64 327/300, Muncie M20, 3.36 rear
When I'm driving maximally the car loses power in higher RPMs. It feels like the timing is not advancing enough. This has been going on since the first time I drove the car when mom won it in a raffle in 1995. After the car was passed on to me in 2005 I went to work on the issue. My first thought was the timing curve was whacked. I replaced points, set dwell and retimed the ignition (both static and dynamic) to no great effect. I replaced the vac advance unit - no change. I played with springs and weights in the distributor to only make it worse. So I rebuilt the distributor hoping new bushes would be the answer, retimed, played with springs and weights. Nothin'. Put in a Pertronix ignitor and retimed. Thought it helped a little, but not really.

Then I rebuilt the Carter AFB - which was actually for an automatic car. No change so I installed an Edelbrock 600CFM Performer (1405) carb with electric choke. It sure ran better at low RPM's, but the high rpm issue persisted. So I finally gave up and keep it below 4500 rpms 99.5 percent of the time.

This year I am driving the car a lot more because my regular car is going through a major turbo upgrade (and taking up my wife's spot in the garage) since May. While I'm waiting for parts for the Audi project I'm tinkering on the Sting Ray and I got back to this problem.

Could this be valve float, and would new springs be enough if I'm lucky? If not valve float, other thoughts?

 

You haven't said at what RPM the engine stops wanting to rev. I notice you say it's a 327/300. Built to spec it would have a flat tappet hydraulic cam. It wouldn't surprise me to learn the engine wants to lay down at anything over ~5000 RPM.

One other thing, if that engine retains its original, weak connecting rods, "driving maximally" is NOT recommended.

 

Valve float is one of those things that you can't describe, but you know when it happens. The car just kind of lays down, it stops accelerating.
As mentioned above, you don't say what RPM this happens.

Could be weak springs, could be air inlet restriction to the carb, could be exh restriction, could be other things. Could be an RV cam, who knows what is in the motor

Valve springs do weaken over time.

If you want to drive like i do, You need to have the motor pulled, taken apart, analyzed and rebuilt for performance motor. 2.02 heads, 10.5:1 CR forged pistons, Crower free floating rods and an L82 350/350 cam (zero lashed) is a good start. full 2.5" exh with low restriction mufflers and Holley 6210 650 CFM spreadbore carb will really wake that motor up. The M20 and the 3.36 will work fine with that cam.

I had valve springs weaken over time, years and years ago. New GM purple springs (for the 350/350), made it spin to 7K again. But, i had good rods.

Doug

 

Replace the fuel filter and bench test the coil. I had an alternator go offline and actually cause a voltage issue at around 5K RPM and cause a miss that felt like valve float, which was not the issue with a solid cam. The two time I had coils take a dump, the first symptom was higher rev cutout.

 

Thanks Jim. It starts lose power around 3800 RPM. It'll climb slowly to 4500 and it won't go much more unless you have time for a sandwich. It's actually very rare for me to hit 3800 RPM in the Sting Ray anymore. I just want it to run right.

I know what you mean about the rods, so 'maximally' in this car is quite different from the 7000 RPM Audi Quattro, which should make @380 bhp when the turbo/intercooler/intake manifold/exhaust/ fuel pump upgrades are completed and tuned this July. It'll be on stock internals, so we'll keep the torque around 340ish. But that's another forum.

I do not know any history of the Sting Ray before mom won the raffle, and taking the intake manifold off for paint is the deepest I've been in the engine, so any guess on internals is as good as the next right now.

What I know to be not original on the car:
* My work:

1. Carb, fuel pump, fuel line from pump to carb, air cleaner
2. Rebuilt guts of the Muncie M21, converted to M20 (shop work)
3. Luk 11" clutch (shop work)
4. Pertronix ignitor
5. Braided stainless brake lines at the wheels (now that's a stiff pedal)
6. Engine dress ups: valve covers (leaky crap chinese stamped chrome pos's from Eckler's), chromed bolts, braided hoses. If I ever get the car back to original colors this crap goes in the trash.
7. Floor mats
8. Battery, tires and other maintenance items
9. Heater bypass (leaking core at hose connector braising)
10. Rebuilt parts: distributor, master cylinder, speedometer (Bob's speedometer), battery meter, a bunch of dash wiring
11. Repaired/replaced parts: pinion gear (bad casting, but took 45 years to break); right rear trailing arm and shock mount; date correct 64 AM/FM radio replaced the 63 AM in the car
12. Reproduction stuff: firewall interior heat shield (that was a pain); antenna shield; grounding straps; Quantum gas tank and new sending unit; rear wire harness; front wire harness; alternator; headlight switch; capacitors; interior bulbs; turn signal cancel cam; shifter; a couple bits of chrome radio shielding in engine bay; PCV tube (replaced incorrect tube); body mount cushions, a bunch of incorrect bolts

