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I was using a 6.5 power valve, 2 inches lower than my idle vacuum.
Makes sense. Dumb question, where are the channel restrictors? Does that just involve drilling the metering block?
mopowers has good picture. A couple of brass set screws, some small number drills and a tap and you can tune the power valve system down to a gnats behind.
#36 drill and 6-32 tap, be care full, go about 3 turns then check your depth.6-32 x 1/8” brass set screws.
Although this is an aside, here's the only explanation I've ever seen for how we got to low vacuum enrichment on race carbs. Take it for what its worth. Posted by Tuner, 12-15-2006, 01:00 AM The history of the power valve hysteria has a few different influences behind it, corporate, media spin and international events that affected Big Oil. The corporate influence came with Colt Industries when they bought control of the Holley Corporation in the late 60’s. When the genuine "old-school" boys were out of the way, the new wave of MBA’s started telling the ME’s how the business would be run instead of the other way around. Read that as profit became more important than quality. I was employed in speed equipment sales at the time and the declining quality was evident from that point on. One cost cutting measure was a reduction of maintenance of the injection molds and machines that made the parts. Die makers are highly skilled craftsmen, almost able to name their own price if they’re really good, especially in the days before CNC. We started noticing main bodies with more core shift, etc. Every time a part is made in an injection mold, the molten zinc or aluminum dissolves some steel from the die cavity and carries it off in the part. As more aluminum is alloyed into the zinc, this process is accelerated. Zinc alloy higher in aluminum content is less expensive initially, but it increases maintenance cost. After a few 1000’s of parts are made the die cavities and the parts they make are no longer the original size but they look the same because the cavity erosion is uniform. I trust you see where this is headed. The metal part of the power valves wandered off the blueprint but the diaphragm didn’t. The crimp process that assembles the casting, the diaphragm and the retainer washer wasn’t maintained well either, so some were loose and some were too tight, cutting the diaphragm. You can see how a backfire can cause a leak with a valve compromised like that. They pretty well have a handle on it now. A normal healthy valve can stand backfires, for that matter, nitrous backfires. The low number valve for radical cams thing was influenced by this situation too. Smog regulations began dictating how engines were tuned in 1966 nationally (a few years before that in California) and by 1968 they were getting pretty lean and retarded. Before that almost all high output factory engines used #85 or higher power valves. The `65 425HP 396 Chevy used a #105 primary and #85 secondary. The purpose of that is to get it rich before it has much load on it so it won’t knock. Because the HP carbs had high number valves, that was what people wanted and what we sold. The low number valves were for dump trucks and economy cars. However, when the debacle with the poorly manufactured valves began, the valves we (and speed shops and warehouses across the country) had in stock that didn’t leak were the old pre-Colt Industries low numbered ones that didn’t sell before because they weren’t romantic enough. People didn’t want a Rambler power valve in their "Fueler 327" with the 950 3bbl. How this got tangled in the valve timing is it was in the same era when cams started getting much more radical. In 1965, most people thought a big cam was 250º at .050". By `70 or `71 durations had grown to pretty much what’s available today. When a low numbered power valve fixed a radical cam engine that wouldn’t idle folks fixated on the number, they might not catch that the valve they replaced was leaking. Brand new carbs came with valves in them that leaked. Once somebody jumped to that conclusion and the enthusiast media picked it up it spread by plagiarism and became lore. OK, so there’s the corporate influence and media spin, now on to international events and Big Oil. Starting in `71, the highly leaded fuel was phased out down to "low-lead" (.1gm per gal.) by `72. That EPA mandate and the 1973 oil crisis, during which OPEC cut off the supply of middle-eastern oil to the US, brought about a change in the chemistry of the gasoline. American industry had agreed on standards describing materials compatibility for fuel and fuel systems that could not be met with the crude available and the amount of gasoline necessary to be made from it. This affected more than just power valves. The soft parts in fuel systems, hose, pump diaphragms, gaskets, o-rings, tips of inlet needles, floats, you name it, if it was soft the gasoline dissolved it or soaked into it and made it swell up. Inlet needle tips would swell and push the float to the bottom of the bowl. The floats soaked up with gas and sunk. (That was a big part of the bad rap the media played on the QJet.) The rubber would just sort of rinse off the diaphragms. If you handled a wet accelerator pump diaphragm it would smear off on your fingers. Needless to say, power valves made with the early materials leaked. The metering block gaskets expanded into the air and idle channels on the face of the block and plugged them. All the carb components had to be changed. Like a flammable game of musical chairs, the new parts had to push the old parts through the system. It took a couple of years for stuff for the less common carbs. Still, once in a while someone shows up with an old carb kit and it’s like déjà vu all over again. originally snipped from: deadlink:innovatemotorsports.com/forums/showpost.php?p=29053&postcount=9 Archived here: Trouble with vacuum signal to power valve in Holley
Very interesting. This is why I always tell Tuner to write a book. It would be great reading and would clear up a bunch of things. Guy is a walking encyclopedia of interesting knowledge.
Bump to the top.
Haven't tried drilling the power valve channel yet, but it is on my to do list. The car did run well on my 3 hour round trip to Moparfest.
Dam good thread.
