Jetting 36mm carbs for Lemans 2

[Murraydali]

I'm replacing the carbs on a Lemans 2 I bought recently. It came with 40mm Dellorto racing carbs - no pumpers or chokes and not practical for street riding.

I've decided to go with 36mm Dellortos and will put an order in soon. But I need some help deciding what jet sizes to order with the carbs. I would go with standard LM2 jet sizes, but the engine is not standard - so there may be a better starting point.

The engine has had all bearings, gaskets and seals recently replaced and is in great shape. It has a 950 kit (88mm barrels), P3 cam, LM heads with 44mm inlets and 37mm exhaust, Lafranconi comp mufflers. I will run it with K&N filters.

I'd really appreciate some good advice from you guys.

Paul

 

[Morizzi]

Paul,

The chances of me picking the correct jetting for your bike are very, very small but I can give you a few factors to consider.

Not all PHF 36's are the same. LM III's came with lean burn carbs. They have a small brass shroud around the needle jet in the venturi. If you get these then the jetting will be smaller than if you get the older style. There is no real point in jetting up lean burn carbs too much as all you will do is waste fuel and wash the bores. For eg OEM main jet on LM II 140, LM III 115.

Wear and compression varies per bike.

You also have a bigger capacity engine than standard. This means that the air velocity through the venturi is faster so the pressure differential is greater so more fuel will be fed in, richer mixture.

For a non lean burn PHF 36 my start point would be something like:

Main 135
Idle 55
Slide 60/3
Needle K5/2 (Stick with what you have to start with.)
Atomiser AB265 or 268 depending on needle.
Choke enrich Whatever you have now will probably work.

There is no way I'll get this right first time but it should run.

The bike should be a bit more like a LM 1000 and the PHM 40's should be easier to dial in though be less responsive.

If so you should be around
Main 145-150
Idle 55-60
Atomiser AB268

Food for thought for you.

Rod

 

[Murraydali]

Thanks for the reply Rod.

I originally left the bike with a mechanic who was having a lot of difficulty getting the current 40mm carbs working. He just couldn't get the bike to idle or run smoothly and the labour costs were starting to wound me.

I brought the bike home and just pulled the carbs off and think I pegged the problem. One of the carb springs was binding and the slide was not shutting properly most of the time. This was compounded by bell top cranks and a slightly notchy action, plus a throttle well past its use by date. Now I'm thinking I'll keep the 40mm carbs and change out the springs and swap the bell cranks for flat tops. I'll leave the jetting as is and just get the thing running and work from there.

I've also posted this issue up on another forum.

 

[Morizzi]

No problems Paul,

I actually saw your post on WG after I repled here. I don't go there and post very often. Too many opinions and too little knowledge. :roll:

I'm in Oz too, on the Sunshine Coast. I'm a LM III person and I have a couple but I have an SP and a few other things too.

Vasco is very good. He works on Guzzis professionally but nobody can just pluck a bunch of numbers out of the air and give you the answer you would like. I can only try and interpolate between the information you give, OEM settings and my experience with similar models. It is, at best, an educated guess and you haven't even given full settings and symptoms. Just 'it doesn't run well."

You need to give us all the carby settings, needles, atomisers, slides, the lot and even then we can only make an informed guess. Every bike is different. unless it is brand new just off the assembly line.

Small valve 1000's use PHF 30's. Mid valve 850's use PHF 36's and big valve 1000's came with PHM 40's.

You will be able to get your bike to run with any of these set ups. Bigger carb, more top end. Smaller carb, more responsive. I answered you previously and from what you indicated as your style I suggested the 36's. I still hold to that but if money is an issue you should be able to get it to run with 40's.

172 mains! WTF?

They can suppy 150% of the fuel a LM 1000 gets from its 140 mains! 150%!!!!!!!!!! No wonder it didn't run.

Go back to the LM 1000 jetting with PHM's and start from there.

Normally you can go a bit up on mains but if you go too far it makes it hard to dial in the rest for road use. Racing is a totally different ball game.

Idles should be big enough that the mixture screw are around 1 1/2 turns out for best idle, give or take.

