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Carb Tuning...what should my pwr valve be?

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#21 ·
Mark, I'm off tomorrow and plan to do some experimentation. Now that I have the misfire figured out ( and eliminated), will reinstall the mech advance limiter, reset the timing and get a new baseline vac reading. Prior to this larger cam install, I was running a 8.5 PV based on the prior cam's 11" vac reading. I do agree that 2" below idle vac (in gear with an automatic) is probably best for street driving. A 4.5PV may be great for racing, but you probably do want to start the enrichment a little sooner on the street. If I get off my butt and weld in the bungs, we can throw the LM-1 on there and do some real tuning. I can see this thread hanging on for a while.
 
Discussion starter · #22 ·
Mark, I completely get your point. I am very new to this even though I am 40 yrs old. I understand the trial and error and not a big deal to try different things for my set up. But on the knowledge side....trying to understand the point of the pwr valve. It seems that if I was to error I would be better off going a little smaller to help with idling. Would that be the correct assumption. I have a 6.5 in mine right now because I was told to run that in a BG850 set up for E85. The problem is that my idle vacuum is around 7-8 hgs. My car is not idling near as well as it was when it was on gas. I was thinking my pwr valve may be opening when pulling in and out of the driveway causing excess fuel. I am way off or am I understanding this correctly? If I am understanding it correctly I figured that it wouldn't hurt to go to a 5.5. What I worry about is will that hurt WOT when I eventually run nitrous?

Thanks for being patient. Some of you may feel we are beating a dead horse and I appologize but I have already ruined one motor and don't want to repeat. Plus I want to know why I am doing something. Everyone has done a good job of explaining.
 
#23 ·
Sinister said:
.Thanks for being patient. Some of you may feel we are beating a dead horse and I appologize but I have already ruined one motor and don't want to repeat. Plus I want to know why I am doing something. Everyone has done a good job of explaining.
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I think this is an excellent topic and am most curious to see what everyone says. My 454 just got a Holley 750cfm vac sec stuck on it last weekend and I was wondering the exact same thing. :confused:
 
#24 ·
LOL, there's never a dead horse in the vicinity when carb tuning is the topic.

Note that your PV "should not" come into play at idle, and I wouldn't think moving it on the driveway or cruising would have anything going on with it either - it either feeds or is fed by the main well, which won't come in off the idle circuit and typically not until you're a little higher in rpm's, like 2000 or so if I remember this correctly.

Plus, running E85 may be a whole 'nother ball of wax, throwing out some of these so-called "rules". But if it's too rich at idle, I assume you can close the idle screws completely and the car still runs? If so, you are getting gas from somewhere else. If it does shut down with the screws closed, I think the carb may need a little more work like changing the air bleeds or similar. I am assuming you've already tried running it with the screws closed more with no effect, right?

I'm actually thinking you may have something else that's off with your carb, like a leaking accelerator pump circuit, etc. See if it runs with the screws turned in first (sorry if you already answered that question, can't look at the first page right now). I'd hate to give you any advice that isn't correct for E85 but would work for gas, I'm not at all familiar with it.

Note the Innovate forum is a potential spot for more info on this - but be prepared for long periods of no responses, way fewer people on the site, and abbreviations up the wazoo you'll need to learn. Nonetheless, it is another resource for the E85 aspect of your idle issues.

Grn, that was a great post you had using that screw for the partial timing lock (I think it was you, but wasn't positive). I did one so far, with too little mechanical play, I'll take out the one I did and add another screw this weekend on the other side to try and add more. The engine absolutely LOVES the added timing, it's pretty amazing really. I just don't want it totally locked out for starting, thought your idea was great.
 
#25 ·
Actually Vince (69-CHVL) had done the post. He figured it out and posted the pic. I just did mine using a 11/32" hex socket screw and locknut (bought at Home Depot). Worked really great, but as I had said, I had removed it chasing a misfire - thought maybe it was causing interference. Turned out to be a cracked plug, so the limiter is going back in for another try.
 
#26 ·
GRN69CHV said:
Actually Vince (69-CHVL) had done the post. He figured it out and posted the pic. I just did mine using a 11/32" hex socket screw and locknut (bought at Home Depot). Worked really great, but as I had said, I had removed it chasing a misfire - thought maybe it was causing interference. Turned out to be a cracked plug, so the limiter is going back in for another try.
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Thanks, I want to bookmark that puppy! Couldn't find it doing a search.
 
#28 · (Edited)
As already stated mult times correct p-valve is somewhat setup dependent .

