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hello all---I've been off the forum for a few years....I am running a stock M37 at 8k-12K feet in Colorado. During the summer, I vapor lock, so I have solved this problem by rerouting the fuel line and using an electric fuel pump.

Now, i find a loss of power at the high altitudes, so timing advance, leaning the idle screw and removing the oil bath intake helps restore the power. I am thinking of removing the stock elbow intake from the carb, and using some kind of after market air cleaner directly into the the carb base. Anyone have a recommendation to repace the oil bath and elbow intake assembly and substitue a shorter intake---"make it easier to breath?'

Thanks

Hi Craig. John here. Are you my neighbor here in Mountain Valley? If so, you have seen my truck and the air cleaner and fuel line routing I have done. I have a short piece of straight pipe with a K&N filter on it. I have some of the same pipe left over if you want to try it out, or you could bolt on my current intake to your truck for a test drive. Let me know. It would be pretty easy to weld up another intake

On a standard day, your power is ranging from 25% to 44% of sea level power. Less on a hot day. Have you considered a repower with a larger displacement engine?

Hi john, yes I am your neighbor in MV.....I would like to have a section of your pipe, and try it with an after market air cleaner I have.....I chose to use a electric solenoid fuel pump because it seemed like a better idea to "push" fuel in a vapor lock situation, rather than "pull" the fuel. Bypassing the mechanical pump did solve the vapor lock problem.

Your rotary vane pump , pressure gauge and regulator seems like it might be a better set up for high altitude. The higher I go, the "richer" the air/fuel mix, and the less power I the truck has. Can you reduce fuel pump pressure/flow with your set up?

JimC wrote:On a standard day, your power is ranging from 25% to 44% of sea level power. Less on a hot day. Have you considered a repower with a larger displacement engine?

Hello Jim.....it feels like I am running 25-44% of power....how did you figure the power drops to that range?

craigca wrote:Hi john, yes I am your neighbor in MV.....I would like to have a section of your pipe, and try it with an after market air cleaner I have.....I chose to use a electric solenoid fuel pump because it seemed like a better idea to "push" fuel in a vapor lock situation, rather than "pull" the fuel. Bypassing the mechanical pump did solve the vapor lock problem.

Your rotary vane pump , pressure gauge and regulator seems like it might be a better set up for high altitude. The higher I go, the "richer" the air/fuel mix, and the less power I the truck has. Can you reduce fuel pump pressure/flow with your set up?

Craig, I can adjust the pressure. I have it at 4lbs now I think. Tinkering with the pressure is the reason I have a pressure gauge permanently installed.

I used my digital calipers to measure the inside diameter of the carb intake, and bought some 1/4" wall thickness pipe with the same inside diameter. I also have some steel plate to make an intake flange with if you want a straight pipe intake. You can borrow my current intake to see if it makes a difference. I have the distributor vents and oil filler vent hooked up to it, so it will probably fit your truck. No brake or gas tank vents though.

Craig, 25% and 44% are the density ratios of the standard atmosphere for the two altitudes you mentioned. I ignored temperature deviations from standard atmosphere and ignored humidity.

Hi Jim. I was a little puzzled by the number you posted so I looked up the pressures using this converter http://www.altitude.org/air_pressure.php. At 8000' we have 76% of seal level air. At 12,000' there is 65% of the oxygen present at sea level. Is this a different way of saying the same thing you mentioned? We do not have 25% or 44% of oxygen, but we might be down that amount at 8000 or 12,000 feet

No, it isn't the same thing. I said density ratio. You are reporting pressure ratio.
% oxygen doesn't change with altitude. It remains about 20.95%.

And I screwed up. Got off on the wrong line of the chart. Wish I had said more so I could take it back too.


8000' density ratio 0.7861
12,000' density ratio 0.6933

Since I know perfectly well that my planes generate 75% power at 8400' DA, that's proof I had my brain out of gear. Sorry.

You are right. I knew the oxygen ratio didn't change, but the pressure does, along with the amount of oxygen in a given volume.

I ride a 300cc two stroke dirt bike, and it can be a handful up here at 8000'. I always wondered what bikes like that are like at sea level, with a third more horsepower than I am used to.