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457 Posts
Discussion Starter · #1 ·
Well i posted a thread up here a few months ago about the fact that i was getting 8mpg. So at that point i replaced the distributor as the vac advance was not working. Since then i have been getting better mpg's but still bad. More or less i have been distracted and haven't dealt with that. So here is what i am getting. The best i have gotten which was all highway miles was close to 16mpg and generally i am at 11 mpg (half city half highway). It smells rich and puts a decent amount of wet black soot out the back.

Like i said i replaced the distributor and the timing is set to 8 btdc. I know that my ignition system probably isn't perfect and doing the team rush upgrade would probably help but at this point i have to think that something is awry with mixture. Kind of confusing because the plugs really don't look rich. Also, it does generally like to start without the choke (manual) in warm weather (engine cold). I have 150psi + or - 2 in each hole so comp is not a problem.

So my question is is it possible to tune the carter yf?? From what i have read the only adjustment it has is an idle mixture screw. Would that effect my mpg that much if it were out of wack. It idles really smooth.

I have searched and cannot find any info about tuning a carter yf other than adjusting idle mixture which does not seem to be my problem. Any advice on what to do here would be good. thanks

2,814 Posts
You have a prestolite ignition.... not the best ignition.

Ignition, carb, headers, intake are the best bang for the buck stuff to do. You did not say what distributor you used for a replacement and the BEST choice at that time would have been to use the DuraSpark Distributor from a 1978/1979 259 and use a different box for the spark generator. The best option now a days since the price had droped sooooo much is use a MultiSpark CDI and the DuraSpark... The DuarSpark Distributor is $50 rebuilt from all auto stores and the CDI is $150 from Summit Racing.

I would be evaluting your Ignition if you do not light it you cannot burn it... if your ignitionis weak the fuel put in is so lean to get it to burn right and then your pep and mileage suffer.

I would also try some aux grounds and that might help some too.

Here are some links.

Ignition Options and evaluation by JeepHammer
Jeep Ignition Upgrade Paths, DuraSpark vs Hei Compare by JeepHammer

Grounds & tricks
Jeep Aux Grounds by JeepHammer

Making Battery Cables by JeepHammer

Aux Grounds

Punch List for CJ problems and good results

Ignition Test Steps by JeepHammer

Premium Member
11,196 Posts
Fred is correct about the YF carb.
The best thing you can do is take the tag off the carb and get yourself a rebuild kit...

Once the carb has good everything inside, and it's working properly, you CAN adjust the carb in a number of ways...
But the most important thing is to get the carb working with the ignition!
If you aren't burning the fuel the carb is pumping in, you won't have power and you won't have fuel economy...

YOU WILL get carb cleaner blown back in your face,

There is no excuse for stupidity like that, so get yourself some COMPREHENSIVE SAFETY GLASSES!

Beg, borrow or STEAL a digital camera with resolution and take images from all angles!
These things can be a nightmare to get the linkages, hoses, ect. back on correctly, so LOTS OF PICTURES both before, and WHILE you are taking the carb off and cleaning it!


Lay the parts out in ORDER as you take them out!
This will give you a record of what/how things come apart, and it will keep you from 'Forgetting' to get things back in the order they came out!

Carb cleaner is designed to hang around a while, soften up shellac and varnish left by fuel, and you shoot things down, then go back in a few minutes and spray them clean.

Brake cleaner is designed to dislodge oil/crud and evaporate cleanly.

Then use brake cleaner to clean out the moisture/carb cleaner...
Then used DRY air (Canned air works well) to evaporate the brake cleaner.


Most of the YF carbs I run into (not many!) are just gummed up inside, usually full of crap and crud and need to be cleaned out.

YF carbs are VERY SIMPLE to work on, and you won't have to do a bunch of disassembly,
Just separate the top plate from the main body,
And the main body from the base plate (Throttle valve 'butterfly')
And clean out all the passages.

