[chuckies76ta]

Engine: 473, Kauffman 300 cfm aluminum heads, Kauffman 236/242 hydraulic roller, '72 stock intake ported to match heads, open plenum. Doug's 1 3/4" headers, 3" exhaust w/ H pipe.

That engine must make at least 500 hp if not a bit more. I would try a different Carb and rule it out. Are you running a return line on your fuel system?
You can get it running and run a +12 volts wire right off the battery to the distributor and do a test run.

[Mike Fowke]

Originally Posted by chuckies76ta

Engine: 473, Kauffman 300 cfm aluminum heads, Kauffman 236/242 hydraulic roller, '72 stock intake ported to match heads, open plenum. Doug's 1 3/4" headers, 3" exhaust w/ H pipe.

That engine must make at least 500 hp if not a bit more. I would try a different Carb and rule it out. Are you running a return line on your fuel system?
You can get it running and run a +12 volts wire right off the battery to the distributor and do a test run.

If the springs check out, I'll check the voltage and see if I can swap the carb. The fuel pump is "returnless":

https://aeromotiveinc.com/product/carbureted-phantom/

[lust4speed]

Is it WOT or higher RPM breakup? While they usually occur together, if it is RPM related a weak spring could be allowing lifter to not follow the ramp and also contribute to some additional wear on the lobe. Valve not closing would definitely promote backfiring.

[Mike Fowke]

Originally Posted by lust4speed

Is it WOT or higher RPM breakup? While they usually occur together, if it is RPM related a weak spring could be allowing lifter to not follow the ramp and also contribute to some additional wear on the lobe. Valve not closing would definitely promote backfiring.

It happens consistently at WOT, under load, at higher RPM. For example, punch it in 1st or 2nd at low speed, it will accelerate, reach a certain RPM and break up. It doesn't nose over or die. It just starts backfiring. If I build some speed and ease into WOT it usually takes off with no issue, and I can wind it up. So, I think it's load dependent and not RPM.

If the problem persists when I get it back together, I'll post a video. I'll also verify the RPM where it breaks up and if I can reach the same RPM easing into WOT.

The machine shop hasn't returned my call yet about measuring the springs.

[Mike Fowke]

No broken springs, but I had one measured: 100lbs closed, 300lbs open. The machinist and I agreed that more spring rate is needed. Suggestions?

1.890 installed height. .594 lift (.360*1.65). Kauffman heads. 236/242 hydraulic roller. Johnson hydraulic short travel lifters.

[tom s]

I run 150 on the seats on my hyd roller cams!Tom

[Mike Fowke]

Thanks Tom.

The cam was made by Comp Cams, and they recommended spring 924-16 which will give me 126 closed and 334 open at my installed height.

[25stevem]

In the case of the broken inner springs I was taking about, 2 of his retainers where the issue.

The nose of the retainer on those two where machined about .010" oversized from the others, and the resultant press fit into the top of the inner spring is what after about 10 passes busted out the top of the spring.

You could not even tell the spring was busted with the retainer in place!

[25stevem]

150 on the seat like Tom posted would be what I would shoot for !
You will drop off between 10 to 15 psi after after 3 to 5 full throttle runs and then be right in that 125 to 130 range .

Also I would not run that 924 spring .
The flat wire damper that they have can become a nightmare for you .
I would use there 929 spring.

[Mike Fowke]

I ordered the 929s. I should have it back on the road in a couple of weeks, and I'll post the results. Thanks for the advice.

I wonder if the lobe damage may be from bouncing or skidding due to low pressure.

[Mike Fowke]

Heavier springs didn't fix it. Back to the drawing board.

[Scott65]

Check ignition voltage. Low voltage to the coil will definitely cause the symptoms you're describing.

[Mike Fowke]

[QUOTE=Scott65;6354068]Check ignition voltage. Low voltage to the coil will definitely cause the symptoms you're describing.[/QUOTE

I did a quick check after the initial test drive. 14-15 volts at idle.

[455dan]

Any chance the Q jet still has a factory filter in the inlet?

I have had a few act like what you describe when the internal filter starts to clog.

They are pretty small filters also so not much capacity to absorb much before restricting flow.

The fuel pressure would also read normal through a run since it is in the Q jet itself.

[Mike Fowke]

No filter in the inlet.

[455dan]

Originally Posted by Mike Fowke

No filter in the inlet.

OK, no inlet filter

Have you tried increasing the secondary flap spring tension to slow the opening


Accelerator pump giving a good full shot to both venturis?

Maybe like some others have posted, time to source a known good test carb just to eliminate your Q-Jet as a possibility.

[Mike Fowke]

The secondary opening rate or transition isn't the problem, but thanks for the suggestions.

I don't have a known good carb to swap, but I know a mechanic who works on hot rods. I may just take the car to him. I've been chasing this for longer than I care to admit, and I'm out of ideas.

[Mike Fowke]

Guys this is resolved, and I'm embarrassed to say it wasn't the engine. In January 2021, I installed a SpeedTech torque arm which includes a two-piece block of aluminum that clamps to the differential case just behind the yoke. The bottom of the block bolts to the torque arm. The block worked itself loose and was apparently bouncing off the case when the suspension was loaded. The noise transmitted from the back to the front of the torque arm where it mounts to a bracket behind the trans. Most of the sound made it's way through the trans tunnel and up the firewall. It sounded like it was coming from the engine bay through the speaker hole in the dash.

I took the car to a friend who builds some nice cars. He rode with me and didn't think it was an engine issue. We put it on his lift and nothing looked out of the ordinary as the support block was hidden by the torque arm. His buddy took a ride and felt the vibration when he happened to lean his leg on the trans tunnel. Another friend took a ride with his hands held on the trans tunnel and confirmed the vibration. He also heard the noise from the same place I was. We all thought something was going on in the clutch or trans (TKO) even though nothing felt weird in shifter. So, I took the car home and had a drink or several. The next day I put it on jack stands planning to pull the trans and clutch. I had to remove the torque arm to get the drive shaft out. That's when I found the loose block.

So, my diagnostic skills need some work. If I try to make chicken salad out of this mess, at least I found out my valve springs were weak, and my fuel pump was likely bowing out. The engine seems to make more power at high RPM now likely due to the heavier springs which is nice. In hindsight, I should have paid more attention to the fact that the engine continued to pull and perform through the noise.

Thanks for the suggestions. Hope this helps someone else diagnose weird noises.