Power valve

I have noticed for awhile now that my car runs excessively rich when I first crank it up. In my mind it was "big cam" and "big carb" is going to cause this until it warms up. In reality however it is still a bit on the rich side for instance idling in the garage after being drove enough for it to warm up. The 4 idle mixture screws on my Holley HP 750cfm DP are only backed out ¾ of a turn each. My spark plugs are black and sooty and never burn as clean as I would like them. So what I have is....

Ancient Mopar cam .474/.474 280/280
10:7 compression 30 over 360
Performer intake
1" open spacer
TTi headers
Stock mopar distributor with Accel 300+ ignition (rev limiter)


The motor runs fine, has great torque, and will burn the tires.... but she is a little rich. I don't ever see black smoke when I get on it or it idling however. Just that eye burning fumes.

Now the other day I was browsing around and came across a thread (that I'm subscribed to) and read something interesting. A fellow member was running the same cam. I moved the total timing to 36 so now vacuum at idle is 8hg and climbs ever so slightly to 9 This is a manual transmission car.
Go here and scroll to post 46 and you will understand a bit more clearly.

http://www.forabodiesonly.com/mopar/showthread.php?t=81499&highlight=power+valve&page=2

Now after reading that one post I have ordered 3.5 hg power valves and going to try that. I will tinker with the springs later in the distributor. Also going to try a 1" 4 hole spacer. I have always wanted to try the 4 hole.

So I believe I am heading in the right direction with the power valves since the stock power valves are 6.5

Other than torque what else would I gain by going with the 4 hole instead of the open? I'm thinking it will narrow down the circuity of the carb if you will.
thanks,

With 36 total what the initial timing?

Probably going to want in the 15-20 range. If it's less you have to open the butterflies more to get it to run at idle in gear.

What happens when you ease in the idle screws? Did you actually adjust them for "best idle" or what?

Don't forget that power valves can "seep" and actually leak through the diaphrams. I used to have this trouble a lot--sometimes on others cars, I used to "kit" carbs in the moonlight. I've found more than one power valve bad, right out of the box, and these were good, brand name kits like Holley, Blue Streak, and Echlin.

Make sure the float isn't too high. Can you see anything dripping out of the boosters at idle?

Ancient 70 340 here, same purple stripe. Initial 18 degrees, total 36.
Rebuilt 780, crap; replace p/v with a 4.5 (vac at 10 at idle). Nope, not responsive enough. Borrowed a 650 DP (auto tranny, don't know about this, not on the road yet). Response good, but idle rich.Put in a 3.5 p/v, and seems good.

Sounds to me like you need basic tuning. That cam has a ton of overlap so you'll always have the fuel smell at idle and it has litle to do with the actual burn. For a base setup I'd say go with 38° total timing w/o the vacuum figured in. Initial 16°-18°. Centrifical should be "all in" around 2800. Once the timing is close, the carb.. Set base idle to the best lean idle at around 850rpm. When that's done, take a vacuum reading in gear. The power valves should be 2-3 numbers lower than that rating. It should get much more crisp as you get it leaned out.

crackedback said:

With 36 total what the initial timing?

Probably going to want in the 15-20 range. If it's less you have to open the butterflies more to get it to run at idle in gear.

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I think it is 16 or 17 with manual transmission.

67Dart273 said:

What happens when you ease in the idle screws?

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The motor will start wanting to shut down

67Dart273 said:

Did you actually adjust them for "best idle" or what?

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No not best idle but achieve the highest vacuum reading.

67Dart273 said:

I've found more than one power valve bad, right out of the box, and these were good, brand name kits like Holley, Blue Streak, and Echlin.

Make sure the float isn't too high. Can you see anything dripping out of the boosters at idle?

Click to expand...

What is the proper method of checking PVs? I always hold them to my mouth and see if there is any air getting through.

Float level is correct with no dripping from boosters.

green1 said:

Ancient 70 340 here, same purple stripe. Initial 18 degrees, total 36.
Rebuilt 780, crap; replace p/v with a 4.5 (vac at 10 at idle). Nope, not responsive enough. Borrowed a 650 DP (auto tranny, don't know about this, not on the road yet). Response good, but idle rich.Put in a 3.5 p/v, and seems good.

