Bog and pop due to thermoquad, vacuum advance or old distributor?

[Northcoast300h]

I'm a tinkerer of everything but an expert at nothing. After 15yrs of this , I think it might be a very old distributor but not sure. Original 440, 150K which originally had holley which ran bad all the time. Car was abused before I got it. 15yrs ago while restoring car, I bought a rebuilt 1972 TQ to 440 spec which has been an excellent carb during normal driving in all weather conditions, easy starting, no stalling, no constant adjusting and great gas mileage. The only problem is when I floor it (which is seldom). Most of the time, it bogs out to the point where it sounds like I turned off the car, then I let go of accelerator and stomp on it again, then it usually takes off, (sometimes I have to do that twice) Yes, I've played around with the secondary spring screw to the point where the secondaries open very slowly (so no power), checked vacuum dashpot, metering rods, mix screws, etc and I still get the bog and pop. Sometimes it bogs a little, then backfires thru the carb, then takes off. One time it exploded in the mufflers and now car sounds pretty cool. However, car accelerates better after I stomp on it first 1-2 times. Usually bog or sputtering is less to none for whole day until next time I take it out for a ride. I changed to a Petronix in original distributor which may have some play on shaft, not sure how to check. Runs smoother but didn't help bog and pop. Installed crane adjustable vacuum advance but how do I adjust to stop bog and pop? Anyone know if I used correct vacuum nipple on the carb for vacuum advance? (see pic) Will a new distributor fix this? Thanks for your help? Timing is a little advanced since it seems to start and run the best that way. Anyone know what I'm doing wrong?

 

[Blish]

I had to replace the distributor CAP on my 400 because of a similar issue. It would just bog at low rpm but any other time it ran great. Even after a carb rebuild it would bog. Turns out the cap clips weren't holding it tight. So it would actually pick up the cap somehow and momentarily lose connection.

Other then that I see no reason to be bogging down. May just need a carb rebuild. It happens when they sit, all the rubber pieces dry rot and the metal freezes up.

May just need to tune on the carb a little too, or change your jet sizes.

 

[Davea Lux]

Has the accelerator pump ever been replaced. Ethanol blend fuels eat up the leather parts and rubber parts. Popping on a hard stomp is a classic symptom of a bad accelerator pump. Without the squirt of fuel the engine leans out.

Dave

 

[thethee]

That is the correct vacuum nipple for vacuum advance. I was able to fix my bog by substantially richening the metering rods.

 

[Mr C]

How much slop is in the timing chain? Ever replaced?

 

[halifaxhops]

I had to replace the distributor CAP on my 400 because of a similar issue. It would just bog at low rpm but any other time it ran great. Even after a carb rebuild it would bog. Turns out the cap clips weren't holding it tight. So it would actually pick up the cap somehow and momentarily lose connection.

Other then that I see no reason to be bogging down. May just need a carb rebuild. It happens when they sit, all the rubber pieces dry rot and the metal freezes up.

May just need to tune on the carb a little too, or change your jet sizes.

Agree on the cap and the clips. Also make sure your mechanical in the dist is free, And vacuum advance is on ported vacuum and functional. Easy check for the Mech advance cap off and see if the rotor will turn a bit freely as the springs insie stretch and return by itself. No one ever oils them. If needed I have all the parts available.

 

[CBODY67]

I concur on the accel pump issue. Other than having a drilled passageway, the accel pump output on the TQ runs though a plastic TUBE to the accel pump shooter, from the pump itself. Same on the TQ-like Street Demon carb! Not sure how that TUBE reacts to ethanol'd fuel blends.

Distributor and timing chain might be contributing factors, but likely not. It's easy enough to install a full-electronic distributor from E-berg, Mancini, etc., if needed.

ONE thing that is NOT at play with the TQ, but can be with a Rochester QJet is sticking primary metering rods. On the TQ, there is a flat plastic paddle the bottom of the primary power piston (which the metering rods attach to), which is run be a cam on the primary throttle shaft, which manually moves the power piston up and down against manifold vacuum, which keeps the power piston at the bottom of its travel against the paddle.

