Valve Float or what? - Pontiac GTO Forum
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post #1 of 14 (permalink) Old 09-12-2019, 04:16 PM Thread Starter
 
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Valve Float or what?

I am experiencing what I believe to be valve float at 4500 RPMs on my 66 GTO 389 with Tripower. I get stuttering and the engine seems to lose power under heavy acceleration at and above 4500 RPMs. I have done all I can to eliminate spark and fuel as the culprit. I didn't build the engine so compression, bore and stroke are bit of a mystery but I assume it's relatively stock based on its performance up to 4500 RPMs. Read below let me know your thoughts please.

Stuttering and losing power at 4500 RPMs under load (WOT)
Compression, bore and stroke unknown

Baseline
1. Plugs replaced
2. Wires replaced
3. Coil replaced in
4. Pertronix ignition replaced with new
5. Distributor rebuilt
6. 12v to Pertronix coil from fuse box - resistance line still present but not connected
7. Installed New 068 cam and lifters installed with roller tip rockers – Cam advanced 4 degrees - 2019 (old cam lobes showed wear but problem persists after cam swapped)
8. Valve springs were not replaced. Reused old ones
9. Initial timing 5 degrees. Mechanical advance appears to be working.

Testing
I can ease it over 5000 RPMs with foot lightly into pedal but it stutters at 4500 RMPs under heavy load (at or near WOT)
Tried disconnected secondaries and ran on 2bbl. Same results at 4500 RPMs under heavy load (WOT)
It holds rpms okay without stuttering just below 4500 RPMs indicating it's not likely a fuel issue.

Overall it runs fine and for the most part I dont push it above 4500 RPMs. I would like to occasionally but well...
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post #2 of 14 (permalink) Old 09-12-2019, 05:06 PM
 
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MY 68 gto convert wouldnt rev much over 4000

did about the same things u did ....

took a good buddy for a ride and he mentioned to hook up his dwell meter ...

ended up being my hood tach calibration on the ole original was off

at 4000 it was at 55oo on his sun tune machine

yup ,,, I was beatin the ole WT up ........



Scott
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post #3 of 14 (permalink) Old 09-12-2019, 05:38 PM
 
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Originally Posted by Mikereps View Post
I am experiencing what I believe to be valve float at 4500 RPMs on my 66 GTO 389 with Tripower. I get stuttering and the engine seems to lose power under heavy acceleration at and above 4500 RPMs. I have done all I can to eliminate spark and fuel as the culprit. I didn't build the engine so compression, bore and stroke are bit of a mystery but I assume it's relatively stock based on its performance up to 4500 RPMs. Read below let me know your thoughts please.

Stuttering and losing power at 4500 RPMs under load (WOT)
Compression, bore and stroke unknown

Baseline
1. Plugs replaced
2. Wires replaced
3. Coil replaced in
4. Pertronix ignition replaced with new
5. Distributor rebuilt
6. 12v to Pertronix coil from fuse box - resistance line still present but not connected
7. Installed New 068 cam and lifters installed with roller tip rockers – Cam advanced 4 degrees - 2019 (old cam lobes showed wear but problem persists after cam swapped)
8. Valve springs were not replaced. Reused old ones
9. Initial timing 5 degrees. Mechanical advance appears to be working.

Testing
I can ease it over 5000 RPMs with foot lightly into pedal but it stutters at 4500 RMPs under heavy load (at or near WOT)
Tried disconnected secondaries and ran on 2bbl. Same results at 4500 RPMs under heavy load (WOT)
It holds rpms okay without stuttering just below 4500 RPMs indicating it's not likely a fuel issue.

Overall it runs fine and for the most part I dont push it above 4500 RPMs. I would like to occasionally but well...

Appears you got it covered with all the parts you have already replaced. So here are a few suggestions to add to your quest.