The #1 problem I run into is the seat for the accelerator pump check ball SEAT is corroded letting the carb draw fuel into the venturi body all the time and waisting fuel.
(I just did one last week that had a corroded check ball, and as soon as you 'Tipped' into the throttle, the accelerator pump 'Shooter' nozzle starting dripping raw fuel into the carb!)

The second biggest problem I run into is the Phenolic Spacer between carb and intake is WARPED,
Giving a HUGE vacuum leak at the baseplate of the carb...
(This particular one had 5 gaskets stacked up trying to get rid of the leak! and failing!)

That is EASY to fix...
First off, make sure your Intake Mounting Studs don't have any rust, gasket/sealer residue collected around the base of them at the intake...
This will keep the gasket/spacer from seating on the intake!

Lay a sheet of plate glass down, tape a piece of finer grain sand paper to it.
Sand BOTH SIDES of the phenolic spacer until the surfaces are FLAT.
Sand in a figure '8' pattern so you don't get sand lines or low spots.
You will be able to tell when it's flat because all areas will be sanded...

Use a VERY THIN coat of FUEL SAFE gasket sealer on the intake, bottom and top of the spacer when you put the thing back together.
If you use too much sealer, it will get into the vacuum passages of the baseplate of the carb, so DO NOT use sealer on the base of the carb, just a VERY LIGHT coat of sealer on the top of the baseplate gasket...
And I mean VERY LIGHT, just a whisper of sealer there!

That will stop the baseplate/spacer leaks.

DO NOT take the linkages apart!
You DO NOT have to remove the throttle linkage from the bores or take the throttle blades off!
Just clean that external linkage with carb cleaner and use a little light lubricant on it.

You need to remove the accelerator pump from the carb,
and replace the rod and diaphragm assembly,
Remove the check balls and check the seats.
Using the old ball (if it's not rusted/corroded) and a punch to 'Tap' the CLEANED seat flush again is a very good way to get the seats working again.

Some things you need to know...
1. Make sure the SEAT & Ball is clean!
You don't want to stamp in the imprint of dirt, corrosion, ect into the carb body!
That will cause another leak that will be VERY HARD to fix!

2. Make sure the kit has a NEW ball before you hammer on the old one!

3. Make sure you have the 'Check Ball' and NOT the aluminum slug they give you to seal up passages!
If you DO NOT have a new check ball in your kit, then DO NOT hammer on the old one!

4. Make sure your float is still good!
Most floats have weight limits, so weight your float on a small Mail scale and compare to the chart.
A leaking/saturated float will be heavier than the specifications and/or if you have a brass float, you will be able to hear fuel sloshing around inside of it if it's failed.

5. YF's usually have a spring loaded inlet valve (needle & seat type), so make sure you get a float level reading with the valve CLOSED, then one with the valve spring COMPRESSED...
And do this BEFORE you take the old float off of the carb top plate...

The spring is in the linkage to keep the float from bouncing open or bending float arm when you hit washboard ruts or pot holes, and you need BOTH readings written down before you take the old float off.

Otherwise, the internals are VERY EASY to take apart, clean and reassemble.
Make sure you take the idle mixture emulsion tube out (Looks like a 'Jet' in the top of the main body) and clean it!
They LOVE to get plugged if you have had unfiltered fuel through it anytime in it's life (VERY LIKELY!)...

Once you get that done and the carb reinstalled,
Then set the idle mixture for a reasonable amount of fuel when you idle...
The 'Proper' way in the manual is to turn the screw 'IN' until it bottoms out on the seat LIGHTLY,
Then back out about 4 turns.

Then screw the idle mixture screw back 'IN' until the RPM drops about 500 RPM and you are done.

You can also set the idle mixture with a vacuum gauge...

Make sure the 'High Idle' isn't set and holding the throttle open when you are doing this.
It should be done on a 'Warm' engine with the choke/high idle pulled 'Off'.

