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Old 06-21-2011, 03:42 PM   #1
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Carb Help!!! Burning gas!

So I have a '90 I6. I replaced the Carter Carb with a Motorcraft 2100 that I bought off ebay. Engine starts fairly easy but I seem to be getting less than 5 mpg. The engine is just burning through gas. It also accelerates poorly especially if I am in a high gear and getting up to speed (ie if I was in 3rd going 35mph, slow down to 10mph for traffic but stayed in 3rd then accelerated back to 35mph). Overall, I think the engine needs to be fine tuned so I will appriciate any help. I also took it to a few mechanics but no one wants to work on carbs. They discriminate - only work on fuel injected now. Anyone know a good mechanic that can work on carbs in the Chicago Land area? Thanks for any help!

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Old 06-21-2011, 03:55 PM   #2
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What do your plugs look like?
Where is your base timing set?
Where is your timing at 3000rpm?
Did you check your vacuum lines for leaks?
How does it idle?
What was your MPG before, for reference?

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Old 06-22-2011, 07:42 AM   #3
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Quote:
Originally Posted by Mr. Sinister View Post
What do your plugs look like?
Where is your base timing set?
Where is your timing at 3000rpm?
Did you check your vacuum lines for leaks?
How does it idle?
What was your MPG before, for reference?
Thanks for Reply -

Changed the spark plugs, wires, etc at the same time as I replaced the carb. I think those are ok.

Haven't checked timing at all - sounds like that might be the next step. Haven't checked for vacuum leaks.

Idle's well - around 600rpm. Not sure on MPG before because Within a few weeks of purchasing the car, I replaced the carb (the previous carb was that bad).
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Old 06-22-2011, 07:58 AM   #4
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Give me your email and I could probably email you some repair instructions for the carb. Or even something to help you diagnose your issue.
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Old 06-22-2011, 08:59 AM   #5
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Quote:
Originally Posted by mogarber View Post
Thanks for Reply -

Changed the spark plugs, wires, etc at the same time as I replaced the carb. I think those are ok.

Haven't checked timing at all - sounds like that might be the next step. Haven't checked for vacuum leaks.

Idle's well - around 600rpm. Not sure on MPG before because Within a few weeks of purchasing the car, I replaced the carb (the previous carb was that bad).
Pull a plug and see what it looks like. Since they're fresh, they'll give you a good idea of what is going on in the cylinders. Post back with what they look like.

Always set timing before you make any adjustments to the carb.

Since the jeep is new to you, I'm wondering if the old carb was masking a bigger issue, or caused a bigger issue. Of course, what's to say the carb you got from ebay isn't having it's own problems. Maybe pick up a rebuild kit for it, just to be safe.
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Old 06-22-2011, 09:22 AM   #6
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Give me your email and I could probably email you some repair instructions for the carb. Or even something to help you diagnose your issue.
Thanks for the help - my email is my username at gmail.com.
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Old 06-22-2011, 09:24 AM   #7
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Pull a plug and see what it looks like. Since they're fresh, they'll give you a good idea of what is going on in the cylinders. Post back with what they look like.

Always set timing before you make any adjustments to the carb.