If the bike surges when idling along at low revs through a town or has a hesitation off idle most hink it is too lean but often it is too rich. Drop the idle jet down and see if it improves. Plug chops help too.

It appears to me that you don't fully understand carbs and the holistic attitude you need to take. Don't take offence. You are in a very large club. Even riding style will affect your optimum settings.

I read the few replies you got on WG and you know my opinion. The unanimous opinion was that you are over jetted.

You now know which way to go.

Cheers

Rod

 

[ARVENO]

I am sorry but didn't a Le Mans 2 comes with 40's from the factory ?
Why put 36's ? I don't get it.... :?:

Just open them up, and put everything back to stock spec, that will be a good starting point.
I would never put 36 carb on a Le Mans.... but that just me....

Good luck

 

[Morizzi]

I am sorry but didn't a Le Mans 2 comes with 40's from the factory ?
Why put 36's ? I don't get it.... :?:

Clearly!

PHF 36's were standard fitment on all 850 Lemans, I - III.

There is always the trend by those that only think in one direction that bigger carbys are better. :roll:

Paul has a LM II that has had its capacity increased to 950cc. He can run 40's but it will also run with 36's. In one of his earlier posts he described his riding style. IMO the 36's will suit his described style better.

Bigger carbs means more air, so you can get more fuel in there when WFO. In the real world where the police and speed cameras exist this may not be where you want to improve the running of your bike.

Smaller carbs means the venturi is smaller. This consequently means the air speed through it for any given piston capacity must be faster. It is the air velocity going through the venturi that creates the pressure differential that is the reason carbs work. The faster the air the more responsive the carb is to throttle changes.

Rod, who has 2 LM III's, both with PHF 36 lean burn carbs, both with jetting not that far away from OEM.

 

[ARVENO]

I am sorry but didn't a Le Mans 2 comes with 40's from the factory ?
Why put 36's ? I don't get it.... :?:

Clearly!

PHF 36's were standard fitment on all 850 Lemans, I - III.

There is always the trend by those that only think in one direction that bigger carbys are better. :roll:

Paul has a LM II that has had its capacity increased to 950cc. He can run 40's but it will also run with 36's. In one of his earlier posts he described his riding style. IMO the 36's will suit his described style better.

Bigger carbs means more air, so you can get more fuel in there when WFO. In the real world where the police and speed cameras exist this may not be where you want to improve the running of your bike.

Smaller carbs means the venturi is smaller. This consequently means the air speed through it for any given piston capacity must be faster. It is the air velocity going through the venturi that creates the pressure differential that is the reason carbs work. The faster the air the more responsive the carb is to throttle changes.

Rod, who has 2 LM III's, both with PHF 36 lean burn carbs, both with jetting not that far away from OEM.

no no i am not one of those bigger is better.....LOL!
I was ( i was wrong ) sure the Le Mans model fits 40mm carbs , but your right , they fit 36 , don't know why i would have swear they were 40's , now i see more sense on the direction you want to take.
Yes, even if the bike is a 950, 40's are too big anyway....
I was looking to get 36's for my bike and found this website sell them cheaper than others website
290 and 295 $ brand new . ( i thought i would share the info...)
Yes, they are jetted for a 1000 sp but the jetting may woks well for a LM too.
http://www.dellortodirect.com/Carbureto ... 2hukupcon1
I don't think will be easy to score 2 good used carbs , the few used i found were too expensive anyway, so i thought i better off with new carbs.

If you ebay the 40's you should get good bucks for them...

good luck..

 

[Morizzi]

I've had a little epipheny! :shock:

I just remembered a few of the 950 bikes came out with mid sized valves and PHF 36's. A few late Cali III's, Stradas, Late 1000S's and maybe some Mille GT's though I'm not 100% on that.

Once I remembered that the base OEM settings were easy to find.

Main 130
Idle 50
Slide 60/3
Needle K18/3
Atomiser AB268
Starter jet 70 but really, who cares?

You have a few different things on your bike and I'm sure you will be able to jet up slightly but that is it in a nutshell.

Cheers and good luck.

Rod

 

[Murraydali]

Rod

Thanks for all the really helpful advice and for taking the time to respond. And no offence taken - my knowledge of carbs is rudimentary, but I am aiming to learn more.