The 'Idle Mixture' screw is located just under the float bowl in the front of the carb.

The CURB IDLE screw is accessible from the PASSENGER SIDE of the engine, and you will find it in back of the carb on the throttle blade shaft just outside the base plate.

Once your idle mixture is set, then adjust the 'Curb Idle' screw for proper engine idle speed...

YOU WILL NOT get proper 'Idle' mixture with the timing set way off!

Check your timing and make sure the timing is set to factory specifications BEFORE you set the idle mixture!

Timing will effect the IDLE SPEED, so make sure you timing is set correctly before you try and set the curb idle, or you will have to do that particular chore again once the timing is correct.

Have you considered an ignition upgrade away from the Prestolite ignition to something a little more reliable and powerful?

If you are interested in getting that done, let me know and I can walk you through that also...

Premium Member
11,196 Posts
Once you get the idle mixture, ignition and accelerator pump working together,
You can start with Jet Sizing & Timing.

Sounds like most of your problem is at low speeds around town, and my guess is that you have plugged this and blocked that with leaking something else...
Someone has richened up the Idle Fuel mixture to help with those issues and now you are consuming/wasting WAY too much fuel at idle or just off idle during transition between Idle circuit and main jet circuit.

Once you get the vacuum leaks and fuel leaks plugged up, get the carb working correctly again,
Get the timing correctly set, then get the idle mixture and curb idle adjusted, you can move on to getting the main jets adjusted if they need any adjustment...
(Most of the time they won't)

Main Jets have to be tested with the throttle WIDE OPEN, HARD PULL, as fast as the vehicle can go,
And to do that without a FAST air/fuel mixture meter of some kind, you are just going to have to take several sets of plugs out of a long, straight road, preferably up and incline so the engine lugs a little.

Switch in the clean plugs, give the engine what for!
I mean get up to speed as quickly as you can, standing on the throttle in every gear!
Once you make a good hard pass,

You are trying to preserve the plug reading on the plugs, and if you use the engine as a brake,
Or let the engine idle at the end of the run, you will kill your plug reading!

Pull the plugs and have a look.
You are looking for a light tan to wheat straw brown on the porcelain part of the center terminal insulator.

It's just fine for one side to be discolored and the other side to be white.

What you DO NOT want to see is BLACK ANYTHING.
Dark brown or black mean the plug is getting WAY too much fuel during wide open throttle.
(The ONLY time the main jet is exclusive. At lower RPM or when working the throttle you are getting enrichment from idle mixture and accelerator pump, so you want a LONG, HARD PULL IN HIGH GEAR to get 'Main Jet' color on the plugs)

If the color is about right, then you are CLOSE, and you can usually adjust the NEEDLE in the main jet to get where you want to be... (you want just a little lighter tan color)

But if you are showing dark brown or black, then it's time for a new jet and maybe a new needle.

457 Posts
Discussion Starter · #5 ·
Okay thanks for that info. That was really what i was looking for to get this carb tuned well. I do a lot of tuning on jet ski carb's just not familiar with the tuning on this guy since it does not have a high speed needle.

I would be surprised if my main jet was to large as i have basically stock comp and the carb as far as i know is stock.

You are probably right about the idle mixture being off.

So if i have stock ignition would 16mpg (90% highway be expected?

Premium Member
6,924 Posts
The best thing you can do is take the tag off the carb and get yourself a rebuild kit...
Just remeber to put the tag back on, or leave it there and write the numbers down... One PO of mine left it off, now I'm guessing...

Super Moderator
23,817 Posts
Okay thanks for that info. That was really what i was looking for to get this carb tuned well. I do a lot of tuning on jet ski carb's just not familiar with the tuning on this guy since it does not have a high speed needle.

I would be surprised if my main jet was to large as i have basically stock comp and the carb as far as i know is stock.

You are probably right about the idle mixture being off.