Since the jeep is new to you, I'm wondering if the old carb was masking a bigger issue, or caused a bigger issue. Of course, what's to say the carb you got from ebay isn't having it's own problems. Maybe pick up a rebuild kit for it, just to be safe.
Timing will be checked this weekend (or sooner if I can get to the mechanic). Carb was brand new so don't think it needs a rebuild kit. Could be masking a bigger issue but not sure where to start looking. Hopefully timing is off and that will be an easy fix.
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Old 06-22-2011, 09:44 AM   #8
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I emailed you a lot of information that may help you diagnose your issue. Also, just verify that all of the vaccum lines are hooked up correctly.
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Old 06-22-2011, 09:51 AM   #9
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Originally Posted by mogarber View Post
Timing will be checked this weekend (or sooner if I can get to the mechanic). Carb was brand new so don't think it needs a rebuild kit. Could be masking a bigger issue but not sure where to start looking. Hopefully timing is off and that will be an easy fix.
Yeah, it SHOULD be ok, but probably still needs some adjustments made. You can usually bolt-on and go with most carbs, but they're typically far from optimized. Good luck with it!!
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Old 06-22-2011, 10:08 AM   #10
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Yeah, it SHOULD be ok, but probably still needs some adjustments made. You can usually bolt-on and go with most carbs, but they're typically far from optimized. Good luck with it!!
Optimizing is where I am getting lost! deathphoenix99 sent me some info so going to read through that and see if that can lead me in the right direction. Will report back when I get to do some work on it. Thanks!
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Old 06-22-2011, 10:57 AM   #11
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Good luck man.
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Old 06-22-2011, 10:59 AM   #12
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I would have told you to get a little Holley or Edelbrock, but since you already have the carb....
It's probably something out of adjustment or a bad/incorrectly placed vacuum line. Those old 4.2's are tough engines, but only got about 15mpg when brand new. As drastic a difference at the 5mpg you're getting now is, it doesn't take much with a carbed engine to really throw the fuel economy off. If I was in the area, I'd be happy to help. I had to basically teach myself carbs, since like you're finding, knowledgeable carb guys are few and far between, and those that have a lot of knowledge, don't seem really eager to share.
I'd imagine part of what he sent you are how to tune the carb with a vacuum gauge?
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Old 06-22-2011, 11:03 AM   #13
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I just emailed him the Mitchell's repair guide that mechanics would use to test it, I also sent a bunch of other information including how to rebuild it, and the engine, along with the specs for the motor. If you'd like anything, feel free to PM me.
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Old 06-22-2011, 01:53 PM   #14
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I have an '87 with a 4.2 and due to what I think is carberation "issues" I am thinking of getting the same Motorcraft carb. So I will be paying very close attention to what is said in this thread. (carberation issues= black smoke, low gas milage, stalling , sputtering, ect.)
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Old 06-22-2011, 06:04 PM   #15
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Originally Posted by mogarber View Post
So I have a '90 I6. I replaced the Carter Carb with a Motorcraft 2100 that I bought off ebay. Engine starts fairly easy but I seem to be getting less than 5 mpg. The engine is just burning through gas. It also accelerates poorly especially if I am in a high gear and getting up to speed (ie if I was in 3rd going 35mph, slow down to 10mph for traffic but stayed in 3rd then accelerated back to 35mph). Overall, I think the engine needs to be fine tuned so I will appriciate any help. I also took it to a few mechanics but no one wants to work on carbs. They discriminate - only work on fuel injected now. Anyone know a good mechanic that can work on carbs in the Chicago Land area? Thanks for any help!
you really should take mr. sinister's advice and check the plugs. This will show you alot more than you think. You can google reading spark plugs and see pictures of what it all means. Also, just out of curiosity, has the vehicle been nuttered? Don't remember if you said you had the 4.0 or the 4.2, but depending on your areas emission laws, a nutter bypass may help you and you also may not be able to adjust timing depending on which engine.
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Old 06-22-2011, 06:06 PM   #16
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90 should be a 4.2. i believe 91 was the first year for the 4.0
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Old 06-22-2011, 06:16 PM   #17
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Thanks, I think you are right, couldn't remember if it was 90 or 91. Really didn't figure someone replaced fi with carb, but it has happened, lol. So, Op, as long as you don't have emissions to pass, you may want to look into the nutter as there are still things being controlled by the computer even after your mc install.
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Old 06-22-2011, 06:19 PM   #18
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Timing will be checked this weekend (or sooner if I can get to the mechanic). Carb was brand new so don't think it needs a rebuild kit. Could be masking a bigger issue but not sure where to start looking. Hopefully timing is off and that will be an easy fix.
Pick up a timing light, it is a great thing to have. Hard to see paying a mechanic every time you want to check timing, big money saver being able to do that yourself.
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Old 06-22-2011, 06:21 PM   #19
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I'd agree with the above. If you don't have to get emissions inspections (or general inspections that check for emissions equipment), I'd consider ripping all the smog equipment off of your Jeep. A clogged convertor could also be causing your issue. Yank the convertor, smog pump, egr, all of it.
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Old 06-22-2011, 07:40 PM   #20
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I don't mean to hijack your post, but I'm having a similar problem

