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Could This Be True?

I have been driving a 1987 Toyota Land Cruiser FJ 60 wagon since bought new. It now has about 158,000 miles on it. I now live at 7,880 feet in the Southern Rockies. My mechanic, a good one, did something while tuning the engine to make it perform a little better up here. This a carburated engine. He told me to bring it in before attempting to drive it a lower altitude or I will burn the valves. I am planning such a trip but cannot imagine this is true. I do not think he rejetted the carb as it is sealed from the factory- there is a plug. He may have advanced the timing ahead of OEM spec. The car does run better with higher octane fuel which is odd given the altitude and the low compression of this engine.

Does anyone have an idea as to how true what he said is? How likely is it that damage would occur at lower altitide? I would ask him exceopt he has relocated to another dimension.

Trust his word on this one. Oxygen at high altitudes is sparce, few molecules per cubic foot. What he did was “lean out” the gas mix so that the engine wouldn’t push too much gas into the sparce inflo of air, enabling a more balanced mix for better operation.

Should you now move south, the lean mix might cause hot cylinders, and that can lead to preignition and ultmately to engine damage. It’d be like using a bellows in a fireplace, the flame gets hotter.

It’s unortunate that he moved. Folks versed in the eccentricities of carburators can be hard to find these days.

A real life situation embarresed GM, the supplier of vehicles for the 1988 winter Olympics in the Rockies. Their SUVs could not reach the top of Mount Allen, the start of the downhill events, to take dignitaries to the site. The cars could not get enough air/fuel.

They ended up with Fords who were easily adjusted for this high altitude.

Engines that are naturally aspirated loose 5 per cent horse power for each 1,000 feet of altitude. The calc is not a straight line; i.e. the calc is 5% of 95%, 5% of 90%,etc. I have had trouble above 11,000 feet.

I do not know if he rejetted the carb since the factory plug is still in the carb. He may have leaned the mix screw but I think this might have been sealed too. If he advanced the timing would that overheat valves at lower elevation? I guess it would be easiest to put a timing light on it and see for myself.

I will double check the carb for that plug. I could take it to lower elevation, run it awhile and then check the spark plugs for color. If he did adjust the carb for the altitude I wonder if replacing with “cooler” spark plugs might help preserve the valves and pistons from burning?

I could just take it to a Toyota dealer service department. They will crap in their pants when they see the other engine modifications I have done and may not want to touch the engine.

If he advanced the timing, that could cause damage due to preignition. And yes, it could manifest itself as burned pistons and perhaps burned valves. But, as you said, that’d be easy to check and correct.

I’d look for an owner-operated shop owned by an older fella rather than a dealership. Most of the young kids at the dealership might not have the desired experience with this situation. Most of them probably have not touched a whole lot of carbs.

That would also explain why it runs so much better on 92 Octane rather than the 87 Octane Regular here. This is especially odd since octane requirements are supposed to reduce with altitude and the stock compression of this engine was lower than most vehicles as it was sold mainly in Third World countries where fuel was not the best quality.

I own a '73 FJ40 and the factory manual states that the stock carburetor comes equipped with a spare jet to install when being used at higher elevations. The extra jet is in place right beside the one being used. The mechanic could have just installed this higher altitude jet. I believe he’s correct in the burning of the valves.

If the spark was advanced significantly the engine will ping when driven at low altitude under load. If re-jetted the engine will lack power under load. The result will be evident. If driven at low altitude on a hot day at high speed, especially if heavily loaded, all manner of damage is possible. Cracked pistons would likely be the most common result.

He likely advanced the ignition timing for the higher altitudes and yes it is quite possible that engine damage could occur at lower altitudes depending on how much advance he dialed in.

Advancing the timing has the same affect as changing to smaller carburetor jets; it leans the system out.
You should have the timing set back to the factory spec before going to lower altitudes rather than take a chance on holing some pistons; just in case your mechanic dialed in a bit too much advance.

BTW, for years the Japanese imports had an adjustment on the distributor for high altitude. It was plainly labeled and easily advanced and retarded.

As a kid I remember one vacation in the Sierra Nevadas, some passes approached 14,000 ft and we were towing at tent trailer. Our vehicle was a 1966 Dodge van with the 225 slant six. We did Ok.

Why do you suspect your mechanic did not simply tell you what he did? why all the drama?

I realize I was 11 years old at the time but I knew every spec about that Dodge van,how could this be as we have adults write in that don’t know to pull over when the engine has oil pressure or temp problems?

Here’s a really novel idea. Why don’t you ask him what he did? The jets can not have a sealed plug, they wouldn’t work. The plug covers the screw for the mixture adjustment.

An '87 Land Cruiser probably had a feedback carburetor whose mixture was controlled by an ECM…An oxygen sensor in the exhaust pipe controlled the fuel mixture, not the main-jet…Few mechanics messed with these carburetors…

If there is no oxygen sensor, rejetting the carb is still a longshot as main-jets were not readily available for these carbs…If the mechanic somehow managed to change the main jets, he should have returned your standard jets to you as finding replacements today will be impossible unless you buy a rebuilt carb, which may or may not be possible…

What impresses me is the fact that there is still a mechanic around that can touch a carburetor.
End of sarcasm, sorry couldn’t help it.