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Le Mans MKIII Jetting

Sevant1

Just got it firing!
Joined
Aug 23, 2011
Messages
3
Hi. I own an unmodified 1983 MkIII Le Mans (UK registered) and am overhauling the carbs (Dell'Orto PHF 36). I see that there's a big difference in the UK- and US-jetting that was used for this model: main jet 132 (USA 115) idle jet 60 (50 USA). Can anyone help with the following questions, please?

1. Why does the US-spec bike use smaller jets (for emissions maybe?)
2. What jetting actually gives the best performance for this bike?
3. Is the standard 265AB atomiser best for the bike, with the K18 needle on third notch?

Thanks in anticipation,

Craig
 
No!!! absolutely not!

The Mk III's use 'lean burn' carbs identifiable by the shroud around the atomiser orifice. If I'd been given $100 for every time I've fixed a Mk III that had been 'De-Rstricted' by a retard by fitting earlier jetting to combat the 'Enviro Nazis' I'd be sitting on a beach with a bevvy of Brazillian swimsuit models bringing me drinks with umbrellas in them.

The lean burn carbs, if set up right, will deliver much crisper performance than the earlier models. If your bike is stock I suggest leaving it that way. The Mk III is one of the very best bikes to EVER come out of Mandello. Most things done to them spoil them.

Pete
 
yes, the US had different emissions standards. In answer to your other questions;
Retain the standard UK jetting
Retain the standard UK jetting
Retain the standard UK jetting
Retain the standard UK jetting
Retain the standard UK jetting

if in doubt, Retain the standard UK jetting
 
Thanks very much for your advice, guys. I get the message: keep it standard!

Cheers,

Craig
 
It's best provided the bike is indeed standard i.e. standard exhaust and intake system. On this site there are other LM3 jetting threads you can search for to broaden your reading
 
Generally speaking unless there have been major changes to capacity or most relevantly the camming I find that mk III's need little in the way of alteration from stock. Even pod filters and open pipes rarely seem to neccessitate more than a minor increase in main jet size and maybe raising the needle a notch.

If the carbs are lean burns mains 115-118, atomiser AB268, needle K18 slide is I believe a /6, if it hesitates off idle remove half a mm from the bottom, not the cutaway, of the slide and that should fix it. I usually turn the pumps off or very close to off as they don't seem to do a lot in road riding conditions except increase fuel consumption.

pete
 
Thanks again for the tips guys. I'm going to keep it standard for now but I'll look up the jetting threads if I make any changes in the future. Nice tip also about the slide if the engine hesistates from idle, which I'll bear in mind.

Cheers,

Craig
 
OK, I'm getting down to the last bit of fettling! 1983 LeMans III with stock intake and exhaust. I am using stock jetting (with the exception of AB268 vs AB265 atomizers), and the accelerator pumps are disabled.

AB268 (manual says AB265)
K18 needle, 3rd notch
60/3 slide
118 main
50 idle

The bike runs very well, BUT, in the throttle "transition" phase, in the zone of 1/4 to 3/4 throttle, it doesn't pull with much authority -- noticeable in the 4000 to 5000 rpm zone. I NEVER whack the throttle open, always progressive. I improved things by moving the needle up, i.e. going from 2nd to 3rd notch. Should I go to the 4th notch, or look for AB265 atomizers? WFO works great!

Thanks for your input.
Chris
 
Update: I haven't changed anything, but it seems to be running better after a couple of rides. I think it was just a matter of getting used to it.
 
pete roper said:
If the carbs are lean burns mains 115-118, atomiser AB268, needle K18 slide is I believe a /6, if it hesitates off idle remove half a mm from the bottom, not the cutaway, of the slide and that should fix it.
pete

I have this problem on my LMIII with stock jetting. How do I do perform this procedure? Scribe a line, and then use a file?
 
How do I do perform this procedure? Scribe a line, and then use a file?

Yes - the number is the size of the cutout in mm. So the difference between a 5 and 6 is 1mm of material.
 
Chris R said:
How do I do perform this procedure? Scribe a line, and then use a file?

Yes - the number is the size of the cutout in mm. So the difference between a 5 and 6 is 1mm of material.

OK, just to clarify: if I remove 1mm from the bottom of my 60/3 slide then it becomes a 60/2?
 
nope !! It becomes a 50/3
the /3 or /2 or ...is the acc pumps ramp designation
the 60 is 6.0mm and so on therefore taking 1/2 a mm of the bottom makes it a 5.5mm or 55/3. try taking the .5 mm at a time as you can't add it back on!!
Have a detailed read through this and you should have a good grounding in these carbs, you'll still possibly have Q's esp about the lean burns but ...http://thisoldtractor.com/gtbender/dellorto.htm
Particularly http://thisoldtractor.com/gtbender/mg_manuals/dellorto_manual_a_guide_1.1.pdfwhich describes the function of various parts and hence tuning technique

try a search in tonti and loop section!

hope this helps as this stuff has been discussed many times ;)
 
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