85 Silverado

Discussion in 'General Chevy & GM Tech Questions' started by Dogtrainer, Jan 10, 2011.

  1. Dogtrainer

    Dogtrainer New Member

    First of All HI,

    Second, Heeeeeelllpppppp. Yanked the worn 305 out of the 85 for my daughter as a gift for graduation and installed a brand new (rebuilt) vortech 350. Had it machined and bored out and we painted and installed the motor. Put a Eldabrock performer intake, vortech heads, shaved, and headers. Rebuilt the Quadrajet and installed a electric fuel pump. I assisted with the help of my dad. Dual exhaust and new distributer with accell coil.

    Truck runs good at idle but under load or when you get to about 35 it bogs down. anything less than just a little accelator causes it to bog down under load...or hitting a hill.
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    I'm assuming you have the HEI distributor in it. What is your initial crank timing? With the L-31 Vortec cast heads you want about 8 to 12 degrees initial, 10 mechanical advance & another 10 vacuum for best power. These heads don't like more than 30 to 35 total from all the tests that have been done on them. If you're looking for mileage, you'll want 40 to 45 total, but from the sounds of the build you're looking for performance so disregard the 40-45 & go with 30 total.
    If all that is in order, then you have a carb problem. You are getting a big gulp of air & no fuel to burn with it. It sounds like it's fine on the idle circuits, & accel pump, but the transition between the primary fuel & midrange isn't working properly. Go back to your rebuild instructions & double check your adjustments. Also can you get it to falter while it is sitting & you rev the snot out of it? If you can, then pull the aircleaner so you can watch the secondaries while you snap the throttle open. If they're trying to open but are stuttering, or if they are trying to open too soon, this could also be your problem.
  3. Taddpole

    Taddpole Member

    Dogtrainer heres somthing else to try. I'm not sure what type of fuel tank your truck has but sometimes the screened pickup in the tank will clog up. The symptoms are like yours. If you check the fuel pressure it seems ok but there isn't enough volume to keep the carb. filled up under a load. I used to clean the screen by blowing a shot of air back into the fuel line towards the tank (be sure to take the gas cap off). Thats assuming you changed your fuel filters including the tiny one inside the q-jet input fitting. Maybe this will help.

    P.S. The wife & I Gunned at many a dog trial, pointers, setters & retrivers.
  4. Dogtrainer

    Dogtrainer New Member


    originally running 7psi but changed it back to between 41/2 to 5 after talking to the rep for the eletronic fuel pump. he said you need volumn not psi. Crazy thing is in 1 2nd, or 3 geat runs a little better and wont bog down as much,. in overdrive it runsh like crap and any slight hill make it bog down. shift down and you have better response. Pressure gage shows plenty of fuel getting to the carb. maybe need to throw the Quad oout and get a simpler carb.

    Second thing is after swapping out motor getting booster vacum leak when i step on the brake only. changed the booster out before the big cold snap and still have booster vacuum noise??????? when brake is depressed. Are they tied together.
  5. stephan

    stephan Rockstar 4 Years 5000 Posts

    I wouldn't pitch the carb just yet, but if you do, Holleys are really simple, a whole lot easier to work on & are very tuneable.
    A vacuum booster that makes a vacuum noise/hissing sound when you apply the brakes is an indication of a leaky diaphram so replacing it was the logical thing to do, but I don't understand why your new one does the same thing. I know you've checked the obvious like the valve, grommet, hose connection etc. Was this replacement a first line manufacturer?
    Yes, a bad vacuum leak could cause your running problems, but unless this booster leaks all the time it shouldn't be the cause of your running problems. It should only be dumping vacuum when you're on the brakes. Just for test purposes, you could plug the line & go test drive it (caution: your brakes won't work worth a damn so be carefull) This would eliminate the booster as being a contributor, & also tell you if it was leaking all the time.
  6. Chris Miller

    Chris Miller Rockstar 100 Posts

    That could definitely be the cause. Did you change the little check valve that plugs into the booster?
    There's a risky test you can try. Disconnect the booster and plug the line going to the intake. Find a deserted stretch of road and see if it drives any better.
  7. Dogtrainer

    Dogtrainer New Member

    More on 85

    JUst had it put on the scope and tersting it and the distributer and wires are not breaking down. Plenty of pwer.

    Carb may need a little adjustment but this is what my dad wanted to ask you guys. Runs great at idle. revs up great at idle. 1 2nd or 3 gear runs pretty good. Overdrive bogs down.

    Questions is: I am running headers and straight pipes, very little exhaust and they think that i dont have enough back pressure. That this motor required some or more back pressure to run right.

    Could this be the case?

    ---------- Post added at 04:36 PM ---------- Previous post was at 04:35 PM ----------

    brake booster is fine now. need help on the motor question!!!
  8. stephan

    stephan Rockstar 4 Years 5000 Posts

    NOPE, the only thing back pressure does is moves the torque curve up or down in the RPM band. If you had too much back pressure such as a plugged muffler or cats, then yes that could cause this.
    Have you ever checked your timing as in my first post? Did they check the timing today? If so what were the initial, mechanical, vacuum & total
  9. Dogtrainer

    Dogtrainer New Member

    no, crazy old men think i need to stuff potatoes in muffler to cause backpressure. will answer question tomorrow.
  10. stephan

    stephan Rockstar 4 Years 5000 Posts

    try sticking some taters in your old mans exhaust pipe tomorrow, then ask em about that backpressure :)

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