PabloCruise
SILVER Star
I thought stoic AFR was 14.7:1?
FJ40 Aisin Jets
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That is correct. I'm bouncing around. In order to get stoic @ high RPM, your idle AFR should be somewhere in the 12.X range. If you tune for stoic at idle, you run rich at high RPM. Edited.
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Stoich is always 14.7:1 for gasoline fuels.
Max power (different from stoich) is in the 12.5-13:1 range.
My mistake on the temp. I thought you were saying EGT went up with rich. CHT or coolant temp does increase when the engine is loaded and running at max power AF. Running lean does not increase the heat of the combustion process. Running lean will incinerate valves & pistons because there is superheated free oxygen in the endgas, just looking for something to go bessemer on.
Max power (different from stoich) is in the 12.5-13:1 range.
My mistake on the temp. I thought you were saying EGT went up with rich. CHT or coolant temp does increase when the engine is loaded and running at max power AF. Running lean does not increase the heat of the combustion process. Running lean will incinerate valves & pistons because there is superheated free oxygen in the endgas, just looking for something to go bessemer on.
Don't confuse temperature and heat. Lean mixtures burn at higher combustion temperatures. Rich mistures can produce more heat because they burn more fuel. Temperature is in units of degrees C or K; heat is power (Watts or Joules per second). Heat is also temperature times mass.
PabloCruise
SILVER Star
Hmm... Feeling less intelligent with every subsequent post.
Good stuff in here.
Good stuff in here.
Yup.
And I've got 3 carb rebuilds on my to-do list...
Question: Would slightly increased displacement (2F, bored 50 over) affect my ideal jetting?
77mustard40
SILVER Star
This beginner says optimal mixture is not a function of displaced volume.
PabloCruise
SILVER Star
I am no Wyle E. Coyote Super Genius, but my instincts say that the physical configuration of the carb will determine the max flow and the carb's ability to service your displacement.
I *believe* (watch out!) that the lower vacuum and increased air flow through the venturis will pull the fuel at the ratio the jet dictates. So I think the carb would max out flow before the jets do?
Now that I type that I realize I have no idea what I am talking about.
Wait for Jim's response!
I *believe* (watch out!) that the lower vacuum and increased air flow through the venturis will pull the fuel at the ratio the jet dictates. So I think the carb would max out flow before the jets do?
Now that I type that I realize I have no idea what I am talking about.
Wait for Jim's response!
Carburetors work via pressure differential. Air moving over an orifice (the inside of which is atmospheric pressure) creates a negative pressure zone. That negative pressure draws air/fluid toward the stream (think of those old-timey perfume bottles with the squeeze bulb). In a carburetor, this is taken advantage of via the bowl. Fuel in the bowl is at atmospheric pressure, and there are various channels with apertures leading to the carb throat through which air is flowing at speed by means of the vacuum created by the retracting pistons in their cylinders. Venturis and throttle plates are used in the carb throat to control, manipulate, and optimize the air speed (and thus the pressure differentials) in the carburetor over these various apertures at various engine speeds. Meanwhile, the airflow to a given cylinder effectively has a very limited and finite time to be properly charged with fuel. That time is dictated by the cam and defined by the amount of time the intake valve is open. During that time, the carburetor controls how much air is allowed in (it's always a bit restrictive in order to have sufficient rate of air flow to create enough vacuum to draw fuel from the bowl), and the number, arrangement, size, etc. of the holes in the carb throat dictate pressure differentials at that occur between the bowl and the throat. The jets come into play by metering the amount of fuel allowed to flow (like an hour glass) at various pressures from the atmospheric pressure of the bowl to the low pressure zone of the throat during the intake stroke.
There is a lot more involved, but that's about the minimum to explain the role of jets.
To answer the minor displacement question in perfect vacuum/perfect sphere terms, yes it will affect your jetting because of the difference in flow rate of air and gasoline and the delta in flow rate created by the increase in displacement. This can be ascertained by comparing the average viscosity of air at your altitude and temperature compared to the average viscosity of your chosen variety of fuel at the same, then calculating the difference in the flow rate of the previously displaced volume of air compared to the newly displaced volume of air over the same period of time, then (reading back through, I can barely follow this and I doubt anyone really cares... time to truncate) do some calculator magic and increase the size of your jets according to the result. There are very thick books available on the subject with lots of complex formulae. This will likely result in small differences, and necessitate a set of VERY fine jet reamers.
Alternatively (and an infinitely more common approach), start with the factory recommended baseline, then get a bunch of jets in a range of sizes and swap them out until all RPM ranges react how you want them to with minimal adverse effects.
Or in more practical terms, assuming the general difficulty of obtaining jets for these specific carburetors and the general insensitivity of the engine, no.
There is a lot more involved, but that's about the minimum to explain the role of jets.
