Power Valve Issue?
 

I have a drivability problem. I have used an O2 sensor to tune in my 4150HP Ultra (950 cfm) on my FE (bored and stroked to 527). The problem is at just touching the gas to keep speed at 1800-2500 rpm, it sounds and feels like a 2 cylinder John Deere tractor, but in the same rpm range giving it 1/4 thorttle or more, it sounds and drives fine, but also accelarates quickly out of that rpm range. I even bogged it down to 1200 rpm up hill in 5th and at that range with minimal throttle, it does that "flutter" but with 1/4 throttle, pulls right out. I am thhnking that this flutter is the power valve, because this is what I was told by the Holley guy on the tech line.

changes to carb are: power valves were changed to 8.5 (stock was 6.5). this is contrary to what I have been told of half the idle vacuum, although, I have not measured the vacuum, but I made this moved based on the mixture being perfect across the board, except at 3/4 throttle, where it was way lean. So it made sense that I wanted the power valve to open sooner. I was also told this by the Holley guy on the tech line. WOT was perfect mixutre, so changing jets would have made the car rish all the time. (perfect to my engine builder is 12.5-13.5 a/f). In that same area of just touching it, it was around 11 - 11.5, but was never affected by jet size or power valve change.

The cam is long duration, lots of overlap and 633 lift, so I assume it would have low vacuum at idle.

The interesting thing is that I did not notice this funny running when I first put in the 8.5 power valve, but then I was concentrating on 3/4 to WOT and was not just cruising.

I did keep finding little pieces of Holley blue gasket inside the bowl each time i opened it to change jets. I cleaned them out, but I suppose something in the metering block could also be clogged? I changed to a diffeent brand gasket and it no longer leaves bits behind. I will blow air though the passages and see if this does anything as well.

I ended up with jet size 75 all around. The carb came with 78.

So my question is: could this be something clogged in the metering blocks or the wrong power valve? If so how do I fix lean at 3/4 and good mixture at wot?

You're just starting to exit the idle circuit when you lightly touch the throttle.

Try this first. Open the idle air adjustment screws an additional 1/4 turn and see if the stumble goes away. Some engines just don't like to run at the highest vacuum level and require a slighly richer idle circuit to transition properly.

Also, make sure the accellerator pump arm is starting to move the same time the throttle is being cracked. Look down the bore at the squirters and make sure they start squirting the moment the throttle is cracked.

I "richened" the idle and it did not make any difference. I checked the accel pump and it is set properly with the proper "gap" at WOT and instant response any any thottle movement. The problem is not just initial "tip in" but cruising at a1800 rpm with super light throttle. I was undering if it could be sometihnh clogged or the power valve "fluttering" because it wants to open too soon. I suppose I can put the stock power valces in and see. . . ..

That's where I would go next. These carbs usually run fairly good (rich) right out of the box.

Did you make the changes prior to running it first?

Your jetting seems way lean for that engine/carb combination.


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according to the o2 sensor, it was right on, see the attached.

With a 482ci, I'm running a Pro Systems 4150 DP with mech secondaries, approx 860cfm, with jetting at 80 for primaries and 88 for secondaries. 6.5 PV on primary side, secondary PV is plugged. 50cc pump on the front and 30cc on the back.

Other than the occasional adjustment for the weather, it runs pretty well throughout the range.....

I've been down this road many times with big cam motors in Offshore boats and Cobras. If you have a big cam you have very little vaccume which is allowing the power valves to open and give your engine too much fuel. You were right on your assumption that going to 8.5 was the wrong direction. In the Fountain offshore boats I was running 2.5 and in my Cobra's with big cams 3.0 to 3.5. Drop the power valve from 6.5 to 5.0 and see if that makes a difference and down from there. They are cheap and easy to replace....good luck.

I have a Pro Systems 950 HP on my 408/W:eek::eek: ( I know, I know BIG CARB ) that,s what he built me when I gave him my spec's.
I had what sounds like the same thing you have. After trying numerous suggestions from helpful members & none solving the problem I put a lot of thought into it & concluded it was not transitioning correctly. Soooooo I started with decrease idle air bleed from .076--.073. Then increased idle feed restrictor from.035---.038. Much better but not quite there, still a very slight lean spot when breathing on the throttle @ cruise. :(:(
Then with everything set as good as I could get it I pulled the carb & checked the transition slot, YIKES!!! only .010 exposed.

I set the throttle so there was .020 exposed put it back on with no further adjustments---YAHOO-- it idles a little faster than I would like, but I can live with that because now I have instant response when I breath on the pedal @ cruise 1500-2300rpm. :):) A/F will run from high 14's low 15's when cruising & drop to mid to high 13's when I even think about increasing throttle.

Hope this is of some help

Craig

I also run with a 02 LM1

First instinct is to say, you have the wrong carb. Square jetting and two power valves, that's a race carb. I'd say you'e be very lucky to get the results you want on the street. Also 8.5 PV's will never cut it on the street with that cam. Where is your O2 sensor located??

right collector

Location is OK for sensor, as long as it's not at the end of pipe. Other than that, I can't offer any advice, like I said, IMHO you have the wrong carb.

Another aspect, for what that carb sells for, you could have had Patrick at Pro-Systems build you a carb, exactly for your car...Price would have been pretty close.

live and learn. . . .

Ditto That!!!

How do I contact Pro Systems?

Also, I plan to check the vacuum of the inatake at idle and change my 8.5 power valve accordingly. However, when I do so, it may fix my 1800 rpm issue, but will probably lean out 3/4 throttle again. So, any thoughts on how to richen up 3/4 throtte without flooding it else where, where the mixture was already good?

One more thing. . . . at 3/4 throttle, the front and the back butterflies are both half open right? so jetting changes should be done to both sides to richen it up or just front or just back?

It all starts with getting the right power valves in place, based on vacuum readings and nothing else. As others have mentioned, the dual PV/square jetting setup is not going to be suitable for street diving. Buy a Holley PV plug and rejet to 77P 85S and see what happens. How do the spark plugs look?

Elmariachi is right on the money!!
Start there.

which one do I plug, rear (secondary)?

i couldn't bring up your a/f chart, but if it is good then your metering is good. put a vacuum gauge on it and find out where your vacuum is. nothing wrong is dual pv's.