Author Topic: The lost art of tuning a carb - a primer?  (Read 129 times)

Offline pencilrunner

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The lost art of tuning a carb - a primer?
« on: June 30, 2021, 11:01:13 AM »
Hey fellas, need an assist - recently got my '90 YJ on the road.  It's got an AMC 258 with a older, non-stepper Carter BBD I got off a wrecked CJ.  I rebuilt the carb with a quality kit, and it is rough-tuned well enough.  The issue is I know it is still running a bit rich, but could use some authoritative advice on how to completely dial-in my carb and timing.  The engine is stock (save for a CRT HEI dizzy), with a V-belt setup (power steering and alternator, no A/C), using Manifold v. Ported Vac for the vacuum advance.  If it matters, I'm going to swap out the thermostat from the stock temp to a 190 temp (seems better for carb'd engines), but that's all.

If there are any fellow leaf-spring-guys/gals out there, who have been down this particular rabbit hole, please lend me your experience.

Thank you all!
Proud father of a 1990 YJ, AMC258, non-stepper Carter BBD, CRT HEI, AX15, a tape-deck radio, two V-belts and two seats.

Expectant father of a 1999 XJ, 4.0/AW4.  Currently refurbishing with new/refreshed steering/suspension/drivetrain/electrical.  Building to 'Upcountry' specs (minus skid plates).

Offline Gr8Dain

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Re: The lost art of tuning a carb - a primer?
« Reply #1 on: June 30, 2021, 12:01:46 PM »
I am no expert for sure.  I had a Holley 4bbl carb on my 258 that did very well, until I was off camber on trails.  I ended up swapping it for EFI.  And was very happy that I did. 

I know that carbs can be made to run great and that it is a skill to know to get them dialed in.  Good luck. 

My 258 is gone now and I am loving my 4.0 with stock fuel injection. 
Dain

1984 CJ-8 Scrambler
4.5" lift and 35s
Trail toy now

1949 CJ 3a - Stock - Garage queen

Offline R3

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Re: The lost art of tuning a carb - a primer?
« Reply #2 on: June 30, 2021, 07:05:13 PM »
Hey fellas, need an assist - recently got my '90 YJ on the road.  It's got an AMC 258 with a older, non-stepper Carter BBD I got off a wrecked CJ.  I rebuilt the carb with a quality kit, and it is rough-tuned well enough.  The issue is I know it is still running a bit rich, but could use some authoritative advice on how to completely dial-in my carb and timing.  The engine is stock (save for a CRT HEI dizzy), with a V-belt setup (power steering and alternator, no A/C), using Manifold v. Ported Vac for the vacuum advance.  If it matters, I'm going to swap out the thermostat from the stock temp to a 190 temp (seems better for carb'd engines), but that's all.

If there are any fellow leaf-spring-guys/gals out there, who have been down this particular rabbit hole, please lend me your experience.

Thank you all!
Where to start. Jets are the easiest thing to start with. Check the adjustment screws most turn them down to tight and deform. They should be under the screws, then have a number on to tell you what size they are. Head to a Napa store and find a could a size or two smaller...might need a couple different sizes.

If it has metering rods... There are adjustments there as well.  Float bowl... Check the needle there too. Seat could be too big as well. All are replaceable.

Just stuff off the top of my head for now.  Hope it helps.

Cheers

Sent from my Moto Z (2) using Tapatalk
Cheers

17 JKURR Daily/rock crawler/overland
2013 Victory XCT hot rod.

Offline orange150

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Re: The lost art of tuning a carb - a primer?
« Reply #3 on: July 19, 2021, 10:44:06 AM »
How do you know it's running rich?

Best way to tune it, other than installing an afr gauge with wide band 02 is to read the plugs. You can play with jet sizing until you're dialed in.

For ported vs manifold: if you've removed all the emissions stuff you can switch it to manifold, otherwise keep it on ported to make sure the emissions control equipment is operating properly.

FWIW, I swapped mine to an MC2150 a few years ago. Easier carb to work on and tune as they utilize a lot of off the shelf Holley parts. They also have about the best off road manners you can get from a carb with a few tricks.
If I were to buy another carb, I'd get the less complicated MC2100, 1.08 venturis, from someone like National Carburetor. But I'm swapping to EFI. I'm done fiddling with carburetors.
'82 CJ-8

Offline REFYVD

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Re: The lost art of tuning a carb - a primer?
« Reply #4 on: July 19, 2021, 03:56:51 PM »
^What Orange said

That Carter is gonna be nothing but a source of irritation and aggravation

I've got a 2100 I'll give you just to save you the heartache

Offline OneArmedSaint

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Re: The lost art of tuning a carb - a primer?
« Reply #5 on: July 21, 2021, 04:36:47 PM »
I'll give a third bump to ridding yourself of the Carter. There are much better, inexpensive options. That 2100 would be one.

Offline Gr8Dain

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Re: The lost art of tuning a carb - a primer?
« Reply #6 on: July 21, 2021, 04:47:36 PM »
I'll give a third bump to ridding yourself of the Carter. There are much better, inexpensive options. That 2100 would be one.
Or go the expensive route like i did and swap to a fuel injected motor. 
Dain

1984 CJ-8 Scrambler
4.5" lift and 35s
Trail toy now

1949 CJ 3a - Stock - Garage queen

Offline orange150

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Re: The lost art of tuning a carb - a primer?
« Reply #7 on: July 21, 2021, 04:53:32 PM »
If you want to go the route of doing an EFI swap on your 258 I have a good 7120 head, wiring harness, computer, intake, exhaust, and the little water soluble packing peanuts that I need to sell. I've changed directions for mine.
'82 CJ-8

 

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