Initial Timing 83 GMC 350 M code engine

[CoMtGuy]

I'm new here.... I'm curious about timing. I have an 83 GMC K2500 with an M code engine 8600 GVWR so minimal emissions gear, just a smog pump, dont see any EGR, no O2 sensor etc. QuadraJet carb.

My emission label says 4 degrees BTC (dist plugged off).

When I bought the truck my engine was after firing and popping out the exhaust. It stunk of raw fuel idling. I put in new plugs, wires, dist cap, rotor, new ECV's, new ICM, and replaced vac hoses where needed. I found the timing at about 12 degrees, and set it back to 4 degrees per the label. I adjusted the 2 idle mixture carb screws to lean it out, by dropping off the idle (turning in) 200 rpm then re-setting the idle speed screw to 700rpm like the label said. It idles better, starts easier and sets high idle pretty easy, doesn't want to come off high idle until temp reaches above 180, then kicks down easy enough, but I still have after firing only until the engine reaches temp, and when the engine is warm I get popping/gurgling in exhaust on initial accel in each gear. I can let off the throttle slightly and popping stops then accelerate and it goes smoothly up to high rm with no popping, with an occasional pop between gears when letting off the throttle during clutching. This makes me think the timing is off, that as soon as I let the mechanical decrease when letting off the throttle the advance drops and stops the popping... like it's too far advanced, which leads me to believe my label is perhaps not enough BTDC for my engine.

My question is... why does the label say 4 degrees BTDC when everything I read here says 8-12?! Should I push the timing to 8-12 and see if the after firing while cold (and popping between gears when warm) disappears?

 

[79K10]

im no timing pro, but i would def advance it some more, i dont think 4 degrees is enough. did you hook up a vacuum gauge and see where youre at while you move the dizzy? my truck never ran off what was under the hood, i tuned it how it liked to be tuned... but for smog, i guess youve gotta get it running off the specs to pass, then id dial it back to where it runs best

 

[firebane]

10-12 is fairly standard these days. 4-8 was more common for fuel emissions.

The nice thing about timing.. its easy to adjust and see how your truck responds.

 

[CoMtGuy]

im no timing pro, but i would def advance it some more, i dont think 4 degrees is enough. did you hook up a vacuum gauge and see where youre at while you move the dizzy? my truck never ran off what was under the hood, i tuned it how it liked to be tuned... but for smog, i guess youve gotta get it running off the specs to pass, then id dial it back to where it runs best

Thanks guys for the feedback, in relation to the above post.... what would I be looking for using my vacuum gauge? That I'm 15-20 when timed right (whatever "right" is for running smooth)? Or am I looking for a break point where the vac decreases/increases?

It's certainly easy to push it to 8-10 and run it there a week to see if the after-firing disappears, and as for passing emissions, I don't need to do that for a couple years since it passed a few months back. How it passed running so poorly and so rich it stank, but in Colorado anything 8600 GVWR and over doesn't have stringent standards. I'll let you know how it works out.

 

[rich weyand]

A stock HEI on a low-compression 350 can usually run all the way to 18-20* BTDC without any problems. More advance gives you more power and torque, and better fuel mileage, until you hit the knock limit. That's why they have knock sensors on modern engines. The computer advances the timing right to the knock limit and holds it there.

I am running 16* BTDC and have been for years.

Read posts #27 and #30 on this page. Also, read the corvette forum thread pointed at in #27. http://www.gmsquarebody.com/forum/showthread.php?t=2231&page=2

Oh, and disconnect the AIR pump and hook your vacuum advance to manifold vacuum, not that ported vacuum nonsense.

 

[CoMtGuy]

Thanks Rich, that sounds like good advice. I need to get a damper timing tape... that goofy tab only goes to about 10 or 12 BTDC. I know I'm now set (as of this morning) around 12, maybe a degree or two more since the damper mark is just off the tab. No popping or gurgling, nice and smooth, no after-fires during decel while in gear... for now I'm happy.

I noticed the PCV hose going into the carb plate is in poor shape and has some cracking... didn't seem like it was leaking - tried the starter fluid test. Old trucks, there's always something they need!

I also need to check bolts on the intake and carb, and do some hunting... the vacuum is low, around 12-13, steady though, drops a little when cracking the throttle then restores.

I appreciate everyone's advice!

 

[rich weyand]

Where is your vac advance can connected?

 

[CoMtGuy]

Rich, it's connected per the under-hood diagram; hose from the dist can goes to a vac delay valve, then to another valve that is switched by the thermal coolant vac switch, the source through the coolant switch ultimately comes from the top driver side on front of the carb.

