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Tuneup of '75 GMC K25

Discussion in 'Chevy Trucks' started by 75gmck25, Jul 29, 2003.

  1. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    I've been driving this truck for the past few months and it starts and runs fairly well.
    However, it has two major problems:
    - It sucks gas - this might not be easily fixable.
    - It is very gutless in the mid range, which makes it a challenge to merge into traffic. I can get a 3-2 downshift when I merge, but the power increase is minimal and acceleration is very slow.

    Background - Its a '75 K25 with 350-4V/TH350/NP203, 4.10 gears and 245/75R16 tires, HD emissions (no egr), HEI ignition, dual 2" exhaust with low restriction mufflers.

    I plan to start my tuneup with the ignition and am looking for suggestions or corrections to the following plan.
    - Replace plugs, cap and rotor with stock Delco parts.
    - Check timing vacuum advance and mechanical advance.
    Static timing is supposed to be about 6-8 BTDC, but what am I looking for on vacuum and mechanical advance at what RPM and what the best procedure for checking it? Do I need a timing tape, or can I estimate it with marks on the damper?
    - Check plug wires. I noticed that my inductive timing light has trouble getting a signal on the number one wire. Does this indicate a problem with the wires?

    I'm going for ignition work this weekend, and will attack the Quadrajet later on. My power problems may be carb related, but I want to check the ignition first.

    Thanks in advance for any comments/help.

  2. wyldman

    wyldman Member
    Messages: 3,265

    Ac Delco or Delphi R43TS plugs,gapped at .065-.080 thou.New cap,rotor and wires.Use lots of dielectric grease on plug boots and coil insulating washer.

    Get an advance curve kit,and install it when doing the rotor.Use the two medium springs in the kit to start.Make sure the weights are free,and the advance mechanism turn freely.

    I'd start with 10-12 initial timing,and you want the timing to start coming in just off idle.Total timing should be in the 38-42 range.Advance initial,and try the lighter springs until it just starts to ping,and then back it off a degree or two.Do all testing with vacuum advance disconnected.After you get the dist and timing set up,then insatll an adjustable vacuum advance,and adjust it to get max timing at cruise,with just a hint of pinging on light throttle tip in.Make sure vacuum advance runs from ported vacuum (no vac at idle,comes in gradually off idle),and that it runs through a temp switch to cut off advance when cold.

    When setting up the timing,you can use timing tape,but you must have the correct size for your balancer diameter.It's easier with an advance timing light.Borrow or rent one if neccesary.

    The timing light should still work with old wires,as long as that cylinder is firing.Are you sure it's got good power and grounds ? Is the inductive clamp on the right way ? Some have an arrow which must point towards the plug end.

    If you still feel the carb is a problem,then don't go too aggressive on the timing until you get the carb fixed or setup.It sounds like you may have sticking air valves,or the air valve tension spring is not setup right.
  3. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    Thanks for the suggestions, but I have a couple of questions.

    You recommended the R43TS at .065-.080, but I think the stock plug is an R44TX gapped at .060. What's the theory behind switching plugs and gap. I would guess that the wider gap gives a better spark, but I assume the heat range is also different on the R43TS. I live in San Antonio, so most of my driving is during hot and warm weather.

    You also mentioned an adjustable vacuum advance with a temp switch cutoff. I've seen adjustable advances, but I don't remember a temp switch. Is this a fairly standard item to buy?


  4. wyldman

    wyldman Member
    Messages: 3,265

    The R43TS is pretty much a standard small block Chevy spark plug,and is the correct heat range for most stock,and slightly modified or tuned motors.That's what I'd use,as the 43 is slightly colder than the 44,which responds better to more timing,and won't ping as much.The S plug has a slightly longer reach,to get out into the combustion chamber a bit more.The gap can be .065-.080.Start at .080,but if you start to see a lean miss sometimes,then drop the plug gaps down to .065,or even .060.

    Don't use anything but AC Delco or Deplhi (same plug),and don't go with the rapid fire's or any platinum.Not worth it.

    The temp switch usually screws into the t-stat housing or intake (any coolant passage) and block off vacuum advance when cold.This will prevent cold drivability problem.You may already have this setup now.A lot of vehicles have one right from the factory.If you don't have one,there are hundred's in the wrecking yards,or buy a new one from a parts store.

    You don't have to have it right now,as most of your driving will be done with the engine hot.It can be added later to help cold drivability.
  5. wyldman

    wyldman Member
    Messages: 3,265

    With the wires,you get get a set of Accel Super Stock universals (but for HEI),which you can route better than the OEM stuff.If you have the wires that go down behind the heads,and under the exhaust,you can reroute them down over the rocker covers out of harms way.

    If you need the part number for the correct wires,let me know,I have it somewhere.

    Now would be the time to do any leaky rocker cover gaskets too.