I want to toss out a caution about equating mileage to mixture. Many things
affect mileage and mixture is only one of them. How you use the car, how hard
you drive, how often you accelerate hard or run full throttle, what rpm ranges
you ten to run in, the advance curve of the ignition, the accuracy of the
methodology of the mileage "estimates", all this and more will affect mileage.
For example carbs that are properly tuned (i.e. the mixture is optimized at all
ranges and under all operating conditions) but has an improper advance curve, or
is set with too little initial advance (Webers and Mikunis like a lot more
initial advance, a faster curve, and less total advance than SUs seem to like or
than most manufacturer's distributors offer) will get worse mileage than a
similarly set up car with a proper ignition advance profile. If the owner of car
A (with the bad advance profile) talks to the owner of car B (with the proper
advance profile) he may decide his carbs are not jetted properly and that he
needs to lean the carbs out to get the same higher mileage as the owner of car B
gets. BUT, his mixture was set properly to start with, and by leaning the mix
out he will make the car run too lean. This combined with a retarded spark can
make the car overheat and in a worst case scenario can lead to a burnt piston.
I for one have never been able to get the sort of mileage that others do, even
when we drive the same car. I drive hard, often accelerating hard. This brings
the acceleration pumps into play more often and for longer periods of time than
would be the case for someone with a "softer" right foot. And that means more
raw gas dumped into the engine (with an accelerator pump type carb). The SUs
mitigate this and are less sensitive to how they are driven than a Mikuni setup
would be, at least as far as gas mileage goes.
So my point here is that you need to look at much more than just jetting when
you compare mileage figures. Mileage is not a good way to asses the mixture
settings on a Mikuni carb setup. There are too many other factors that will
affect mileage. Also each phase of operation on a Mikuni carb can be tuned
independently of the other phases (not possible on an SU really) and so you must
identify when the carbs are going rich and then adjust the jets based on that
info. If the carbs are rich at idle and you change the main fuel jets, you will
not make any difference. In fact you can remove the main fuel jets entirely (you
can just remove the whole fuel block, main jet, air jet, emulsion tube and all)
while the engine is idling and it will not affect the idle at all.
Gerardo Magana wrote:
>
> ----- Original Message -----
> From: Gerardo Magana
> To: datsun-roadsters@autox.tem.net
> Sent: Friday, July 20, 2001 11:00 AM
> Subject: Solex, running rich??
>
> I recently just got my car back on the road. I have only had one long trip
> with it so far. I have noticed that the exhaust is putting out black
> condensation on warming up?? Is this unburned fuel. The mileage I get with 44
> pph Carbs is at about 20 mpg. Do you guys get the same mileage?? I am
> thinking that it is running rich, what do you guys think??
> Gerardo Magana
> 67.5 2000
--
Marc Sayer
82 280ZXT
71 510 2.5 Trans Am vintage racer
|