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Thermo-quad? Vibration at WOT underload, runs smooth when revving in neutral and on jack stands

Triple Black 73

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You guys have been great in the past and wonder if y'all can help me again. I got a issue at WOT underload.

The engine revs up fast and smooth in neutral from idle all the way to red line. it's the same way on jack stands and in gear: fast and smooth. And I can hold it any RPM without a vibration.

However when underload there's a vibration at WOT. It just gets worst the faster the engine spins. If I had to put it in words it would be something like "vooooooom" without load and "voovoovoovoovoom" underload.

I had a bog going from part throttle to WOT which I was able to tune out with the idle mix and timing. Now I'm trying to dial out this WOT issue. It's always been there. The spark plugs look good. Maybe I don't have enough timing? I have no clue what spark knock sounds like so I can't tell you. Maybe I should I get a o2 sensor? I have a A/F gauge in the spare parts crate; I just need weld in a bung and wire it up. Maybe the stock 45 gph fuel pump can't supply the carb with enough fuel? I read that adjusting the floats help. I don't really don't know much about this old school stuff as it is still very new to me. But I want to learn everything I can about it.

Here's the setup in the '73 Charger:
  • 383 bored 60 over
  • 452 heads
  • 9:1 compression
  • '72 Thermo-quad 800cfm carb
  • Stock dual snorkel air cleaner
  • Holley Street Dominator single plane intake
  • Stock ignition box and coil
  • MSD 8.5mm spark plug wires
  • Bosch 4236 Platinum, Gap 0.035"
  • Compcam Xtreme Energy XE256H
  • 1-3/4" headers into 3" dual exhaust, no H-pipe
  • Stock 45 gph fuel pump, 7/16 fuel line, no return
  • Timing 12 degree initial, 38 all in
20181031_075126.jpg
 
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The only thing I have seen from experience that may cause something like a drone or "vibration" is lean out conditions at WOT. I am certainly no expert on thermoquads, but have messed with them a lot. If you were able to tune out the bog with timing, it makes me wonder if at WOT your secondary's are locked out, and with a big block this could cause lean out condition as you are pretty much running only on the primaries, and your engine is starved for fuel, and lean under load would be noticeable. I am running the large bore thermoquad on my 340, and I had this happen, secondary's were not engaging but after I adjusted the lock out lever, they engaged, which gave me a bog at WOT that had to be adjusted out with tightening of the air door spring so it doesn't go to full wide open until higher RPM. When you are revving or doing full throttle in park, have you seen the secondary's opening full? When throttle lockout is properly set, both the primaries and secondary's should be wide open at the same time with WOT. With that said, if both are open, you should easily be able to tell as the secondary air door should open as well as it opens by air movement into the secondary's. Now, it is possible that you are getting too much air, and the air door needs to be adjusted to open just a bit later as this can cause lean conditions under WOT as well. If you have the service manual for your car there are very detailed instructions on making these adjustments to your thermoquad.
 
The secondary are opening fully and the air door is opening freely. My dad locked out the secondary from fully opening back in high school to slow the car down. So that was one if the first things I checked.

Maybe the engine isn't getting enough air with the stock air cleaner? I heard they are very restrictive. Would a rich condition cause it to act like this? I don't have another air cleaner laying around so I would need to buy one. Flipping the lid trick won't work because the hold down bolt isn't long enough and it won't clear the hood anyways.
 
If it was running rich, your plugs would be much darker in color. Looking at your plug, it appears to me to be on the lean or hot side. I was looking at your plugs and maybe that range is too hot for your engine configuration, but I don't know what year the motor is that you have there. For reference, using 72 383 specs, your plug crosses over to champion 18 series, where stock plugs are 14 or 16 series and some lower performance engines use plugs are 11/12 series, and with the cam you are running, you might want to try one range lower and see if that helps because running too hot of a plug can cause lean conditions.

Regarding the air door, I meant, that maybe it needs the spring adjusted to restrict the air just a bit, if you tap on it lightly and it opens easily that may be part of it.
 
Try a larger secondary jet. You can always change back.
 
Thermoquads secondarys are different than other carbs that just use interchangeable jets, they use jet tubes with a metering jet sleeve that does vary in size and I don't believe anyone is making reproductions or selling any different sizes.
 
Based on your description of the vibration and sound you get at WOT, it sounds like it might be improper driveline phasing. Especially if this is happening at a particular speed and RPM. Just food for thought.
 
Going out on a limb here...I have seen cars with bad camshafts run fine under no load but pop and sputter as RPMs increase under a load.
Here is what is going on: The exhaust lobes are not pushing all the valves open evenly so whatever cylinder has a failing lobe will be where the problem lies. At idle, in neutral or cruise, the engine sees very little load so even though the exhaust lobe is only opening the valve 1/2 or 1/3 the way, it is still enough to evacuate the cylinder. As cylinder pressure increases with greater RPM or load, the opening of that valve becomes a restriction. The exhaust has to go somewhere, often times it will travel back up through the intake valve during overlap. This contaminates the fresh fuel/air coming in.....causing a loss of power. In some cases, the cam lobe is so worn, you can actually hear crackling/popping coming up through the carburetor.
I hope that this isn't the problem. Replacing a failing cam often means total disassembly to clean all the debris from the engine.
 