To answer the minor displacement question in perfect vacuum/perfect sphere terms, yes it will affect your jetting because of the difference in flow rate of air and gasoline and the delta in flow rate created by the increase in displacement. This can be ascertained by comparing the average viscosity of air at your altitude and temperature compared to the average viscosity of your chosen variety of fuel at the same, then calculating the difference in the flow rate of the previously displaced volume of air compared to the newly displaced volume of air over the same period of time, then (reading back through, I can barely follow this and I doubt anyone really cares... time to truncate) do some calculator magic and increase the size of your jets according to the result. There are very thick books available on the subject with lots of complex formulae. This will likely result in small differences, and necessitate a set of VERY fine jet reamers.
Alternatively (and an infinitely more common approach), start with the factory recommended baseline, then get a bunch of jets in a range of sizes and swap them out until all RPM ranges react how you want them to with minimal adverse effects.
Or in more practical terms, assuming the general difficulty of obtaining jets for these specific carburetors and the general insensitivity of the engine, no.
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Exactly, thanks for the clarification. Running overly lean is definitely worse than running rich, but running rich isn't good either. There are also differences in performance across the same range (as previously stated). Given a choice between lean and rich, I'll pick rich; given a choice between rich and correct, I'll take correct.
The OP asked about jetting for a difference in altitude between somewhere in Arizona:altitude unknown/presumably appreciably lower than Denver and Denver (presumably Denver proper):5,000'-5,500'. My response is that if it were me, I would do my best to run the jets most appropriate to the altitude (which are not the same as sea level).
77mustard40
SILVER Star
spectre6000 said:
Thank you for the concise response, I stand humbled
PabloCruise
SILVER Star
Ah! I was right!
j/k, thank you for taking the effort to post the theory involved, I appreciate it.
Back to the original question, which is do you need to re-jet because of a slightly higher displacement as a result of a rebuild. As you say, the higher displacement will result in a slightly higher air flow through the carb. It seems to me that as far as the carb is concerned, this looks the same as a slightly higher RPM. A 50 thousandths overbore is about a 3% change in displacement which will result in (slightly less than) a 3% increase in airflow which looks to the carb like about a 3% change in RPM. For a tractor engine, I think that you can safely stay with the same jets after a rebore, or for that matter moving to a new location that is 50' higher elevation, or going from an average temp of 50 to an average temp of 70.
There have been two major questions posed thus far: rejet for altitude? and rejet for marginal increase in displacement?
For the latter question, edwjmcgrath, that is correct. A Carburetor, despite it's many virtues, is a device of averages. It's a compromise. The best you can do is come close. Differences in RPM are handled by the different circuits, but each circuit is really only good for one specific RPM (i.e. 800, and 2500 say). Jets are absolutely correct only for a single altitude at a single temperature (generally sea level and something around 70 IIRC). Anything outside of that scope is less than ideal (the spheres become ovoid and the vacuum starts to get a little breezy). Higher altitude means less air pressure pushing down, means less air density, means less available oxygen for combustion, means less fuel is to be metered. Higher temperature is the same, and so on. A 3% delta would be like going from a 55 slow to a 56.7... Pretty sure you're breaking out the reams for that. That sort of tuning is beyond what I even mess with, and I have a big box of all sorts of jets to play with (but only for the air cooled stuff...). When you get to that point, assuming you know the hypothetical 55 to be dead nuts as good as it gets, you decide which side of good that 55 is, and err high or low. I.e., if the 55 was dialed in in winter (cooler denser air, more available oxygen), and your ideal moves to 56.7, and summer is warmer, keep the 55 as it'll run a skosh leaner in the winter and a skosh better in the summer (assuming you're tuning that finely). As has been stated though, these engines aren't THAT sensitive.
50' is trivial (the difference in available oxygen is only 0.15%). The general rule of thumb for changes in altitude is that there is a decrease in oxygen available for combustion of 3% per 1,000'. Going from sea level to where I'm at (just under 8,000') means I have 24% less oxygen available for combustion; Denver is 15-20% deficient. Generally you want to drop a jet size or two for every 2,500, but that's in no way a hard fast rule.
For the latter question, edwjmcgrath, that is correct. A Carburetor, despite it's many virtues, is a device of averages. It's a compromise. The best you can do is come close. Differences in RPM are handled by the different circuits, but each circuit is really only good for one specific RPM (i.e. 800, and 2500 say). Jets are absolutely correct only for a single altitude at a single temperature (generally sea level and something around 70 IIRC). Anything outside of that scope is less than ideal (the spheres become ovoid and the vacuum starts to get a little breezy). Higher altitude means less air pressure pushing down, means less air density, means less available oxygen for combustion, means less fuel is to be metered. Higher temperature is the same, and so on. A 3% delta would be like going from a 55 slow to a 56.7... Pretty sure you're breaking out the reams for that. That sort of tuning is beyond what I even mess with, and I have a big box of all sorts of jets to play with (but only for the air cooled stuff...). When you get to that point, assuming you know the hypothetical 55 to be dead nuts as good as it gets, you decide which side of good that 55 is, and err high or low. I.e., if the 55 was dialed in in winter (cooler denser air, more available oxygen), and your ideal moves to 56.7, and summer is warmer, keep the 55 as it'll run a skosh leaner in the winter and a skosh better in the summer (assuming you're tuning that finely). As has been stated though, these engines aren't THAT sensitive.