I just did the Motorcraft swap aswell. The first trip around the block the Jeep bucked and coughed like crazy! She'll rev up to 3500rpm with no probs sitting in neutral in the driveway all day. I 've only got PCV, Break and vac advance hooked up for vac all with new lines. PCV to the large port on the back of the carb, break boost stayed in the same port and vac advanc in the port beside it.

Hopefully you guys could help both of us out with your advice. Thanks all, and sorry for butting in mogarber.
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Old 06-22-2011, 07:49 PM   #21
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the vacuum advance needs to be plugged into a timed vacuum port (not full manifold vacuum, like brakes or pcv). i'm not real familiar with the 2100, so i'd say verify the port you have it connected to is ported vacuum.
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Old 06-22-2011, 11:57 PM   #22
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This is actually fairly common after initially swapping on the motorcraft carb. Its an easy carb to tune, but anytime you start moving anything under the hood of a 21+ yr old vehicle, things break.

Start by setting your timing to *8 BTDC with the vacuum advance line plugged at the distributor.

Second, ensure the port you have the vacuum advance plugged into is ported vacuum (no vacuum at idle, steadily increases as RPMs increase). This should be the vacuum port on the passenger side of the carb under the choke.

Next, ensure your choke is functioning properly. Make sure it closes fully and is FULLY AND FIRMLY open after being on for a few moments.

At this point it should atleast be idling. Now you need to check for vacuum leaks. This is where the statement about moving things under the hood of a 21 yr old jeep comes in. Any number of vacuum lines could be broken. Check everything and make sure anything not being used is plugged. Also check the vacuum port on the rear portion of the intake manifold, this one gets broken frequently when installing the carb. Use carb cleaner or something similar and spray all the vacuum lines, the base of the carb (where the carb bolts to the intake), the throttle shaft of the carb, and where the intake manifold bolts to the head. (The latter isnt likely, but you may as well check, I found a leak here at this point in the process.) If you have any leaks, fix them. If the leak is between the intake and carb (either between intake and adapter or adapter and carb), you can buy fuel-proof RTV and use that on the gaskets to fix the leak
(this is what I ended up doing).

Once all the leaks have been found and fixed (I'm sure you'll have atleast one), get a vacuum gauge. The cheap one for $15 from Harbor Freight works perfect. Connect it to a manifold (full vacuum at idle, decreases as RPMs increase) vacuum source (the large port for the PCV hose works well). The vacuum gauge needs to be connected ON the carb, or as close to it as possible. The farther from the carb, the farther off your reading will be. When idling your vacuum should be between 14 and 26 depending on the condition of your engine and the altitude your at. On the front of the carb, on the bottom, are two screws. Start with either one of them. While the jeep is idling slowly turn it in, before it seats, the jeep should stall. If it does, thats a good thing. GENTLY seat the needle (turn the screw in until you get light resistance) then turn it out 3 full turns. Do the exact same with the other screw then start the jeep.

Now to tune it:
Watch your vacuum gauge closely throughout. Start with either screw and turn it in half a turn then pause and watch the needle on your vacuum gauge. If the vacuum increases, turn the other screw in half a turn. The vacuum should increase further. If vacuum decreases as you turn the screw in, then back it out half a turn back to where it was, then back it out another half a turn. What your looking for is PEAK vacuum. The two screws need to be turned together so they're the same number of turns out. Eventually your going to need to start turning the screws in 1/4 turn increments. Once you hit peak vacuum (eventually you'll reach the point where turning the screws has no effect on the vacuum, this is why you need to wait 15 to 30 seconds between each 1/4 turn), adjust the screws so your idle is about 1in under peak.

