1961 364 Nailhead Valve Float

I am not new to cars and engines but I am new to Nailheads. The engine in my LeSabre is a rebuilt stock (as far as i know) 364 low compression. It has a 4 bbl intake and Edelbrock 600 cfm carb on it. Timing is at 30 degrees, all in with vacuum plugged. Distributor is connected to the ported connection.

I finally got to get out and drive my car some today and decided to "get on it" a little and see what she'd do. What she does is about the time she starts making good power, the valves float. Fortunately, it doesn't appear that it did any harm. I don't have a tach, so I can't tell you what RPM it's floating at but I believe it's less than 4000, maybe 3500. The engine would rev willingly if the valve springs didn't give up the ghost. Is this normal for this engine? How close are the valves and pistons?

I am trying to optimize the governor in my TH350 but it looks like that is not gonna happen unless I fix this other issue. Advice?

 

if you really think the valves are floating just put new springs in. you can do it with out pulling the heads. how you know it not an ignition miss or running out of fuel

 

Do you have a recommended valve spring compressor to do this?

I kinda doubt your getting valve float at that low of a rpm as well. Valve clearance is tight in these engines tho.
If you have the butterfly valve at the exhaust manifold (heat riser), make sure it's not stuck closed.

 

I would suggest this type of compressor possibly to get accessibility to the rear springs by the heater box and brake booster clearance issues.

 

mine was like this, it was a pain to do but it can be done, i didn't have power brakes if that matters i don't know.

 

like this

 

You might have to much gap in the points!

Bob H.

 

It has a points conversion and a new fuel pump. I guess it could be fuel but it sure seems like valve float. I am capable of replacing the springs and have the tools. I was just curious if that was a common symptom. I may upgrade the fuel pump first. An electric will make hot starts a little easier.

 

doe's it feel like a miss fire breaking up or more like a surge.

 

Are you sure you floated the valves,
This if it did take place it would accompanied with Valvetrain noise and once the motor first dropped back to close to idle rpm would be running ruff for a bit.

did this take place?

If yes, you do not want to keep bouncing the lifters off the cam lobes!

the valves at full stock lift on a nail head are still a long way’s away from the piston tops, so no worries there.

 

It feels like it just falls on its face and then runs rough for a few seconds after i get off the throttle and then straightens up.

 

I am going to try to eliminate ignition and fuel from the equation before going any further.

 

Well, I am 99% sure its valve float. I recurved the distributor to get all in by 2500 rpm. Full mechanical advance is 30 degrees.

With my digital timing light and no load, all it would do is 4500 rom. Once it got above 4200, it started to miss and just wouldnt go higher than 4500. After each run like this, it idled rough for a minute or two until the lifters normalized. I guess I am going to have to do some engine work that I didnt want to do.

 

You weren’t revving it to 4500 rpm in neutral were you?

 

Park

 

Same thing, bad thing!

 

So, how would you have ascertained that it was valve float? I eliminated ignition and fuel, as much more fuel is required under load. I didn't over rev the engine and I didn't do any damage.

 

You have to drive it, the engine has to be under load.
You can wind it out in first gear to whatever rpm it happens.
Revving it to 4 or 5000 rpm in neutral is a killer on rod bearings.
Yes, rpm is rpm, but there’s a difference in the TYPE of rpm, under load, or free rev.

 

In terms of those rebuilt heads you have with what might be reused 61 year old springs that you do not know the history of, I can relay this info.

I have rebuilt a lot of those heads in the early 70s in the shop that I worked in.

Those 364 springs are marginal at best and if the motor was ever overheated, well you can stick a fork in them because there DONE!!

if they are shot I bet that on a good number of them you can use two Thumbs to apply enough force to get the valve to move up & down a bit.

 

I would check what the Springs are at for tension open and closed. Eliminate the guess work, use a proper gauge.