Posted September 8, 200222 yr comment_15240 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? Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/ Share on other sites More sharing options...
September 9, 200222 yr comment_15302 Maybe your points are bouncing? Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15302 Share on other sites More sharing options...
September 9, 200222 yr Author comment_15309 hmmm I've never heard of that one before... can you please elaborate a little? Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15309 Share on other sites More sharing options...
September 9, 200222 yr comment_15310 The bouncing point idea is good. Have you considered moving to a magnetic type system? They have conversion kits available for pretty cheap. Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15310 Share on other sites More sharing options...
September 10, 200222 yr comment_15359 Does this happen in every gear, or just first??? Could make a difference. Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15359 Share on other sites More sharing options...
September 10, 200222 yr comment_15397 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. Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15397 Share on other sites More sharing options...
September 10, 200222 yr comment_15399 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. Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15399 Share on other sites More sharing options...
September 10, 200222 yr Author comment_15400 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 Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15400 Share on other sites More sharing options...
September 10, 200222 yr comment_15419 posted above Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15419 Share on other sites More sharing options...
September 11, 200222 yr comment_15431 Maybe there is a imbalanced part that at 5500rpm it reaches resonance and causes an oscillation (have I got that around the right way?) Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15431 Share on other sites More sharing options...
September 11, 200222 yr Author comment_15481 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 Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15481 Share on other sites More sharing options...
September 12, 200222 yr comment_15536 Do you feel the car lose power during that period... If so you may have made an improvement, but not completely cured the problem... Link to comment https://www.classiczcars.com/forums/topic/2966-high-rpm-stumble/#findComment-15536 Share on other sites More sharing options...
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