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  1. 75gmck25

    350 Water Pump Woes

    I used the cheapest Stewart high flow water pump and it’s been in my truck for at least 15 years. Keeps the truck cool without any problem. I also use a Stewart high flow 195 degree thermostat. My pump is the small block Stage One, and Summit has them for about $110. More expensive than a NAPA...
  2. 75gmck25

    Changing brake fluid

    If you have a newer design brake booster with a screw-on reservoir cap, it’s easy to connect a pressure bleeder and just bleed each wheel, one at a time. The bleeder both provides pressure and ensures you don’t introduce any air. Older trucks like mine have a flat brake booster cap with a wire...
  3. 75gmck25

    Adding 81-82 bumper fog lights

    If you want to add more lights, use a relay to connect it into the headlight or marker circuits and use them only to activate the lights. It puts a load on the battery, but no more amperage is drawn through the old lights wiring. Swapping a 94amp 12si v-belt alternator for your old 63 amp (or...
  4. 75gmck25

    Nothing happens when turning key. Better results when in neutral. Tried changing the Neutral Safety Switch

    Ignition switch moves a toothed rack in the column, which then moves a rod that runs up and down the column. The rod is connected to the electrical switch mounted lower on the column. Very common to have the pot metal rack break or crack so it does not move the rod correctly. GM ignition...
  5. 75gmck25

    Starter wiring problems

    The threaded terminal on the back of the alternator is the Bat terminal and it must be connected to battery positive. - original wiring for the charge wire ran from the Bat terminal along the top of the passenger side valve cover, down the back of the engine through a protective (from exhaust...
  6. 75gmck25

    Headlights flicker and radio cuts out when driving over a bump

    The wires with round barrel-shaped plastic around them are the fusible links. They are used on those wires instead of fuses. FYI - On my ‘75 they did not yet use the plastic barrel link - it’s just fusible link wire that burns up if the current is too high. You can get replacement wire ends...
  7. 75gmck25

    4.1 mpg?

    Make sure you are reading the vacuum advance specs correctly. Example - VC700 83 8-10 11.5@19-21 This means it - starts to advance at 8-10 inches of vacuum, and - maxes out at 11.5 degrees of distributor advance when it reaches 19-21 inches of vacuum Crankshaft timing is double the distributor...
  8. 75gmck25

    Picture Please: No Spare Tire Holder

    The stock under truck tire carrier works okay, but it’s an abomination that is a real PIA to use. I usually end up putting my spare onto a small floor jack to hold it up, so I can get it in to position and bolt the carrier arm back in place at the same time. My spare’s exact position is...
  9. 75gmck25

    Dual Tank Issues

    I’ve been working on my ‘75 GMC K25 for over 20 years, and this is the best and most complete description I’ve ever seen if of the squarebody dual tank system. My truck was originally a ‘75 single tank with a dealer installed aux tank. I used pick and pull parts to convert to the dual tank...
  10. 75gmck25

    parking brake lines and exhaust interfering

    GM changed the design of the parking brake cabling over the years. My ‘75 uses a single cable that is routed as an upside down U shape, with the legs of the U connected to each brake drum. On the drivers side it is pulled into the corner by the emergency brake pedal cable, and on the...
  11. 75gmck25

    What is this??

    If you have dual batteries, it could be the battery relay. On my K20 it connects the 2nd battery when the engine is running, so it will chsrge, and then disconnects it when you turn off the ignition, so it can power a camper.
  12. 75gmck25

    Rear leaf spring bracket to crossmember support?

    If you are talking about the one between the top and bottom of the frame, I think this is part of the bed support. It serves to strengthen the spring hangar where it bolts on, but it may not be listed as part of the hangar. LMC does have a part called a reinforcement brace for that spring...
  13. 75gmck25

    Gauge ground location

    Missing cluster ground will cause a variety of dash problems with lights and gauges. but for some reason the fuel gauge might not be affected. As mentioned, there is a ground post on the emergency brake pedal mechanism, and a black cluster ground is connected to it. On newer trucks they...
  14. 75gmck25

    Clean old windows- years of gunk!

    I read recently that a clay bar and lube kit will work. They sell them at the auto parts store, but I always assumed they were only for cleaning up dingy paint.
  15. 75gmck25

    Heater Core how to

    I would have to check my parts bin to get a parts number, but I once tried using a vacuum operated valve to bypass the heater core. I connected it into the vacuum line for the recirculate vacuum pod, so if I put the A/C into max/recirculate it shut off the water flow to the heater core. I...
  16. 75gmck25

    16 in Rims?

    I replaced the original 16.5” split rims on my ‘75 K25 with 16” rims from a newer squarebody. Used a pick and pull yard where I could verify they came from a squarebody truck and they were all the same width. Then bought GMC 16” hubcaps from somebody on eBay. 245/75R16 10 ply tires are about...
  17. 75gmck25

    Fusible link burning up key off

    Most likely cause - battery hooked up backward. Usually burns the link at the junction on the firewall next to the brake booster. Luckily it probably didn’t damage anything other than the fusible link. You don’t even need to ask why I know this can happen.
  18. 75gmck25

    Brake Confusion

    FYI - master cylinder has a relatively simple piston inside a tube that is pushing the brake fluid. One common failure on an old system is that as the cylinder heats up inside, a worn piston seal gets soft, and it no longer seals and pushes fluid. It may be fine when the brake fluid is cool...
  19. 75gmck25

    How do YOU set timing?

    Sounds like the cam will want quite a bit of timing , but with iron heads and 10:1 compression you will need premium gas to prevent pinging. Stock distributor with 20 degrees of vacuum advance may be too much for cruise (it may ping at light throttle) if you raise base timing. You may also...
  20. 75gmck25

    Charcoal Can

    GM seemed to change the vapor recovery design multiple times over the years, and for your ‘73 the original vacuum diagram may have been on the air cleaner, not the radiator support. The diagram is often missing. My ‘75 350 has a two port canister, and the 2nd line just ran to a dedicated...
  21. 75gmck25

    Starter wires near exhaust manifold

    Does your engine have a metal tube (about big enough to fit your thumb) bolted on the back of the engine near the firewall? On my truck that metal tube is bolted to one of the bell housing bolts and it provides a channel for the wires to the starter. That keeps them away from the exhaust...
  22. 75gmck25

    Rear leaf suspension 73C10 Stepside

    Total spring length is either 52” or 56”, and you can move (usually need to drill out rivets) the rear hangars to use either length. Some folks switch from 52” to 56” because it may ride smoother. You also need to verify the spring eye diameter, but fitment issues can usually be fixed by...
  23. 75gmck25

    HHHEEEEELLLLLLLPPPPPPP!!!!!! Update- problem unsolved again.

    How do you know the mixture goes super lean at highway cruise? Could it just be starved for fuel? Did you change anything on the fuel system when you changed the distributor? Is there a brand on your “full performance distributor”? HEI module, advance mechanism and other parts could be bad...
  24. 75gmck25

    91 Suburban Rear Speaker - Which Ohm?

    There is also a product called DeOxit, and it works well to clean up the contact on a reheostsat (volume control speaker fader,etc,) or a slide switch. IIRC, older vehicles in the 50s and 70s used mostly 8 ohm speakers, but at some point the industry transitioned to 4 ohm speakers. I don’t...
  25. 75gmck25

    1977 K20

    I would say Dana 44, although ‘77 was the year they started switching to the corporate 10 bolt. Both axles were used that year.

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