High RPM Problem Help!

[PutPut6]

I rarely take my engine over 4,000rpm but when I do, I have noticed that while in gear driving, the engine has a significant loss of power at 4,300rpm and starts running real rough. When this happens I let off the gas or take it out of gear and the engine still runs rough regardless of rpm but smoothens out after about 5 seconds. While sitting in the driveway at neutral I can rev it up to about 4,500-4,600 until it does the same thing. Does anyone know what in the world could be causing this? Thanks in advance for your help.

 

[CoupeBoy]

Sounds like a fuel delivery issue. How long has it been since you checked/replaced the fuel filter? Double checked the float level? What sort of fuel pressure/volume is your fuel pump pushing out?

-ron

 

[PutPut6]

The fuel filter is about 7 months old. I probably should check the float level, I can't remember if I checked it when I rebuilt the carb. I have no idea how much pressure my pump is pushing. I'll get a gauge and check that. Thanks for the help ron. If anyone else has any other ideas don't hesitate to chime in.

 

[CoupeBoy]

Its probably not this but when does valve float normally begin? What sort of behavior does that incur? I don't think this is the problem but figured I would toss it out there.

-ron

 

[PutPut6]

I thought about valve float but after hitting around 4,300rpm and the engine starts running rough, if I let off and let it idle it still runs rough for a couple seconds at idle. Valve float wouldn't do that would it?

 

[CoupeBoy]

plulgged exhaust? Maybe your muffler baffles have come loose and its plugging itself off? I once backed into a sidewalk and accordianed (technical term) my muffler and the car whistled real bad and would not build power until I had it replaced the next day..

But in your case it could be building up pressure then retaining pressure for a while until it clears out..

Just tossing ideas out.. hopefully somebody can be more useful.
-ron

 

[nitrousnick]

sounds like the lifters are pumping up and it will take a few seconds for them to bleed down and run smooth again.

 

[Stubby]

When valve float occurs the spring can no longer keep the lifter in contact with the cam. When the lifter jumps off the backside of the lobe, the lifter can pump up and when it gets on the base circle of the cam it will hold the valve open like it is adlusted too tight. It will blead down shortly and smooth out. It could be valve float. You would have to check spring pressure to rule this out. I would try to rule out everything else and be careful when testing with your foot. Valve train instability can cause lots of problems. If you know someone with a tester you can check the pressure without pulling the head. You can't hardly check open pressure on the head but if you have the specs on the springs you can verify seat pressure if it is still ok then open pressure should be also. And make sure they are good for that rpm.

 

[PutPut6]

Openned up the carb and the float was a tad low. I Readjusted it and it seems to have helped some, maybe can go another 100rpm, but not much. I can rev up to about 4,500 in first without much roughness, can barely get to 4,300 in 2nd before it starts to run rough. I shot it up to 5,000 and it ran rough for about 10secs. I had to pump the gas a couple times to get it to smoothen out. The exhaust sound was kind of like a "put put put" when it runs rough... i duno, that's how I thought of it. Hope this may help figure out what's going on. I'll get a new filter tomorrow and a fuel pressure gauge.

 

[PutPut6]

I don't know anything about the springs, they were on the engine when I got it (remanufactured longblock). This engine is only 7 months old after being remanufactured, would valve float at 4,300rpm be normal for an engine like this? I figured I'd be able to do atleast 4,500 on completely stock stuff.

 

[hasa68mustang]

One question is there anything done to your motor? when mine was stock it wouldnt make much power after 4000 rpms... now its more of something like 4600 but I have been able to do 5300 rpms but didnt notice float... but I have no clue what springs the machine shop put in, I told him I was looking into doing a turbo setup in th future but I dont know if he installed stock springs or something better... something better would be nice... It is a racing shop.. MAYBE THEY INSTALED RACING SPRINGS SO NOW I HAVE 800 HP... ok so I can dream...

 

[PutPut6]

I haven't done much to it. As for the longblock, it should be just plain stock unless the remanufacturer decided to put something interesting in there... who knows. New exhaust and a Pertronix II is pretty much all I have done to the engine so far.

 

[Anonymous]

I shot it up to 5,000 and it ran rough for about 10secs. I had to pump the gas a couple times to get it to smoothen out.

Was this in neutral also like this

While sitting in the driveway at neutral I can rev it up to about 4,500-4,600 until it does the same thing.

Doug

 

[JackFish]

Maybe a combination of things, including valve float if you used stock springs. I don't have a tach, but above 60mph my engine vibrates. I KNOW my springs are shot, especially since I just had the u-joints replaced and the driveshaft balanced. Try that.

Also if you used a stock timing chain and advance your car is probably better at pulling trailers than G's above 55mph.

With my kit I ordered a set of springs from Mike that have dampners to eliminate "harmonic resonances" at high revs. As a musician, that makes sense to me. There's a lot of parts vibrating around in a motor and it's natural that there would be "resonating frequencies". And it seems that the valve spring would be perhaps quite susceptable to that kind effect.