I have stripped down one of the carbs and am in the process of writing down all the details and making a list of what I need to get it rebuilt. The mains will definitely be downsized though. I reckon before I'm done I'll be asking a lot more questions!

You don't post on Guzzi Exchange do you? I saw a thread which discussed the bike I just bought.

Paul

 

[Chris R]

First rule of tuning: create a baseline: make sure the timing and valves are adjusted properly. Check that the floats are set to the correct level. Change one variable at a time i.e. don't change a jet and a needle and then adjust the valves and reset the ignition timing. You'll never figure out what effected what.

Get the Dellorto tuning manual - it explains how the carbs work and is very practical. Then it is a matter of methodically working through throttle openings from idle to WOT.

Steady state tuning of the dellorto is done by picking the right idle jet, slide cutout, main jet, needle and atomizer (rarely mucked with). The idle jet is easy: too big or too small and the bike won't respond to adjustment. The slide cutout can be modified with a file if you need a bit more fuel.

If the steady state tuning is correct the bike will perform normally when you smoothly open the throttle without any accelerator pump (you can remove the actuator if you want to be sure it is not part of the equation).

Once that is sorted you tune the accelerator pumps which are adjusted by setting the stroke, jet size and slide ramp (in practice no one changes the ramp or jet size). The accelerator pumps are very much a matter of the way you ride - i.e. if you love giving it a big helping of wrist you increase the stroke. If the engine bogs you turn it back a bit until it doesn't stumble. In fact some racers disconnect them completely - I don't FWIW.

How do you know when you are in the right range?
Old school method 1 is to do plug chops: run the engine for at least a couple minutes at a particular opening, pull in the clutch and hit the kill switch and close the throttle (Stops fuel washing the plug). Remove plugs and 'read them'. The problem is that reading plugs is a bit of an art and is now more difficult due to unleaded fuel and alcohol. There are plenty of pictures out there of what the plug should look like: the problem is that to really read a plug you have to look at the whole ceramic insulator portion of the plug which requires taking the plug apart (The late great Smokey Yunick showed me this) and even then it is not easy.

Old school method 2 is to look at exhaust temperatures like the aero boys. The problem here is it is relative to where and how you mount the sensor so you have to develop your own scale as to what is rich and what is lean. This is a good backup to method 1 as it gives you some confirmation of which direction you are going.

New school method 1 is to take it to a dyno with a four gas meter. This is the most expensive and, of course, the fastest and most accurate way to come up with the proper jetting. For some reason, most people prefer to buy go faster parts then invest in making them work together correctly. You also need a dyno guy that knows what he is doing - there are a lot of BS artist out there. No doubt Todd has some good suggestions in his area.

New school method 2 is to fit a lambda sensor to the exhaust and a meter. The problem here is that the sensors give you only a single parameter and tend to have a slow response time and are not necessarily accurate. They are improving though.

 

[Morizzi]

Rod

You don't post on Guzzi Exchange do you? I saw a thread which discussed the bike I just bought.

Paul

Yep, I go there every now and then.

If it was the post regarding the sidecover badge then I read it. I even posted a reply.

As suggested the Dellorto tuning guide is very useful if not a bit over simplistic. It cuts the jetting issues down to nicely definable sections but that is simplistic at best. The overlap areas between the jets/slide/needle is HUGE!!!!!

It can be downloaded off the net. Just google it but I vaguely remember one being on a good Ducati site. Ducatimecchanica or something similar.

Melbourne is a bit devoid of good Guzzi mechanics. Was Brad the bike boy there or in Sydney? If in Melbourne he may be helpful.

The Victorian Guzzi Club (MGCoV) is quite active. I've even been to one of their pub meets when down there and I've been to the Spaghetti rally a couple of times with the long distance awards to prove it. :blink:

They have lots of good guys who will be happy to help. Far easier to do it face to face than over the net. I'm happy to help if you need but I normally limit my input on a forum. I've got tired of those 'smarter than me' coming in and confusing the issues.

Enjoy the LM. Great bikes. Its a pity you weren't able to experience it in a more original condition.

Cheers