GMC 250 Inline 6 Tuneup

[68gmcdude]

Hello everyone. I am looking for help with tuning my 250 inline 6. I am looking for the steps to take but most importantly, the order I need to do them in. The carburetor and distributor have both been rebuilt. Spark plugs, wires, points, cap and rotor have also been replaced. Where do I start? Setting RPM? Points? Timing? I have a service manual so I know what the settings should be, but I'm looking for the order I should do these in. Thank you for your time!

['68OrangeSunshine]

Points .019 gap. [New]
Plugs .035 [for points]
Timing on Cyl 1 at TDC/0
153624 Firing order.

[68gmcdude]

Thanks for the reply. I'm also looking for the order in which I should do the tuneup. Do I start with the timing? And how do I make the adjustments to the RPM's and fuel mixture on the carburetor... thanks!

[geezer#99]

First thing.
Does it start and idle right now?
If yes, warm it up and then set the timing.
Adjust carb after that.

[68gmcdude]

It runs very rough. My guess is the timing is off but I want to check everything

[geezer#99]

Plug off the vac advance on the distributor and set your initial timing at 10 degrees.
Your manual likely says the timing is lower but for diagnostics set it at 10.
Then hook the vac advance back up and let us know if it runs better.

Did you set the point gap with a feeler gauge?

[Ol Blue K20]

Make sure to set th points before setting the timing.

[68gmcdude]

I did set the points gap to .019. Just curious, why set initial timing at 10 degrees? I thought timing on these 250's was 0. Also, I forgot to mention that my 250 has the factory California smog pump. The truck is all original so I'm keeping it on.

[Carl Spangler]

In my experience the most critical 'tuneup steps' are to set the timing and dwell. The dwell is very important............ try to get it at least 32 degrees. Some aftermarket point sets don't have enough adjustment to get 30+ degrees. If that is the case, replace the points or file the hold down slot open a bit.

Carb adjustment after the timing is set.

[geezer#99]

Originally Posted by 68gmcdude

I did set the points gap to .019. Just curious, why set initial timing at 10 degrees? I thought timing on these 250's was 0. Also, I forgot to mention that my 250 has the factory California smog pump. The truck is all original so I'm keeping it on.

10 degrees is so it will run better for testing purposes.
If 10 degrees doesn’t change the rough idle then either your point gap is wrong or it’s your carb.

It’s just a process of elimination getting your tune right.

['68OrangeSunshine]

68gmcdude -- Do you have a Dwell Meter? Some were sold as Dwell/Tach/Volt Meters. Very handy for tuning with points.

[RichardJ]

>>I did set the points gap to .019. Just curious, why set initial timing at 10 degrees? I thought timing on these 250's was 0.<<

The reason some are suggesting a Dwell meter is because many have trouble setting .019" accurately. It takes a delicate touch. Without knowing your skill level, you should understand that many guys are ham-fisted and will end up with a gap much smaller than .019" without realizing. It's best to pull the feeler out of the set and hold the single feeler.

Any change in the dwell will move the timing mark, so dwell must be done first.

Timing 2nd.

Idle speed 3rd. Set idle speed screw to 600 or as close to 600 as you can.

Adjust mixture screw. Readjust speed screw back to 600. Readjust mixture screw if needed.

Set the timing to 0 or 10 as needed? If it pings @10, retard the timing. If you start driving with the timing @0 , you can experiment by adding a couple degrees each time you drive it.
If it pings when you lug the engine, don't lug it, downshift, or get higher octane gas, or retard the timing a couple degrees, or all the above. Just don't let it continue to ping. Downshift.