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Old 08-26-2009, 07:09 AM
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Holley 750 DP

First off sorry if this gets long, I just want to be thorough to avoid confusion.
Car is 1977 corvette, weighs about 3,400#'s
350 chev, bored 30 over to a 358, flat top pistions
Trick flow bolt on heads
Comp cams 122-122,(230 dur. & 480" lift) Has 1.6 full roller rockers
3000 stall and 390 rear end
Perf RPM AIR GAP intake, 1 7/8" full tube headers
Holley 750 dp, mech secondaries, no choke tower, 50cc accel pump
Vacuum at idle is around 8-9", goes down to about 5" in gear, Power valve is 6.5 (i'm sure to big), primary jets are 73, and primary squirter is a 40.
Initial timing 18 deg. and 34 total all in by 3000 rpm, no vac advance.
Took it out for a test drive after full warm up, motor ran fine through the gears but once I slow down and come to a stop, its chugging and sounds like its flooding, I'm sure it was cause of the black smoke.
The problem always seems to be after its been running a while, after full warm and then out on the main road. If I cruise it around the neighborhood and drive civilized it seems fine but once I get the rpm's up and cruise up to 50-70 mph and then slow back down I start to see all these issues with it wanting to stall out?
You guys all helped me out earlier this year and I'm very thankful.
Any advise would be greatly appreciated.

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Old 08-26-2009, 07:30 AM
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is the carb a new one or one thats been on the car awhile?

your PV could be blown if you had a backfire through carb.
it can also be a fuel level problem or your idle mixture is out.
but if its an old carb it might just be that you need to rebuild carb.
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Old 08-26-2009, 08:15 PM
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The carb is not new, I purchased it new maybe 10 years ago, it has been taken apart and rebuilt several times and has performed well over the years. I have played with a couple different pwr.valves just recently, they were all brand new and the motor hasn't had any backfires at all. The one thing that I keep questioning is this is a new Perf. RPM Air Gap that I put on before spring, I have always run the perf. RPM in the past, could this intake be the cause of so many tuning issues?
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Old 08-26-2009, 09:10 PM
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Possible dirt/grit infection of the carb causing the needle and seat to leak ( flooding) and or the power valve to leak at idle when it should be closed.
(replace the fuel filter). Possible fuel pressure fluctuation (hot engine) relief valve sticking in a mechanical fuel pump or just overheated fuel in the fuel lines or carb bowls.
The holley power valve must have a vacuum rating that allows the thing to stay closed at idle in gear. If you have 5" manifold vaccum at idle in gear , use a 4.5" rated PV. It will idle much cleaner.
Possible intake manifold gasket vacuum leak that gets worse when the motor is hot.
Possible carboned up combustion chambers. When you hoof it, carbon deposits are getting lose in the chamber and infecting the spark plug gap.
possible sticking valve or lifter. Cylinder head and exhaust valves getting too hot form the lack of proper vacuum advance at cruise. This results in the ex valve and guide expanding enough to grap the valve stem and the ex valve sticks up creating a missfire right after extened hyway running.
Hook up the vacuum advance and use it. 10-12-15deg of maxumin vac advance is typical for this type of motor. 20deg ca be a bit much. The rate has to be found/optimized by trial and error. The engine will run much better and use less fuel.
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Old 08-28-2009, 08:35 AM
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F-Bird, thank you for your reply, I will check into these.
I do have one question regarding the PVC valve; I have heard before that running the unburnt fumes mixed with hot oil fumes back into the carb can hurt a number of things, like performance and feeding an unclean mixture into the motor can't be a good thing. Is this totally false or is the benifits of using a properly set up PVC outweigh some of the negatives? I have no problem hooking in back up if it truly a benifitial thing? I see many cars at the 1/4 mile track that don't run a PVC, many of them are streetable type cars running 11 & 12's ?
Thanks again.
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Old 08-28-2009, 10:50 AM
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PCV can only benifit
without it you can cause all kinds of oil leaks through your gaskets.
PCV also helps piston ring seal.
But by all means ask F bird lol.
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Old 08-28-2009, 12:31 PM
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Aw thanks Dawg! You are a great help too!
Ya know the only bummer here is that I have these tall aluminum fabricated valve covers and only one side had the baffle and grommet for a PVC so I guess I will need to drill out the other side for a breather. Would that be something you all would recommend ?
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Old 08-28-2009, 01:09 PM
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try the PCV in the hole you already have.
I used to run a breather on passenger side and PCV on drivers side.
until a race engine machinist set me straight.
he gave me a small plastic bag.
said put it between your index finger and thumb making a hole.
he said now suck on the hole.
bag collasped in on itself.
he then put a hole in the bottom and said try again.
i did and nothing happened.
So I only run a pcv on one side now and have not had any oil leak issues and seem to get a better ring seal.
vacum always present on lower end.
BTW i dont get the oil sucking up the PCV at all either now.
but i did with breather on passenger side.
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Old 09-16-2009, 07:19 PM
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O.k. here is the latest update. I took the 750dp all the way apart, cleaned everything with carb cleaner, compressed air, new 4.5 pwr valve, 71 prim jets, 80 sec jets, 50cc accel pump with 31 primary squiter. I am now running a dedicated hose to a functioning pcv valve and full manifold vacuum to trans and brake booster. During the rebuild I set the sec. butterfly's to the "little square", turned out idle mix screws one full turn. The motor started up and idles good, except at about 1400 rpm, with a fairly solid 9hg of vacuum (better than it ever was). I put it in gear and idle comes way down to about 500-600 and it nearly wants to stall out. I did drive it around to get it to operating temp, I then put the vac guage on and began to adj. idle mix screws to see if I could get anymore vacuum, didn't really get any more and the screws are about 1 1/8th out. Got back in car, idle still high and the idle set screw isn't even opening up the butterfly's, so I turned the secondary butterfly's close just a hair and the idle came down to around 1000-1100, put the car in gear and stalls right out?? I'm not sure if I mentioned this earlier but the stall is a 3500 Hughes unit and seemed to work with this combo before. All of these tuning problems I am experiencing seem to have started when I switched to the performer rpm air gap? Confused !
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Old 09-16-2009, 10:26 PM
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There are three reasons it could be stalling when put in gear along with the high idle.