50' is trivial (the difference in available oxygen is only 0.15%). The general rule of thumb for changes in altitude is that there is a decrease in oxygen available for combustion of 3% per 1,000'. Going from sea level to where I'm at (just under 8,000') means I have 24% less oxygen available for combustion; Denver is 15-20% deficient. Generally you want to drop a jet size or two for every 2,500, but that's in no way a hard fast rule.
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so spectre, do you have any specific jet numbers for the denver/metro area for a daily driver/colorado mountain wheeler? as mentioned, pighead and i have three carbs that need to be done. one was swapped in colorado to a colorado engine (but not sure if it was jetted properly at the time), the other is off a junker (no history on motor) and replacing a w$%#r, and the third will be a rejet from sea level.
that's pretty much what I had planned to do, hoping threads like this can help cut down the guesswork. I do have a small handful of jets. I've taken apart a few Aisans and kept the small bits, so I have a spare or two of everything. As I said, I have 3 very similar carbs to rebuild, and 3 fairly similar 2Fs to put them on.
First up is a November 6 1974 that came off of a junked 40 that was sitting in the driveway when I arrived in Denver. It had been sitting outside for 10 years or so. I was bored one day and started it up...
http://www.youtube.com/watch?v=btJOhff_qKs
So I know the carb works...
It's going on NuclearLemon's Blue Pig, to replace the weber that's on there now. Going to have to convert the Aisan to cable throttle to. It's got a recent rebuild 2F, pretty much stock inside, headers and no smog. It's a strong runner now. But smells rich and would just be better with the Aisan.
I'm thinking I can't go wrong with stock jets to start, and see how it goes.
Second is the 1/'72 on the 2F in the FJ45SWB. No idea of the history of either except that it hasn't run in 10 years. No reason to go with anything other than stock just to get it running.
Lastly, the 8/'73 carb on TWRECKS, my '74 Pig with the slightly built 2F (some details in my sigline link). This is where I start wondering...
I'm starting with a JimC rebuilt carb, which He did about 8 years ago, when I was building this engine. I don't even know what jets are in it now, I have barely touched it since I got it.
(Spectre, no offense, neighbor, but if Jim C says old sea-level jets are the new 5K jets, I'm gonna listen to JimC)
And I was thinking of tweaking the jetting in it slightly leaner, just to see. Now, I'm not so sure. Also wondering if just leaning the primary while tweaking the secondary linkage so that the secondary opens sooner...or lean out just the secondary by one size.
Nuclearlemon, unfortunately I don't unless you want to tune a VW or early Porsche. I haven't had enough functional Land Cruiser pieces in one place at one time yet to do any fine tuning on these specific carbs (the theory is universal). I have searched between here and the Rising Sun forum, and there are a number of threads discussing Denver area jetting. Right now I have the closest I could get to those recommendations from the jets that were in my carb and a core that I bought for the sight glass. I'll be using that as a baseline, and dialing it in from there.
Pighead, do what you want. The theory that modern gasoline is energy deficient enough that sea level jets are now altitude jets (appropriate for 5,000'-5,500') assumes that a given volume of gasoline contains 15-20% less energy.
A little background on me beyond some random guy on the internet that knows how carbs work, I started my career in experimental alternative fuel feedstocks and I know a thing or two about fuels. If you are running E85 in your 40, sea level jets might actually get you close (straight ethanol has about 70% the energy content of gasoline). Otherwise the difference between the average modern regular pump gas and the average pump gas of the 70s is the additives. The additives from the past allow engines to run a much poorer tune (modern cars take the art out of tuning and the range of variables that need to be accounted for are much narrower), but the energy content isn't appreciably different. Even if your pump gas actually contained 10% ethanol (which you can pretty well be assured it doesn't, those pump stickers are more or less to facilitate implementation of and reduce the overall cost of the RFS Act), you'd only be jetting up about half a size. That said, do what you want. It'll run.
Where is "Pigsty, CO" that we are neighbors? I'm fairly new to the area, but I'm pretty sure that's not a place... I'd like to meet some mudders if there are any nearby.
A little background on me beyond some random guy on the internet that knows how carbs work, I started my career in experimental alternative fuel feedstocks and I know a thing or two about fuels. If you are running E85 in your 40, sea level jets might actually get you close (straight ethanol has about 70% the energy content of gasoline). Otherwise the difference between the average modern regular pump gas and the average pump gas of the 70s is the additives. The additives from the past allow engines to run a much poorer tune (modern cars take the art out of tuning and the range of variables that need to be accounted for are much narrower), but the energy content isn't appreciably different. Even if your pump gas actually contained 10% ethanol (which you can pretty well be assured it doesn't, those pump stickers are more or less to facilitate implementation of and reduce the overall cost of the RFS Act), you'd only be jetting up about half a size. That said, do what you want. It'll run.
Where is "Pigsty, CO" that we are neighbors? I'm fairly new to the area, but I'm pretty sure that's not a place... I'd like to meet some mudders if there are any nearby.
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pighead and i are in the north denver area, so not far away