So if i have stock ignition would 16mpg (90% highway be expected?
Actually that is incorrect. The YF carb does have a "High Speed" mixture adjustment.

The YF carb has a mixtures needle that travels up and down inside the main jet. The deeper this needle sits in the main jet the leaner the mixture. The height of the needle during driving is depended mainly upon the vacuum in the engine and position of the throttle. So the more load on the engine, the richer the mixture. THere are conditions, like WOT that the needle is pulled further out of the main jet further to force a rich mixture.

A lot of people think that the accelerator pump is just for shooting in a steam of gas during acceleration but it actually controls the mixture of the carbs also. A small amount of vacuum is bled into the bottom side of the accelerator pump and this pulls the mixture needle down. If you had a bad vacuum leak or a plugged vacuum passage then the carb would think there is no vacuum and richen your mixture up as far as you can.

Two things important here.
1. Make sure the accelerator piston is fully pushed down before tightening the housing.
2. When adjusting the needle height, make sure your pushing down on the accelerator assembly again.

Turning the needle height screw one way or the other will richen or lean out the main fuel mixture.

Don't get gasket sealant down inside the vacuum passages. I use a very thin coat of HT axle grease on mine. If the surfaces are flat, that is all you will ever need. Most mating surfaces have a little bead going around the perimeter that helps with sealing the carb. Don't file off this bead.

Remember having a no vacuum leaks is important for economy otherwise the YF will run rich all the time.

JH mentions the Ball in the accelerator pump circuit, remember there should be two balls or one ball and a loner counter weight. I have seen some kit include what looks to be an extra float valve but is also used to replace the ball and counter weight. If using one of these DO NOT use a ball to re-seat the accelerator check 'ball'.

See 8 , 46 and 47

I could not even get my YF adjusted with the Pestolight ignition system. When I switched over to an e-core and Duraspark ignition, it was easy to adjust and typically get 18-20 mpg out of the Jeep during the summer.

4 Posts
Hello from a new member!

Hello All from Austria,

Thanks for the great forum, I have recently come into the possession of a 1974 CJ5. It is completely stock and has been sitting for the past 3 years in a shed.

I have everything working - except the carb which was running way too rich.
Thanks to this post, I have been able to disassemble the carb and find the cause of the over rich mixture. The accelerator pump diaphragm was perished an allowing un-metered fuel to be drawn by vacuum down into the inlet manifold alter the butterfly. In several posts people have refered to reconditioning kits for the carbs which are available.
The carb is a single barrel Carter YF - there is no tab present on any of the float bowl screws, but stamped on the side of the carb body is YF6431S.
The 1 may be an 'i' but I am not sure.

Can someone kindly, tell me
If the YF6431S carb number is correct?
Is there a supplier of carb reconditioning kits in Europe?
If there is not, can someone please recommend a kit supplier in the US?

Many thanks for your reply in advance.


4 Posts
Hi Fuel Tank,

I offer you hearty thanks for your help, in response to my question.
I was also pointed to a company called daytona parts.

Funny, the carb co. seems to be none too fond of people emailing them.
It actually took a minute or two to figure out what their email was.

Anyways, you are a gentleman who sustains the mantra of there being no such thing as an unhelpful Canadian.


From the EaglesNest.

122 Posts
"Turning the needle height screw one way or the other will richen or lean out the main fuel mixture."
John Strenk,
Do you know which way leans the mixture and which way richens the mixture? i.e., turning the adjusting screw clockwise will lean the mixture, and counter-clockwise with richen the mixture.

Thanks in advance,

4 Posts
In response to your question

turning the adjusting screw clockwise will lean the mixture, and counter-clockwise with richen the mixture.
This is correct, there is a standard forward thread on the mixture screw, so the more you screw it IN (clockwise) will reduce the size of the hole, therefore LESS fuel will be allowed pass into the Venturi at idle.


Eagles Nest
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