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Discussion Starter · #1 ·
Hi, I've been following the thread at http://www.impalassforum.com/vBulletin/showthread.php?t=247741 because the problem sounds a lot like mine. Engine stumbles at high rpm, just before changing up, then pulls hard till the next change. I've got a 383 low compression (9:1), supercharger, 50lb injectors, new Racetronix\Walbro pump and harness, MSD 6, MSD optispark and coil. Tune by Brian Herter. Having read the previous thread I strapped a fuel pressure gauge to the windshield and fitted the air\fuel gauge.

Took the car for a run. Pressure gauge rock solid at 45psi, but air\fuel dropped from 14.7 to 11.1 at WOT with engine stumbling.
Any thoughts\advice?
George
 

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Hi, I've been following the thread at http://www.impalassforum.com/vBulletin/showthread.php?t=247741 because the problem sounds a lot like mine. Engine stumbles at high rpm, just before changing up, then pulls hard till the next change. I've got a 383 low compression (9:1), supercharger, 50lb injectors, new RacetronixWalbro pump and harness, MSD 6, MSD optispark and coil. Tune by Brian Herter. Having read the previous thread I strapped a fuel pressure gauge to the windshield and fitted the airfuel gauge.

Took the car for a run. Pressure gauge rock solid at 45psi, but airfuel dropped from 14.7 to 11.1 at WOT with engine stumbling.
Any thoughtsadvice?
George
Welcome! What spark plugs are you running , and what gap are they set at?
 

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Discussion Starter · #7 ·
what rpm is it stumbling at and what valve springs are you using?
Valve springs are Comp Cams. Rest of spec below.


Block: Thermally cleaned and Stainless Steel Shot. Inspected by MPI process. Decked, bored, honed with torque-plates and pressure washed. Clevite cam bearings and pioneer brass freeze plugs installed.
4-bolt main caps and ARP main studs. Block is O-ringed

Crank: Eagle 4340 forged steel, internal balanced front and rear

Rods: Eagle 4340 forged steel H-beam, 5.7” length floating pin.

Pistons: SRP forged blower piston

Rings: Hastings Moly ring set

Bearings: Clevite H-Series Main and Rod Bearings

Main Studs: ARP Performance main studs

Lifters: Comp Cams Hydraulic Roller

Pushrods: Comp Cams Magnum

Valve Springs: Comp Cams

Rocker Arms: Comp Cams Pro Magnum 7/16 Full-Roller 1.5 Ratio

Timing Set: GM LT4 roller timing set

Timing Cover: Factory Aluminum timing cover

Oil Delivery: Melling high volume pump, pickup tube and HD Drive

Oil Pan: New factory 5-quart

Gasket Set: Fel-Pro Performance

Head Bolts: ARP Performance Hex Head

Cam:
Manufacturer: Comp Cams
Type: Hydraulic Roller Supercharger
Operating Range: 2300-6400 RPM
Duration: Intake: 214˚@ .050”
Exhaust: 224˚@ .050”
Lift: Intake: .531”
Exhaust: .568”
Lobe Separation: 115˚

Heads:

Type: GM Aluminum LT1, Five angle performance valve job, Lloyd Elliot (LE2) porting, valves back cut and new valve guides

Combustion Chamber: 54cc
Intake Runner: 205 cc
Intake Valves: Manley 2.00”
Exhaust Valves: Manley 1.56”

george
 

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Discussion Starter · #10 ·
Forgot to answer all your question in my last post. Starts stumbling at about 4500rpm and continues to gear change.
George
Correction Just checked again and it starts arrround 5,000 and continues to gear change. Shift points are set at 1 - 2 5,200, 2 - 3 5,200 and 3 - 4 at 5150.

Not sure if the 11.1 air\fuel ratio is cause or effect. It shows 11.1 when it stumbles, but that could just as easy be unburned fuel as a result of the misfire. Up to the point that it misfires it's in the low 12's

I've got LT1edit. Would it be safe to drop the shift points to say 4000 and see if the problem follows, to rule out transmission issues?

george
Thread: Engine stumbles at WOT Reply to Thread
 

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Had high RPM stumble years ago. Turned out to be corrosion at the Coil wire/coil connection. Easy to check and clean.

HTH
 

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Discussion Starter · #12 ·
Had high RPM stumble years ago. Turned out to be corrosion at the Coil wire/coil connection. Easy to check and clean.

HTH
Checked out and looks OK. Everything replaced within the last 2,000 miles when the new engine went in. (and car doesn't go out if it's raining) I think I've had this problem since the new engine went in, only really noticed the stumble when I started taking it to the strip. Best I've managed is 14.2 @ 102mph and I was doing better than that on the old 350.

george
 
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