One is that the mechanical advance springs are too weak to keep the weights pulled in at idle so the idle flares up, then when it is put in gear and the idle slows down the springs can pull the weights in, timing retards, and the car stalls. Have you checked timing at an 1200 rpm idle and then at a 800 rpm idle to see if it retards when slowed down?? If the springs are too light(and most are that will allow full advance by 3000 rpm), you will have a hard time getting a stable idle without altering the internal mechanical advance slot.

Two is that the secondary barrels are still open too far, negating any adjustability in the front barrels. Close the secondary down until the front needs to be opened with the idle speed screw to obtain an idle. I think you have "out-tricked" yourself with too many changes. Reason Two plays right into Reason Three, it's too lean.

Three is that the idle circuit is just too lean before it reaches the mixture screws. The screws only deliver a larger or smaller amount of a pre-mixed idle fuel and air mixture who's ratio is decided by the fuel bleed in the metering block and the idle air bleed in the main body. This may require you to alter the size of the idle fuel feed restrictor(bleed), but I would think a DP would already be rich enough. A dual plane will show too lean symptoms sooner than an open plenum will however.

Why is there a 50cc pump on the front of this carb?? It still has a 30cc pump on the rear??

You could have a vacuum leak at the head/intake gasket, either external or into the lifter valley.
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Old 09-17-2009, 07:33 PM
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Im definitely going to check those advance springs, Im certain there are two light springs in there. I put the 50cc accel pump on the primary because of a flat spot off idle, I left the secondary accel pump alone. The flat spot is gone and there seems to be more power as well.
I will make sure the secondary butterfly's are closed up as well and then see what happens.
Thanks again for your help!
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Old 09-17-2009, 08:51 PM
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First, I am not a Holly guy, I wont even ty to help there. But I will say, I really don't think any problems are being caused by the Edelbrock RPM.
Though I will throw a few questions out at you:

#1 Did you change anything else when you changed manifolds?
#2 Any possibilty of an air leak on the manifold or carb?



Edit: sorry eric72nova I didn't read the last line in your post.
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Old 09-17-2009, 10:01 PM
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FBird posted the fix here, it needs more ignition advance at idle.


Quote:
Originally Posted by F-BIRD'88
...Hook up the vacuum advance and use it. 10-12-15deg of maxumin vac advance is typical for this type of motor. 20deg ca be a bit much. The rate has to be found/optimized by trial and error. The engine will run much better and use less fuel.
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