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That's what I hope to achieve.

moper said:

Sounds to me like you need basic tuning. That cam has a ton of overlap so you'll always have the fuel smell at idle and it has litle to do with the actual burn. For a base setup I'd say go with 38° total timing w/o the vacuum figured in. Initial 16°-18°. Centrifical should be "all in" around 2800. Once the timing is close, the carb.. Set base idle to the best lean idle at around 850rpm. When that's done, take a vacuum reading in gear. The power valves should be 2-3 numbers lower than that rating. It should get much more crisp as you get it leaned out.

Click to expand...

Yes sir you are correct. I will get the 4 hole spacer on there and tune as you described. Except I have a manual transmission so I can't put it in gear. Im really thinking that the 3.5 PVs is going to help... I hope I get to use them after taking the reading.


Thanks everyone!

Looks like the thread is well underway and your getting good info. Follow mopers advice. And absoultely understand the cam has enuff overlap that the tail pipe will have a rich smell almost no matter what you do. But the eye burning, Yaow!

What jet size's are in there now? If for example, there 88's, try 84's.

You also need to have the vacuum gauge hooked up to the full time vacuum port and the distributor hose pulled and the port pluged.

The idle screw seem to be in very tight @ 3/4 of a turn. I see mostly 1-1/2 to 2-1/2 turns out of them outwards.

From the 4 corner idle carbs I have worked with, 3/4 turns out all the way around is not uncommon.
My 440 with a single plane and 750 DP, is about there or less and runs great.
And as stated, certain cams will most always give a stinky rich idle.

Good point on the turns outward. There is more fuel available from 2 more corners. Just seems a bit tight though.

3/4 out seems that there is another problem, like too rich from the primary circuit. Acell pump adjusted correct? with gap?

Had a similar problem with mine. So rich at idle your eyes would burn in the garage.
Some one here had me check vacuum at idle, I have a very lumpy cam, I only had 3 inches of vacuum. I put in a 1.5 power valve and was able to finally get adjustments out of all for corners of my carb.
Have you put a vacuum gauge on it?

kocuda said:

Had a similar problem with mine. So rich at idle your eyes would burn in the garage.
Some one here had me check vacuum at idle, I have a very lumpy cam, I only had 3 inches of vacuum. I put in a 1.5 power valve and was able to finally get adjustments out of all for corners of my carb.
Have you put a vacuum gauge on it?

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Yes its around 8 right .... I got my 3.5 power valves yesterday from Summit. I'm going to remove the carb today and just go ahead and install them then give it a try. Right now the car has 6.5 PVs

rumblefish360 said:

Good point on the turns outward. There is more fuel available from 2 more corners. Just seems a bit tight though.

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FYI, whether you have two or four corner idle adjustments there is still a secondary idle circuit, its just that on a two corner carb you can't adjust the secondary side.

FWIW, the power valve does not have an impact on the idle circuit. Changing it will not fix a overly rich idle.

I would be looking at more initial timing and ensuring that you don't have the transfer slots uncovered.

Well, well, well... not to good.

Installed 3.5 PVs. Installed 4 hole spacer. Cranked the motor, let it get hot, exhaust pipes are aimed out the garage door, not long eyes are burning.

Initial 14
All in 38


750 Holley HP dp
Accel squirts 31
Front and rear jets are 73
Outer bleeds 78
Inner bleeds 36

Idle RPM 900

Float levels are a shade below the screw hole (I can see the fuel)

Fuel pressure is 7.5 at idle w/ stock FACTORY type fuel pump???

All four idle mixture screws are turned out a shade over ¾ turn. Any less than ¾ when I ease the clutch out it wants to shut off.

Forgot to note vacuum but all of this is where Im at right now.


In the link above the fellow member noted that the "dragster" sound is a result of the 6.5 PVs but I still have the same idle characteristics with the 3.5 .......... right now Im taking a break from it.... hadn't made much progress.

Don't drive it, but advance to 18 degrees initial, and play with the carb.