Fuel delivery issue. Tri-Power when opened needs gas. Are the outboard carbs fully opening off the center carb linkage? What size gas line? Fuel pressure? Fuel filter? Air cleaners - heard some time ago there was a type of air filter that choked the air flow on the tri-power. Maybe try a blast without filter on.

I know the factory initial advance is 5 degrees, but just for fun, step it up to 9-10 degrees and see if any change. Just back out of the gas if you hear any detonation/pinging under load.

Most cams already have a built-in 4-degree advance - is this what you mean, built in advance of 4 degrees from the manufacturer? If you added another 4 degrees to the cam, you may now have 8 degrees which may be your problem right there.

What oil weight? Some lifters will not work with heavier oil per the manufacturer of the lifter, like 20W-50. The oil is not thin enough to drain back fast enough keeping the lifter pumped up and thus holding your valve open at the higher RPM's.

You did not mention exhaust. If still factory cast, one side of the exhaust manifold has a "butterfly valve." It uses a bi-metal spring that opens and closes the valve - which is used for faster warm ups in winter. These rust closed or partially shut.

You used the old valve springs and they could very easily be weak, allowing valve float (bounce). The "068" is not radical, and it has the same Pontiac lift most Pontiac cams have - .406", but your valve springs may not be strong enough. With some aftermarket cams, ie the Comp Cams Xtreme units, they have very fast opening and closing ramps. This requires their heavier and recommended valve springs to keep the valves seated or they can bounce on th seat. I would also assume other brands may also use the faster ramps to "enhance" the already good features of the "068" cam. Some barns cams are not exactly factory "068" and will provide all the opening/closing & duration numbers of the "068", but add a little more lift putting it around .447".

If the heads are still original, they will have bottleneck rocker arms studs. They simply get torqued down to 20-25 ft lbs. The problem here is that if any block milling, head milling, or thinner than factory head gaskets are used, it is possible that valvetrain geometry is off, ie your pushrods are now too long and could be holding the valve(s) slightly open at the higher RPM's. If aftermarket pushrods were used, often the heavier wall pushrods are selected for greater stiffness, but the trade off can be a heavier pushrod. It takes additional spring pressure to control as RPM's go up.

If you pull a valve cover, you should see the pushrods spinning because the lifter spins on the cam and that spins will be transferred to the pushrod. if you don't your rocker arms may be too tight or a cam lobe could be bad.

You might want to try to make your valvetrain adjustable by getting a set of 3/8" poly locks - not the 7/16" used for the Big Block 7/16" screw-in stud swap. By doing this, you can "zero lash" your lifters and perhaps compensate a little if the heads have been milled rather than get new pushrods. This will not fix your problem IF it is a valve spring issue.

So, one of these suggestions may or may not solve your problem.
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post #4 of 14 (permalink) Old 09-12-2019, 07:52 PM Thread Starter
 
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Thanks Jim. That’s a lot to consider. I believe we can eliminate air, fuel and spark. I’ve got RA manifolds. The tripower and fuel delivery setup is to spec.

The problem was present before the new cam was installed. We suspected worn lobes since we’d been through everything else. Turns out replacing the cam was a good idea. The one the guy who built the engine used was nothing special and it looked like the previous owner didn’t use ZDDP. The problem carried over. The setup on the cam is the same the guy I worked with has on all his GTO’s so I’m comfortable there. The only part of the valve-train not replaced was/were? the springs. Poly’s were used along with the roller tip rockers.

I was hoping there was something simple I was missing. So thanks for all the great info. Replacing the springs seems like the easiest next step. You agree?
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post #5 of 14 (permalink) Old 09-13-2019, 08:35 AM Thread Starter
 
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Tested that BLK. I't about 200 RPMs high. Thanks
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post #6 of 14 (permalink) Old 09-13-2019, 07:03 PM
 
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Originally Posted by Mikereps View Post
Thanks Jim. That’s a lot to consider. I believe we can eliminate air, fuel and spark. I’ve got RA manifolds. The tripower and fuel delivery setup is to spec.