The vac advance is adjusted via an Allen wrench which will go inside of the nipple on the vac advance can, just barely. There is a similar Allen nut which it indexes with to change the spring pressure which determines how fast or slow the vac advance comes in. There should have been some instructions, somewhere, on how to do this, I suspect. MANY aftermarket replacement and probably some OEM replacement vac advance cans CAN tend to be adjustable, just that we are not told that, I suspect.

The vac advance should be attached to a PORTED vac source. That means the vac nipple which has very little vac at hot base curb idle, but gets lots more vac as the throttle is increased for cruising and such. Like over 1000rpm and such. You can check which is which with a finger test at idle, usually.

The issue with the accel pump output tube has NOTHING to do with "professionally rebuilt" or similar, but with the quality of the tube itself. As I recall, it was a hard plastic tube, which is internal to the carb float bowl area. I also suspect that in many cases, it might not even be touched or replaced in a rebuild.

Also, check for the correct carb base gasket and its integrity. Hopefully, you are using a spreadbore intake which would not need an adapter to make the TQ work on it.

Just some thoughts,
CBODY67

 

[GJS]

Second the timing chain ... if it's never been replaced it's over due by about 50K miles on a 100k mile engine..... this is not to say that there aren't a host of other issues but this is almost definately one.

 

[halifaxhops]

Just noticed the elepant in the pic. Change the vac line onto the other nipple you are getting full vacuum advance from the start.

 

[thethee]

Your current vacuum connection is correct and is PORTED vacuum (says 1976-1977 but applicable to earlier years):

 

[Lefty71]

Will a new distributor fix this?

If your car is utilizing that small retard box (black?) on the side of the distributor, then yes, it can have an effect on the low speed performance. I changed mine out years ago and it made a huge difference in off idle performance on day one (on a '71 440).

 

[70bigblockdodge]

I'm guessing the distributor vacuum pot has a hole(not holding vacuum).

 

[lemondana]

That is not a 72 440 TQ. 440's did not get the Thermoquad till 73. That is a later model carb. What rebuilder did it come from? You could have mismatched carb parts-base-bowl-and top horn, they won't work as they are supposed to ,having mismatched parts. Post the number on the rear left foot of the carb, so we can see what you actually have.

 

[Paul Clark]

In my experience with the TQ carbs you will get the the famous bog w/back fire when the secondaries open to fast. Just my opinion but have fixed(or have seen) many fixed with that simple adjustment. If that does not fix it I have seen the plastic bowl warp or crack causing issues too.

 

[Paul Clark]

That car should have an AFB on it if I am not mistaken.....

 

[70bigblockdodge]

Went back and reread. I'm going to say you have a ignition problem. Carb problem with age on it it usually too much fuel (flooding) not what you have due to bog. Exploding muffler is because unburned fuel it in the exhaust, didn't burn in the cylinder.
I would start with a complete tune up, especially if it has not been done in a while. Sounds like point gap problem, but you have a petronix lawn mower repair system on there so ?

 

[CBODY67]

(Pertronix lawn mower system?)

 

[Lefty71]

That car should have an AFB on it if I am not mistaken.....

I didnt see where he said what car lol.... but given the avatar, i would say AVS, not AFB...... I had an OEM carb chart here somewhere....

 

[70bigblockdodge]

(Pertronix lawn mower system?)

It's how you fix your lawn mower when the condenser fails. You don't have to pull the flywheel. Works on chain saws also.
You would not want to fix the points properly, just keep adding switches till it runs.
Amazing how tv shows influence so many.
Rant off.

 

[Northcoast300h]

I appreciate everyone's help. I've done the tune up, played with vacuum and tried a spare Holley and the bog problem is still there (but a little better than TQ.) So if there is too much play in the 51 yr old original distributor shaft, timing chain and valves, with 150K on it, car will still runs great on primaries 2 barrel, but will bog out and backfire thru carb when secondaries 4 barrel opens up at WOT? I thought when those things were worn, any car would run like crap at any speed? My engine doesn't make any weird noises or pings. If it was you, what would you replace first?