Engine runs great, starts easy, pulls real good until about 5200 - 5400. Then it feels like its stuttering. Hard to describe, but it acts like its hitting a rev limiter. No backfires, no popping thru the exhaust, just runs real ****py at high rpm. The ignition checks out good; used a digital scope to watch it and the duraspark/MSD is making big sparks. There should be enough fuel and air going to the engine. Plugs look a little rich, but not bad. I'm going to borrow a wideband sensor to check air/fuel ratios, but I'm not convinced that's the issue. The only other thing I can think of is valve springs limiting the rev range. This engine should be good to 6500, but struggles past 5500. What does valve float sound like and is there a way to know for sure?
In my experience, foating valves will not really have a specific "sound", but you will "feel" it. The engine just runs out of wind and goes "flat". You can push harder on the pedal...but it doesn't gain any RPM or speed. When the valves are floating...you are basically dumping compression out the exhaust system, which decreases power. Are you running compe***ion valve springs, or stock? I'd suspect that the springs are weak or improperly shimmed to restore the correct seat pressures.
Several motor back I used heavy adjustable lifter in one of my flatmotors.Whinding it up too high made a noise like 1,000 very quick intake backfires.I use a rev limiter now,plus lighter valve train compondents.
Truth, sounds like the motor just runs out of air when its hitting a high note sustained for a long time.
If the engine has hydraulic lifters, one sure sign is that when you let off, the engine will continue to run poorly for several seconds, even at idle, until the lifters "recover" & "re-lash" themselves, i.e. allowing the valves to shut properly again. After a few seconds it settles down & idles OK. With solids, it's a little more of a guessing game; typically, if the exhaust is quiet enough, you can hear the engine flatten out & begin misfiring...possibly even clattering....or just flatten out if the exhaust masks it. With roller lifters, after the first or second time you float the valves, you typically hear the sounds of your valvetrain di***embling itself without your ***istance, followed by the sound of your tires quietly rolling to a stop.
Make sure your float level is correct in both float bowls - ***uming you're running a Holley 4 barrel. Sometimes if float is low, especially the secondaries, it can feel like you've hit a wall with no more go even though you should have more rpm left. Basically staving for fuel when you hammer it but not noticable just driving around.
i floated the valves in my old mans newyorker one time just sitting there reving the **** out of it, the last rev the engine had a sound as if it was just became very free running, then it stoped and wouldnt start for over an hour till the lifters blead off.
On the positive side, if you are running weak valve springs, don't mess with it. You can run the new oils without flattening a cam lobe.
It's a hydraulic cam and it returns to idle with no problems. Maybe it's not valve float. At first I thought maybe I had a high speed ignition misfire, but I think I've ruled that out. I've adjusted the floats and those are good. Fuel pressure is good, filter is clean. Dammit, peculiar engine problems can be so frustrating sometimes!!
Once had a 69 chevelle with a 307 and did exactly as you originally described - cause was weak valve springs from 100k+ miles on them.
Depends on the motor... On an OHV, you'll usually float the intakes B4 the exhaust, and depending on the degree of float, she'll either nose over (power loss) or in extreme cases, you'll get some backfiring before metal to metal contact causes the motor to eat itself. On a flathead, you can float 'em 'till the cows come home - you run the risk of some busted valvetrain parts, but since the valve and the piston never occupy the same space - no major damage. If the exh valve is open when the cylinder is lit off then it sounds like Groucho sez. do keep in mind that your spark voltage and gap is a function of compression ratio - if you've a high ratio, you can experience spark blowout with either low voltage or a large gap.
If it recovers to idle just fine, then I'd examine the following: Insufficient fuel pressure/float level too low/needle & seat too small (basically, insufficient fuel volume) Inadequate valve springs Choke pulling itself shut at high rpm Ignition issues It sounds like you feel as though you've eliminated or crossed off most of these issues. At this point I would test the spring pressure. The easiest way to do it (though not perfectly accurate, it's a ballpark figure) is to use an on-engine tester, ***uming it's OHV. Beyond that, pull 'em. Goes fairly quickly once you get the rhythm. Or...try this.... One thing I purposely neglected to mention earlier, is that, depending on your hydraulic lifters & their bleed-down rate, the engine may recover more quickly than a few seconds. Rhoads-type lifters act more like solids in that respect. So, try adjusting your hydraulics so that they are barely lashed- almost at the clip- & see if it makes a difference. If so, get better springs. Most of the so-called "anti-pump-up" lifters are/were regular hydraulics, with stronger snap rings/Tru-Arcs....or not even that. BTW...what do you have in terms of an exhaust system?
This is an inline six. It's an Aussie Ford aluminum, Cleveland-style, canted valve, crossflow head adapted to a 1966 200 cube Ford block (the original engine in my Mustang). The intake has a common plenum fabricated from tubing and holding a 350 Holley 2bbl. The exhaust side is headers with 1 1/2" primaries feeding two 2" duals with gl***packs, one out each side. Cam is a mild hydraulic 220/220 @ .050 with a .484 lift. I'm running it straight up. Ignition is a recurved Ford duraspark with an MSD6A and a TFI coil. As you can see in the photos, there is still some tin bending to be done and the exhaust will get re-routed, but we've been go-karting around the neighborhood like this. The car is very, very fast until you hit that upper rev limit just above 5000. Then it goes flat and sounds like its choking. Let off the gas and it heads back to a normal idle. No overheating, backfiring, or popping thru the mufflers. I'll double check the float level and fuel pressure. I'll also relook the timing and the ignition.
Umm... the problem may be your carb mount. fuel slosh on a side mount holley will cause the front jet to run dry - eventually. the trick would be to slowly take the revs up to see if you can get past the magical 5K mark - if you can, then it's a fuel feed issue. Just went thru this with a buddy of mine's car - ran fine on the street, but would **** the bowl dry at about 80 MPH at the track - nosed over and fell off.
Valve float sounds like marbles in your intake.... or a machine gun like Groucho mentioned. Unfortunately you may not be able to hear it over the wind and exhaust noise. If possible put in a new set of springs and/or step up one level of spring pressure to see if it disappears.
Similiar to a flat cam lobe on a SBC or when the inside of a double wall exhaust pipe got loose on a 70's car? Usually will see some excessive fogging in the carb when it happens
Mmmm, still thinking fuel. I agree that G's under hard acceleration could take fuel away from jets in that setup. I'd back the sneak up on the rpm theory too. Is this a new problem or has it always been the case? Tripple Webbers.
Chuckspeed, This one "noses over" while revving in the driveway as well as under load. It seems more rpm dependent than load dependent. It doesn't seem to have a problem while cornering, but I haven't taken a corner with the engine going 5000 rpm yet either! I'll see if I can get a video up over the weekend.
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