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MG MGB Technical - Timing / Carbs High Altitude/Elevation

know this has been beat to death and I have looked up (I think) most of the available material but....

What is everybody doing for timing? For my car (69 - 18GH engine) Haynes shows 14 degrees BTDC Dynamic, Bentley shows 20 degrees BTDC @ 1,000, Teglerizer site , same as Bentley and Paul Burgess comments that should be 14 degrees at idle. So I am planning move from 14 up to 20, then check for pinging under load.

Any impact at altitude of 5,280 here in Denver? Seems like it might take longer to compress the air to combustible point perhaps needing to retard timing.

Currently at 14 Degrees BTDC and runs reasonably well but starts hard when cold and dies at 9,000 feet. That was before replacing fuel pump. Also, just replaced the vacuum unit on the dizzy. Had a slow leak and wouldn't hold vacuum. Will use a Colortune to check rich/lean adjust, etc. this afternoon.

Plan on taking it to our cabin near Pikes Peak, doing a little more tuning then running up the Peak as an acid test.

Thanks for any help.
J.T. Bamford

J.T.

I live in Manitou Springs and I set the timing at 15 degrees BTDC. My 68 GT seems to run very well at all altitudes around here. Replacing your vacuum advance should help out some.

Are you going to Glenwood Springs?

Steve Sletta
Steve

Don't know if I will be there for whole deal but may stop in on the way to Delta to visit friend.

Will email you offline. Will be in Green Mountain Falls this weekend with the B.

JTB
J.T. Bamford

I ran my car on regular unleaded 85 octane and advanced the timing 1 or 2 degrees until the pinging under load stopped. Then I checked it with my timing light. 20 degrees at idle. I've taken the car to Aspen and Breckenridge with no problems whatsoever. If you normally run a high octane fuel you can get away with a much lower timing advance. 15 degrees or so sounds about right. I switched to K&N filters and used the next size richer needles in the carburetors to get the most out of the thinner air here. My car ran fabulous in Green Bay, WI at 14 degrees BTDC and standard needles. Moving here, the car lacked the punch it had in WI at 400 ft above sea level. It's amazing what a mile higher does for the power of your engine. The K&N's definately make an improvement, although you will probably have to change needles. Well worth the effort, IMO.

Scott
Scott Wooley

Good luck you guys, I just ran a quick calculation for density altitude. This is the effective amount of oxygen your MG has available. At 8000' above sea level on a 65F day, you will have a density altitude of about 10,000'. This means the car will have about 23-25% less oxygen to breath with a proportional loss of horsepower. Most cars need to be leaned out at altitude. In aircraft engines, we have a mixture control to get the best performance.

Time for boost or efi.

Edd Weninger
Edd Weninger

The old rule of thumb was that you could advance ignition timing one degree per 1,000 feet of elevation change. Thus, a car with a factory specification of 14 deg at 1,000 rpm would be set to 20 deg at 1,000 rpm at 6,000 above sea level. As with any other change, you do need to check it out after making the change to verify it is correct for your car. Worked well for me in Cheyenne.

As to carbs, you may need to go to a leaner needle than standard. As Edd notes, the air is less dense and this is a function of both altitude and temperature. Thus, with standard needles, you will be running too rich a mixture which can both waste gas and, if excessive damage the engine. Scott may want to re-examine his use of a richer needle after the K&N switch by doing a plug check. Normally, I would expect the increased flow volume of air to, perhaps, allow the use of a standard needle with the K&N filters (at altitude). This, again, should be verified by testing. Les
Les Bengtson

Something I've often wondered is, why couldn't you just turn the mixture nuts 1 or 2 flats lean when you're at altitude? I've heard stories that the reason the MG distributor has the vernier adjustment is so you can adjust the timing according to the quality of gas (during war time when it varied) but unfortunately that only allows for 1 or 2 degrees. So why couldn't you just adjust the mixture too? And then at lower altitudes just set it back where it was.

If you don't mind the drive all the way to Ft Collins, give Bill Wardlow a call. His number is on his website at www.motorwayltd.com He's done jetting on both Weber and SU carbs.
Ken T
Kenneth Thompson

I meant leaner needles NOT richer. Sorry for the confusion. Fatter needle, leaner mixture.

Scott
Scott Wooley

Scott. Not a problem. I use AAA needles in my 68 prior to K&N filters and did not have to change after installing the filters. This may indicate that I was running a little rich. However, in spite of the fact there are hundreds of SU needles, there are very few which fit the proper profile for out cars. When I needed just a little leaner needle, I could only find two profiles that would work. One was too lean to allow the engine to run properly and the second was just capable of allowing the car to get to the emissions testing station and home before being replace by the AAA needles. They are now labled "Emissions Testing Needles" and are only installed for the test.

Ken. The raising of the jets a couple of flats will have a slight influence on the idle and cruise portions of the needle, but not a great deal. The needles needed at sea level are considerably thinner (richer) than what is needed at altitude. Most of the air is at 8,000 feet and below. At 5280 feet, Denver's air is rather thin. Go up higher and the standard needles can cause problems. When I left my MGB with my wife, while she went to school in Denver, I had the carb (79 with Z-S) re-needled by a shop near Garth Brothers/Sports West on Broadway. The needle was so lean that, when I picked up the car and drove it back to Florida to have my household goods packed and shipped, the car barely ran. Switched out to the factory needle and all was fine. We had similar problems with our MGAs and Corvettes up in Cheyenne. The As needed leaner needles and the 'Vettes needed smaller jets to perform properly. Les
Les Bengtson

J.T. I'm here in Arvada. 20deg. is where you'll want to be. I've had no problems running in quite a long time. I also run a Weber witn good sucess.
victor gardino

Les, my 74 originally had HIF carbs but last Fall I bought a new set of AUD465's that were used on the 70-71 models. On Saturday I went with the British motoring club of northern CO up over Trail Ridge and it did just fine with the needles that came in the new carbs. The timing is set according to the Haynes manual, I'm using Crane XR700 igntion and a 91 octane fuel. The car lugged and pulled up the steep slopes just fine. The temp gauge showed about halfway between N and H but I think that is a problem with calibration. I have a 160 thermostat in it. I didn't notice any pinging or any other problems. It was probably 40 degrees outside temp at the top of Trail Ridge at 12,000 ft. And YES, all of the club vehicles had their tops down! Out of the 20 or so vehicles on the trip, there was only 1, a TR4, that vaporlocked at about 8,000 ft, just west of Estes Park. They tried leaning it out but he finally just turned around and went back. Too bad, it was a helluva fun trip!
Ken T
Kenneth Thompson

This thread was discussed between 23/05/2003 and 28/05/2003

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