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Unread 08-23-2013, 12:14 AM   #1
drespecki
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Motorcraft power valve and jet size most are running?

Hi...Need some info on who is running what with their motorcraft 2100/2150 carb on the 258. I am at 2500 feet and run up from 2500 to 8000 once in a while but am at 2500 85% of the time... What is the best power valve size and jet size for my area? (Tucson AZ)

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Unread 09-17-2013, 12:57 PM   #2
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Anyone?
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Unread 09-18-2013, 11:23 PM   #3
drespecki
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Have the carb off and see that I have #47 jets and a 7.5 power valve.. Should I try #46 jets and a 6.5 power valve at my altitude of 2500+ feet?
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Unread 09-19-2013, 12:43 PM   #4
Jon In Tucson
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Not on a Jeep, but about a million years ago, my son and I had a '51 Chevy 1/2 ton with a newer GM 235ci. The carb was a 2 bbl Holley, and I believe the jets were 46's. Too much water under the bridge to remember the power valve. How do the spark plugs look with the combo you're running now? Do you need to pass emissions? Or just wanting to improve performance? No matter what you do tuning wise for 2500', up on the mountain it will run rich. I would tune towards where you do the majority of your driving, in this case the desert floor. God bless.
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Unread 09-19-2013, 03:48 PM   #5
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Are you having any problem with it? I'm running 47's in one Jeep, 46's in the other, and honestly I can't really tell a difference. I'm around Memphis TN, whatever elevation that is........
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Unread 09-19-2013, 08:26 PM   #6
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What venturi? Hopefully 1.08. What's your vacuum pulling? You want a power valve which is rated for half of the inches of vacuum at sea level I believe- mine pulls 21" of vacuum so I run a 10. For the jets, start at 47 and work down as needed.
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Unread 09-20-2013, 12:15 AM   #7
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Back for a few minutes....Seems that I was around 17 or 18 on the manifold vacuum but did not drive it to get the true reading (think there was something else I needed to do to get the proper PV size?)..Just pulled out the 47's and am putting in 46's and a 6.5 pv to see how it responds. Might opt for a Weber 38/38 or a 32/36. If I was rolling in dough, the Howell FI would be the ticket but alas, I am a bit shy of that price range.
Am down right now, rolling monstaliner this week and she is all apart for a few days.
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Unread 09-20-2013, 09:00 AM   #8
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Quote:
Originally Posted by CrazyHorse6 View Post
.....You want a power valve which is rated for half of the inches of vacuum.....
That's a good starting point but to really nail down which PV you should have you need to monitor your A/F ratio with a wide band O2 sensor. Most people don't but that is the best way to really dial it in.
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Unread 09-20-2013, 12:36 PM   #9
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Quote:
Originally Posted by drespecki View Post
Back for a few minutes....Seems that I was around 17 or 18 on the manifold vacuum but did not drive it to get the true reading (think there was something else I needed to do to get the proper PV size?)..Just pulled out the 47's and am putting in 46's and a 6.5 pv to see how it responds. Might opt for a Weber 38/38 or a 32/36. If I was rolling in dough, the Howell FI would be the ticket but alas, I am a bit shy of that price range.
Am down right now, rolling monstaliner this week and she is all apart for a few days.
I've had a 32/36, and like the MC2100 much better. The 32/36 did perform better than the MC2100, but not by much, but the main reason I like the MC is once you get it dialed in, it stays dialed in for a long time.

I had to monkey with my Weber ALL the time to keep it running right. With the MC, I bolted it on, spent a little time on the choke and mix adjustment screws, and haven't had to mess with it for a lonng time.
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Unread 09-30-2013, 11:09 AM   #10
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Well, the monstaliner will be finished this week, need to put the 2150 back on and guess I need to pick a power valve..... 6.5 7.5 or 8.5
Anymore opinions on which one to go with on my worn out old 258?
Thanks for all the input, I am trying to absorb it!
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Unread 09-30-2013, 12:15 PM   #11
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You will have to put on the mc2100, and find out what your PTC vacuum is.

Find a flat, long road and hook up your vacuum gauge with a long enough line to get it in the cab with you. Hold a steady speed somewhere between 45-55mph, and read what vacuum you are at. This is your Part Throttle Cruise vacuum. Your power valve should be about 2 below this number.

If I remember correctly, my PTC was 10, so I went with a 7.5 power valve.
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Unread 09-30-2013, 12:19 PM   #12
drespecki
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What was your in park idle vacuum? (i have an automatic) I need a rough gauge to go by to put one in so I can go down the road and do that vacuum test... Mine was about 18hg in park at idle..
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Unread 09-30-2013, 02:46 PM   #13
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Mine was about 18.5 in neutral (standard).
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Unread 10-01-2013, 07:33 PM   #14
drespecki
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Would regular manifold vacuum be okay to do the test with or do I need carb baseplate vacuum closer to where the power valve is for a more accurate reading?
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Unread 10-02-2013, 08:48 AM   #15
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I went with regular manifold vacuum.

Not because it is the best reading, but because I didn't know if there would be a difference in the reading closer to the carb base plate.
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