Upper RPM Miss?

Great68 · Sep 24, 2004

Upper RPM Miss (5400+rpm)?

If you would believe, since I did the headswap I haven't really pushed my car... Until tonight since the insurance expires in 4 days and the car goes away for the winter.

I took it out to a nice stretch of road and floored it, I noticed that at 5400rpm the motor had a miss, like a stutter and I was too afraid to push it higher. Below 5400 it's fine, no problems and really hauls ass.

Is this a fuel starvation issue or what? I have a Performer RPM mechanical fuel pump, so I don't think that's the problem. Although my fuel line from the tank to the pump is stock. (If that's the issue, then I have a nice winter project )

6Stang7 · Sep 25, 2004

Hmmm, maybe there is an issue with your timing. What is your timing advance curve like?

mfp4073 · Sep 25, 2004

May be an issue related to fuel, but not neccesarily starving due to fuel pump or lines. I had some problems with my car leaning out at high rpms (almost where you said). Solved it by fixing a partially sticking thermostate and better grade fuel. Try some 93 octane and see if its still there.

Ozsum2 · Sep 26, 2004

Timing advancing too far.

thehueypilot · Sep 26, 2004

Your fuel bowls could be running dry. Check your the fuel bowl level front and rear at idle it should just be coming out the inspection hole.

Max Power · Sep 27, 2004

Wouldn't timing be fully advanced at 3000-3500 rpms? If it was advancing too far wouldn't it be a problem at a lower RPM than above 5000?

I would suggest either fuel starvation or ignition components that can't keep up with the rpms.

Ozsum2 · Sep 27, 2004

Max Power said:
Wouldn't timing be fully advanced at 3000-3500 rpms? If it was advancing too far wouldn't it be a problem at a lower RPM than above 5000?

I would suggest either fuel starvation or ignition components that can't keep up with the rpms.

Not this time bro..............

PickupMan · Sep 27, 2004

I have the same problem at the same RPM. I've tried the timing thing, fuel you name it with no luck. I spoke to a friend of mine and i thought this could be it as well, I have valve float. I never changed the springs when i did the cam swap due to a performance cam was installed from previous owner. You said you did a head swap. What did you swap to? Did you install the right spring for the 262 cam?? Some thing to think about if you didn't

Great68 · Sep 27, 2004

Sorry for the late reply guys, I was in Portland for the weekend performing at the Portland Polish Festival.

Anyways,

6Stang7: Timing is 14* initial, 35* Total, all in by just under 3000 RPM (The 2 light blue springs are in the distributor). I have a vacuum advance connected.

mfp4073: I'm running 94 Octane currently.

thehueypilot: My float levels are exactly halfway in the sight glass

PickupMan: I have AFR 165's with whatever springs they put in them (Which are a dual spring, good to .550" lift, and have 120 lbs. pressure on the seat). According to comp cams, the cam requires either 942 springs which are a single spring good to .500" of lift, and have 115 lbs. pressure OR 972 springs which are a single spring good to .600" of lift, and have 124 lbs. pressure.
I would figure that the since the numbers on the springs that come with the heads are in between the two recommended by comp, that I should be allright.

Right now I have Autolite 3924 plugs gapped at 0.045", My wires are 8mm. MSD's, and my coil is a new MSD blaster coil.

It's weird, it almost sounds like there's a rev limiter at 5400 rpm, but I know there isn't one in there haha.

So if it's a timing issue, is it because I have too much timing? or too fast of an advance rate?

Thanks for the help guys!

Dark Knight · Sep 28, 2004

try running it up to 5400 or higher without flooring it and see what happens..

my vert would do that, ended up being a bad coil..

you might also close the spark plug gap some.. .035-.040

Max Power · Sep 28, 2004

Well, if you think it is timing advanced too far, it would be real cheap and easy to find out!

Ozsum2 · Sep 28, 2004

Great68 said:
Sorry for the late reply guys, I was in Portland for the weekend performing at the Portland Polish Festival.

