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Old 04-27-2013, 12:23 PM   #1
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need carb tuning experts on a 90 4.2 yj

rebuilt 4.2 inline 6 on a 90 yj. HEI distributor 8.5mm wires autolite plugs. Offenhauser intake and MC2100 carb.

Im having trouble tuning and want to know the best way to tune it. it's timed at 8degrees TDC without vacum plugged in and i can get it to idle great but driving it the jeep act like it misfires. more like it sputtering but idling fine.

I just want to know the best way step by step to get it tuned right. I've turned fuel/air mixture screws all the way in then backed them out 2 full turns and worked from there and i've had them turned really far out but it's hard to get them just right and wanted to know if maybe there could be something wrong or if im just not doing it right.

It's getting enough fuel for sure. New fuel pump and filter was installed and i can see fuel spraying in the carb.

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Old 04-27-2013, 01:13 PM   #2
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are you getting correct voltage to the HEI distribtor?

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Old 04-27-2013, 02:08 PM   #3
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Tune it by ear....warm up the jeep...set idle ....then turn your air /fuel screws out one at a time until the motor starts to rev up...then turn them about an 1/8 turn in....then reset your idle....then you can adjust your distributer until it sounds and runs good...hit the throttle a couple times to make sure it will run good while stomping the gas...and then road test...if its not good just make small adjustments advancing and retarding your distributer until its perfect .....another thing to check would be your valve adjustment ....they can cause it to run like crap as well
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Old 04-27-2013, 02:12 PM   #4
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I work on harleys for a living and never use a timing light....its about a waste of time unless your building a race engine where your making 1° timing adjustments for max power....every vehicle is different so I prefer tuning by ear to find that engines sweet spot
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Old 04-27-2013, 06:33 PM   #5
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IF you are satisfied that your ignition timing is OK, then:

The most basic parameter that needs to be set first before you tune is fuel level. If the level is off, then you'll have a very difficult time getting it to run correctly. Pull the top off by 4 screws & measure from the tip of the float to the top edge of carb body casting (while the bowl is full of fuel)...it should measure right around 7/16".

If this is known to be good, then you have to figure out if you have a rich, or a lean condition. Pull a spark plug or two & see what they show.

If you have decent idle, the idle mixture screws are fine & won't do much else.

Does the poor driving occur under hard acceleration....or cruise?

Hard acceleration troubles indicate your acc pump, power valve or main jets may be off.

Cruise troubles point to fuel level & to a lesser extent, main jet size.
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Old 04-27-2013, 09:27 PM   #6
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check you firing order again, I set the mix screws with a vacuum gauge, turn in and out to most Inches of vacuum, as far as timing goes {with age things change, get wear and slack } mine runs best no vacuum 12* BTDC.

A new tight motor should time 8 to 10. Hope this helps
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Old 04-28-2013, 11:05 AM   #7
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Quote:
Originally Posted by bigtoysforbigboys View Post
Tune it by ear....warm up the jeep...set idle ....then turn your air /fuel screws out one at a time until the motor starts to rev up...then turn them about an 1/8 turn in....then reset your idle....then you can adjust your distributer until it sounds and runs good...hit the throttle a couple times to make sure it will run good while stomping the gas...and then road test...if its not good just make small adjustments advancing and retarding your distributer until its perfect .....another thing to check would be your valve adjustment ....they can cause it to run like crap as well
I work on harleys for a living and never use a timing light....its about a waste of time unless your building a race engine where your making 1° timing adjustments for max power....every vehicle is different so I prefer tuning by ear to find that engines sweet spot

Well i can say that was great advise on the timing and tuning. I have been able to get the jeep to rev up great and idle great but it's the in the middle throttle is jumpy and poping.

Any other advise on that? just keep messing with it?
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Old 04-28-2013, 11:07 AM   #8
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IF you are satisfied that your ignition timing is OK, then:

The most basic parameter that needs to be set first before you tune is fuel level. If the level is off, then you'll have a very difficult time getting it to run correctly. Pull the top off by 4 screws & measure from the tip of the float to the top edge of carb body casting (while the bowl is full of fuel)...it should measure right around 7/16".

If this is known to be good, then you have to figure out if you have a rich, or a lean condition. Pull a spark plug or two & see what they show.

If you have decent idle, the idle mixture screws are fine & won't do much else.

Does the poor driving occur under hard acceleration....or cruise?

Hard acceleration troubles indicate your acc pump, power valve or main jets may be off.

Cruise troubles point to fuel level & to a lesser extent, main jet size.

I haven't been able to really cruise around with it. IT revs up great and idle great but the in the middle throttle response is horrible. Like just giving it little bit of gas and it pops and jumps around.
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Old 04-28-2013, 11:08 AM   #9
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check you firing order again, I set the mix screws with a vacuum gauge, turn in and out to most Inches of vacuum, as far as timing goes {with age things change, get wear and slack } mine runs best no vacuum 12* BTDC.

