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Carter YF/YFA tunability

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tomj View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Topic: Carter YF/YFA tunability
    Posted: Apr/20/2022 at 12:45pm
I'm back on loving Carter YFs on rambler sixes, after my brush with the unreliable bureaucracy of the aftermarket EFI.

I bought a 1976 Gremlin Carter YF at the parking lot sale of an AMC meet in 1999, I think Cactus Cruisers? I forget. Bought it on the last afternoon, for one dollar, "I don't want to take it home" type sale. In '99 few cared about one barrel carburetors from the 70's. 

Since then it's been the carb I use to start up engines for the first time, because it's so simple and always works. Turns out there's a big hint in there...

Anyway I'm stuck with it now since the rest of the OEM carb choices are crap. And the cheap clones of them from China are pretty good, and under $100 for a new carb is a good deal, and parts are available including modern-fuel-resistant floats, accel pumps etc. It's easy to jet, other than the PITA of partial disassembly on-car to get at the jet and metering rod.

But I "discovered" a feature of the carb I'd not seen mentioned anywhere. One of the available adjustments-by-screw is the metering rod lift, that affects the transition from low-speed (idle jet) to high speed (main jet). The rebuild setup instructions have you depress the plunger and adjust the screw tip to zero clearance, then one more turn in (lift). More turns in (lift) from there makes open-throttle enrichment start earlier, etc.

But you gotta take the top off the carb every time. Except that you don't -- I was thinking 'wouldn't it be nice to have a hole in the carb top right there...' and lo! there is one, from the factory.



There's a steel plug in a hole in the right location. Pop it out with a small screwdriver etc from below. I made a gasket that covers it while tuning, and when done, put the plug back in. Voila!












1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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tomj View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Apr/20/2022 at 12:54pm
I updated my crappy carburetor/induction pages to be less crappy.


And I found a Carter YF ---> THEORY OF OPERATION <--- factory manual, PDF link on that page, and below.

It explains how the feedback carb works (easy: 10 Hz PWM, ON=lean). So now I want one -- over the years I'd fantasized about a closed-loop carb computer, but now I think the way to think about it is as a manual altitude control:

* set a feedback YF up for correct jetting at seal level (los angeles).

* when I drive to the mountains, Flagstaff through Santa Fe etc, turn a knob that leans out the cruise mixture. 

DONE. I could write the code and make the electronics for that in one day and have it work first time guaranteed. A simple box with one knob, R to L. R is basically off ("sea level"), L is solenoid open, max lean, and half way is 50% PWM. Look at the AFR meter cruising at 60 mph flat, turn the knob til the AFR is 15, look at it again later. No sensors, no nuttin.


CARTER YF/YFA SERVICE MANUAL #3608B


1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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tomj View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Apr/30/2022 at 5:42pm
Man, is it easy tuning (jetting, adjusting) a YF for correct A/F ratio. I bought the cheapest AFR meter kit (AutoMeter, $220 from Summit) stuck it in the hole in the panel (that someone cut years ago). The '76 7112S YF I have was running 15.5 - 16 A/F at 60 mph flat cruise, a bit lean, but plugs looked great.

The jet was .100" (possibly .101). I had a .109" jet so stuck that in. Now it's 14:1, a bit rich for cruise, so I will take a spare jet I have, .09x, and drill it out .104" and try.

I bought some metering rods from eBay thinking I would fine tune with that, but a closer look at them and I realized it's a waste of time, mostly. The fat end (cruise) doesn't vary much, and are largely various tapers from .080" down to about .060", and having tried two fairly far apart, very hard to discern any difference. The tip though affects WOT and there, maybe...

But whatever, I'll probably get to 90% perfect with just poking the jet and let WOT fall where it lies. Maybe then find a fatter-tip metering rod to maybe lean WOT a bit, it's 12:1 now.

With the .100 jet and in around-town surface streets for 15 gallons of fuel, I got 14.5 MPG, not good. Richer may improve that. I was getting low 20's on log highway trips with it. Still tuning though.


FYI, I do my tuning notes in a spreadsheet (google Sheets) and so forthehellofit I calculated effective jet size, including the area of the jet blocked by the cruise portion of the rod vs the tip. One rod to another just doesn't make more than a few percentage difference. This is probably why tuning via metering rods isn't a thing. 

The screw adjustment for rod lift also somehow affects transition from cruise to WOT/partial throttle, but fiddling with it an extra turn wasn't hugely noticable. I do have a bit of a lean dip transitioning from idle to throttle (main jet) so maybe it will affect that once I get close enough to care.


A larger point being, the Carter YF is a fine carb to stick on old motors. 


1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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Ollie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ollie Quote  Post ReplyReply Direct Link To This Post Posted: May/01/2022 at 7:01am
I am rattling my brain here trying to wake up some cells from a few years back. But seems to me that when I had my 199 with the YF , that I put on it,  the best jet was a 101. Almost sure I got it from Mikes carb. I could not find any available rods. I think I remember I increased the jet size from .098. That gave me the best mpg and it was 20-22 highway.

Like what you are doing, and follow your posts.

I have learned that when dealing with this crappy ethanol fuel, bigger jets is better. More fuel = better mpg. Sounds weird but true.

Not alot of room to wiggle with a yf, but probably one of the best and simplest carbs ever made. 

Having AMC Fun,
Ollie
1966 American Convertible -- "The Rambler"..SOLD
1974 Postal Jeep -- "Rapid Delivery"...SOLD
1969 Rambler 220 post car--"Road Warrior"
1989 Jeep Comanche Pioneer, 4.0L, auto, 2wd
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2022 at 12:07am
Thanks Ollie! I swapped out the .100 for a .110 I had and it runs 14:1 afr. A bit rich. So tomorrow I'll drill out one of my extras for .104.  

So I'll be pretty close to your 101. 

I've been having erratic idle screw adjustments... And found that the screw, tough out about 1.5 turns, wiggles. So maybe mine is worn out. One of the top cover tapped holes is stripped too.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2022 at 1:17pm
as a further verification that the YF is a good, if not great carb, back in the IMSA Radial Challenge days, they restricted the AMC teams (Team Highball, etc) to use of the OEM YF on the 232 6.  And they made a bunch of power.  
73 Gremlin 401/5-spd.
77 Matador Wagon 360/727.
81 Jeep J10 LWB 360/4-spd
83 Concord DL 4-dr 258/auto

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