PHP: WHAT IS YOUR FUEL PRESSURE when the engine is running bad? I tried this weekend to install a mechanical 0-15 PSI fuel pressure gauge so I could view it while driving but was unsuccessful. The hose would not fit through the cowl to my windshield without getting a severe kink when closing the hood. I am trying to find a local shop with a rolling dyno so I can test the fuel pressure under load.
Just pull off the hood for a few minutes, but use a tiny bit of paint to mark where the hinge sat on the hood for allignment.
Probably won't be a fuel pressure problem but more like a jetting problem. I have a 750 on mine and was at 78 fronts and 82 rears and thats when it started the stumble. I then went way down to 72 fronts and the problem is gone. You could also have too much timing in and that can cause a stumble also....how much total advance do you have at the stumble rpm ? Mark
Is the carb you are using the street advenger? I've sceen just what you are describing with them alot and various vehilces up here.
The problem does not live in the carb. I put a BRAND new carb on this weekend and still have the same problem.
UPDATE INITIAL PROBLEM SYMPTOMS (2 Weeks Ago): 1. Strong acceleration under full throttle up until around 3500 RPM then the motor stumbles HARD and falls flat on its face. When I let off the gas it returns to driving normally. 2. Idles low but with no signs of a "miss" due to a bad spark plug wire, etc. 3. Car still has enough power below 3000 RPM to lay a good 20 foot patch. 4. Under light throttle the stumble is not present but the throttle response is a lazy compared to when it has a good tune up. WHAT I'VE DONE SINCE THE PROBLEM STARTED: 1. NEW Fuel Pump . Old pump was leaking badly. New pump seems to have good flow at idle. I have a see through filter and fuel is flowing good at rest. 2. NEW soft fuel lines at tank, pump, and carb. 3. NEW Edelbrock 600 CFM Performer Carb installed and tuned (Same as old Carb). 4. NEW points & condensor, NEW rotor, NEW spark plugs & plug wires, 5. FULL TUNE UP - Set timing 10 BTDC, verified mechanical timing and vacuum are working properly using a dial-in timing light. 6. Checked for Vacuum leaks at idle. None found. 7. NEW MSD Blaster 2 coil. (Same as old) CURRENT PROBLEM SYMPTOMS (AFTER EVERYTHING I'VE DONE): 1. POOR acceleration under partial or full throttle. Under WOT at around 3500 RPM the motor stumbles HARD and falls flat on its face. When I let off the gas it returns to driving normally. 2. With the transmission (TH350) in D1 the engine stumbles under medium throttle and falls flat on its face under WOT at 3500RPM. It does exactly the same thing with the tranny in drive. 3. Idles decent with no signs of a "miss" due to a bad spark plug wire, etc. 4. Under light throttle there is a slight stumble. I feel a slight miss / hesitation at around 30% throttle and BIG hesitation at full throttle. 5. At 45 MPH in third gear when I give WOT the transmission shifts to first at just "bounces" at 3500 RPM but does not accelerate at all. It feels like a when you hit a REV limiter in a modern car. NEXT STEPS: 1. Fuel pressure gauge is on the way. I'm going to install it at the carb inlet and check pressure under load. 2. ????????? ANY INPUT IS GREATLY APPRECIATED! Thanks!
two things come to mind. have u checked the distributor endplay. meaning is there alot of up and down of the distributor shaft. if so u would need to shim. second, i know u have checked for vacuum leaks, but how about the intake manifold gasket. i remember an old phr magazine article where one of the editors had same problem as urs. it turned out to be intake gasket. keep at it and good luck:TU: ralph sr.
How many inches of mercury does it pull at idle? Try this... disconnect the transmission modulator and the brake booster and plug the origin source and check it and see if it's different..
I don't have a clue, but have learned a lot reading these posts.:gp: Can't wait to see how it turns out, like watching a soap opera only a lot better!
Try getting on it with the gas cap removed to eliminate the possibility of a tank vent problem. With everything else you covered it's most likely fuel delivery or weak valve springs as mentioned earlier. I've seen the springs require replacement on 40k mile cars before, I consider them a wear item. Wouldn't hurt to pull the valve covers and check.
Regarding the valve springs, they seem to go bad the less the car is driven from what I've found. Daily drivers are usually ok while low milage cars need them more often.
Valve springs will not fail from storage in a compressed condition, even at coil bind in a vise unless they were manufactured/heat treated incorrectly. A spring must be pushed past it's elastic limit to lose it's properties. The biggest factor in spring failure is "spring surge", or spring motion due to harmonics and not the simple mechanical actuation. Not positively locating the springs, spring rub between inners/outers, excessive rpm are all contributors. The new "beehive" design is supposed to do a really good job of controlling harmonics. Severe heating cycles can also take their toll on springs. Another factor many don't know is that the cam break-in period is doing a number on the springs via heat, and that they greatly benefit from a cool down after that 20-30 mins of higher rpm operation. Devon
Top killers of valve springs are revving quickly before they are at temperature, and over revving. Happens of the beehives too, that's standard issue on LS series engines.
This does seem to sound like a dist. problem. This could also be a gas problem also but I think it is not because if it was running out of gas at 3500rpm when you let off the gas the car would stall. You had said that when you let off the gas you would go back to idle with engine runing. So I would go back to the dist. and see if you do have the end play in it. Maybe try a different distributor and see if that fixes it.
The only other thing that you could check is the sock in the gas tank. Mine was smashed shut but you could still blow thru it. I still replaced it.