Anyways,

6Stang7: Timing is 14* initial, 35* Total, all in by just under 3000 RPM (The 2 light blue springs are in the distributor). I have a vacuum advance connected.

mfp4073: I'm running 94 Octane currently.

thehueypilot: My float levels are exactly halfway in the sight glass

PickupMan: I have AFR 165's with whatever springs they put in them (Which are a dual spring, good to .550" lift, and have 120 lbs. pressure on the seat). According to comp cams, the cam requires either 942 springs which are a single spring good to .500" of lift, and have 115 lbs. pressure OR 972 springs which are a single spring good to .600" of lift, and have 124 lbs. pressure.
I would figure that the since the numbers on the springs that come with the heads are in between the two recommended by comp, that I should be allright.

Right now I have Autolite 3924 plugs gapped at 0.045", My wires are 8mm. MSD's, and my coil is a new MSD blaster coil.

It's weird, it almost sounds like there's a rev limiter at 5400 rpm, but I know there isn't one in there haha.

So if it's a timing issue, is it because I have too much timing? or too fast of an advance rate?

Thanks for the help guys!

My thinking is too much total timing.

pabear89 · Sep 28, 2004

Lifter float?
The springs may be too stiff for the lifters at that rpm.
or the lash is too tight.

Just a thought.

PB

wildstang · Sep 28, 2004

Possibly your flame is being blown out at higher rpm, may need to upgrade your ignition with a stronger coil and related hardware.

302 coupe · Sep 28, 2004

Ozsum2 said:
My thinking is too much total timing.

If total max timing is in by 3000 rpm, it should start missing by 3000 rpm. Once it advances to 35, thats it, there is no more advance. As rpms climb over 3000, timing stays at 35. Not questioning you Oz, just trying to understand your theory behind it.

I have heard that AFR's are happy with slightly less timing than other heads, and its easy enough to take a few degrees out. It would be worth trying it just to eliminate a possibility if nothing else. I'd also check valve lash as it will cause some issues at higher rpms if it isn't set right.

Max Power · Sep 28, 2004

That's what I was thinking 302 coupe.

Too much advance, in theory, should effect an engine more at 3000rpm than 5000 rpm, as piston speeds are much slower. I would also think you would hear detonation first before a miss, but there are probably a lot of things you can't hear at 5000rpm on the track.

Ozsum2 · Sep 28, 2004

302 coupe said:
If total max timing is in by 3000 rpm, it should start missing by 3000 rpm. Once it advances to 35, thats it, there is no more advance. As rpms climb over 3000, timing stays at 35. Not questioning you Oz, just trying to understand your theory behind it.

I have heard that AFR's are happy with slightly less timing than other heads, and its easy enough to take a few degrees out. It would be worth trying it just to eliminate a possibility if nothing else. I'd also check valve lash as it will cause some issues at higher rpms if it isn't set right.

Is it possible that the timing is too far advanced, and just wasn't being noticed at an earlier RPM, and then when things are strained to the limit, that the driver notices it?

Great68 · Sep 28, 2004

Ozsum2 said:
Is it possible that the timing is too far advanced, and just wasn't being noticed at an earlier RPM, and then when things are strained to the limit, that the driver notices it?

I don't know, it's quite distinct when it does happen. I think i'm gonna play with my timing a bit and see what happens. If it doesn't go away, then I'm going to leave it until next spring when I can dyno tune it.

SadbutTrue · Sep 28, 2004

well, the 262 cam creates power to 5600 rpm I believe. So i think it might just be maxxing out (max power will be several hundred rpm less than the advertised max, since Compcams rates their cams with a wide power band).

Not saying that your engine isn't missing or anything, but you should factor in that its possible/probable that at 5400 rpm your cam is about done.

Dark Knight · Sep 29, 2004

not sure what the total timing is on my 84, but my AFR's like 17* initial.. any more or less cost HP on the dyno..

I'd still close the gap some on the plugs.. and you might need a better set of valve springs..

Upper RPM Miss?

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