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Old 03-07-2012, 04:18 AM
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High rpm miss?

I purchased an 83 Z28 for next to nothing. Clean rust free car. Its got a rebuilt 350 from a 70's era vehicle with stock but ported out heads , 10:1 and a solid lift cam. Long story short, I have it running pretty decent and it has that awesome nasty idle, but it starts "breaking up" in the upper rpm's once warmed up. I changed plugs, wires, cap, rotor, and put a new accel coil on it. I used the stock delco plugs and it has an HEI distributor. The carb is an Edelbrock 600 with .098 secondary jets and .100 primarys orange springs and 7047 metering rods. It also has a new stock mechanical fuel pump. The timing is it 10 degrees initial and vacuum advance is full manifold vacuum. I think I'm around 7"-8" manifold vac at 900 rpm. It seems happy there but I'm at a loss over the upper rpm (4500 and up) missing. Distributor, fuel? The cam specs is unknown so I never messed with valve lash. Maybe its out of wack? Whats a safe lash adjustment, .024? I do know the choke is open fully once warmed up. Not enough carb maybe? Any advise on what to check next would be greatly appreciated.

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Old 03-07-2012, 04:37 AM
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Usually this problem is either one of two things or sometimes both. It's either running out of spark or running out of gas. However, I just reread your post carefully, the problem is the stock fuel pump. The stock pump on the early 80's Z28 as well as the monte carlo and other vehicles would barely keep up with the demand of the weak kneed 305, no way it can keep up with a healthy 350. Carter makes the "muscle car" series pumps and they are pretty cheap. However I don't know how well they perform. Holley, Edelbrock, and many other companies make fuel pumps that will provide more than enough fuel for your small block. Before you install the next mechanical pump, make sure you pack the spring area of the pump with moly lube. The SBC does not have direct oiling to the fuel pump and if the pump is installed dry the lack of lube causes heat buildup which can greatly weaken the spring as well as cause other problems. After you install the new pump you'll more than likely have to retune the carb all over again. Most aftermarket pumps will also require a fuel pressure regulator so if you don't have one it's a wise idea to get one and a fuel pressure gauge as well.
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Old 03-07-2012, 10:22 AM
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I checked out the Carter "muscle series" pumps and I have a question. It says it flows 40 gph and the Edelbrocks, Holley etc flow about 100 gph. Would the Carter be good enough for my application because I do like the price tag much better?
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Old 03-07-2012, 01:13 PM
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This pump should be all you need and the price is right.
http://www.summitracing.com/parts/SUM-250000-1/
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Old 03-07-2012, 05:44 PM
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I was looking at that pump too....do I need a fuel pressure regulator with that pump? I think the carb needs 5.5 max if I"m right?
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Old 03-07-2012, 05:48 PM
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10 degrees initial isn't likely enough for a large lumpy cam. Try bumping it up looking for more vacuum at idle.
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Old 03-07-2012, 05:50 PM
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Even if it says it doesn't I would still get one as I've gotten pumps many times that said they would not put out over 5 psi and they put out 8.
The basic holley cheapy fuel pressure regulator is only about $30 bucks.
I think the summit brand regulator that is identical to the Holley unit costs even less.
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Old 03-08-2012, 04:20 AM
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Thanks for the info guys. I will play around some more with the timing and change out my pump. I will keep you posted. Thanks again for some direction.
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Old 03-08-2012, 09:31 AM
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Ron is right. Your timing with vacuum advance disconnected and plugged should be 12 degrees before top dead center. Once set, lock the distributor down. Now reconnect vac advance to a full time manifold source, one that pulls vacuum full time, including idle. When you do so the first thing it's gonna do is idle up. Bring it back down to normal with the idle speed adjustment. Now recheck timing, it should be 18 to 24 degrees before top dead center. Next is reset the idle mixture screws with a vacuum gauge. Set each side to where it pulls the highest vacuum.
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Old 03-08-2012, 03:42 PM
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Well, I took a ride out to Summit Racing and picked up the Carter, part number GM6626, fuel pump and installed it. I set my initial timing at 12 degrees and the vacuum advance is full port. Mixture screws are at 1 and quarter turns out and idles excellent. I replaced my step up springs with blue instead of orange. I'm only getting around 6-7 of vacuum thats why I changed the springs out according to the Edelbrock manual. It seems to like it but its pouring rain out so a good cruise is out of the question. It does seem a lot better though. Thank you again guys!
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Old 03-08-2012, 03:44 PM
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I will keep you's posted when I get a chance to really romp on it! The weathers getting a little crappy again so it might be a week or so.
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Old 03-12-2012, 06:55 AM
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Well the car ran great till it fully warmed up and the breaking up and popping at high rpms is still present. With the new pump I had to make my accelerator pump leaner but that was a given. Im going to recheck my valve lash but I have a couple questions. Do I adjust it cold or hot? .024 a safe bet for lash with an unknown mechanical cam? Im thinking maybe my distributor is worn?
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Old 03-12-2012, 09:42 AM
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I set the lash at .025 with the motor cold. It had almost no lash what so ever. Now I'm getting around 8-10" of vacuum at idle! Much more than before. Its raining so a good joy ride is going to have to wait. Could this have been my problem?
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Old 03-12-2012, 10:39 AM
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Quote:
Originally Posted by danzigs
Could this have been my problem?
Actually, I was thinkin' valve springs, since you said the problem shows up at 4500+ rpm's.
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Old 03-12-2012, 12:35 PM
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Do you think there too weak? Ive been chasing ghosts on this car for awhile now. I'm almost ready to do my own build just so I know what parts are what and I'll know its done with right stuff.
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