high-rpm stumble

[BadDog]

I'm getting a little stumble right after 5,500 RPM (I can even hear the exhaust crackle). It then immediately smooths out and keeps revving, so I don't think it's for lack of fuel...

My setup:

stock '73 engine, round-top SU's, stock ignition, motorsport equal-length header. I'm running the timing a little advanced, about 9 or 10 BTDC...

Any ideas of what to check?

[Ed]

Maybe your points are bouncing?

[BadDog]

hmmm I've never heard of that one before... can you please elaborate a little?

[Mike]

The bouncing point idea is good. Have you considered moving to a magnetic type system? They have conversion kits available for pretty cheap.

[smg1404]

Does this happen in every gear, or just first??? Could make a difference.

[Ed]

Though the points are a good concept at moderate RPM's they tend to "bounce" at highter RPM's usually around 5000-5200. What they actually do is they can't keep up with the RPM's and stay open. I 'm sure someone has a better description.

[smg1404]

This would be a good explanation, if the rpm did not go on up past the 5200 point.. But the description said that it only hesitated, or stumbled at 5200, then accellerated fine on up past 5200 rpm... I tend to think that it is more likely an ignition break down, ie weak coil, wrong or worn plugs. It could be caused by a combination of heat and pressure in the combustion chambers, causing the resistance of the plugs becoming too great to allow a reliable spark. In my experiance this has happened to me in about this rpm range. I was able to correct it with a better set of plugs, and better wires.

[BadDog]

Hmmm i see... so is that something you can see with a dwell meter?

I've been thinking about getting one of those points replacement kits anyway. Pertronix is usually the kit that people go with, right?

Yes, it happens in first and second gears... I don't usually get much opportunity to wind out 3rd ;-)

***edit***Well, I *did* gap my plugs a tad on the wide side (.0375 or so?) but only because when I had them at .0325 or so they closed up on me

[smg1404]

posted above

[Ivan]

Maybe there is a imbalanced part that at 5500rpm it reaches resonance and causes an oscillation (have I got that around the right way?)

[BadDog]

Well I re-gapped my plugs back down to .035 and it got rid of the audible popping and stumble at ~5,500... I still have a wierd phenomenon, not so much a problem: At about 4,500 to 5,000 RPM under WOT the tach kind of slows in it's pace, then speeds back up again after ~5,000 RPM. Since it happens so fast, it's hard to tell if tach is not keeping up with actual RPM's at that point, or if the ignition is really causing the engine to slow in it's acceleration, then surge again after ~5,000 RPM.... I think I'll just replace the points with something like the Pertronix kit before I worry much more about this. Could be a cam timing/valve clearance issue too, I haven't either since I bought the car in March

[smg1404]

Do you feel the car lose power during that period... If so you may have made an improvement, but not completely cured the problem...