Warm days and air/fuel mixture

M

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• May 3, 2011

• #1

Finally, the days are warming up here in western Virginia (not to be confused with West Virginia). I took the TR3 out yesterday for a 150-mile trip over and along the Blue Ridge Mountains. A wonderful ride through beautiful early spring territory.

But although I had carefully set the air-fuel mixture a month or so ago, the car dieseled whenever I shut it off (running too rich, I assumed). I did several road-side adjustments and finally eliminated the dieseling.

But I wonder, can warm weather affect the mixture that much, so that adjustment is necessary season to season?

 

Twosheds

Darth Vader

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• May 3, 2011

• #2

As temperature and altitude increase, the mixture richens.

Enough to require an adjustment? I dunno.

 

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M

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• May 3, 2011

• #3

Dr. John,

Well... it was certainly warmer than when I first adjusted the mixture, and I was up in the mountains. So perhaps that explains it. The dieseling wasn't too bad.

 

TR3driver

Great Pumpkin - R.I.P

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• May 3, 2011

• #4

With carbs, fuel mixture is always a compromise, rarely perfect for conditions. They basically mix by volume, so anything that affects the density of either air or fuel will change the mixture, including not just altitude and intake air temperature but seasonal changes in fuel composition. So touching up the mixture from time to time is a reasonable thing to do, IMO.

But I've had enough trouble with dieseling in the past that I've developed the habit of always killing the engine with the clutch. Just leave it in gear, right foot firmly on the brake, turn off the key and just as the engine should be almost stopped, let out the clutch.

 

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M

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• May 3, 2011

• #5

Randall,

Now that's an idea. If the dieseling keeps up, I'll try your solution.

The pollen level is so high around here right now, that I'm almost tempted to say that perhaps it is contributing to making the mixture rich by cutting down on the air... ha, ha.

 

T

tr8todd

Jedi Knight

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• May 3, 2011

• #6

Double check your timing as well. If the dizzy isn't locked down tight, a little movement can also be a contributing factor.

 

TR3driver

Great Pumpkin - R.I.P

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• May 3, 2011

• #7

High idle will do it too.

 

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M

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• May 3, 2011

• #8

We set the timing about two weeks ago and locked down the distributor. Probably not the problem, but will check it out.

UPDATE: I checked the distributor and it is rock solid -- no movement at all. So I don't think there's any drift there.

Also, my idle has always been around 1100-1200 rpm. That's where it is now.


UPDATE: At one of my road-side adjustments, I ran the adjusting nut all the way up and down nine flats. That seemed to take care of the dieseling. Many of the manuals say to screw it down twelve flats before fine-tuning, and that is what I may have done before. Perhaps that was too rich. At any rate, at the end of the day yesterday, there was no more dieseling.

 

TR3driver

Great Pumpkin - R.I.P

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• May 3, 2011

• #9

A word to the wise : the distributor clamp grabs a fairly weak part of the distributor body. Even though it's a fairly big bolt, you should tighten it only enough to keep the distributor from turning. Otherwise you can break the casting.

12 flats down is merely a starting point, it's not likely to be anywhere near correct except by sheer coincidence. (And the factory workshop manual suggests starting at 15 flats instead of 12.) IMO roadside adjustments are much better made by either doing the "lift the piston" test, or listening to the exhaust note at idle.

The weather has gotten a bit warm here, so I backed off a flat myself after lunch.

 

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M

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• May 3, 2011

• #10

Randall,

Well, actually, I started nine flats down and then did the "lift the piston" test with the piston lifting pin. I don't think I had to do any more than two or three flats more to fine-tune the carbs.

A mechanic friend of mine tightened the distributor clamp, and it looks like he got it pretty tight, but I can't see that anything has been broken. Which casting are you referring to: ---the distributor clamp itself?

 

TR3driver

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• May 3, 2011

• #11

Don't worry, Ed, if it seems tight then it's not broken.

The clamp is not a casting, it's bent from heavy sheet steel and not likely to break. But the distributor body is a "pot metal" (zinc alloy) casting, and it can break.

 

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M

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• May 3, 2011

• #12

Randall,

I suspected that was what you were warning me of. I looked carefully at the distributor body -- what I could see of it -- and all seems well. At any rate, it is tight and doesn't turn.