Humidity did it, runs like a rocket!!
Thread Starter
Burning Brakes
Joined: Aug 2000
Posts: 805
Likes: 0
From: Vasteras, Sweden
Humidity did it, runs like a rocket!!
:party:
Yesterday the BB stumble when I hit it, but today it runs like a rocket and I havn't made a thing!
Can anyone explain what the differens is and how I adjust the carb to make it run like this in dry weather, without the stumbles?
The sparks is cigarette filter colored(brown tan) / concrete-gray.
:hurray:
[Modified by Lasse, 8:12 PM 5/30/2003]
Yesterday the BB stumble when I hit it, but today it runs like a rocket and I havn't made a thing!
Can anyone explain what the differens is and how I adjust the carb to make it run like this in dry weather, without the stumbles?
The sparks is cigarette filter colored(brown tan) / concrete-gray.
:hurray:
[Modified by Lasse, 8:12 PM 5/30/2003]
Drifting
Joined: Oct 2000
Posts: 1,825
Likes: 56
From: Shawano WI
Re: Humidity did it, runs like a rocket!! (Lasse)
There are generally four weather conditions that affect carburator tuning
1: Temperature - as the temperature decreases the air becomes more dense and the carb will have to be richened to compensate.
2: Altitude - as you go to a higher altitude the air becomes less dense and the carb will have to be leaned out to compensate.
3: Humidity - as the humidity rises water displaces oxygen and the air becomes less dense and the carb will have to be leaned out to compensate.
4: Barometric Pressure - as the barometric pressure rises the air becomes more dense and the carb will have to be richened up to compensate.
There are some on here that suggest that you try to run your air/fuel ratio as lean as possible to make more power but because of changing weather conditions that can be disasterous if you run across a cool/dry evening and the carb becomes so lean that it leads to detonation/pinging. It is always safer on an engine to run a little on the rich side.
[Modified by Pete79L82, 5:16 PM 5/30/2003]
1: Temperature - as the temperature decreases the air becomes more dense and the carb will have to be richened to compensate.
2: Altitude - as you go to a higher altitude the air becomes less dense and the carb will have to be leaned out to compensate.
3: Humidity - as the humidity rises water displaces oxygen and the air becomes less dense and the carb will have to be leaned out to compensate.
4: Barometric Pressure - as the barometric pressure rises the air becomes more dense and the carb will have to be richened up to compensate.
There are some on here that suggest that you try to run your air/fuel ratio as lean as possible to make more power but because of changing weather conditions that can be disasterous if you run across a cool/dry evening and the carb becomes so lean that it leads to detonation/pinging. It is always safer on an engine to run a little on the rich side.
[Modified by Pete79L82, 5:16 PM 5/30/2003]
Race Director
Joined: Oct 2000
Posts: 15,104
Likes: 79
From: central FL, near the beach.
Re: Humidity did it, runs like a rocket!! (Pete79L82)
Pete
Can you put some numbers on this.
i am looking thru my books and i found a ride from sea level to 5000' needs 9% leaner jets.
How about just 50 to 90 degrees?
How about 35% to 90% humitidy constant temp?
thanks
matt
Can you put some numbers on this.
i am looking thru my books and i found a ride from sea level to 5000' needs 9% leaner jets.
How about just 50 to 90 degrees?
How about 35% to 90% humitidy constant temp?
thanks
matt
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: Humidity did it, runs like a rocket!! (Pete79L82)
This is one of the reason I am switching to FI. In MI I can drive it in early spring/late fall in 30 deg weather or in mid summer traffic at 120 deg, how can a carb adj to that? With FI the system corrects itself. If you think about it with our higher hp engines having FI is even more critical when driving in a wide range of conditions. Take a 200 hp eng and have it lose 10% peformance do to cond's and you lose 20 hp, take that same 10% from a 400hp and your a looking at 40hp. With FI you might lose 1 to 2%.
[Modified by Fevre, 9:39 PM 5/30/2003]
[Modified by Fevre, 9:39 PM 5/30/2003]
Burning Brakes
Joined: Oct 2001
Posts: 1,185
Likes: 0
From: New Wilmington PA
Re: Humidity did it, runs like a rocket!! (Fevre)
:iagree:
That's why I went EFI too. Altitude, no prob. Temp, no prob. Humidity, no prob. Once set up your car gets what it needs when it needs it regardless of outside conditions.
I just got the preliminary tuning done today and look forward to getting it on the road next week.
That's why I went EFI too. Altitude, no prob. Temp, no prob. Humidity, no prob. Once set up your car gets what it needs when it needs it regardless of outside conditions.
I just got the preliminary tuning done today and look forward to getting it on the road next week.
Drifting
Joined: Oct 2000
Posts: 1,825
Likes: 56
From: Shawano WI
Re: Humidity did it, runs like a rocket!! (Matt Gruber)
Matt :
The problem is you can not put straight numbers to it. Rarely does the humidity change without the tempuratue changing or the barometric pressure changing. So you have to take all of them into account. When I dyno a motor I have to take weather calculations before every test and the corrections can change every 15 minutes.
Here is a good artical that helps to explain weather factors on an engine.
http://www.quarterjr.com/members/index.htm
Here is a calculator to help jet correction. It is only for holley style carbs (holley, BG, Demon, ect) because that is what the majority of race carbs are.
To use it first put in your weather and jet# and correct it "to standard". That will give you a baseline jet for standard weather correction.
