Holley carb stumbles

Discussion in '1974 - 1978 Mustang II Talk & Tech' started by itsaMustangtoo, May 18, 2006.


  1. Blue Thunder

    Blue Thunder Active Member

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    More compression = greater efficiency. Also, 3.4" stroke of the 347" motor has more leverage than the 3" stroke of the stock 302, therefore, yes, the motor has a lot more low end torque, which is where you need it to be for fuel mileage.
     
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  2. itsaMustangtoo

    itsaMustangtoo New Member

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    Ok, here it goes. Ran the car to my buddies shop for inspection. While I was there I checked it with his vacuum gage. Guess what? 13" @ idle in park @900rpm. I had my gage along and adjusted it to match his. I put the .035 squirter in and moved the orange cam to pos #2 . We both agreed that I should move the timing to 12 degrees initial and any small flat spot will probably vanish+ I might pick up another 1" or 2 of vacuum. But as it is now I think I can stand it for awhile. The thing just screams :D Thanks guys- hope to see some of you @ the all ford nationals @ carlisle june 2-3-4th OH and check out the Pro street II on eBay:hail2:
     
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  3. Dano78

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    Well according to this Haynes Holley Techbook (which is a really good read too) It seems as though the Powervalve is a vacuum controlled bypass valve designed to add more fuel than the main jets are set for. Used in such cases as when you open the throttles but the vacuum isn't quite enough to pull the needed fuel (such as fast acceleration, climbing steep grades, etc...) and is closed at idle. So it's spring loaded open and idle vacuum holds it closed. When a weak signal is appearant, it opens the Power valve. (so you were right :nice: ) Now... there are 2 different restrictor ports (idle air bleed and idle fuel feed) in the idle circuit that are unchangeable (unless you drill them) and 1 restrictor port (PVCR Power valve Channel Restriction) unchangeable on the power valve port. All three are in the metering block and could possibly be altered to change the fuel ratio. I suppose one could be ballsy enough to alter these in a scenario where the engine needs "more" rather than "less" than what was determined at the factory. The book says more or less to leave it all alone. I wish i had a scanner, these are pretty good diagrams in this book.
    Not trying to call you out or anything. Actually the wife picked this book up at Goodwill last week. Until then, like you, I've been learning tuning holleys the old fashioned way... word of mouth and lotsa tinkering. :nice: I never really understood how it all actually worked its harmony, but I'm starting to see the light.


    I've got a Weiand 2x4 Tunnel Ram with 2- 450cfm Holleys. I just got 2 390 cfm holleys and when i get the chance, install those and see how it does. JTS has a pair of 390s on his tunnel rammed II and seems to not have any major troubles. The 450s are more likely just too big. However, after reading this thread, I may have just whizzed into the wind in the way I went about tuning them too. I may just start all over at factory spec. on the carbs and try again. I have a huge nasty dead spot whenever I ease into the throtte at crusing speed, and it wasn't really there when i began the tuning fiasco. :shrug:
     
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  4. itsaMustangtoo

    itsaMustangtoo New Member

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    Are the 390's 2 barrels??? 2-2barrels would be great don't ya think . When I sold my 74 I took the 4 bbl , nitrous & all that crap off and put a stock 2 bbl & intake on. It was insane off of idle to midway through the rpm's ( then it flattened out) But not to worry the kid had it 3 days before he wrapped it around a pole.
     
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  5. itsaMustangtoo

    itsaMustangtoo New Member

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    Yeah , I thought I had 10" ( wait that didn't sound right:D )of vacuum which is why I went with the 55 power valve. John @ my local speed shop said take your idle reading , divide it in half and add a half. 10" divided by2= 5 + 5 so a 55 or 5.5 whatever. I think a 65 still would have been ok. Well I know it's OK now.
     
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  6. Dano78

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    No, not 2 bbls, but I have thought of that! And probably would have done it before long. They are the 390 cfm Holley 4bbl carbs you still see from time to time. P/N 8007. I also got my hans on a pair of older p/n 6299 holley 390 cfm carbs. They are suppose to have a leaner Idle circuit than the 8007 carb.
    The car runs insanely well when you gat all 8 barrels open, or on the high end, just can't get it to behave crusing around (in the realm of 1800-2400 rpms). I know the tunnel rams are known for this but I think its far worse that it should be.