Running 1in HG UNDER peak vacuum is the key.

Once you've got that done, try driving the jeep. It should be running MUCH better assuming the carburetor was the initial problem. The vacuum gauge can also help diagnose whats going on, so if its bouncing or slowly drops in vacuum, et cetera be sure to let us know.

Also, keep in mind that even though its running well, that carb wasnt designed for that motor, so it's likely not set up properly for this application. You may have issues with it flooding or stalling on hills / hard acceleration. If you do, we'll have to adjust the float settings and such.

Start with that, and post back with your results.
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Old 06-23-2011, 08:05 AM   #23
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Wow! Thanks everyone for the help and suggestions. Sunday is "car work day" so will be attempting a lot of this - will report back with what we find.
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Old 06-23-2011, 08:27 AM   #24
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What tslroper said, but cap the vacuum port on the CARB, not the distributor. The carb is the vacuum source, and it pulls against a diaphragm inside the distributor to advance timing. The distributor does not make it's own vacuum. Capping it off does nothing, but leaving the port on the carb open causes a vacuum leak.
You set timing with the vacuum advance port on the carb plugged so you are reading pure mechanical base timing. This is what is controlled by the weights and springs inside the distributor. The vacuum advance is load-based, that's why it uses vacuum to control it.
As far as base timing, try 10* first. You want the most mechanical advance you can get without pinging and without making the starter labor. Modern fuel is harder to burn, so a little more advance helps greatly with idle and performance. You can probably get away with 10-12* on 87 octane without pinging.

Once you get it all sorted out and you're ready for the next step, I can tell you all about how you want to run full manifold vacuum over ported vacuum. Emissions crap necessitates you run ported vacuum for now, but if you do decide to ditch it all, I can help you really wake up that little 4.2L.
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Old 06-23-2011, 10:04 AM   #25
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Yeah, I probably should have clarified. I pull the line off the distributer and stick a screwdriver or golf tee in the end of the vacuum line.
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Old 06-29-2011, 09:39 AM   #26
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Sorry everyone that I haven't posted sooner. Had a busy weekend and even busier start of the week.

I wanted to fill you in on what we have done so far.

Vacuum - tested all vacuum with a gauge and looks to be right - no issues that we found.

Timing - was way off. Adjusted the timing to about 10 BTDC at 1600 RPM.

We kind of ran out of time after this so will be following more of your advice after the holiday weekend. Didn't notice much of a difference in driving it yet but only put on about 5 miles. And for the same reason, haven't noticed any gas mileage difference yet. Will keep working on it and filling you in.
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Old 06-29-2011, 09:43 AM   #27
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can't wait to hear how it turns out.
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Old 06-30-2011, 12:38 PM   #28
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Question carburator problems

i have an 89 wrangler with the 4.2 i have just purchased and it had a after market carb on it. When driving down the road and going up little hills it starts spuddering and i can push in the clutch and gas it it will clear up for a few minutes. I have taken it to a mechanic and have replaced plugs, fuel pump, and rebuilt the carburator and still not any better. The mechanic told me the carburator was to big for the motor. It is a 2 barrel carb. Is this possible or is it maybe something else ...please help getting real frustrated
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Old 06-30-2011, 02:00 PM   #29
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sorry to be on your post im still learning the forum
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Old 07-01-2011, 04:20 PM   #30
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Thanks, I think you are right, couldn't remember if it was 90 or 91. Really didn't figure someone replaced fi with carb, but it has happened, lol. So, Op, as long as you don't have emissions to pass, you may want to look into the nutter as there are still things being controlled by the computer even after your mc install.
I am having the same problem but i know for a fact that it is the vacuum system no im where i live so some hack job to a saw to the vacuum lines and i don't know where to go from there would like to rip all emissions out and start from scratch

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