CAUTION: I have no idea what I'm talking about.
Random link:
Road Star Clinic has been a great source of information.
Since my
last thread failed to gain a response. I am going to assume no one knows. The rule of thumb that I've seen spread is: go up a pilot jet size if the idle mixture screw is more than 3 turns out
Is there a similar rule of thumb for going down a size?
I assume we're using
Lancer's Straight & Simple Carb Tuning as our tuning procedure.
There are several reasons for this thread. First, to gather my thoughts in one place. Second, to find out if I'm being rational or not.
Assumption: At idle the pilot jet, idle mixture screw, needle jet, jet needle, idle speed adjustment screw, and throttle plate have an effect on the air/fuel mixture. The pilot jet and idle mixture screw have the largest impact. The impact of other components may or may not be significant or measurable.
Assumption: For any size pilot jet the idle mixture screw can be adjusted to produce a theoretically perfect air/fuel mixture. This is obviously not completely true, but it may be true for the three sizes normally considered (52.5, 52.5 with bleed holes, 55).
Proposal: With a theoretically perfect air/fuel mixture, a fixed volume of reactants (air/fuel) is required for the engine to produce a specific RPM.
Consequences: A smaller pilot jet will produce a lower volume of reactants at a theoretically perfect air/fuel mixture. Therefore an adjustment to the idle speed adjustment screw to open the throttle plate wider will be required to reach a specific RPM. Therefore, with a smaller pilot jet the needle jet and jet needle have a more significant impact at idle. That role may or may not be significant or measurable.
There are 3 easily observable results of adjusting the carburetor. First is a change in fuel economy. Second is the afterfire at shutdown. Third is the afterfire at throttle close.
The afterfire at shutdown is often attributed to a lean idle circuit.
The afterfire at throttle close is often attributed to a lean mid to idle transition.
Why am I blathering on like this?
With no other changes, by using no white spacer I was able to eliminate the afterfire at shutdown, the afterfire at throttle close, and decrease the fuel economy.
With no other changes, by increasing to one half a white spacer I was able to continue to have no afterfire at shutdown, but reintroduced the afterfire at throttle close, and saw a relative increase in the fuel economy.
An additional thought: Given two S40s with no difference other than the pilot jet size, assuming each carburetor is adjusted appropriately, will they have a difference in fuel economy? If so, why?
The Road Star Clinic mentioned above has the following passage:
Quote:A lean mixture tends to burn too slowly and causes hesitation to throttle input, reduced mileage, intake backfiring, exhaust backfiring, surging at steady throttle cruise speed, and/or engine overheating. A rich mixture burns faster than a lean one, but may cause poor mileage, smell of unburned gasoline, and/or reduced power.
I was under the impression that surging at steady throttle cruise speed was primarily impacted by the "mid circuit" (needle jet, jet needle, and throttle plate).
I did experience surging at steady throttle cruise speed with a 55 pilot and full spacer. I did not experience it with a 55 pilot and no spacer. I'm still evaluating one half spacer, but I believe there is some light surging at very specific throttle openings.
I'm somewhat surprised to learn that the 55 pilot may be too lean and I could be over compensating with a richer mid circuit. Thoughts?
Edited: 07/19/09 at 08:05:01
Turns out I didn't read far enough.
Quote:A lean condition, on the needle will give less than optimum acceleration, may result in surging during cruise and give poor gas mileage.
So lets blame both and call it a day.
This turned into a really long post. Hopefully, the helpful people here have some experience in these areas and can help me understand what I've observed and proposed.
If these topics have already been discussed, here or elsewhere, please feel free to post a link.
Thanks.
-D. Dwarf
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