* Previous "restorer's" work:

1. Paint, seats and carpets - should be Daytona Blue on Silver (which I would love, love, love)
2. The right head is from a '73 something, iirc.
3. The side cover on the Muncie is from later (don't remember) - the rest of the trans castings are correct
4. Incorrect radiator
5. All hoses - previous restorer replaced with non-period correct hoses...everywhere, so I put braided covers on the eye sores
6. Side pipes
7. Knock-offs
8. Top

 

Jiminy crickets, Doug, that's a dream build for me. I'm actually building out my sedan right now for 380bhp/340tq on a forced induction, direct injected 2.0L 4 banger. Heavy car, but a Quattro drivetrain and a killer factory suspension. I'm keeping the torque down to 340-360 neighborhood because it's stock rods too (until I bend one).

 

The alternator was replaced a few years ago after my rebuild attempt failed miserably, so the coil is very interesting - got my spidey senses tingling. It has never been out of the car, but it will be as soon as we get back from vacation (leaving tomorrow, back mid July). It would be awesome if the coil is the culprit considering I'm blowing this year's car budget on another project. Fingers crossed. Thanks!

 

Just a thought with everything you've tried. Are you sure the tach is reading accurately? Maybe you are closer to 5k then 3800 when it falls off? Only talking 1200 diff.

 

That's a good point, and another one to check is your harmonic balancer, maybe it has slipped & your timing is way off?? Verify TDC on #1 is actually correct as 0°on the balancer.

 

There was no performance head in 73 so whatever casting it is is not a good choice. I would do a compression test on all cylinders for starters. Then I would look at the ignition and as mentioned the fuel filter. You can buy this tool for checking valve springs on the car. There is a class of dirt racecars that have to run stock springs and they use this weekly to check springs.


https://www.speedwaymotors.com/Valve...ester,601.html

 

Good point. The tach has developed the bouncies over the last couple of years. However, when I was first diagnosing the problem the tach was steady and spot on with my dwell meter and digital timing light.

 

Yep. Checked and good to go. I do wish Chevy included a bolt there, though. I've seen some guys drill and tap to add a bolt.

 

I need to double check the casting. I was going of memory for my post, and the recall is not what it used to be these days. I'll do the compression test when we're back in town. It's very strong in the low RPM range, but I'd still like to know the numbers.

Thanks for the tool tip! Definitely worth ordering.

 

Did you check the valve cover top to carb inlet clearance?

You didn't say what air cleaner or carb you had.

Fuel delivery problems IMO usually results in surging.

Corvettes had special harmonic balancers and timing covers, a mismatch will throw your timing off 10* either way, depending on what is mismatched.

Doug

 

I always double check my balancer on a “new-to-me” engine, and witness mark TDC on the pulley, just in case the outer ring slips…

You rebuilt and then replaced the carb…. Are you sure you are getting full throttle? If you aren’t, the secondaries won’t open and could act as you describe…

 

A few things also come to mind. How many miles on the engine? Has the timing chain been inspected and or changed? Lastly, the later Pertonix, ignition triggers have an RPM limiter. It can be adjusted. Jerry

 

they did on SHP engines

 

You don't suppose it's a fuel supply problem like a restricted fuel sock in the gas tank and the engine simply runs out of fuel at high demand.

 

Someone PM'd me about the spring pressure. He asked if I thought recessed seats would cause this. My answer absolutely which is why I suggested first thing to do is check the compression then the springs with the checker I suggested. With valve recession the valve sinks into the head you have virtually no spring pressure you have no ticking so you don't suspect a valve train problem. This was exactly what happened to the member that asked about it. He had seats installed an no more issues. I'm not saying this is your problem but it's very possible. Flame away!

 

Good show on valve float on Engine Masters this week on Motor Trend TV. Check it out.