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4spta
Tribal Scout

USA
20 Posts

Posted - 11 Oct 2017 :  9:12:59 PM  Show Profile  Reply with Quote
My brakes work fine around town normal driving but when I have to step on them really hard the motor vacuum and RPM drop and it almost wants to die what causes this and how do I correct it could just be a carburetor issue thanks everyone

400/467cu.310cfm kauffman D-ports w/1.65 steel roller rocker arms. XE274-H cam.Forged steel Ohio crank.Eagle rods. Icon pistons.Balanced.HSD intake.750mech.sec Holley.1-3/4in hedders.3 in collectors.2-1/2in exhaust.Flowmasters.ST-10 tranny.4.10 gears. 6 point cage.subframe connectors.every bushing polyurethane. F/R drag shocks line lock. Lightened up front end. Appox.3500lbs

Bill Boyle
Horse Feathers (Charter Member)

USA
4794 Posts

Posted - 12 Oct 2017 :  07:49:00 AM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
Jeff--I've experienced perhaps a similar behavior when auto crossing the TA. On a few occasions where I was hard charging and had to do a panic stop, the brakes worked like they should but the engine almost stalled when the car came to a stop. As I recall this didn't happen in tight turns but when racing full steam toward the last electronic light at the end of the run and there was a very short distance to brake. I attributed this to the carb. I don't recall it ever happening on the street, but once on the highway when traffic came to a sudden unexpected stop. It could of also been attributable to me being "late" pushing in the clutch to disengage from the engine.

How does this differ from you're situation? Is there a difference? Please elaborate further.
Bill

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.
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Blued and Painted
Chief PONTIAC

USA
3405 Posts

Posted - 12 Oct 2017 :  11:05:44 AM  Show Profile  Reply with Quote
It could be a leaking brake booster. Is it new?
With car stopped but running, mash the peddle firmly then switch off the vehicle. The peddle should not move. If the peddle moves upward when switched off, the booster or check valve is leaking.

It's helpful to enter a brief discription of car and power train in the signature line.
Thanks, B&P.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 12 Oct 2017 11:22:30 AM
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4spta
Tribal Scout

USA
20 Posts

Posted - 12 Oct 2017 :  4:08:01 PM  Show Profile  Reply with Quote
It happens during last minute braking comming to a quick stop. Or if u have to jump on the brakes during a panic stop at 40 mph or less

400/467cu.310cfm kauffman D-ports w/1.65 steel roller rocker arms. XE274-H cam.Forged steel Ohio crank.Eagle rods. Icon pistons.Balanced.HSD intake.750mech.sec Holley.1-3/4in hedders.3 in collectors.2-1/2in exhaust.Flowmasters.ST-10 tranny.4.10 gears. 6 point cage.subframe connectors.every bushing polyurethane. F/R drag shocks line lock. Lightened up front end. Appox.3500lbs

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4spta
Tribal Scout

USA
20 Posts

Posted - 12 Oct 2017 :  4:14:07 PM  Show Profile  Reply with Quote
Blue ill try your suggestion with the booster test and see what happens. Sure seems vacuum related.

400/467cu.310cfm kauffman D-ports w/1.65 steel roller rocker arms. XE274-H cam.Forged steel Ohio crank.Eagle rods. Icon pistons.Balanced.HSD intake.750mech.sec Holley.1-3/4in hedders.3 in collectors.2-1/2in exhaust.Flowmasters.ST-10 tranny.4.10 gears. 6 point cage.subframe connectors.every bushing polyurethane. F/R drag shocks line lock. Lightened up front end. Appox.3500lbs

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Blued and Painted
Chief PONTIAC

USA
3405 Posts

Posted - 14 Oct 2017 :  12:19:50 PM  Show Profile  Reply with Quote
Symptoms above could also be "brake fade". Where the peddle pressure remains the same but brake efficiency goes away.
This can b caused by worn out friction material, overheated friction material, budget/cheep pads or shoes, drums or rotors milled past minimum thickness, rear flex hose blocked/defective.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 14 Oct 2017 12:26:49 PM
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Phil
The Great White Buffalo

USA
7215 Posts

Posted - 14 Oct 2017 :  12:28:04 PM  Show Profile  Reply with Quote
Sure sounds like a carb issue. What carb are you running? I've had center hung Holleys do this, for example.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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4spta
Tribal Scout

USA
20 Posts

Posted - 15 Oct 2017 :  9:29:36 PM  Show Profile  Reply with Quote
Hi Phil. Im running a 750 mech sec. Holley w\ #74 main jets and #80 for the secondary.Stock squirters and cam.6.5 pwr valve. Pretty simple set up.

400/467cu.310cfm kauffman D-ports w/1.65 steel roller rocker arms. XE274-H cam.Forged steel Ohio crank.Eagle rods. Icon pistons.Balanced.HSD intake.750mech.sec Holley.1-3/4in hedders.3 in collectors.2-1/2in exhaust.Flowmasters.ST-10 tranny.4.10 gears. 6 point cage.subframe connectors.every bushing polyurethane. F/R drag shocks line lock. Lightened up front end. Appox.3500lbs

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