported.