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Old 06-29-2013, 03:54 PM
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Main jets and fuel economy relationship..??

I'm wondering if jetting down a bit could improve fuel economy..

at part throttle, where most of city driving is done, do the jets actually flow at full capacity? I say no way, so what would be the point of installing smaller jets?

jet size should only matter at wot where all the carb circuits are flowing at max capacity, at part throttle while cruising I believe most of the fuel is flowing trough the transfer slots and idling circuits, the power valve also should be shut most of the time.

so, would it be a waste of time installing smaller jets to improve fuel economy on a daily driver car that very seldom if ever sees wot?

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Old 06-29-2013, 04:10 PM
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Quote:
Originally Posted by Augusto View Post
I'm wondering if jetting down a bit could improve fuel economy..

at part throttle, where most of city driving is done, do the jets actually flow at full capacity? I say no way, so what would be the point of installing smaller jets?

jet size should only matter at wot where all the carb circuits are flowing at max capacity, at part throttle while cruising I believe most of the fuel is flowing trough the transfer slots and idling circuits, the power valve also should be shut most of the time.

so, would it be a waste of time installing smaller jets to improve fuel economy on a daily driver car that very seldom if ever sees wot?
This is where spark plug reading comes to the fore. After warming the motor, make a full power pass down the drag strip or abandoned section of roadway, chop the ignition and coast to a stop. Pull the plugs one at a time and inspect with a good plug-reading light/glass. If you tune for best power, you will also be tuning for best mileage. Here's the tool.....
::rag Racing Online::: It's New to Us - 3/26/2012
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Old 06-29-2013, 04:33 PM
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Tuning idle is important, as is transition tuning but your main jets will have an effect at mid rpm cruise. Your power valve should in no way affect cruise.
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Old 06-29-2013, 06:23 PM
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jets

Hi all

Main jets or any jet is a restrictson.So a larger jet will flow more fuel than a smaller one.As for fuel mpg. it may get better with a larger jet.The factory tunes engines on a balance of mpg. and emissons.With more fuel you can push the ing. harder.But it will not run as clean.Most of the time smaller jets will not help.In the late 70`s gas mpg. took a nose dive big time.I retuned many with nice results going from mabey 10mpg to 15mpg and boxes of thank you letters from happy custs.Running a lower thromsat also helps.Good Luck with the retune.These suggestions are for off road use hehe.

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Old 06-29-2013, 09:15 PM
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Quote:
Originally Posted by techinspector1 View Post
This is where spark plug reading comes to the fore. After warming the motor, make a full power pass down the drag strip or abandoned section of roadway, chop the ignition and coast to a stop. Pull the plugs one at a time and inspect with a good plug-reading light/glass. If you tune for best power, you will also be tuning for best mileage. Here's the tool.....
::rag Racing Online::: It's New to Us - 3/26/2012
I've already done a plug reading and it's got that nice brick like color, but that is at wot, a condition it seldom if ever sees this car, it's got a Ford I6 300 cid engine, brand new 1bbl carburator, new electronic distribuitor, free flowing 2.5 inch exhaust with one glasspack only, (sounds cool btw) but at part throttle I get the feeling it runs on the rich side.. I've spent lots of time fine tuning the carb, lowered the float 1/8 of an inch, but I can't do anything to the main jet, which has a needle inside, like q-jets do, because nobody seems to have jets for this carb,

but anyway I have my doubts that changing the jet will do anything at part throttle simply because not enough fuel is flowing through it at that speed, if say for instance only half the fuel was flowing, half of the jet's area is empty, what would be the point in using a smaller jet?, this would only restrict flow at WOT, it will run lean at WOT.

what could I do to lean it out a bit at part throttle? (no vaccum leaks please )
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Old 06-30-2013, 06:59 AM
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You apparently don't understand how a carb works, which is fine as most people don't. For tuning at cruise though you can't "feel" an air fuel ratio. If you're concerned about it hook up a oxygen sensor, wide band is best but even a cheap narrow band can help.
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Old 06-30-2013, 08:13 AM
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carb work

Hi all

In stall a copper wire in the jet.The wire should be about 2" long.Wrap the extra lengh around the outside of the jet to hold it in place.Start with about .010 and change as you see fit.Engines that run to lean smeal funny.You changed the exhaust that will also lean out the engine.


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Old 06-30-2013, 10:22 AM
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Originally Posted by ap72 View Post
You apparently don't understand how a carb works, which is fine as most people don't. For tuning at cruise though you can't "feel" an air fuel ratio. If you're concerned about it hook up a oxygen sensor, wide band is best but even a cheap narrow band can help.
Carburators were banned from production vehicles when you were 2 years old, at that time I already had earned a ASE certificate ans was already graduated in Auto and diesel mechanics, I've been racing and tuning carbs more time than you have been breathing and I can tell you for sure I do feel how an engine performs, I can listen it and also smell it, most of the guys who were born in the pre efi era will agree with me.
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Old 06-30-2013, 10:28 AM
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The only real way to adjust the cruise mixture is with an 0 2 sensor and even this needs to be understood as it is not perfect. You need a very thorough understanding of how the carb works. Buy a book on your style of carb....then memorize it.

The exhaust system needs to be air tight , no leaks as these affect the AFR.
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Old 06-30-2013, 11:54 AM
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now here this

to the guy that hates me

I am 60 years old.I have a Master`s N.I.A.S.E in all 8.I only try to help people.Sorry I can`t help You.
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Old 06-30-2013, 12:24 PM
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Quote:
Originally Posted by Augusto View Post
Carburators were banned from production vehicles when you were 2 years old, at that time I already had earned a ASE certificate ans was already graduated in Auto and diesel mechanics, I've been racing and tuning carbs more time than you have been breathing and I can tell you for sure I do feel how an engine performs, I can listen it and also smell it, most of the guys who were born in the pre efi era will agree with me.
If you're such an induction genius why ask such a simple question? Being old and certified apparently doesn't mean much as there are several teenagers that can answer this stuff.
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Old 06-30-2013, 12:33 PM
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With just one carb feeding 6 cylinders you will not get a perfect tune on all 6. You are tuing fgor the WORST cylinder. That can/will change with carb throttle position angle too.
The leanest cylinder misfire is your limit. Somewhere around 15.5:1-16:1 delivers the best mileage.
But every engine is different and every engine is imperfect. Leaner AFR's allow and want more spark timing (vac advance) EGR is critical and allows this more timing and actually adds engine efficientcy and mileage, while reducing pinging at lean cruise. The amount of EGR flow amy need to be tuned.
What works at one throttle angle may not work at another. SO find the happy medium.
Watch all the plugs. Not for colour so much, but excessive heat.
The O2 sensor is just a average of a 6 cylinders and does not actualy read AFR . It reads o2 content in the exhaust. But it helps. So does a vacuum gauge.

Yes at cruise you are still on the idle/transition circuit or on the idle and main circuit.
With tapered rod in jet you can adjust the rod height up down too. And or add to the rod cross section with solder. Have a spare rod in case you screw it up. Adjusting the main jet on most carbs does have a slight triming effect on the idle circuit too.
The engine will show you what it likes. But watch the plugs for excess heat.

Gas mileage savings: lower idle speed.
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Old 06-30-2013, 07:57 PM
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If you're such an induction genius why ask such a simple question? Being old and certified apparently doesn't mean much as there are several teenagers that can answer this stuff.
because hearing other people's opinions increases your knowledge, nobody in this world knows everything, but getting a "you don't understand this matter" for an answer is not an opinion is an insult.
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Old 06-30-2013, 08:13 PM
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Originally Posted by 2x4 Barrels View Post
Hi all

In stall a copper wire in the jet.The wire should be about 2" long.Wrap the extra lengh around the outside of the jet to hold it in place.Start with about .010 and change as you see fit.Engines that run to lean smeal funny.You changed the exhaust that will also lean out the engine.


2x4`s
I usually do this trick you say but in this case is not possible due to the needle that goes inside the jet, here we are at 6.600 feet of altitude and changing jets to lean out the mixture is a must in all carburated engines, but for some you can't get any parts so a piece from a multi strand wire saves the day.

btw.. I don't hate you
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Old 06-30-2013, 08:24 PM
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Quote:
Originally Posted by F-BIRD'88 View Post

Yes at cruise you are still on the idle/transition circuit or on the idle and main circuit.
With tapered rod in jet you can adjust the rod height up down too. And or add to the rod cross section with solder. Have a spare rod in case you screw it up. Adjusting the main jet on most carbs does have a slight triming effect on the idle circuit too.
The engine will show you what it likes. But watch the plugs for excess heat.

Gas mileage savings: lower idle speed.
moving the rod down a bit is a pretty good idea, actually is what is done in motorcycle carbs, why didn't I think of it before? thanks for the idea.

I have done the solder in the needle but I don't like it much because it wears down pretty easy, specially on q-jets that hang inside a hole with edges, moving the needle is the correct trick here, so is for any carb with needles.!

the plug's color tell me that it can be leaned out quite a bit at part throttle, but you don't get a good reading of plugs at other than WOT.

I was thinking of leaning out the cruise range by enlarging the air bleed holes but this usually will requiere a larger jet to correct the top end.

what do you think of sligthly closing the transfer slots? maybe by peening them a bit on the top side?
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