If it was running rich, your plugs would be much darker in color. Looking at your plug, it appears to me to be on the lean or hot side. I was looking at your plugs and maybe that range is too hot for your engine configuration, but I don't know what year the motor is that you have there. For reference, using 72 383 specs, your plug crosses over to champion 18 series, where stock plugs are 14 or 16 series and some lower performance engines use plugs are 11/12 series, and with the cam you are running, you might want to try one range lower and see if that helps because running too hot of a plug can cause lean conditions.

Regarding the air door, I meant, that maybe it needs the spring adjusted to restrict the air just a bit, if you tap on it lightly and it opens easily that may be part of it.

What about Autolite AP85 Platinum? I need to order a purge valve for the '14 Ram 1500 and Rock Auto have them on sale with a rebate; $1 each after rebate.

Based on your description of the vibration and sound you get at WOT, it sounds like it might be improper driveline phasing. Especially if this is happening at a particular speed and RPM. Just food for thought.

The car is smooth at 60-70mph in neutral and the engine idling. There is a slight vibration when cruising because of the exhaust, tires, ploy-mounts, and skipping on the $650 crank balancing job. I'll ask the alignment shop to check the angle and phasing when I get an proper alignment done.

Bad drivers side motor mount?

I have poly mounts. I feel everything!

Going out on a limb here...I have seen cars with bad camshafts run fine under no load but pop and sputter as RPMs increase under a load.
Here is what is going on: The exhaust lobes are not pushing all the valves open evenly so whatever cylinder has a failing lobe will be where the problem lies. At idle, in neutral or cruise, the engine sees very little load so even though the exhaust lobe is only opening the valve 1/2 or 1/3 the way, it is still enough to evacuate the cylinder. As cylinder pressure increases with greater RPM or load, the opening of that valve becomes a restriction. The exhaust has to go somewhere, often times it will travel back up through the intake valve during overlap. This contaminates the fresh fuel/air coming in.....causing a loss of power. In some cases, the cam lobe is so worn, you can actually hear crackling/popping coming up through the carburetor.
I hope that this isn't the problem. Replacing a failing cam often means total disassembly to clean all the debris from the engine.

Hmmm, you know, it might be worth it to remove the valve covers and measure each rocker movement with a dial gauge.

The valve seats are deep in the head. Maybe this is creating flow issues? We had to put .060 shims (!!) under each spring to get them to the correct installed height. How bad was the valve spring setup? Well, the worst cylinder compression was just 130psi with 45 year old springs. All cylinders was around 165psi with new springs and spacers.

I'm about done with this '71 383 engine. It has been nothing but trouble out if since we first tired to start it up. I would like to rebuild the original 400 big block with a 440 source 452 stroker kit, stealth heads, roller cam, a Holley Sniper EFI, and a 5 speed manual but the wife keeps talking about saving for retirement and the kids college fund. Anyone what to buy a slightly used '71 383? Selling it cheap!

Another note: The vibration really kicks in around 3500 rpm regardless of speed. I did take out a few degrees of timing today. The engine ran a little better above 3500 but worst below 2k rpm. I ran the car yesterday without an air cleaner (by mistake) and it was like a different engine. Much more power and smoother, but pinged a bit at WOT which was the reason for taking out timing. The rain started coming in today before I could test it further.
 
Ap85 crossover to the same heat range as what you already have installed.
 
I had two big issues going on.

1) The fuel line was old and rusted. It flowed enough for low load but not enough for high load.

2) I installed the ballast resistor upside down so the high volts with to the ECU and low went to the coil. Instead of connecting the coil to 1.2 ohms side , I had it connected to the 5 ohms side. Thus, I didn't have enough volts to charge the coil at high rpms.
 
Way to go! Another problem resolved with perserverance. Thanks for posting back what you found.
 
Actually the ballast resistor two resistors are start and run. With start being the lower resistance (since voltage drops during cranking).
So you may want to check that again, it may have been correct before.
 
Actually the ballast resistor two resistors are start and run. With start being the lower resistance (since voltage drops during cranking).
So you may want to check that again, it may have been correct before.

I found this wiring diagram. I had the low voltage side (5 ohm) going to the coil instead of the control unit. It was only getting maybe 5 volts instead of 7 volts.

mopar_charging_crucuit.jpeg


Good 'ol Santa bought me a Sniper EFI for Christmas because Summit Racing was offering 0% interest for 18 mouths, 20% off all things Holley, and $250 in various discounts. Santa couldn't say no to such a good deal. However, Santa couldn't get the Sniper CD ignition because Holley is sold out until February. The plan is to make a harness to run 12v from the starter relay to the coil through a relay triggered by ignition #1. I'll remove the coil harness when I get the Holley Sniper CD ignition which I will have pre-wired for.
 
Way to go! Another problem resolved with perserverance. Thanks for posting back what you found.

No problem, mate. I hate looking up a problem with no collusion. Maybe not everyone will be running a rusted fuel line and a backward ballast resister but it helps just one person...
 
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