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MG MGA - Outside Temperature and Performance

Ah, the cooler days of fall. With outside air temps below 70 my '57 1500 runs well, with coolant temp at 180, if with a somewhat fast idle speed. It's the summer that confounds me. As coolant temp passes 190, idle speed drops and becomes rough. My engine doesn't overheat, it just runs poorly; difficult to keep going while standing, rough running, difficult to start after short stops. Outside air of 90 or above keeps me from going out.

I've calibrated my temp indicator, so I'm confident the engine isn't overheating. In cooler weather the exhaust note sounds good and the car runs smoothly with good acceleration. Starts with little cranking with choke pulled.

I would love to put bushes in my worn carb shafts, but it's a bit pricey. Could this possible air leak contribute to my warm-weather woes? My Lucas Sport Coil is still shiny from being new 7 years ago. Should I suspect it is overheating? I don't suspect vapor lock because the condition gradually gets worse as the temperature increases. Am I off base? Is living at 6000ft altitude just making MGA driving not possible? This condition is only a couple years old, so I don't think altitude is my problem. Any other ideas?
Andy Gerhard

Are you sure it is related to the coolant temperature and not just the air temperature. High, Hot and Humid are the worst conditions for running an engine and in the summer you have them all. Cold dry air is denser than warm moist air so in the winter your engine is getting an extra dose of oxygen. The mixture will change with air temperature, so maybe you are running too rich in the summer. Not an expert on SU carbs, but perhaps because of your altitude you need to run different needles in your carbs.

Jeff Schultz

Andy,
My 1600 does the same as yours. Anything above about 85F and the car does not like it. The car will run at 180 to 190 temp all day long when the ambient temp is less than 80. The moment the temp goes above 80, I have to start watching the temp gauge. At 85 to 87F the car does not like to run at all. I have messed with the carbs (Rebuilt) and timed it and checked the plugs, wires etc. All to no avail. There is a guy in our club that put a 5 row alluminum radiator in his A that is also running a 1800cc B engine. He has driven that car in heat that I can only dream about and had no trouble.
JP Jim

Do you have the carb heat shield in place? Hot underhood temps can cause the fuel in the carb bowls to come close to boiling from heat. The heat causes the fuel to expand in the bowls to a height that can cause raw fuel to run out of the jets in the carbs. This will give a rough idle as it floods the engine, when turned off the same thing happens and the pool of raw fuel makes restarting hard. Try lowering the level in the float bowls by adjusting the float height, this may reduce your problem.
J Heisenfeldt

At an altitude of 6,000 feet, you are rather high. Most of the air is below 8,000 feet and thins as one gets above sea level. At your altitude, you need to set the car up very differently than at sea level. Add in elevated temperatures, meaning, effectively, less available air (higher density altitude) and you need to be running significantly leaner needles than at sea level and slightly leaner needles than perform best in cold weather. (Standard temperature, as I remember it, is 60 deg F. and standard conditions are sea lever, barometer at 29.92 and 60 deg F.)

The engine does not burn the mixture as cleanly, or as rapidly, at higher altitudes. Back when I was living in Cheyenne, Wyoming, I used an MGA as a daily driver. Box stock 1600. As I remember it, we used leaner needles in the carbs and advanced the ignition timing one degree per 1,000 feet above sea level, or a total of six degrees beyond standard. Never had a problem with over heating in the summer, including in mountain driving, and the MGA would start when many of our American cars would not. Good, reliable car when properly tuned.

Les
Les Bengtson

High altitude means less barometric pressure and fuel evaportates easier--meaning it could be doing it in your carburator bowls. Alcohol that is often used in the lower octanes makes this evaportation even worse. If you are running stock needles you must change to a leaner needles and consider running fuel with no alcohol--I think that you will find the high octane fuel does not have alcohol added, at least that is the way it is here in Wisconsin. You probably don't need the high octane ratings if you have a standard compression engine, but you do need fuel without the alcohol.
James Johanski

Andy,
Off the subject, how did you calibrate your temp. gage?
Ssteve
Steve Meline

Calibrate - I removed the temp sender from the head, and the gauge from the dash and took off the front glass. Boiled a quantity of water, which is 200 at 6000ft, and placed the sender in the water, along with an accurate thermometer I use for brewing. I compared the temp on the digital thermometer with the temp gauge, noting the adjustment necessary. I carefully pried off the needle, and returned it to the temp of the digital thermometer. I reheated the water to 200 and repeated the process until satisfied. I seem to recall it was a one shot deal. I have no idea if it is accurate at lower temps, but I'm pretty confident that from 180 to 212 it is darn close.

Thanks all for the feedback. Heat shield and insulating spacers are in place, as are air ducts and felt dam. I had adjusted mixture for best idle, so I can understand that standard needles may result in too rich when at speed. I'd discovered that timing needed to be significantly advanced. I'd not thought of float level, other than to try to get it spot on per specs. I'll try lowering the float next spring, along with a leaner needle. I'll leave the carb spindle bushing for my lottery winnings and ignore the coil.

I went to a Triumph gathering in the mountains a couple of years ago, driving over Loveland and Hoosier passes across the Continental Divide, both over 11,500 feet. Getting passed by semi's on the way up was humiliating, but going down was a blast. An early evening drive through Park county is truly spectacular.
Andy Gerhard

This thread was discussed between 29/10/2007 and 31/10/2007

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