You may not have to. Test first, drill second. It all depends on the combination and the carb. I've told this story before. Had the car on a chassis dyno and just made jetting changes and experimented with a little more timing (no good). Two pulls were about a equal so my on the spot decision was leave with the slightly smaller PMJ setup. On the freeway type highway, the engine started revving and then dying. It only happened when over 60 mph or so. So my solution was drive home at 55-60 max (which was OK on that road). Moral of the story was those jets were the lean limit for cruising. Additionally, while maybe just as much power on the dyno, likely a little too lean for the track. So on that carb and application, the PVCR could be left alone. ------------------------------------------------------------------------------------------------------ With Respect to finding PV opening by checking for increased acceleration when crowding a vacuum. Here's an example: Blue line is vacuum. Black is RPM, Orange is forward accelation. Violet is AFR derived from WBO2. The first two steps down in vacuum are matched with step increases acceleration and rising rpm. In this example, done at relatively low speed (25 - 50 mph or so) the next increase in throttle, dropped the vacuum as expected but there was no increase in acceleration. This dead spot around 12" HG was not because the AFR was lean. In this case it was because the timing was too advanced and possibly also due to an incorrectly located set of kill bleeds. I've corrected the timing but haven't had a chance to put that carb back on. Maybe some time this fall.
Interesting. That's one of the issues you run into with any form of static testing. What looked good on the wheel dyno may be lean in actual practice. The dyno (any dyno) can never accurately duplicate actual load and road conditions. The dyno assumes flat road, no aero drag and some other things. You always have to verify dyno tuning by actually using the car.
Well I did drill out the PV channels one drill size to 1/16. I tried going down one more size in the primaries from 70 to 68. The cuda didnt like it. Yes it got the A/F into the 14's but it was hunting and surging. I came back and put the 70's back in and it was happy again. The A/F ratios were back in the 13's and part throttle response was good. I think I found my leanest setting.
So you have 13's at a cruise or WOT??? Sometimes you have to live with what the O2 sensor says. You are tuning to the leanest cylinder and the sensor is giving you the average of the cylinders on that bank.
Yes 13’s at cruise. Part throttle on the primaries stayed in the 13’s. Wasn’t able to try full throttle, roads were wet. Also tough to watch the gauge at full throttle.
Chicken!!! Just point is straight, mash on it and look at the gauge!!!!!
It would start off straight anyway. With over 600 lb/ft of torque it gets sideways quickly. It won't break loose in 3rd, but it does get illegal quickly, so I have to pick my spots.
I decided I am going to bolt the drag radials on before I take it out for a test drive. They are brand new, so I am kind of interested in trying them out anyway.
Issues here with the test method which can give misleading results. * Part throttle cruise AFR is vague. At least to us on this side of the screen. Load is unknown, throttle opening is unknown, circuits supplying fuel are therefore somewhat unknown. Somewhat because if changing jet causes lean surge, we can conclude the primary main circuit is involved. Although throttle position is helpful, my experience has been that a steady throttle cruise condition at 30-45 mph is probably on the transition slots, and at 65 or 70 mph almost certainly on the main circuit alone. * Aiming for cruise AFR numbers we're aiming at the wrong target. Those things off the widebands are just references numbers so we know more about the change made. From what you've written, you are probably correct to conclude the jetting for cruise is as lean as it will tolerate at the moment. If the e-holes and MAB are changed (for example to get a flatter AFR) then the jetting may change. *To test for best fuel restriction at WOT its neccessary to get the engine at the maximum load it will see and give it and the fuel system time to stabilize for the that load and then go from air flow at that rpm through the rpm of maximum horsepower. * Aiming for WOT AFR numbers aiming at the wrong target. The target is max torque and power, best measured at 1/4 mile track. When comparing WOT jetting to cruise jetting, its important to see how flat the AFR is at 100% load. In other words, high gear through max hp. And if its not flat, then the emulsion or airbleeds need to be worked with. Just look at the trends shown here. Because even in 3rd or 4th gear, AFR from a WBO2 is going to jump around depending on the sampling frequency and speed of the system. (If its really jumpy, that could be timing or the logger wiring etc.) The jetting shown above is probably close, but once a flat AFR is established, then the jets may have to be changed until we get out best ET and MPH. Then it wouldn't hurt to check if the AFR is still flat. The more drastic the fuel restriction changes are, the more likely air bleed sizes may need some adjusting. Above was a dyno run. Below is short test on the road with rolling start. First gear is useless for establishing jetting/PVCR. Second gear is flat and suggests, but doesn't guarentee, that the air bleeds may provide a flat AFR delivery through 6000 rpm. It does provide some confidence the e-holes are about correct since they tend to most effect the begining of the fuel curve. Finally, remember the WBO2 interpretation is just that. Change the timing and the AFR may shift. Well of course it probably hasn't. A carb delivers whatever the engine's airflow demands. Its just the burn has changed such that the left over O2 has changed.
Stuff like this should be put in a sticky. It's that important. Great post matt Edit: there was a thread (may be this one but I don't remember) where someone wanted some pictures of some cutaway metering blocks I said I had. At that time, I couldn't find them. Yesterday I found them. If I post pictures of them can Matt or someone who is good at this crap point out all the stuff on the blocks with arrows and such? I don't know how to do it. But I can post a picture. I may be easier to see some of this stuff with the cutaway blocks.
Sure. I use MS Paint for drawings and IrfanView for marking up jpg photos.
Ok. I'll get some pictures today and post them here. You can paint them up. Thanks
quick google search
I have two different cutaways. You can see more with what I have, I think.