What are the symptoms of valve float and ignition point bounce?

[Kern Dog]

I just pulled a 360 from my 67 Dart and replaced it with a freshly rebuilt one.
The old 360 used a points distributor and stock valve springs in the heads. The engine was strong right from idle and midrange but didn't pull very strong at higher RPMs. I have no tach but for the sake of conversation, let's assume an RPM of 5000 where it feels slow to accelerate.
The combination was as follows:
Stock bore and stroke 1976 360, honed and cleaned in the early 2000s. Back then, new rings, bearings, oil pump and cam. I got the short block in 2005 with no heads and a cam with a couple failing lobes. I put rebuilt #308 heads on it and a new Mopar Performance 280/474 cam too. Since then, not much is different. Weiand 4 barrel mid rise intake, Holley 750, cast iron exhaust manifolds and 2 1/4" exhaust with an H pipe.
I've wondered if the higher RPM sluggishness could be the points bouncing or valve float but I don't know how to determine that. The engine makes no popping sounds and does not feel like it cuts out or misfires at "5000 RPMs", it just feels like the throttle response goes away.
I guess it could be carburetor related but with vacuum secondaries, it isn't like I can see the secondaries opening as I'm driving.

 

[Oldiron440]

What did you use for valve springs and what did you use for the installed height?

 

[junkyardhero]

if they were stock 308's with 1.88 valves that 5000 rpm with the 484 cam was probably where it was just done flow wise.

308's are good heads for about 400 and have tons of potential, but need work to get there.

 

[4mulas]

What springs did you use kern? Also, think your heads might be stifling your combo..

 

[PRH]

I suspect it’s valvetrain related, based on the stock springs/474 cam pairing.
That being said, I’m sure you have an electronic ign laying around you could wire in as a troubleshooting tool, easier than swapping out springs.

 

[CRUZE 418]

Wouldn't valve float or ignition have a missfire?

 

[Murray]

If you're experiencing valve float or point bounce you would certainly know it. The engine would be breaking up and misfiring something terrible. My 383 was dyno'ed at 6k and points with no problem. Points are good to a degree.

 

[TT5.9mag]

Valve float will not make the engine misfire. It will just all of the sudden stop accelerating or making any power. Point bounce or unstable ignition you will likely feel as a misfire. And it should go in and out on the way up the RPM. Either way when one or the other happens you are absolutely done making power.

 

[Oldiron440]

If you’re using forty year old springs that were for a stock cam I would change them out no matter what.
I have no problem throwing out valve springs, they should have gone with the cam you pulled.

 

[PRH]

40 years ago, when I really didn’t know anything, I had a similar experience with my 440 after a cam change.
The motor just felt flat after 5500 or so.
No skipping or popping but some mild surging.
I checked the fuel pressure, and that was fine while the surging was going on.
So, fuel or ignition?
I went to one of the local engine builders and took him for a ride. He didn’t feel it was ignition since there was no misfiring, but said it didn’t really feel like valve float either.
He suggested adding a shim under the springs and seeing if it delayed the onset of the surge to a higher rpm.
Seemed logical, so I added a .060 shim under each spring……… and the surge was gone!

$4 and a few hours labor cured that one.

The heads had aftermarket springs, but I didn’t know anything about “installed height”, and they were just in at too tall of a height.

 

[Kern Dog]

What did you use for valve springs and what did you use for the installed height?

If you’re using forty year old springs that were for a stock cam I would change them out no matter what.
I have no problem throwing out valve springs, they should have gone with the cam you pulled.

Yeah, I thought I stated that I used #308 heads that I had rebuilt but the springs were original to the heads. This was about 20 years ago before I knew anything about the importance of matching the springs to the cam, installed height, etc. I started the engine, it ran great and I thought I was done.
Even today I still don't know that much about it.

Valve float will not make the engine misfire. It will just all of the sudden stop accelerating or making any power.

THAT is what this feels like.
That engine is on the engine stand now. I'm going to pull the oil pan to address a low oil pressure issue so why not change the valve springs too?
I have a different 360 in the car now, it has matched springs in it so it ought to rev just fine.
Thanks guys!

 

[TT5.9mag]

View attachment 1716503385



Yeah, I thought I stated that I used #308 heads that I had rebuilt but the springs were original to the heads. This was about 20 years ago before I knew anything about the importance of matching the springs to the cam, installed height, etc. I started the engine, it ran great and I thought I was done.
Even today I still don't know that much about it.



THAT is what this feels like.
That engine is on the engine stand now. I'm going to pull the oil pan to address a low oil pressure issue so why not change the valve springs too?
I have a different 360 in the car now, it has matched springs in it so it ought to rev just fine.
Thanks guys!

Sounds exactly to me like what happens when you don’t have enough spring to control the intake valve.

 

[Oldiron440]

Sounds like you have it handled…

 

[Kern Dog]

You’re too kind.
I didn’t figure it out, YOU guys did. I am lucky to have people around that are willing to help.
Thanks again

 

[LO23M8B]

I believe we are all very lucky to have such a broad base of experts on this site. Cars, electrical, plumbing you name it. And a big thanks to all who have helped me out.

 

[teringer]

Most points will bounce at 5200 rpms unless you get high performance points and you can pick the bounce point ,like 6000/6500/7000 ect

 

[GGress]

Most points will bounce at 5200 rpms unless you get high performance points and you can pick the bounce point ,like 6000/6500/7000 ect

I don't know where to buy high performance points anymore.

 

[Murray]

One thing I played with in 1968 was the points (BB). I installed a dual point distributor (Prestolite) and used Hemi points from Mopar. These had very different contacts, small tungsten ones and heavy duty springs, ran great. I haven't seen them in years. Are they still available?

 

[halifaxhops]

Those were made for the prestolite transignitor ignitions, usually in IBB distributors. They are around not cheap though. I get $60 each point set and thats really reasonable.

 

[Slantsix64]

I had a similar issue, it turned out to be my ignition cuttting out at around 5k. Swapped over to a MSD box now I can rev it up to 6k and beyond.

Try adding a HEI set up to trouble shoot, that way you can determine if it’s valve float or a ignition issue. It’s a lot cheaper and easier than pulling valve springs.

 

[Kern Dog]

Sorry guys, I forgot to update this thread.
To recap....My '76 360 was honed and reringed around 2001-2002. I got it around 2004-2005 with no heads and a cam with a couple of lobes going bad. I put a 280/474 cam in it along with rebuilt #308 heads, a Weiand intake and Holley 600, later a 750. I retained the stock points ignition.
A couple of years ago I noticed that while the car ran strong from idle, by around 4000 rpms or so the acceleration rate was slower, like a car with freeway gearing and a slant six. It ran smooth, there was no noise, no misfiring, the acceleration just slowed way down.
I wondered if the points were just bouncing, if the valves were bouncing, if it were starving for fuel due to (maybe) inoperable secondaries....
Last Thursday I got the rebuilt 1990 360 up and running using the same points distributor, same carburetor but new fuel pump, reasonably fresh valve springs and whoa.....whatever high speed issues it had are GONE. This new mill is great at all ranges.
I doubt it was just the fuel pump, I suspect it was the valve springs. The heads were rebuilt but the springs were stock. The heads came from a truck and we know that truck engines are designed for torque, not high revving HP. They are late 80s heads meaning that they are almost 40 years old!
Before the '76 360 goes in my next car, I'll address the springs along with the other issues with the engine.
Thanks!

 

[RustyRatRod]

The number one biggest symptom of valve float is a scattered motor from piston to valve interference. If you have any symptoms before that happens, it's probably too late.