High RPM stummbling

[6Stang7]

So while I was out running some errands today in the stang, I got on it hard through 2nd, 3rd, and 4th. Once I hit about 4500 in 4th the motor stummbled and dropped about 500 rpms, then picked back up untill I let off at 6000. My question here is why did it stummble only in 4th and not in any other gears? Does this means that it is more of a fuel deliver then spark? I was thinking that maybe the floats are not adjusted correctly, so by the time I got to 4th they were really low (they are not letting enough fuel in fast enough to keep the bowls at the proper level during high rmp). The motor was only at 110 degrees though, so maybe it was still too cold. Ideas?

 

[mustang70]

Sounds like fuel delivery to me. I had a similar problem in that mine would stumble only in top gear. Raising your floats a bit might help.

 

[1969mach1351]

I'd check float levels and then pump. Do you have a choke and was it still closed? If it was the choke it should have done it in all gears though; just another possiblity.

 

[Max Power]

You're on the right track with fuel delivery. Sounds like you are falling short. Float level can effect this, but I think your fuel filter should be replaced first, then pump output checked.

 

[6Stang7]

I took the choke plate off cause I never have needed to use it before (it never gets cold enough here to where I need to use a choke). When I start her for the first time cold though, see will stumble if you give her a qucik throttle snap, but after about 30 sec to 1 min of running, see doesn't do it anymore.

 

[ForceFed70]

I agree, sounds like fuel delivery.

Replace the filter... then possibly look at the fuel pump. Getting a fuel pressure guage will also help.

 

[mdjay]

Be careful not to run it hard while she's lean... Stay off of the throttle 'til you get this resolved...

 

[HistoricMustang]

How old are your plug wires? A break in the wire core will sometimes not show up until high RPM's and it appears to be a "stumble".

It it is happening at the same point each time this could be the problem.

"Been there and done that".

HistoricMustang
www.historicmustang.com

 

[Ronstang]

Could be fuel delivery but since it only happens in the top of 4th gear it leads me to believe it might be an ignition problem. Like Historic said, ignition problems can sometimes only show at high RPMs + load.....that is why it is only happening in 4th. We just had this very same problem with a 93 coupe a friend and I race at the strip. At the top of 4th the car would stumble/flutter every run. We could not track it down so we pulled the Crane ignition box and coil and sent them to Crane.....they tested bad and we recieved new ones in the mail....problem solved.

 

[ForceFed70]

Ron, I'm not saying your wrong, I'm just confused at your logic as to why you believe it to be an ignition problem because it only happens in 4th?

I agree, it could be an ignition problem... tho to me fuel seems to be a more logical choice because with each gear your time at high RPM gets longer. We all know that fuel consumption increases with RPM and engine output horsepower. It's my guess that he does not see these problems in the lower gears because the fuel in the bowls of the carb act as a bit of a buffer. However once he hits 4th, the buffer isn't enough and he ends up going lean.

Like I said, a fuel pressure guage will quickly tell you if this is a fuel or ignition problem....

 

[Ronstang]

I agree that it could be fuel....but like I said I had the exact same problem recently and it was the ignition.

 

[foghorn67]

had the same problem, ended up being a spark plug wire. you can test the resistance on it before dumping money for a replacement set.
just saying it could be either fuel or ignition.

 

[6Stang7]

I have MSD wires on there right now with a wide cap, pertronix flamethrower coil and solid state ignition system. It does seem to been an intermit problem though. Today on the freeway coming home from class I dropped it from 5th to 4th a couple times and stood on it. Out of the 6 times I did this she only stumbled once for a split sec., but other then that she ran strong up to 6000. I have noticed a bog/stumble in third too, but I am thinking this is due to a different set of reasons. If you drop to too low of an rpm when you are at WOT, will you get a stumble? On a hard start, I will fry that tires all through first and seconded gear, then when I click of 2nd at 6000 and hit 3rd, it will bog down at the start until she gets the RPM's back up. I assume that this is because I am spinning the tires so bad in seconded, that when I shift to third, the RPM's drop too much because I am not going as fast as if I had traction.