Something you might think about if you are changing altitude a lot. I invested in an air density gauge. I have Dave Rebello dyno my engine at sea level in dense air. He records the density and sets the carbs appropriately. I note the mixture settings. Then, I take the car out on the road in the Nevada desert on a hot day at 5000' elevation and re-tune them to the thin air, noting the new mixture settings. Then I graph the mixture settings vs density using the data points to make a line. Of course I do separate lines for each carb. Now I have a means of making very accurate and fast mixture settings. I do this every time before exiting the pits for a track session. I takes only about 10 secs a carb. Believe me this works like a champ! Obviously you won't have Dave do your initial settings but you get the idea.

Blue,

When I drove mine from Flagstaff to NJ in Indiana I had to adjust the carbs because the increased vacuum was collapsing the hoses. When I moved hear that was the lowest elevation I has a Z car at.

In Arizona the Moagleana (sp?) ridge can really mess with a car, you can be rich on top and lean on the bottom in less than mile.

my 240 will for the majority of it's drives..stay above 7000 feet. Home is 7300 and work is @7000.....

Yeah, Blue, that is one crazy video. You can tell that the engine is under load too, not just revving up in neutral. On the road or a dyno?

Chris

It is someone's mini with SU's. Looks like somewhere in Europe.

Something you might think about if you are changing altitude a lot. I invested in an air density gauge. I have Dave Rebello dyno my engine at sea level in dense air. He records the density and sets the carbs appropriately. I note the mixture settings. Then, I take the car out on the road in the Nevada desert on a hot day at 5000' elevation and re-tune them to the thin air, noting the new mixture settings. Then I graph the mixture settings vs density using the data points to make a line. Of course I do separate lines for each carb. Now I have a means of making very accurate and fast mixture settings. I do this every time before exiting the pits for a track session. I takes only about 10 secs a carb. Believe me this works like a champ! Obviously you won't have Dave do your initial settings but you get the idea.

Absolutely sound advice with numbers behind it!

FYI Here is what the 72 FSM gives for temp and altitude compensation:

Absolutely sound advice with numbers behind it!

FYI Here is what the 72 FSM gives for temp and altitude compensation:

More interesting stuff, Blue. If I'm reading this right, it appears that I should enrich the fuel mixture 1/2 a turn, maybe 3/4 turn, for my commute home. A 30 F temperature rise during the day isn't that unusual in these parts.

Chris

wow... same here even more...@ 40 degrees change

Previously I wrote:

More interesting stuff, Blue. If I'm reading this right, it appears that I should enrich the fuel mixture 1/2 a turn, maybe 3/4 turn, for my commute home. A 30 F temperature rise during the day isn't that unusual in these parts.

Chris

Now that I think about it, adjusting the mixture is the last step of my tune-up routine and I'm usually doing it between 3:00 and 6:00 in the afternoon. So I should be going the other way. Lean it out for the morning commute, back to setting for commuting home.

Chris

I am about to stat modeling the flat top SU so I decided to model the jet depth to get my head back into the game. It has been a while since I had such output.

Here is an xls tool that models the graph. I will probably make a visual basic tool shortly for fun and eventually a java version when I get around to setting up a new tech tip website.

Note: This is a Microsoft Excel (XLS) Tool

Enjoy: Hitachi Jet Depth Calculator

240z260zSUJetDepthCompTool.xls

Edited September 9, 201113 yr by Blue

The chart shows the absolute depth of the jet: i.e. The number of turns down from the very top.

For this chart to mach your carbs, you need your carbs set up correctly with factory parts and also in good working order with no worn or out-of-spec parts.

With respect to which way to turn:

1. When it is colder (denser air and more stuff to burn up), you need to add more fuel; so the jet must be lowered to expose more "hole/orifice" gap between needle and nozzle as this exposes more fuel.

2. When you drive higher in elevation (thinner air and less stuff to burn up), you need to reduce fuel; so the jet must be raised to expose reduce "hole/orifice" gap between needle and nozzle as this exposes less fuel.

So in a cold morning you need more fuel than on a hotter afternoon.... so the jet is lower in the morning and higher in the afternoon.

I would just set it in between both and be lazy FYI my opinion on daylight saving time is for everyone to move 30min then just leave it

Edited September 9, 201113 yr by Blue