The problem was present before the new cam was installed. We suspected worn lobes since we’d been through everything else. Turns out replacing the cam was a good idea. The one the guy who built the engine used was nothing special and it looked like the previous owner didn’t use ZDDP. The problem carried over. The setup on the cam is the same the guy I worked with has on all his GTO’s so I’m comfortable there. The only part of the valve-train not replaced was/were? the springs. Poly’s were used along with the roller tip rockers.

I was hoping there was something simple I was missing. So thanks for all the great info. Replacing the springs seems like the easiest next step. You agree?

Check a couple more items before the springs, but my money is on the valve springs at this point - but I could be wrong. The roller tip rockers have a higher ratio than the factory stamped rockers. The factory says 1.5 ratio, but from all I have read in the assorted Pontiac build guides, they are more like 1.48. The roller tips are 5.2, so possible the higher ratio lifting the valve further open and then slamming shut could aggravate the valve bounce. If you do go with new springs, you may want to check the "installed height." If the valves are sunk at all, this could have bearing on installed height and spring pressures could be affected. If not correct, shims can be used to bring them up to the correct installed height.

You have the polylocks. Did you "zero lash them?" Assume you did. How much did you turn the polylock after the "clicking stopped? It is possible you over tightened them and pushed the plunger too deep in the valve body. Different blogs & sites will say anything from 1/4 turn to 1 full turn. I only go 1/4 turn of the polylock and then cinch down on the set screw.

Do you have all your ground wires from the block to the frame and block to the firewall?

Have you ever put a voltmeter to the coil and see what the readings are as you rev the engine up? It is possible that there is a voltage drop the higher the RPM's. If you are not getting enough volts to fire the plugs at the higher RPM's, this could cause problems.

Run the car/engine at night in a nice dark place. Look for any arcing or sparking to see if something is grounding out that you would not observe during daylight.
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post #7 of 14 (permalink) Old 09-14-2019, 05:49 PM
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Overly advanced timing will kill top end revs. I would disconnect the vacuum advance and give it a try. If still laying over, I would retard the timing down to about 4 degrees BTDC and try it. You installed the cam advanced by 4 degrees, which could be the issue. Since this problem happened before the cam change, my gut tells me it's a fuel or ignition issue, but it could indeed be weak valve springs. Check your fuel delivery, and check the ignition coil. You may want to install a stock points distributor to rule out a lazy HEI distributor. (BTDT). You could have inadequate fuel, a weak coil, a bad advance curve, or weak valve springs. Hell, even a restricted exhaust (not likely). You can have the valve spring seat pressure checked to verify weak springs. Keep us posted. FYI, with a tripower, if it's a fuel delivery problem, you can do this simple test: run it up to high rpm on the center carb, and when it lays over, open up the other two carbs. The fuel in the other carbs float bowls will temporarily overcome the lack of fuel in the center carb, and the car should pick up briefly. If no change, probably not a fuel problem.
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post #8 of 14 (permalink) Old 09-15-2019, 03:01 PM
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I agree - just working from what you've said so far, my first two suspects would be

1) Weak valve springs
2) Cam that was installed advanced

Weak springs I'm sure you already know can allow valve float, and advancing the cam shifts the power curve lower in the RPM range. But since you also listed stuttering as a symptom, that points me at valve springs. If it was *just* the advanced cam then it would be down on power in the upper ranges, but it wouldn't "stuffer".

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post #9 of 14 (permalink) Old 09-16-2019, 07:33 AM
 
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I agree with Bear, sounds like valve float to me since it is RPM based (4500+).

I am curious since you said the Tri power is to spec, do you log AFRs? What jetting is in the center carb vs the outer two? (Just curious and I don't think this has anything to do with your issue.)

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #10 of 14 (permalink) Old 09-16-2019, 11:39 AM Thread Starter
 
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Thanks GeeTeeOh. I will update the thread when this gets resolved.
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