TR2/3/3A Weather question

[Member 10617]

After a summer and fall that were warm and humid with the car running well, now with cold dry air, would it improve things to make the mixture a little richer, a little leaner, or just leave it alone.

 

[Geo Hahn]

I'd say leave it alone. I've never noticed much difference in running characteristics once the engine & oil are well warmed up.

 

[Member 10617]

Hi, George,

Many thanks. I suspected that was the answer.

(Hope to be in Tucson in the Spring.)

 

[TomMull]

I ran my TR3 winter and summer here in Northern New England between 1964 and 1974 as it was my only vehicle. The only concession I can remember ever making for winter weather was to put in some lighter oil, a can of ether behind the seat and a pair of snow retreads on the back.
Tom

 

[angelfj1]

After a summer and fall that were warm and humid with the car running well, now with cold dry air, would it improve things to make the mixture a little richer, a little leaner, or just leave it alone.

Ed, i'd let it go. I understand how much fun it is to tinker, but its best to keep the mixture optimized for the climate that the car "sees" most of the time.

 

[martx-5]

Actually, constant velocity carburetors such as the SU are adept at providing a good mixture under a lot of different conditions, however, there are some situations that the carb does not compensate for, and those are weather, season, and altitude. Here's a quote...

"The SU carburetter is of a constant Velocity, variable choke design...that is, by varying the position of the dashpot (12) as a function of the volume of air the engine draws, the cross-sectional area of the throat of the carb is varied, which keeps the velocity at the venturi point constant...and by keeping the velocity constant, it is fairly simple to meter the fuel for a given volume of air induced, although this leads to somewhat of an oversimplification in fuel metering, since the fuel should be metered as a function of mass of the air being ingested and not just volume (as density of the air, which can vary due to things like weather, season and altitude, is not compensated for). That is why I like to adjust the mixture for summer and winter. Other than that, they seem to thrive on neglect, I honestly don't understand why some owners seem to continuously need to fiddle with them!"

Of course, you can always just adhere to the last sentence of that quote and leave them alone. But if you feel the car is not running quite right, you may have to enrichen the mixture.

The quote from this web page...

https://www.sw-em.com/su_carbs.htm


Edit: Also, it really is very easy to adjust the mixture on the SUs. To enrichen a little for the winter, just screw the jets down a couple of flats and that should do it. As long as you keep track of how much an of an adjustment you make, it's a simple matter to go back to your starting point next spring.

Normally, I never drive the car in the winter, but maybe this year I will to see how much the mixture does actually change from season to season. I have a wide band O2 sensor so I can see what effect the colder, denser air has on the mixture of the SUs, and how much adjustment will be necessary to compensate.

 

[TexasKnucklehead]

You know, I recently considered this. In October, we left Spring, Tx (elevation 121' at 70/80 degrees) and drove 2 days to Portage, Pa (elevation 1,800' at 28 degrees) and the TR3 didn't seem very happy about it. (Actually it did ok, but seemed sluggish and not at all zippy on the hills.) I had a float sticking, and after returning found the points wiper had worn until I couldn't see the gap, but I've been wondering about the stalling issue I had while up there. The TR3 ran like a champ down the highways, but it seemed every time I pulled off a highway and stopped for any reason, the car would stall. It always started up as if nothing happened, but no matter how I tried, it would stall if I came to a stop immediately after a highway run. I chose to stick with the later portion of that quote, and before I got home, it returned to normal behavior. Perhaps an O2 sensor would have been helpful, but I'd need some more sense to use it.

I prefer a more definitive approach to adjustments, where something is measurable, like seeing the color change on the color-tune, than waiting for 'pinging in a high gear at load'. My imagination takes over and I don't trust it. For now, I'm sticking to the idea that my TR3 prefers warmer weather. I was a little more sluggish, slower and had trouble breathing up in the cold -the TR3 and I are alike.

 

[Geo Hahn]

Thinking about this as I was in the garage preparing the cars for the Tucson version of winter I realized that I usually use a lighter oil in the carbs in the cold months. I use a very heavy oil in the summer (20w50) so I feel I need to lighten up a bit.

Yeah, I know the engine bay warms quite a bit as you drive but the carbs are possibly not fully engulfed in that heat.

...after returning found the points wiper had worn until I couldn't see the gap...

That will certainly cause poor running as it not only affects the spark but also retards the timing as a smaller gap opens much later. I use a dab of assembly lube on the cam to forestall this -- they used to include a little capsule of lube when you got a new set of points.

 

[achtungeveryone]

ideally if there was just a slight difference in run quality between summer and winter, then it might serve as a sort of "ground hog" day. and one would miss this if they went to fiddlin' with the carbs.

 

[Member 10617]

All of this is very helpful.... Thanks. As cold weather settles in around here, I may enrich the mix by two flats and then "go up" two flats when warm weather returns.