I keep reading the suggestion to put my dist vac to the manifold, but I haven't found a port on the manifold- unless one is available at the back of the carb and I can't see it? There is a large port the PVC hose goes to on the plate between the carb and the manifold. There is another large port the brake booster is connected to on the manifold (which is where I hooked the vac gauge).

 

[350runner]

I'm new here.... I'm curious about timing. I have an 83 GMC K2500 with an M code engine 8600 GVWR so minimal emissions gear, just a smog pump, dont see any EGR, no O2 sensor etc. QuadraJet carb.

My emission label says 4 degrees BTC (dist plugged off).

When I bought the truck my engine was after firing and popping out the exhaust. It stunk of raw fuel idling. I put in new plugs, wires, dist cap, rotor, new ECV's, new ICM, and replaced vac hoses where needed. I found the timing at about 12 degrees, and set it back to 4 degrees per the label. I adjusted the 2 idle mixture carb screws to lean it out, by dropping off the idle (turning in) 200 rpm then re-setting the idle speed screw to 700rpm like the label said. It idles better, starts easier and sets high idle pretty easy, doesn't want to come off high idle until temp reaches above 180, then kicks down easy enough, but I still have after firing only until the engine reaches temp, and when the engine is warm I get popping/gurgling in exhaust on initial accel in each gear. I can let off the throttle slightly and popping stops then accelerate and it goes smoothly up to high rm with no popping, with an occasional pop between gears when letting off the throttle during clutching. This makes me think the timing is off, that as soon as I let the mechanical decrease when letting off the throttle the advance drops and stops the popping... like it's too far advanced, which leads me to believe my label is perhaps not enough BTDC for my engine.

My question is... why does the label say 4 degrees BTDC when everything I read here says 8-12?! Should I push the timing to 8-12 and see if the after firing while cold (and popping between gears when warm) disappears?

little help here... first the A/F screws must be set to an equal amount of turns in or out. Engine off turn them both in until they"seat" do not (crank/apply torque) them as they seat. Second back both of them out 2-1/2 turns. Adjusting the carburetor A/F screws must be done this way with a vacuum gauge. O otherwise it's just like the blind leading the blind. Second? By after firing do you mean when you turn the ignition switch off the engine sounds l like it wants to keep running but can't? This is called dieseling. Typical cause is the throttle being set to high (back out the throttle screw) or initial timing being set too high. Since you've done well by checking the timing and regarding it we can exclude the timing from this check l list

Sent from the dust in front of you!

 

[rich weyand]

By after-firing I think he means popping off in the exhaust system, because he was dumping unburned/rich mixture and he has the AIR pump still on it supplying oxygen.

 

[rich weyand]

Rich, it's connected per the under-hood diagram; hose from the dist can goes to a vac delay valve, then to another valve that is switched by the thermal coolant vac switch, the source through the coolant switch ultimately comes from the top driver side on front of the carb.

I keep reading the suggestion to put my dist vac to the manifold, but I haven't found a port on the manifold- unless one is available at the back of the carb and I can't see it? There is a large port the PVC hose goes to on the plate between the carb and the manifold. There is another large port the brake booster is connected to on the manifold (which is where I hooked the vac gauge).

All that crap is to try to make it run without manifold vacuum.

When people say hook to manifold vacuum, they don't mean to actually hook it to the manifold, just to a manifold vacuum (non-ported vacuum) source on the carb.

I found a pic someone made of the ports on the Qjet. I'm not sure if it's right, but a manifold vacuum source should read 15-20 at idle, while ported vacuum will read zero.

You must be registered for see images attach

I would try bypassing all that crap and connecting directly to the vac advance can from a non-ported vacuum source.

 

[CoMtGuy]

Rich Weyand is right... after firing (popping/burning in exhaust). I was taught back firing is through the carb or in the intake manifold, and after firing is too lean or too rich gas burning in the exhaust.

I was happy yesterday with the setup I achieved (thanks to everyone's advice!) and am still happy today after starting it cold this morning (30 degrees), running it to and from work (64 miles round trip). It was smooth, no popping or afterfiring even on downshifting and decel.

So next on my list is leaning the carb a bit more.... another post for another day. Thanks everyone for your help this last weekend, you helped me get this truck to running as sweet as I think it looks..... Controlled pearl base paints prove you really can make a pig look better with lipstick!

 

[CoMtGuy]

And thanks to 350Runner for the carb adjust advice, and the pic from Rich Weyand - while not my carb, helped me understand ported vs non-ported. As noted... next week or so I'll get to that! Thanks again guys! I'll pay it forward!