But as Mark just stated ALWAYS ENSURE THE TIMING WHEN RUNNING A PERF CAM (EPS BASE TIMNG) ISN'T RETARDED which will cause carb issues due to lower intake vacuum signal due to retartded base timing.

A good/safe place to start for base timing for many aftermarker perf cams is for very mild RV type cam to moderate perf cam is approx 16-18 deg base timing & 18-20+ deg base timing for farily hot to real hot perf cam along with just enough mech adv in dist to obtain approx 36-38 total .

But as Mark also mentioned some setups/real hot cams do better on the street with even more base timing like 30+ and in those cases the mech adv needs to be limited to only 6-8 deg . That setup is more starter /street friendly vs running locked out timing at 38+ deg . Then you could also run a adv for more timing resulting in a better idle/better fuel economy/better throttle responce/cooler running motor

Back to the carb/p-valve.

I have found over the yrs that if for ex the idle vacuum is 10" (in gear with auto trans) i would not go 1/2 with a 5.0 p-valve like holley recommends because i think it's too lean of a setup in many cases.

But in the example case of 10" idle vac i would go with the stock for many holleys 6.5 p-valve in that case which would be a good starting place for dialng it in better later on if need be.

Choosing p-valves that are a little more then 1/2 ilde vacuum like the 6.5 p-valve with 10" idle vac example i used has often worked well enough for me to leave as is without another p-valve change later on.

And if its a case where the motor is new/fresh rbld thats never been fired there's enough cam/engine build exp here in t-chevelle to est roughly/close enough what the idle vacuum would be for a particualr cam in a particular size motor . Then you can go from there for choosing an inital p-valve choice for initial fireup using a little more then half the idle vacuum as a good starting place for te p-valve.

That should be close enough eliminate or at least greatly reduce an over rich mix (caused by wrong p-valve choice)for initial fireup to avoid possible ring seal issues due to excessive fuel wash of cyls durring breakin ,then after that the carb/jetting/p-valve can be dialed in better later on.

Scott
 
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Discussion starter · #29 ·
Scott my motor had already been broken in and run on 93 octane. However, I have recently changed to E85. Tuning for E85 is a little harder to get it to run good on the street. For example. My car ran great around town and ran a 7.5 at the track. Changed to E85, car ran like crap just driving up to the track but ran a best of 7.3 on my one and only shot before I tore up the trans. I have completely gone through the carb tuning it up as has been posted. My car starts right up now and seems to run like it did with the 93. It just is a little off now which may just be attributed to the 30% more fuel flow with E85. My pv is 6.5 and my Vacuum fluctuates between 7-9. I am wondering if this "fluctuation" it may drop to 6.5 Hg's therefore opening my Pv in idle which I don't want. If I am understanding this correctly. I am going to the track tonight if my shift cable shows up. I am also going to buy a 5.5 pwr valve and see if it helps with the idle. Like others said, hopefully, it doesn't affect anything while I am WOT.
 
#30 ·
Here's an update. I reinstalled the limiting screw in the HEI which of course reduces the amount of mech advance. After adding the mech adv limiter, total advance was down to about 33*. Retimed the motor to 38* total, which nets about 30* at idle with the limiter installed. Readjusted idle. Vac is 10.5" at 1050 in in neutral, drops to 8.5" at 950 in gear. So, yes there is more vac to be had by adding base timing.
 
#31 ·
Just a couple points, getting more initial timing is something a lot of people miss. Most factory cars were dialed low initial to meet emission standards, you can almost always put more in and help the performance.
Second, if the engine and carb are tuned correctly, the PV will not have an effect at idle. The main jets and PVCR both feed the main wells, and the idle circuit is fed from the main wells. However the restrictor for the idle circuit (IFR) is much smaller than the main jets, you can remove the main jets and have no effect in idle mixture, again, if everything is working correctly. Now if the boosters are flowing at idle... it will have an effect and also is not working correctly. The only other reason the PV would have an effect is if it is leaking thru to the vacuum side.
 
Discussion starter · #32 ·
I am pretty proud of myself. I tuned my own carb, after setting initial timing, idle, etc... The car ran like crap last time out but ran great at the track. This time it drove beautifully up and back from the track. It ran a new best of 7.202 and new best mph of 98.51 even though the conditions were worse. It also didn't use near the same amount of E85 as last time. So I think everything is correct now. Didn't touch the pwr valve. I left the 6.5 in there.
 
