How to diagnose valve float?

[John Chesher]

I have what feels like a miss at 5000 - 5500 RPMs and, after going through the ignition, I'm wondering if it might be valve float. I don't know what valve float would "feel" like. Can anyone tell me what the symptoms of valve float would be and how to definitively diagnose whether it is happening or not?
Thanks!

[Wayne K.]

Re: How to diagnose valve float?

John,

The last time I experienced valve float was when I was 16. My dad had a 63 Chevy II Nova with a straight six that I beat the snot out of. Valve float happened so you knew it was time to shift to the next higher gear. The hydraulic lifter collapses as I recall and there was a noticeable loss of power. After the event I can remember often times there would be valve train clatter for bit until the lifters recovered. Maybe that's why they called them clicking sixes.

Wayne

[Wayne K.]

Re: How to diagnose valve float?

John,

The last time I experienced valve float was when I was 16. My dad had a 63 Chevy II Nova with a straight six that I beat the snot out of. Valve float happened so you knew it was time to shift to the next higher gear. The hydraulic lifter collapses as I recall and there was a noticeable loss of power. After the event I can remember often times there would be valve train clatter for bit until the lifters recovered. Maybe that's why they called them clicking sixes.

Wayne

[Duke W.]

Re: How to diagnose valve float?

Assuming you have hydraulic lifters, hard valve float causes the engine to "lay down" - almost as if the ignition was shut off, and it takes several seconds to recover. The previous explanation is accurate, except what happens is that the valve gets "tossed" over the top of the lobe creating lash in the valvetrain that causes the lifter plunger to extend fully. As a result, the valves won't close completely, so the engine has no compression and no power until the lifters bleed down and allow the valves to completely close.

Likely your miss is electrical. Free rev the engine to 5500 with a dwell meter attached. If the dwell varies more than 2 degrees from the idle value, the points are likely floating due to weak breaker arm tension, excess shaft end play, or a wobbly breaker plate.

The first time I ever floated valves was on my dad's '59 Caddy that I borrowed for some proficiency training at about age 14 while the folks were out in the other car. I put in in "L" and floored it. (Yes, once upon a time I was a moron!)At about 55-60 MPH I experienced the above effect. I thought I really screwed something up, but everthing turned out to be okay. Hydraulic lifter valve float is usually forgiving.

It was some years later that I figured out what happened.

I don't think I ever floated mechannical lifters, and they can be more unforgiving - like bent pushrods - or worse - like a bent valve from whacking a piston. I recall letting out the clutch too fast on a 1-2 shift in the SWC, which caused the gear selector to hang in neutral while I got back on the power and saw the tach pass 7K before I could react and let off. I heard "noises" all the way home, but the Duntov cam/valvetrain turned out to be okay.

Duke

[Duke W.]

Re: How to diagnose valve float?

Assuming you have hydraulic lifters, hard valve float causes the engine to "lay down" - almost as if the ignition was shut off, and it takes several seconds to recover. The previous explanation is accurate, except what happens is that the valve gets "tossed" over the top of the lobe creating lash in the valvetrain that causes the lifter plunger to extend fully. As a result, the valves won't close completely, so the engine has no compression and no power until the lifters bleed down and allow the valves to completely close.

Likely your miss is electrical. Free rev the engine to 5500 with a dwell meter attached. If the dwell varies more than 2 degrees from the idle value, the points are likely floating due to weak breaker arm tension, excess shaft end play, or a wobbly breaker plate.

The first time I ever floated valves was on my dad's '59 Caddy that I borrowed for some proficiency training at about age 14 while the folks were out in the other car. I put in in "L" and floored it. (Yes, once upon a time I was a moron!)At about 55-60 MPH I experienced the above effect. I thought I really screwed something up, but everthing turned out to be okay. Hydraulic lifter valve float is usually forgiving.

It was some years later that I figured out what happened.

I don't think I ever floated mechannical lifters, and they can be more unforgiving - like bent pushrods - or worse - like a bent valve from whacking a piston. I recall letting out the clutch too fast on a 1-2 shift in the SWC, which caused the gear selector to hang in neutral while I got back on the power and saw the tach pass 7K before I could react and let off. I heard "noises" all the way home, but the Duntov cam/valvetrain turned out to be okay.

