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Unread 11-19-2012, 10:33 AM   #1
CJ51973
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Motorcraft tune

I have been running my Motorcraft for about a 2 years. It runs well and gets good mileage. The only remaining issue is hesistation off idle. It only does this in hi range, choke off and not every time, maybe 1/2 the time (makes me think I am close). I moved the accelerator pump linkage to the top hole for more pump/gas and it helped but did not eliminate it. I have wasted a couple cans of cleaner looking for vacuum leaks and never found one. I never notice it off road/low range. One thing I wondered if it has anything to do with the Offy intake (the one with water) as the Jeep had a non-functional Holley 4 barrel when I got it 3 years ago. Didn't know if the shear volume of this intake would makes some sort of difference. It would seem it would only pull into the engine what it pulled through the carb. Ideas of where to look next?

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Unread 11-19-2012, 10:51 AM   #2
Old4X
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If you are that close with the carb as far as tune, you might try adjusting the vacuum advance can. It adjusts with a 1/8th inch allen wrench in through the snout.

Start by counting the number of turns to fully seated, so you know where you started. Then try 1/2 turn at a time each way from the starting point till you see improvement. Often this will cure that little bog that just will not go away.
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Unread 11-19-2012, 11:11 AM   #3
Beachcruiser
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You might want to check the float level, too.

Ballpark for most 2100/2150s is around 7/16" down from top edge of carb body/fuel bowl to top of float at it's free end. Raising the float (smaller measurement) will make the carb run a bit richer on the lower end of the throttle range.

There are holes on the pump actuating arm that rides on the throttle shaft, too. The top hole provides the most fuel.
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Unread 11-19-2012, 01:20 PM   #4
Old4X
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Quote:
Originally Posted by Beachcruiser View Post
You might want to check the float level, too.

Ballpark for most 2100/2150s is around 7/16" down from top edge of carb body/fuel bowl to top of float at it's free end. Raising the float (smaller measurement) will make the carb run a bit richer on the lower end of the throttle range.

There are holes on the pump actuating arm that rides on the throttle shaft, too. The top hole provides the most fuel.
Also, raising the fuel pressure a few psi raises the float level slightly (reverse is also true).
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Unread 11-19-2012, 02:32 PM   #5
CJ51973
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It is when it transitions from the idle circuit to the throttle plate. That's the accelerator pumps job is I understand it correctly. I have it adjusted where the lever is just in contact with the pump at idle and its in the top hole
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Unread 11-19-2012, 02:37 PM   #6
Old4X
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Originally Posted by CJ51973 View Post
It is when it transitions from the idle circuit to the throttle plate. That's the accelerator pumps job is I understand it correctly. I have it adjusted where the lever is just in contact with the pump at idle and its in the top hole
Yes, the accel pump plays the biggest role. But as you open the throttle, your manifold pressure rises, and the vac advance starts retarding the timing. Retard it too much too fast, and your available horsepower drops (even at the same rpm). So tuning the vac advance can help. If you start pinging, you went too far with the adjustment.
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Unread 11-19-2012, 03:40 PM   #7
MaddBones702
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Don't forget to look at the power valve. It should be rated at 1/2 of manifold vacuum. It plays a big role in hesitation. Commonly a 6.5 is installed as was the case for mine. I chased bogs and hesitation until I went with a 9.5 PV. Made a big difference.
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Unread 11-19-2012, 06:50 PM   #8
rambo3489
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Subscribed. Good info guys. This sounds like a good winter project for me.
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Unread 11-20-2012, 05:31 AM   #9
CJ51973
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Quote:
Originally Posted by Old4X View Post
Yes, the accel pump plays the biggest role. But as you open the throttle, your manifold pressure rises, and the vac advance starts retarding the timing. Retard it too much too fast, and your available horsepower drops (even at the same rpm). So tuning the vac advance can help. If you start pinging, you went too far with the adjustment.
At idle there is no vacuum advanced to speak of, its running on the mechanical. How would it "retard"? I could get where it wouldn't advance with the initial vacuum drop of opening the throttle plate. I did notice on the way into work today while pulling a big hill that there was not rattle. I will try a couple more degrees of timing and see how that goes. If I remember I am at 8 now (HEI). The can isn't adjustable on my HEI.
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Unread 11-20-2012, 08:45 AM   #10
Jax89Jeeper
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Quote:
Originally Posted by MaddBones702
Don't forget to look at the power valve. It should be rated at 1/2 of manifold vacuum. It plays a big role in hesitation. Commonly a 6.5 is installed as was the case for mine. I chased bogs and hesitation until I went with a 9.5 PV. Made a big difference.
Yep... I am constantly reminding people about the PV when they complain of a hesitation under load. Mine came from the ebay guy with a 9.5 and I had to swap in a 7.5 to get things running smooth.
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Unread 11-20-2012, 09:42 AM   #11
CJ51973
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The power valve is more of a engine under load deal, not my issue. Mine is 7.5 inHG BTW. My issue is right off idle. Even if I am rolling down the road, like in a turn or something, and get back into the thottle lightly it will do it. If I fludder the pedal some it will accelerate without hesitation.
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Unread 11-20-2012, 12:32 PM   #12
Old4X
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Quote:
Originally Posted by CJ51973 View Post
At idle there is no vacuum advanced to speak of, its running on the mechanical. How would it "retard"? I could get where it wouldn't advance with the initial vacuum drop of opening the throttle plate. I did notice on the way into work today while pulling a big hill that there was not rattle. I will try a couple more degrees of timing and see how that goes. If I remember I am at 8 now (HEI). The can isn't adjustable on my HEI.

Ported vacuum is for smog setups. You should be running manifold vacuum to the dizzy.
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Unread 11-20-2012, 12:36 PM   #13
MaddBones702
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Just for kicks measure vacuum. A 7.5 PV means your total vacuum is 15. I doubt that's the case. I think you'll be surprised at how much of an impact that PV has
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Unread 11-20-2012, 03:41 PM   #14
CJ51973
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It's zero at idle, jumps to 7 around 1000 rpm, and will hit 15 at 3000 rpm. But that's sitting in the driveway so it really doesn't count anyway. Load is the deal there but the engine is capable of making 15 so I am in the middle. This is Venturi/carb, not manifold.
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Unread 11-20-2012, 04:00 PM   #15
CJ51973
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Old 4x4, I stand corrected (if this link is correct, probably is). Opposite of everything I had read so far. I am old but never got a good understanding of distributors. I will try switching to manifold and see what it does. Probably Friday so I will update.

http://www.jeepforum.com/forum/f12/m...vacuum-994651/
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