View single post by Esprit2
 Posted: 11-01-2005 03:29 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 322
Judson Manning wrote: Tim was kind enough to post Spec10 & Spec11 for me last year on the Lotus Forum while I was jetting the 907/910 hybrid.  According to my notes, both Spec10&11 show 135 mains vs 160 mains for Spec9.

The real question is:  Why does the JH have 130mains, Spec9 has 160, then Spec10 &11 shift back down to 135mains?



There are differences between the carbs other than jetting.   I haven't played with these things on a dyno along with an exhaust gas analyzer, so I can't talk smart about what the differences mean to mixture individually,  or in concert.   I'm just noting that the differences are there.


Specs 1,  3,  5  & 9 carbs do NOT have a power jet.

Spec 10a,  10b,  10c (Excel manual, Excel automatic, Esprit...  there is no Spec-11) all DO have a power jet (as does the Fed. carb-Turbo).   Quoting a Lotus manual,  "At high engine speeds,  an additional amount of mixture flows through the needle jet (52) upstream of the inner venturi (50);  mixture is composed of fuel drawn from the float chamber through channel (56) and metered by the power jet (55),  mixed with air metered by the calibration passage (54) and drawn from the float chamber top (53)."

In other words,  the power jet circuit dumps additional fuel through an extended needle-orifice right in front of the inner venturi's inlet.

With the Power Jet carrying part of the load at high speed,  the Main Jets can be set a little leaner to favor the mid-range mixture requirements.   So, comparing idle and main jetting combinations between Power Jet and NON Power Jet carbs isn't valid.   I tend to conclude the fact that jetting similar to a Spec 10 carb (Power Jet) used in a Spec 9 carb (non power jet) worked for you is coincidental.   Any attempt to figure out a correlation will just strain a neuron.


Spec 1, 3  & 5 have relatively rich Idle Air Correctors (Idle Jet Holders)

Spec 9 & 10 have very lean Idle Air Correctors

Weaker ................................... Normal .... Similar  .......... Richer
   7850.5,     .10,     .9,     .4,     .1,     .3,     .6,     .7,     .2,     .8

.1 is midrange and used on Spec 1.     .6  & .7 are virtually identical,  upper-midrange rich,  and used on Spec 3  & 5 respectively.    .9 is the third leanest and used on Spec 9 & 10.    That "may" be of more significance when you consider that...

Spec 1, 3 & 5 have 10g floats set to 16.5-17.0 mm height.

Spec 9 & 10 have  8.5g floats set to 14.5-15.0 mm height.

Float weight and height must be coordinated.   A heavier float naturally rides lower in the pool,  and a lighter one rides higher.   To achieve the same fuel level,  a heavier float must be set deeper (16.5-17.0) and a lighter float set higher (14.5-15.0).   So...  I don't know what the real fuel level difference is between those two combinations,  but it's not 2mm as one might conclude by casually reading the specs.

However if one fuel level is higher,  that carb will run richer overall.   Leaner jets could be used with a higher fuel level to achieve a similar mixture.


Last edited on 11-01-2005 03:33 am by Esprit2