Topic: Am I Running Way Lean Here? (carb theory question)

[grumpy]

Here's the situation:

On the way home from work I travel down a 3-4 lane freeway across a valley floor where there is almost always a tail wind. Always heavy traffic.

So with the tail wind and the draft I get from the surrounding traffic, I can cruise at 75mph / 5,000rpm with the throttle juuuuust above idle.
There can't be much gas going into the motor. I mean, the engine is spinning pretty fast on only the idle circuit and a tiny bit of needle jet.
Am I running way lean in this situation?

[tlbranth]

Don't think it matters. The engine's not doing any work so it isn't going to overheat.

[TwoTired]

Am I running way lean in this situation?

Can't say for certain with the parameters mentioned.  However, I'm thinking you are confused by terminology.

Lean indicates a fuel air mixture ratio that has more air than the proper amount of fuel need for 100% combustion.

Closing down the throttle (at what ever engine speed)  Closes down both air and fuel. 

If anything, the higher vacuum in the carb throats with an engine at mid RPMs and small power setting, would pull more fuel through the jets while the air inlet is restricted by the slide.  This would result in a richer mixture than needed by an engine not producing very much power.

Are you're plugs showing rich or lean deposits?

[grumpy]

Nope - plugs look fine but the pipes are beginning to blue.

I was thinking that, in this situation, there should be a lot of air getting sucked in by the motor yet the only fuel inlets are the idle jets and the needles (just barely). See what I'm saying? Would those 2 fuel inlets be enough to supply sufficient fuel for that much air?

BTW - K3, stock air box, 120 mains, floats set to 26mm, needles in the middle, new carb rubbers all the way 'round, 341s with new exhaust gaskets.

This is an example of what goes thru my head after an hour in traffic at 95F.

 

[TwoTired]

Nope - plugs look fine but the pipes are beginning to blue.

I was thinking that, in this situation, there should be a lot of air getting sucked in by the motor yet the only fuel inlets are the idle jets and the needles (just barely). See what I'm saying? Would those 2 fuel inlets be enough to supply sufficient fuel for that much air?

You have mechanical slide carbs, where the slide moves directly coupled to the throttle.  Close the throttle/close the slide, and the slide blocks air to the engine.

The throttle does NOT directly control the idle fuel jet flow, its is the vacuum at the idle outlet port the determine the fuel flow into the carb bore.  Closing the throttle while keep the engine RPMs up increases that exit port vacuum and flow, while the slide still shuts off air flow.

The blue pipes could possibly be the fuel that is unburned in the combustion chamber (not enough air) finish burning in the head pipe.  That extra heat would make the pipes blue near the head.

In this case pay extra attention to the base ring spark plug deposits.  I would expect extra thick carbon buildup there after your "tailwind runs".

This is an example of what goes thru my head after an hour in traffic at 95F. 

This could give new meaning to the term "boil over"? 

Sorry, I'm just a mean man, I guess.

[grumpy]

Closing the throttle while keep the engine RPMs up increases that exit port vacuum and flow, while the slide still shuts off air flow.

OK - I get it now: High vacuum does not = high air volume when the throttle/slides are almost closed.
So, yeah. I can see why you would say rich.

I need to see which plugs are in there. I bet I still have hot plugs from when I was pushing oil. That could be why they're not showing signs of a rich mixture.

Thanks!!

Re: Boil over. You ain't kiddin'!!