A new tight motor should time 8 to 10. Hope this helps

Firing order is right. I have no idea how to do a vacum gauge but i don't even have one anyway.
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Old 04-28-2013, 11:10 AM   #10
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Does anyone know if the MC2100 even works right with the Offenhauser intake? Also the engine has been bored .30 over.
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Old 04-28-2013, 11:25 AM   #11
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Check your plugs and see how they look ....are you sure its popping and not cutting out like its starving for fuel? ....my guess would be a clogged jet or float height is incorrect ....that carb should work just fine...any carb will really if you know how to tune them...heck im running a 750 cfm edelbrock on my 258 and it runs like a dream
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Old 04-28-2013, 11:30 AM   #12
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Bore .030" don't mean a thing, when someone does use a vac. gauge there will get the best fuel mix, or should i say best power and best MPG , they are cheap and the way to tune the low side of your carb. so it runs best at low and mid rage RPMs
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Old 04-28-2013, 11:49 AM   #13
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i can check the plugs no problem. and i'll look into getting a vacum gauge.
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Old 04-28-2013, 11:51 AM   #14
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Vacuum gauges are only good for synchronizing multiple carbs to get all cylinders to run the same speed ....it's not gonna help achieve more fuel mileage ....and the fuel/air screw only works at idle ....so if its a problem off idle then it is jet or float related
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Old 04-28-2013, 12:06 PM   #15
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Quote:
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Check your plugs and see how they look ....are you sure its popping and not cutting out like its starving for fuel? ....my guess would be a clogged jet or float height is incorrect ....that carb should work just fine...any carb will really if you know how to tune them...heck im running a 750 cfm edelbrock on my 258 and it runs like a dream
650 edelbrock on a 232 here runs great, but the secondaries almost never open...

beachcruiser is dead on... the mixture screws become completely irrelevant once the throttle is opened, so "tuning" using adjustments is not going to work in your case.

Do you have vacuum advance on your HEI? If the advance is "maxing out" too soon, that would also cause your mid range throttle to suck... Using the wrong vacuum port to supply the distributor advance is a LOT more common than a clogged jet or internal carb issue...
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Old 04-28-2013, 12:09 PM   #16
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Vacuum gauges are only good for synchronizing multiple carbs to get all cylinders to run the same speed ....it's not gonna help achieve more fuel mileage ....and the fuel/air screw only works at idle ....so if its a problem off idle then it is jet or float related

Ok well i think i will definitely look into at least getting a vacum gauge and seeing what it reads.



"The most basic parameter that needs to be set first before you tune is fuel level. If the level is off, then you'll have a very difficult time getting it to run correctly. Pull the top off by 4 screws & measure from the tip of the float to the top edge of carb body casting (while the bowl is full of fuel)...it should measure right around 7/16".

And im guessing this procedure is how i check float adjustments?? Again im not a carb expert at all and i really appreciate the help guys.!

And i assuming that the jets are fine cause this carb is still new. I only got to use this carb right before the engine blew up.

But i could be wrong of course.
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Old 04-28-2013, 12:35 PM   #17
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Ok well i think i will definitely look into at least getting a vacum gauge and seeing what it reads.



"The most basic parameter that needs to be set first before you tune is fuel level. If the level is off, then you'll have a very difficult time getting it to run correctly. Pull the top off by 4 screws & measure from the tip of the float to the top edge of carb body casting (while the bowl is full of fuel)...it should measure right around 7/16".

And im guessing this procedure is how i check float adjustments?? Again im not a carb expert at all and i really appreciate the help guys.!

And i assuming that the jets are fine cause this carb is still new. I only got to use this carb right before the engine blew up.

But i could be wrong of course.

Yes, fuel level/float level are basically the same thing.

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I haven't been able to really cruise around with it. IT revs up great and idle great but the in the middle throttle response is horrible. Like just giving it little bit of gas and it pops and jumps around.
Alright....that tells you that your idle circuit & acc pump are fine.

Assuming that your ignition timing is correct & you have no vacuum leaks & your fuel level is fine (make SURE this is all true before going any further)....then you may be looking at a lean condition caused by too small of a main jet. This will make your plugs look very light tan/white.

Is that what your plugs read?

When you check your fuel level...check & see what your main jets are.

I'm guessing that this is still a timing/fuel level issue....but our engines may need anywhere between a 47-52 main jet, depending on altitude, mods & the tune of any particular engine. Just because carb is 'new', doesn't mean it correctly tuned for YOUR engine.
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Old 04-28-2013, 01:31 PM   #18
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Well, you know, the more I read on this, and how your car is acting, I don't think it's a tune thing at all.
It sounds a lot like a blown Power Valve! But I may be wrong! That or you have so plug-age in some of the internal ports that's keeping it from metering right.
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Old 04-29-2013, 11:11 AM   #19
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well i do appreciate all the help!! i will let you guys know in a couple of days what the problem is. i am getting some hands on help with this so i hope to get it figured out.
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Old 05-06-2013, 10:19 AM   #20
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thanks alot for the help. after really looking at the carb with some good mechanics at a shop we have found that it was just the new intake manifold didn't seel up very well and so we got all new bolts and washers and another new gasket and OMG what a difference and it was so easy to tune up and even without a vacum gauge we were able to tune but we did use a gauge anyway and man this jeep is a beast.

I will make a thread on the build up and post a vid. Thanks again.
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Old 05-06-2013, 10:54 AM   #21
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Good deal, N2!

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