#33 ·
jmarkaudio said:
Most factory cars were dialed low initial to meet emission standards, .
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That might be the case for 70's and 80's cars, but I'm not so sure that it would be the reason for early 60's cars having the ignition timing settings at merely 4 or 6 degrees BTDC. Because I don't think that there were any emissions standards before 1967. Am I wrong?
 
Discussion starter · #34 ·
Billy, since you are on here. Does moving the lower control arm from the top hole to the inch lower hole on the UMI relocation bracket....move the rear end down or up? I need my rear to sit lower. It sits too high right now. My front end moved a lot better but I still don't think it is transfering all that well.
 
#35 ·
Sinister said:
Billy, since you are on here. Does moving the lower control arm from the top hole to the inch lower hole on the UMI relocation bracket....move the rear end down or up? I need my rear to sit lower. It sits too high right now. My front end moved a lot better but I still don't think it is transfering all that well.
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Del, adjusting from one hole to the next has nothing to do with moving the rear end from it's location. :noway:

It's for changing the "instant center" so that the car will have better straight line traction. Moving to the lower hole will shorten your instant center, and give the car more "anti-squat" (preventing the rear end from squating during hard acceleration). This might result in better initial traction from a standstill or off of the starting line at the dragstrip. But depending on the power level of the engine you have, it can also cause the car to have less traction 20 feet off the line too. But it dpends on the power level you're at with this car.
 
#36 ·
jmarkaudio said:
Just a couple points, getting more initial timing is something a lot of people miss. Most factory cars were dialed low initial to meet emission standards, you can almost always put more in and help the performance.
Second, if the engine and carb are tuned correctly, the PV will not have an effect at idle. The main jets and PVCR both feed the main wells, and the idle circuit is fed from the main wells. However the restrictor for the idle circuit (IFR) is much smaller than the main jets, you can remove the main jets and have no effect in idle mixture, again, if everything is working correctly. Now if the boosters are flowing at idle... it will have an effect and also is not working correctly. The only other reason the PV would have an effect is if it is leaking thru to the vacuum side.
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This has, likewise, been my experience: Like jmarkaudio, I can see no reason to pick a PV that opens at half idle vacuum. I don't care what the factory says, it doesn't make sense to me. Your idle feed restrictors, idle air bleeds, and idle mix screws affect your cruise mixture. You don't need PV enrichment until you get to some vacuum below about 9 inches AT CRUISE. A bigger number on the PV means it will richen up sooner. A bigger number than you need will just waste gas and polute your oil. All that being said, 6.5 is a good ballpark number. I guess that's why they put them in so many carbs. Don't hesitate to try something smaller. If you go too small, you will get stumbling. That's how you know where you are, and will need a bigger number.

Of course, all of this leads back to the transfer slots. When the slots are right, a lot of bad things don't happen any more. If the timing is where it should be, and the slots are right, and you still aren't running right, you need to look for something wrong. Float settings, need for jet extensions, plugged in-carb filters, linkage just bent wrong and not closing the secondaries, carb getting too hot, wrong gaskets on your Holley, leaking/sticking floats, carb opening all the way, etc. The usual stuff.

When working on carbs, keep the fire extinguisher, or the garden hose, close. Always. Don't ask how I know this.
 
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#37 ·
kirkwoodken said:
When working on carbs, keep the fire extinguisher, or the garden hose, close. Always. Don't ask how I know this.
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FYI, water FEEDS a gasoline fire and makes it worse. Use a fire extinguisher - good advice to keep it handy.
 
#38 ·
Xtreme70SS396 said:
FYI, water FEEDS a gasoline fire and makes it worse. Use a fire extinguisher - good advice to keep it handy.
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Having been on fire and receiveing skin grafts, and putting myself out with a garden hose, I can tell you that: When YOU are on fire, a hose works. You can not blot out, stop, drop, and roll, to put out a gasoline fire on your clothes. Water will keep your hide from burning off. Being on fire is not as much fun as it looks like.

When your car is on fire, you are better off with a fire extinguisher. When I'M on fire, I'll take water over a fire extinguisher. Your results may vary.
 
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#40 ·
Delbert,great to hear its running great now.

To recap what was the idle vacuum and what p-vlave did you end up with that worked good with your setup so we can all benfit from another known setup when it comes to idle vacuum vs power valve choice that is working well?

I ask that because holleys recommends cutting idle vacuum in 1/2 and using that for an inital p-valve choice which i think is too lean/conservative .

So i like to see what p-valve ends up working well vs idle vacuum in the real world to see how close it comes to the holley rec of 1/2 idle vacuum like for example 5.0 p-valve for 10" idle vacuum.

Scott
 
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