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Old 01-10-2004, 08:16 AM
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383 carburation problems

Just installed a 383 in my truck. 207 casting 4 bolt block. eagle 4340 crank and H beam rods. 18 cc dished Wiseco Pro Tru pistons 9.8:1 compression. Edelbrock E Tec 200 heads and RPM Air Gap Vortec intake. Comp Extreme Energy 270/276 Hydraulic roller Retro fit cam and Crane retro fit lifters. Mallory HEI unit & coil
I started with 10 before baselineand a 4777 650 double pump with original 67/76 pri/sec jets. Surged off idle and at cruising speeds like there was a vacuum leak or an extreme lean condition.
Fattened pri jets to 71 and retarded timing to 7 before for a total mechanical advance of 34 deg vacuum takes it to 45 at 12 in HG. Cleaned up the surge while cruising but still surges off idle.
I have a 4779 750 double pump but don't believe this engine needs that much carb. I was thinking of putting the primary metering block from the 750 on the 650 to get the bigger idle feed retrictions. Maybe that would clean up the off idle surge. Anyone have any suggestions.

Guess I should have introduced myself first then asked my question but Oh Well. My name is Joe, Live In Va Beach. I am a diesel and auxiliary mechanic in the Navy and as such have had to fix everything from 2HP Briggs engines to 16k HP Pielstick diesels and all kinds of stuff in between. I've been racing Motocross and 4x4's since before I had a license. Mostly a Ford guy but I also have a '81 K20 Chevy I'm playing with now. built a lot of engines for both myself and others so I have a fair amount of experience but mostly on BBF, Some FE motor experience and SBC. BTW love the site.

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Last edited by hemiford; 01-10-2004 at 08:16 AM.
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Old 01-10-2004, 09:12 AM
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Have you tried a different power valve yet? Sometimes they leak at the diaphram and cause all kinds of havoc. Going down a few inches helps too, vacuum fluctuations can cause them to pulse and dump raw fuel at idle.
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Old 01-10-2004, 10:43 AM
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Me and a friend of mine built a motor very similar to that but with pro topline heads. I did a little looking around at what carbs worked well on what engines, and it seemed to me that that motor would like a 750, which is what we used. I would try the 750 and see how it works.

Good luck

Adam
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Old 01-10-2004, 11:33 AM
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Joe, you are doing things to correct your surge problems that are beyond my abilities so I'm reluctant to make an input. I have a similar 383 with the ETec 200 heads, 10.2 to 1 compression, but I have the Pro Flo EFI. This has a 1000CFM air valve, but my Ram Air System will only let 800 CFM flow to it. Apples and Oranges as far as carb and EFI. but I don't think your 750 DP is too big for your set up. By the way, my idle timing is 10 degrees, WOT at all vacuums is 36 degrees and at 12 in vac, timing is at 45 degrees. I lean toward 4 Jaws thinking and focus on the power valve.

Trees
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Old 01-10-2004, 12:01 PM
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I guess I could stick the 750 on it as a test. There is a 65 power valve in the carb that I know is good because I tested it. Engine pulls about 14" at idle and cruises at 2500 with about 22". Another question I have is these rockers I'm using. I bought a set of Comp pro magnom CRMO roller rockers and stuck them on this engine. The valve train on this thing is so loud it sounds like a diesel!! Nothing is binding or rubbing anywhere just loud. and it gets louder when you jump on it. Wierd. This is my first experience with these rockers I usually run crane aluminums on chevy's. Any input???
Thanks Joe
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Old 01-10-2004, 12:26 PM
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hemi,
I have two stroked 350's (383 and 385) I also have the Pro Magnum rockers, they do not make any noise. Maybe the noise has something to do with the retainers of the retrofit roller cam?

As far as the carb goes a 750 is not too much for you engine. If it was going to see the track I would even suggest an 830.

The surging is another issue and I am not quite sure what to think of that. Silly question, Are you sure your floats are set right?

To me your initial timing is way low too, I would think you would want 12-14 initial and your total is too high (IMO) you should shoot for 34-36 (in my experience strokers don't like/need too much timing).

Are you sure the "surging" isn't detonation? You can't always here it. Before getting to involved in the carb you might want to recurve/adjust your timing and see where that gets you.

In either case please post back with what cures it. It might help one of us in the future.

Royce

Edit* Are you running vacuum advance? If so try un hooking it and see if the surge goes away (of course you will have to plug the hose and maybe bump up the timing a little).
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Old 01-10-2004, 12:44 PM
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I guess the noise could be the link bars on the lifters. The engine was built to go in a "62 Impala but the deal fell thru on the car so I stuck it in my 5000 Lb K20 4x4. Originally I had a 282/288 XR cam in it but stuck the 270/276 XR cam in for the truck. The main reason I am reluctant to put the big carb on it is it will probably never see over 5000 RPM. If I do go to 750 I will probably pick up a 3310 vacuum secondary to try and preserve some mileage. I do have the vacuum advance hooked up but have the travel limited to about 10 degrees. It helps cruising mileage a lot. I will throw the 750 DP on it and test drive once the weather warms back up a bit and either start a new thread or pick this one back up. Thanks Guys, later Joe
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Old 01-10-2004, 06:48 PM
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Joe, I also have the Comp Cams Pro Mag roller lifters in my 383 and they are very quiet. !.5 ratio, but I did have to go to 0.25 in shorter than stock pushrods to get the roller in the center of the valve stem. Yes 1/4 inch shorter. I am running a Crane hydraulic cam.

Trees
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Old 01-11-2004, 06:22 AM
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I will agree that you have a lean condition if it is surging. You could try bumping the jet size just a touch more. Only other thing I can think of(and it is a shot in the dark) is you could try opening the secondarys a bit at idel to expose more of the transfer slot and then close the primaries to compensate for idle speed. This way, at cruise you could get a little more fuel from the secondary side. Like I said, other than jets, this is a shot in the dark.

Chris
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Old 01-11-2004, 05:05 PM
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Are you using a vacuum advance HEI? If so try disconnecting the vacumm advance on it. I also ran into this sort of problem with my set up. I have a kinda simalar setup with a larger duration cam shaft and iron heads. I cant remember off hand what size jets I am using but I am using a 750 DP carb. If by the time you read this and havent solved the problem reply back to me and ill check out my jet sizes.
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Old 01-12-2004, 03:35 PM
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Stuck the 750 dp on the engine over the weekend jetted 72/76. Bumped the timing back to 10 before. Off idle surging/hesitation is gone. Because of the weight of the vehicle (K20 Pickup) I have to meter the gas pedal though. I can get better acceleration slowly pushing the pedal as R's come up. With that 650 I could floor it and it took off roaring.
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