    When you're taking your vacuum reading.. is the car idling in gear? If it wasn't, it needs to be in gear when taking that reading if it's an automatic and also set to the idle speed you plan to use. :nice:
     
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  7. Wart

    Wart I'm Mad as HELL and I'M not Gonna Take it ANYMORE!
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    But no where near as often as they use to.

    The power valve doesn't controol anything at idle. Only effect the valve has at idle is if the diaphram ruptures and allows fuel to be drawn into the manifold.
     
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  8. joeythesaint

    joeythesaint New Member

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    Dano,

    I'm running the 8007s on my tunnel-rammed 5.0 H.O. and the engine loves them. I idle at about 900. I rebuilt them to spec and they bolted right on; I just set the idles and took off. If you have the older Holleys, install the blowout protectors for the power valves; in a dual-quad configuration the PV's are a PITA to swap. (Are you the guy who built the side-by-side linkage? Someone on a board I frequent did this, and it made bowl removal a snap.) I've done my PV's 4 times, now -- my vacuum advance was sticking, and the backfires would blow the PV's right to hell.

    FWIW, I get a hesitation when I romp it, but then the car goes ape$#!+ a moment later, as soon as the rpm's go up past about 4K -- which, from the feel of things, is about where the secondaries open. I have to kind of ease my foot down and then mash it the rest of the way to really get her to jump. But all 8 barrels is a beautiful thing -- like being shot out of a cannon.
     
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  9. Blue Thunder

    Blue Thunder Active Member

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    That's true, they used to blow EVERY time, now it's maybe half or even a third of the time.

    The powervalve is open, in its resting state. The spring keeps it wide open. Engine vacuum at the rated inches defies the spring and causes the valve to remain closed. The instant the vacuum drops, the powervalve opens, and the fuel flows through the opening. Yes, when the diaphram rips, the vac leak sucks fuel through, but without the diaphram functioning to keep the valve shut, the spring will keep it wide open. So you get the extra fuel from the rip in the diaphram, AND the extra fuel from the open powervalve.

    If you run no powervalve and the hole is plugged, and the jets are increased in size to compensate, then at idle the engine will likely be getting too much fuel, unless it's a serious race motor, which wants a lot more fuel anyway.
     
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  10. Blue Thunder

    Blue Thunder Active Member

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    BG Demon carb trivia:
    A single Demon on an RPM intake made more hp AND more low speed torque than an aftermarket race fuel injection setup. They're awesome carbs. The billet alum. base plate and metering blocks are totally cool, too. The base plate is a hell of a lot tougher than the standard holley cast baseplates, which break the ears off easily. And the billet CNC machined metering blocks are completely non-porous and accurate. The tops are flowed smoothly from the factory, and the throttle plates are razor thin. The hardware is all stainless, although it's a cheaper grade of stainless that does get a few flakes of rust after awhile, it cleans up easily with scotchbright. All the threaded holes in the carb have stainless helicoil inserts, too.
     
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  11. Dano78

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    Yeah I've had my eye on your tunnel ram setup. I saw your success and decided that maybe the 390's were the key. So i got a couple here now. Just need to push crap outta the way and get a few other projects done and I'll be able to swap that stuff.
    No, not the guy that did the side by side carb setup. Was thinking of doing that too just for the reasons you stated! Mine are inline and I did send you pics of my fuel line and linkage setup back when you 1st put it all together on your car.
    Are you running a PCV valve in your car? I tapped one into my upper plenum in the middle of the carbs up underneath but it seems to run better without one. How is yours? :shrug:
     
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  12. Blue Thunder

    Blue Thunder Active Member

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    I hate that PCV. I also tapped into the middle of the plenum(since the Race Demon has no vac ports) I think I'm going to set up some type of electric vac pump to operate the power brakes, and the pcv system at hopefully 18"-20" of vac. My engine struggles enough with only 7" of vac, it doesnt need that other crap leeching vac.
     
