Lean Burn Jetting LM4

[pete roper]

Hi Guys;

my carbs are not lean burn ( ie - dont have the brass "shroud" in the airflow ( dont think we got them in Australia.)

Baz

Yes we did.

[wirespokes]

Ah HAH! Pete knows all about them! Any sage advice for me (us)??? 

[pete roper]

Sorry, not really. What I call the 'Lean burn' type have the (?) 6-ish mm, horseshoe shaped shroud around the atomiser and I've only seen them on 36's. The 40's, from memory, had two types of atomiser surround. One that didn't protrude into the Venturi and one that surrouthe atomiser orifice and rose about 3mm above the floor of the Venturi.

Sorry, that's all I've got.

Pete

[wirespokes]

Thanks Pete. That reinforces the idea I've gotten that the lean burn carbs were 36s. If the shroud reduces consumption, why don't the 40s have them too? Why don't they all have them?

[moto-uno]

  We're these for reduced fuel consumption or reduced emissions ?  Peter

[pete roper]

Probably a mixture of both. The shroud increases the manifold depression above the atomiser orifice allowing leaner jetting but of course this means that more fuel is pushed past the annulus between the needle and atomiser.

Apart from the difference in fin shape there are no significant differences between the roundfin LeMans and the MkIII but a correctly set up MkIII uses less fuel and feels far more refined than its earlier siblings, or indeed, it's later ones. I've always thought the big valve motor was a pig of a thing. Ride a MkIII and a MkIV back to back and there is no comparison. While partly that was due to the adoption of the 'Tall Head' frame and the fact the entire bike suddenly became 'Bigger' with the MkIV a lot of it is down to the horrible combustion chamber design and obese pistons used in the big valve motor.

Pete

[wirespokes]

The shroud increases the manifold depression above the atomiser orifice allowing leaner jetting but of course this means that more fuel is pushed past the annulus between the needle and atomiser.

I wondered about that. The pressure drop is still the same, and at larger throttle openings, it won't make much of a difference. Guess I'll have to play with it to get the right combination. An exhaust sniffer would help a lot, but this is making more sense now.

Got it on the difference between the LM3 and 4. Sounds like the marketing dept requested changes to an already perfect combination and one thing lead to another.

[moto-uno]

  It's certainly been a while since the Le Mans 111 intro , but was there not supposed to have been improved ports in the Le Mans 111 over the Le Mans 11 ?   Peter

[pete roper]

Same size valves, same manifolds, (Shrug?). They look pretty similar to me.

[moto-uno]

  Okay , thanks  . Just thought that with all the exterior changes( to the heads and barrels) there was something other than just cosmetics involved ? Peter

[pete roper]

Not in the way they breathed.

With the MkIII you got the change to the wider stud pattern of the Squarefin motors and the change to alloy for the rocker carriers. The wider stud pattern is what allowed the eventual punching out of the motor to its final '1200' form. That setup, used on the last of the CARC bikes, used the longest stroke possible without the rods biffing the cam lobes and the biggest bore squeezable into the stud pattern.

Pete

[moto-uno]

 Thanks for the update "Pete" . Interesting your remarks concerning the stroking and boring potential with the Le Mans 3 motor . About 2 decades ago I was at the "Sport-bike Rally"
held in Parry Sound , Ontario . While there myself and another fellow ( on a 900 SS) thought we'd have some fun with this modified Le Mans from the States . That was one hell of an eye opener for the 2 of us . On an open stretch of highway around 110 kph we all nailed it . He literally vanished  . Found out he had a machine shop weld up the crank until the rods came to around a millimeter or so from the loony cams he had and bored to the largest sleeves he could install . No idea who did the head work ( porting and valve sizes ). I felt like I was riding a T-3 . Peter

[pete roper]

If you want probably the best of the 2V motors try and get a ride on a Bellagio one day. I know you didn't get them in North America, (An insane marketing decision! Completely unfathomable!) but they have the shortest stroke of any big block and a big wide bore. They are super smooth and spin up stupidly quick. Also, not having a tacho, you just hold the throttle wide open until they go 'Blurp-Blurp-Blurp' and grab another gear. Do that five times and you find yourself going at a cracking pace!

Pete