Hey guys, I'm a little puzzled as to what my issue is here (I'm new to wrenching on sbc's).

I was having issues with bogging when the car was cold. It would idle fine, but as soon as I would try to take off, it would sputter and bog. So I'd have to get the revs up a bit and then dump the clutch to get going from a stop. Once the car warmed up after ~5 minutes, it would be fine, but I'd still get the occasional misfire. I did not have the vacuum advance hooked up and I was getting about 10-12 mpg.

So my buddy came over with his fancy timing light and we timed it based on the mark on my balancer. We checked that all the plug wires were in the correct order and they were.

When we checked what the initial timing was set at, we got a reading of 40 degrees! That was with the vacuum plugged on the carb. Neither of us could believe that it was even running as smooth as it was, but we dialled it down to 12 degrees.

When we took it for a drive after that, it was running beautifully. No bog at all and nice smooth power delivery. I thought we totally fixed it.

But now in the cold, it bogs worse than before, to the point where it will stall unless I let it heat up for 5+ minutes. And it doesn't seem like I can floor it without bogging/backfiring until it's been running for 15+ minutes. It also feels like I'm down on power by quite a bit.

I've got a M4ME Quadrajet (17085581) which I believe came stock on my Canadian 305 H.O. car; the electric choke is not connected. I've got an L31 block 350 that I think is basically stock.

So what's the deal? Is my timing mark wrong? Is the choke somehow operating even though it's not electrically connected? Like I said, once it warms up it runs very smoothly, but it feels less powerful than before.

 

The deal is you set the initial timing at idle with the vacuum advance disconnected and plugged. 8 to 12 degrees BTDC is what the typical SBC likes.

When you reconnect the vacuum advance, it should shoot up beyond the timing tab.

The "real" way to set timing for power is to disconnect the vacuum advance, run the RPMs up until it has achieved full mechanical advance, and set the timing to 34-36 degrees (which will be off of the factory timing tab - you either need to put timing tape on the damper, or get a dial-back timing light). Let the idle advance be what it wants to be. Most factory distributors don't have much mechanical advance built into them, so doing this can cause starter drag/kick-back with the initial advance being high enough to get 34-36 degrees full advance.

Having said all that...

Since your car now has a 350, did you do the swap yourself? Do you know what front cover and damper were used? It's not unusual to get a mix of two types, since one type had the timing mark at 12 o'clock when cylinder #1 is at TDC, and the other had it at about 2 o'clock when cylinder #1 is at TDC - damper and timing tab both have to be of the same type. Both 305s and 350s used both types, and 3rd gens even used both types. It's also not unusual for the damper outer ring to slip on the hub, making the timing mark off to TDC, even if the damper & timing tab are both of the same type.

The best way to know for sure is to get a piston stop and remark the damper for the true TDC.

However, another common cause of off-idle q-jet bog is the thermovac system in the air cleaner not working. Something tells me you have an open air cleaner, meaning you don't have any thermovac system at all.

Not having a functional choke isn't helping you, either. If it isn't electrically connected, it has to be manually adjusted so it fully opens when cold, because engine compartment heat isn't enough to get it to fully open on its own. A partially-warmed/fully-opened-choke engine usually isn't a happy engine.

You shouldn't expect the engine to be fully warmed until it's been running for 10-15 minutes, and you shouldn't be going WOT before the engine is warmed.

 

What five7kid said.

 

Did I miss anything???

 

No, not really.

I'd check to make sure the pump shot from the accelerator pump circuit is working properly.

If the carb is used I'd also make sure the pull-over-enrichment tubes on the secondary side haven't fallen out of the underside of the airhorn casting. (Internal carb issue)

I'd also make sure the vacuum pull-off on the pass side/front of the carb is working properly and is properly adjusted, too.

Aside from that I think you pretty much covered it.

 

I didn't do the 350 swap and I'm not very confident in the skills that the previous owner had either.

I'll check to see if my timing mark is off based on TDC.

I also have a spare Holley 4160 lying around that I might put on for the mean time. I know it's not as good of a carb as the Q-Jet, but I think if I remove the Q-Jet and give it a good rebuild, it will make a difference. It doesn't look like it's have a good cleaning in twenty years.

 

With your L31 having Vortec heads, the ideal total timing is more like 29 degrees, rather than the 36-38 that older heads needed.

 

I didn't see any reference to L31 or Vortec heads, but that is true, they don't need as much timing because of the improved chamber design. However, the best-best way to determine total timing as always is on a dyno to see what the engine likes best.