Fuel Mixture Adjustment Note

Carbs & F.I.

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Amskeptic
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Fuel Mixture Adjustment Note

Post by Amskeptic » Mon Apr 16, 2007 7:04 am

from a PM:

twinfalls wrote:
How come a lean adjust, means a hot running with PDSIT ? Everybody will jump to say: Lean=hot.

How come the heat running at idle, stays at higher throttle, knowing the Solex PDSITs have a completly different way to achieve mixture.
From my understanding of PDSITs, a carb adjust cannot yield a hot running engine.
To be honest, I do not understand why a lean mixture induces a hot engine.
If you have ever used a torch (oxyacetaline) to weld metal, you know that a lean mixture causes a much hotter flame. Some torches have a little oxygen boost lever to let you cut metal with your torch.

Your engine is no different. Oxygen is an amazing catalyst. Too much oxygen will melt pistons and burn valves. Luckily, there is usually a power drop-off with a lean engine, this will warn you that the mixture is lean and it is hot, I promise you.

If you set up the idle mixture too lean, that lean condition will make itself known throughout the operating range of the engine. This is because the carburetors are calibrated to use the fuel flow through the idle fuel ports at all times. They are called transfer ports and you can see them on your throttle bodies, little tiny holes in a row right near the throttle plates. At idle, the little holes above the closed throttle plates are taking in air to mix with the idle fuel that is dumped out below the throttle plates. At higher engine speeds, the open throttle plates mean every hole along the throttle body is now subject to engine vacuum and is dumping fuel into the intake manifold.

With the central idling circuit of our buses, air/fuel is always being dumped into the intake manifolds. That mixture screw on top of the left carb is supplying fuel at all times to the engine as well, but when the throttle plates are open, the vacuum drops off quickly through the central idling circuit. So every mixture screw, left/right/central/ must be properly adjusted. I like slightly lean left/right screws and a slightly rich central idling screw.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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