Then that the baseline jet (the new number you just calculated) and set in the weather you want to correct for and select "from standard"
A word of caution, you must use un-corrected barometric pressure. NOT the number the weatherman give on TV. Un-corrected barometric pressure can usually be gotten from a local airport or from a modified racing altimeter.
http://www.bgsoflex.com/holley.html
Pete
[Modified by Pete79L82, 9:14 PM 5/30/2003]
The problem is you can not put straight numbers to it. Rarely does the humidity change without the tempuratue changing or the barometric pressure changing. So you have to take all of them into account. When I dyno a motor I have to take weather calculations before every test and the corrections can change every 15 minutes.
Here is a good artical that helps to explain weather factors on an engine.
http://www.quarterjr.com/members/index.htm
Here is a calculator to help jet correction. It is only for holley style carbs (holley, BG, Demon, ect) because that is what the majority of race carbs are.
To use it first put in your weather and jet# and correct it "to standard". That will give you a baseline jet for standard weather correction.
Then that the baseline jet (the new number you just calculated) and set in the weather you want to correct for and select "from standard"
A word of caution, you must use un-corrected barometric pressure. NOT the number the weatherman give on TV. Un-corrected barometric pressure can usually be gotten from a local airport or from a modified racing altimeter.
http://www.bgsoflex.com/holley.html
Pete
[Modified by Pete79L82, 9:14 PM 5/30/2003]
Race Director
Joined: Oct 2000
Posts: 15,104
Likes: 79
From: central FL, near the beach.
Re: Humidity did it, runs like a rocket!! (Pete79L82)
Pete
for a guy with a carb on the STREET that link is useless!
Even if we go on a trip to 5000 feet we are not going to stop every 1000 feet and change jets. We will just let it run rich. and wast a few $ of gas.
As a dyno operator you need to give a corrected figure so the customer can come back and compare changes.
But, you can't make a jet change and get back lost power during a T-storm with low air density. You just correct for it, in theory, on an ideal day it would be higher.
The only one i see running lean is norval and he has a blower. I'm scared to run lean(i shoot for 13:1 WOT in the leanest cylinder), but if it's working for norval i am impressed!
The bottom lineis we are not going to get engine damage on a bad hair day, just a little less power. (my lean cyl might go to 13.6:1 due to extreme weather) but probably not :cheers:
for a guy with a carb on the STREET that link is useless!
Even if we go on a trip to 5000 feet we are not going to stop every 1000 feet and change jets. We will just let it run rich. and wast a few $ of gas.
As a dyno operator you need to give a corrected figure so the customer can come back and compare changes.
But, you can't make a jet change and get back lost power during a T-storm with low air density. You just correct for it, in theory, on an ideal day it would be higher.
The only one i see running lean is norval and he has a blower. I'm scared to run lean(i shoot for 13:1 WOT in the leanest cylinder), but if it's working for norval i am impressed!
The bottom lineis we are not going to get engine damage on a bad hair day, just a little less power. (my lean cyl might go to 13.6:1 due to extreme weather) but probably not :cheers:
Drifting
Joined: Oct 2000
Posts: 1,825
Likes: 56
From: Shawano WI
Re: Humidity did it, runs like a rocket!! (Matt Gruber)
Matt, that is my point exactly. On the street you have to keep it a little on the rich side to cover all situations so you don't hurt the motor.
Pete
Pete
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Race Director
Joined: Oct 2000
Posts: 15,104
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From: central FL, near the beach.
Re: Humidity did it, runs like a rocket!! (Lasse)
Lasse
is the vacuum gauge still hooked up?
with 5", if it went to 2" it would run like a rocket. Look into a sticking carb, or the wrong spring operating the secondaries.
Since humidity would cut power, you can rule it out as a coincidence.
is the vacuum gauge still hooked up?
with 5", if it went to 2" it would run like a rocket. Look into a sticking carb, or the wrong spring operating the secondaries.
Since humidity would cut power, you can rule it out as a coincidence.
Safety Car
Joined: Aug 2000
Posts: 3,909
Likes: 26
From: Clymer PA
Re: Humidity did it, runs like a rocket!! (Matt Gruber)
Matt:
Norval has been running his motor like that for some time. In fact he ran all of his motors that way. Motors that run rich are down on power. Sure they are safer to run to a certain extent but you are missing out on power. Not only are you missing out on power but running too rich washes the cylinder walls of oil and can lead to pre-mature engine wear.
I can remember Norval telling me that after a nice highway cruise if he pulls a plug to check it, that it's normally white. It's easy to get a motor to run rich, it takes a good bit of tuning to correctly lean out a motor.
Norval has been running his motor like that for some time. In fact he ran all of his motors that way. Motors that run rich are down on power. Sure they are safer to run to a certain extent but you are missing out on power. Not only are you missing out on power but running too rich washes the cylinder walls of oil and can lead to pre-mature engine wear.
I can remember Norval telling me that after a nice highway cruise if he pulls a plug to check it, that it's normally white. It's easy to get a motor to run rich, it takes a good bit of tuning to correctly lean out a motor.
Race Director
Joined: Oct 2000
Posts: 15,104
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From: central FL, near the beach.
Re: Humidity did it, runs like a rocket!! (bence13_33)
So Shane,
it is your opinion that 13:1 WOT is rich?
exactly what A/F ratio are you using, and what are you suggesting i use?
BTW i run 17:1 at cruise
it is your opinion that 13:1 WOT is rich?
exactly what A/F ratio are you using, and what are you suggesting i use?
BTW i run 17:1 at cruise