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Old 09-07-2008, 09:53 PM
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383 mopar stumble

Hi, So we finally got my cousins 70 sport fury on the rod tonight, she's got the original 383 magnum in it with a torker intake, holley 4777 650 dp carb and headers. So its till fairly stock, it runs good but when floored it stumbles then puts us faceing in the other direction and half in the ditch when going around a corner. I was wondering if this this sounds like too much or too little fuel, i thougth its too much so we where gonna knock down the primarys jets a couple sizes. It has 68 up front and 74 rr and a 6.5 pv. But i havent taken a vacuum reading to see if thats correct yet. The timming is set at stock specs which i think was 18 degree and im not sure what it comes out to at higher rpm. The carb has been cleaned fully and adjusted properly so im just wondering what i should start with, hope this is enough info, i'll get the vacuum reading tommorrow and try the jets anyways to see what happens. Thanks, Jake.

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Old 09-09-2008, 12:20 PM
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Almost time to park the car and tune up the sled...

Jake

I don't think you have an issue with jetting or choice of power valve here, they're pretty close to what the carb had out of the box.

You're asking a lot from the ignition and induction system when you floor the throttle and go from low RPM to full throttle - especially with a mechanical secondary carb on a single plane intake. This on a fairly heavy car with what I assume is stock gearing.

Perhaps do some basic tune-up stuff and ensure there is no slack in the accelerator pump arms, check float levels, make sure you've got a healthy ignition etc... It still may not fix the problem.

Make sure you are running some manifold vacuum to the distributor's advance can, and try dialling in as much initial advance as you can get away with to work with your large induction combo.

You may be able to get away with that 650 dbl pump on the original 2-plane intake if you still have it around, it was likely a square bore intake wasn't it?

Keep playing with that ignition timing...
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Old 09-09-2008, 01:28 PM
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Haven't owned a Holley in years but does yours have an accelerator pump on the secondaries? If so, you may need to ck it for operation or do some cam changes on the pump lever. Just a thought.


Mark
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Old 09-09-2008, 03:10 PM
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It sound to me, He has in automatic with a stock converter and Highway gears. If so, You need gears and higher stall converter. 650 D.P. should handle it quite easy. Also check the vacuum advance. If it's full vacuum on the dis. advance it will cause to stumble when you floor it. There should be no vacuum advance at idle. By the way, 750 vacuum secondary would be better off if the automatic and the rear end gears are stock.

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Old 09-09-2008, 07:44 PM
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well, you guessed it, its completly stock drivetrain also. I think the rear gears are somewhere around a 3.23 ratio. Right now im just trying to figure out if it sounds like its too rich or lean. I'm gonna take a vacuum reading as soon as he gets more premium fuel. It realy pulls hard top end, just that damn hesitation. Also when in gear if you burp the throttle when just creeping around it'll likely stall. The only problem i got is when he put a new timming chain in the chrome cover had no timing pointer, so i cant do much with the timming till we get something to us where the timming is. Although i did check it before and it was set to stock at i think 18 degrees. Hope this helps, Thanks for the replies, Jake
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Old 09-09-2008, 08:16 PM
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Unless there's a major problem with the accelerator pump(s) in the carb it sounds like spark timing is the issue. That Holley is all about pump shot, so it should be giving you plenty of fuel when you crack the throttle.

I assume you've checked for vacuum leaks too...

You're not doing too badly with 3.23 gears if that's what you have. When I was a kid my friend had a '69 Roadrunner with the 383 magnum, auto and 3.23 limited slip. What a crazy ride for a kid under 20 to drive. If the tires aren't too tall (the Roadrunner had 14" wheels) it should work OK.

The camshaft in the '69 383 was pretty healthy too - maybe yours is the same stick.

You could mark the base of the distributor against the block with pencil for a baseline and just keep bumping it advanced a bit at a time until it rattles a bit when you try to lug the engine, then tap it back a tad. So long as you have a mark from where you started you can't get too far lost on the spark timing.

Good luck with your Mopar, sounds like a fun ol' boat.
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Old 09-09-2008, 08:42 PM
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My mopar 383 4 spd with 323 gears I had to up the accell nozzels I run a 33 front and 31 rear to get rid of the hesitation but I run a 750 dp. What ever size your 650 came with you may have to up that size. Make sure the accell shot comes on the very instant the throttle moves. At wide open throttle the arm should have .015 play in it between the bolt with the spring and the small arm it pushes on.
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