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MG TD TF 1500 - Seasonal adjustment of carb mixture
|I rebuilt my SU carbs this Winter and used ColorTune to fine tune the fuel mixture. We are now in Spring and the temperatures have warmed substantially. I recently checked my spark plugs and they had black carbon deposits, indicating that the mixture was rich. I readjusted the carbs today, again using ColorTune. I only needed to adjust them by one or two flats.
I was wondering if anyone else had this experience and if it was seasonal, relating to warmer temperatures. If this variation is due to seasonal changes in temperature, does this mean that I need to check my carburetor mixture a couple of times during the course of the year?
|Once the carbs are set, there should be no reason to ever touch them again unless they wear out. Some states change fuel blends for Winter/Summer but I doubt it would make that much difference. I've never found a need to adjust for it.|
|When you looked at the plugs had you been on a looooong run with wide open throttle, then gone into neutral, and shut off the engine?|
If not, don’t look at the plugs.
are you using ethanol free fuel?
This is a 1930s design engine.
|P G Gilvarry|
|I agree with the above Milton. Set ‘em and forget ‘em.|
|Hello Milton, Never look at the plugs if the car is running well! You can't read plugs like you used to due to modern fuels. On a cold day in Florida (below 50F), I have always had to either keep the choke slightly out, or just give each carb about one flat richer. Then reverse when warmer. Also, the winter vs summer fuel is quite different. The winter gas is more volatile than the summer. That is a sample size of one car, which has been this way since the 70s. George|
|George Butz III|
|George: It certainly is good to hear from you. I still remember and appreciate your gracious hospitality when we visited your home while on our Chapter tour of Florida. I hope all is well with you and your family. |
Gentlemen: Thanks for your comments. Much appreciated.
|My rebuilt carbs haven't been touched in 4 years and no difference than when I put them on. I agree with the notion that looking at the plugs after a few idling sessions the plugs will be black. Had a Model A Ford that was the same way. Reminds me of an old plug that used to be out there that Pep Boys sold, LOYD, that would fire in oil, put them in a couple old 6 cyl Chevys with oil soaked plugs and they took the miss fire out! At night you could actually see them firing! LOL, my how times have changed.|
|Loyd Plugs. Found these on ebay. LOL|
|About 15 years ago I went to a presentation on fuels at a large local car club. The fuel company technician was from one of the largest fuel companies in the world. At that time they produced 16 different fuel brews for 16 regions in Australia, and each region had their brew tweaked every 3 months in line with the change of seasons.
Generally you don;t have to tweak your car, because the fuel is tweaked instead.
I imagine they would have lots of regional brews in the USA and UK too.
|A L SLATTERY|
|The weight of cold air ist heavier than warm air. So the air fuel ratio is different. I installed an air fuel gauge (Lambda probe) . So I see the difference between winter and summer and I tune the carbs to the best air fuel ratio of 14,7:1 (Lambda 1)|
You might try a little richer mixture. Max power is at 12.6 to 1. The excess fuel evaporates and the engine actually runs a bit cooler.
|Hi J Barry,|
you are right. Lambda 1 is my adjustment for idle. When accelerating Lambda changes to richer mixture due of the needle profile.
Not to hijack the thread, but what needle are you using? I just switched to a GS, used for MGA's. The mixture sits nicely around 12.6 to 13. Previous GJ needle was around 15 to 16 at speed. Engine definitely seems smoother and more powerful... I have a TDC overbored to about 1325cc (0.080).
I have an AEM air/fuel meter, but find it difficult to use because of its sensitivity.
|On the subject of needles, I have fitted the factory recommended "lean" needles to all my cars - M-Type, YA, YT, Mk3 midget and a couple of SA's - they all perform much better. Went one plug hotter as well across the board.|
Works for me in all seasons in the sub-tropics.
|A L SLATTERY|
|Hi J Barry,|
I drive a TD also overbored to 0,080 and tuned „Mark II“ head. Carbs H4 with LS 1 needles. While drive without acceleration the gauge shows just between 14 and 14,5 while accelarating it goes down 13 to 11,5.
To Toni: That will be locical. Hot air has less oxigen than cold air so that you need lean needles.
Regards from still frosty Germany (-2 degrees in morning)
This thread was discussed between 30/03/2020 and 04/04/2020
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