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Cashcrazy 11-26-2013 01:13 PM

MC2100 power valve size on 258?
I have touched on this in other posts but have been chasing several problems at once and believe my power valve is the last big issue. I rebuilt my Mc2100 with a GP Sorensen kit that had an unmarked power valve. I have been doing a lot of fine tuning and it runs very well with the exception of the strong fuel smell at idle. I believe the power valve in the kit is not the appropriate size for a stock 258. Just curious what size power valves have worked for others on the same setup. Thanks for any info.

capt205 11-26-2013 01:16 PM

Went from 8.5 to 6.5 on mine, at sea level in FL.

Cashcrazy 11-26-2013 02:10 PM

I am at 550ft so it should be close. How was the difference and what size jets are you running?

capt205 11-26-2013 05:33 PM

47 jets. The pv change made a noticeable difference. Landed on that combo by reading many posts here and elsewhere.

Try it, see if it works for you.

Matt1981CJ7 11-26-2013 06:03 PM

I'm no expert with the MC 2100, but I'm not sure the power valve has much affect on idle quality.

Doesn't the PV open in off-idle situations when there is a load?

Someone please correct me, if Im wrong.


capt205 11-26-2013 06:31 PM

It could cause problems if it's blown

drespecki 11-26-2013 06:34 PM

I am at 2500 feet and for now run a 8.5 pv and 46 jets in my motorcraft 2150... I wish there was a database somewhere that gave the general guidelines at to what is best at what altitudes... That would be very handy!

86cj74.2L 11-26-2013 07:18 PM

The power valve if not bad will be closed at idle. It opens at the vacuum number to richen up the fuel curve under load. So if you have 18" of manifold vacuum............richness is not from PV.

Lack of a CAT or retarded timing at idle will put the skunk in the tail pipe. You on ported or mani for your vacuum advance?

Generally you go half of manifold vacuum at idle to start. Though ford and their better ideas also have a two stage power valve.

jammer1 11-26-2013 09:08 PM

How do the plugs look? If it's not a emissions carb it will have more of a gas's smell. If plugs have a good color and aren't loading up it may be normal.

Cashcrazy 11-26-2013 09:56 PM

The PO removed the CAT. Running manifold with 8 degrees initial advance. The fumes will saturate clothing and wilt anything green. It's strong even condering there is no CAT. Plugs are a nice dark brown.

jammer1 11-26-2013 10:09 PM

Check the power valve to make sure the diaphragm didn't rupture. If ok check the accelerator pump to make sure diaphragm didn't tear. Can you screw in the idle screws without dropping the idle speed. Are the jets the same as when it ran correctly?

Cashcrazy 11-26-2013 10:36 PM

It runs great other than the stink at idle. I have only ran #48 jets, but it's my understanding that jet size doesn't effect the idle. The accel pump is good. If I start to screw in the idle screws the vacuum guage will not be steady and it will begin to run bad.

86cj74.2L 11-26-2013 11:35 PM

2 Attachment(s)
Dark plugs are too rich.

The single is mine. The four together may be matts.

jammer1 11-27-2013 06:55 AM

I've read in a holley book that if jet sizes can effect idle mixture but only if the sizes are way off base. If the jeep ran ok before and something just happened (that's why I asked, incase you just changed jets), I was just trying to rule out. If you screw in the mixture and rpms start dropping and runs irratic your not to far off on the mixture. Is it a new carb to the jeep. If I remember right (god help us), they made the 2100 in different sizes. If it's a new carb to the jeep could it be a higher cfm carb, which could run a richer idle mixture.

swatson454 11-27-2013 08:39 AM

Will the engine die if you screw the mixture screws all the way in? What idle rpm are you running?


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