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  13. itsaMustangtoo

    itsaMustangtoo New Member

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    Without a tester is there a way to tell if a power valve is shot?
     
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  14. itsaMustangtoo

    itsaMustangtoo New Member

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    You sold me on one, but I will have to wait until the money starts flowing again. I am going with a 650 mighty demon
     
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  15. Wart

    Wart I'm Mad as HELL and I'M not Gonna Take it ANYMORE!
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    Thanks for explaning to these guys how a power valve works. I would have used terms like pressure differential and how when the differential (in inches of Hg) aproaches the number stamped on the Holley power valve (which is in inches of Hg with an implied decimal) the valve opens and provides an additional path of fuel flow to the main circuit.

    Yep, I've known about fuel enrichment valves for about 30 years, wasn't till I went to the Evelyn Wood Institute of Aircraft Mechanics that I looked at 'things' from the perspective of pressure differentials. ....

    In the idle circuit?

    Gotta tell you, if the diaphram is ruptured to such a degree the differential needed to keep the valve closed isn't being produced .... That's going to be pretty apparent. Like it will still idle with the idle needle about seated, if not seated and still flooding the engine.



    You sure about that?

    Seems to me if an engines ingesting such copious amounts of fuel at idle to create a pressure drop across the mains that upping the main jet size effects idle fuel flow .... well, that's a pretty extreme engine, or the mains were undersized in the first place. I would say. Even an extreme/abnormal street engine shouldn't need that much fuel to idle. Even in top fuel the difference between idle and full power is akin to the difference between a dribbling faucet and a faucet that's wide open. OTOH I haven't played with something that my moral responsibility to society would keep me from driving on the street so I could be wrong on this one point.
     
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  16. Wart

    Wart I'm Mad as HELL and I'M not Gonna Take it ANYMORE!
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    The vacuum enrichment valve on a ford carb is pretty easy, pull the cover and look for evidence of fuel in the cover or on the valve.

    Holleys ar a bit more difficult as the valve could get wet during dissassembly, but there may be evidence of raw fuel in the valve chamber.

    But this will really only indicate a blown/leaking diaphram. Not if the valve/seat itself is leaking.

    Been my experience you don't need a ruptured and gushing valve to screw you, especially at idle. It only needs to drip ......... drip ............... drip ...............


    Pull the valve, put the dry side to your lips and suck. If you think there is any kind of leakage it's probably bad.

    This powervalve thing opens all sorts of memories. Today these things cost well under $15 each, some under $8. Use to be a carb rebuild kit was $5 ~ 7 without the valve and $15 ~ $25 with a valve. This was in the day of $2 minimum wage. If someones car belched you got ready to run because if it didn't idle right afterward they tended to lose their temper. Even at the good starting wage of $4 / hour they were working most of the morning to replace it.
     
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  17. Dano78

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    Actually got to knock the dust off the Tunnel Ram II today. I decided to take a vacuum reading (couldn't remember what it was...been a while) to comapre to what everybody was having to deal with. My car at idle in Drive idling right about 800 rpms is pulling 6" to 6.5" of vacuum. At 1K idle in stays steadier at 6.5" of vacuum. In park at about 1100-1200 romps I'm pulling about 13" of vacuum. Tunnel rams are vacuum pigs, that's for sure. I can't remember what Power Valve i ended up putting in there. From what everybody's been sayin, a Power Valve of 5"-5.5" would probably be what I should be running, or am I wrong?

    BTW- oddly enough, the power brakes work just fine.... must not require much vacuum?? :shrug:

    I also noticed something else. I thing the vacuum modulator is leaking (vacuum) too. I unhooked that hose to hook up the vac. gauge and the engine seemed to be a little smoother when the gauge was plumbed. Hmmmm... may have more skeletons in the closet that i anticipated.... RRRrrrrrRRRR and I had to put'er back into its cocoon for another day. :( One of these days I'll get that thing all ironed out. :mad:
     
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