Duke

[John Chesher]

Re: How to diagnose valve float?

Thanks for the responses. The lifters are hydraulic and it's more like a miss that instantly goes away as the RPMs drop, so it doesn't sound like valve float, as there is no delay while the lifters bleed back down. I'll have to goe through the ignition system again.
Thanks!

[John Chesher]

Re: How to diagnose valve float?

Thanks for the responses. The lifters are hydraulic and it's more like a miss that instantly goes away as the RPMs drop, so it doesn't sound like valve float, as there is no delay while the lifters bleed back down. I'll have to goe through the ignition system again.
Thanks!

[Myron Sleeva]

Re: How to diagnose valve float?

Going lean due to lack of fuel will also cause a top end miss. Is this problem in first gear or a third gear pull where the engine is under more load and been running hard for a time?

At one time, the most cost effective cure for lifter pump up on the high end of the RPM scale was to just set them at a 1/4 turn beyond zero lash. Kept them quiet plus allowed for the rocker nut to back of slightly before needing to be reset. Since the push rod was not deep into the lifter, as with the normal 1 to 1 1/2 turns beyond zero, the lifter could not pump up causing the valves to hang open.

[Myron Sleeva]

Re: How to diagnose valve float?

Going lean due to lack of fuel will also cause a top end miss. Is this problem in first gear or a third gear pull where the engine is under more load and been running hard for a time?

At one time, the most cost effective cure for lifter pump up on the high end of the RPM scale was to just set them at a 1/4 turn beyond zero lash. Kept them quiet plus allowed for the rocker nut to back of slightly before needing to be reset. Since the push rod was not deep into the lifter, as with the normal 1 to 1 1/2 turns beyond zero, the lifter could not pump up causing the valves to hang open.

[John Chesher]

Re: How to diagnose valve float?

Happens in every gear.Thanks for the tip, but do you mean 1/4 turn LESS than the usually prescribed 1/2 turn after zero lash? Wouldn't that give the lifter less room to pump up?

[John Chesher]

Re: How to diagnose valve float?

Happens in every gear.Thanks for the tip, but do you mean 1/4 turn LESS than the usually prescribed 1/2 turn after zero lash? Wouldn't that give the lifter less room to pump up?

[Duke W.]

Re: How to diagnose valve float?

The Chevrolet Power Manual recommends "zero lashing" hydraulics on a racing engine. This means turning the nut down 1/8 to 1/4 turn from zero lash. Or, don't turn them down at all from zero lash, but this can beat up the valvetrain (in addition to being noisy), and I don't recommend it on a road engine.

The less preload the lifters have, the less the plunger is depressed. The valve will still be tossed at the same speed, but with less preload the valves hang open less, and the engine will take less time to recover from hard float.

The only disadvantage of less preload is less allowance for normal wear. If a quarter turn or less preload is used, the preload should be checked after initial valvetrain run-in (assuming all new parts) and thereafter probably about every 30K miles.

The power on most OE engines will be beyond the useable range at the valve float speed, but if the heads have been massaged, the useable power bandwidth may extend right up to the valve float speed, so using a quarter turn lash or less may be useful.

Duke

[Duke W.]

Re: How to diagnose valve float?

The Chevrolet Power Manual recommends "zero lashing" hydraulics on a racing engine. This means turning the nut down 1/8 to 1/4 turn from zero lash. Or, don't turn them down at all from zero lash, but this can beat up the valvetrain (in addition to being noisy), and I don't recommend it on a road engine.

The less preload the lifters have, the less the plunger is depressed. The valve will still be tossed at the same speed, but with less preload the valves hang open less, and the engine will take less time to recover from hard float.

The only disadvantage of less preload is less allowance for normal wear. If a quarter turn or less preload is used, the preload should be checked after initial valvetrain run-in (assuming all new parts) and thereafter probably about every 30K miles.

The power on most OE engines will be beyond the useable range at the valve float speed, but if the heads have been massaged, the useable power bandwidth may extend right up to the valve float speed, so using a quarter turn lash or less may be useful.

Duke

[Myron Sleeva]

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[Myron Sleeva]

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