4.2 stumble problems

I have an 87 Wrangler with a 258I-6.
Problem I am having is that the rpms will pull strong up to about 2500rpm and begin to stumble and will not continue to gain rpm's. Sometimes while in Neutral the motor appears to increase but when it does the tach will fall to 1500rpm. I was checking the timming and noticed that the timming light will stop flashing reg above the stumble point which is telling me that its loosing spark above the 2500rpm range....my question is why?
Today I replaced wires, cap, rotor, coil, and distributor.....
Nothing has changed.
Also this appears to be an aftermartket Webber carb on it. And most of the emission things are gone....
thanks for any help.
~Rich

I don't know much about this stuff (cuz it aint an XJ), but there must be something controlling the spark, and at that age presumably it isn't points and condenser. I'd look at whatever computer/module/ blah-de-blah is controlling the spark. If this were a points system I'd also suggest looking at distributor shaft and breaker-plate wear ( because a worn breaker plate can cause a sudden change in dwell, for those who even remember what dwell is!) but it still might be worthwhile looking to see if there is something, such as a vacuum advance, that is cutting in at the critical speed and causing the spark to go off. I could conceive of a timing issue such that when the advance goes too far the rotor-to-post timing goes out of range. Conversely, if this is an emissions-era rig it may well have a fairly drastic timing retard at idle, and if so, a failure of the vacuum or centrifugal advance mechanism (or whatever else controls the spark) might leave the timing too late to achieve higher speeds. Have you checked with the timing light to see whether it seems to be advancing as you rev it up?

When you say that at idle the motor appears to increase but the tach falls, do you mean that it is actually not increasing, or that the tach is failing to record it properly? A falling tach is sometimes a first symptom of a failing ignition controller, or of bad connections in the circuits involved. There are probably thousands or even millions of current and former Volvo 240 owners driving around with the covers off the fuse box, so that when the tach drops out they can fiddle with the ignition fuse before it dies.

Thanks but as I stated:
I replaced wires, cap, rotor, coil, and distributor.....
And:
Also this appears to be an aftermartket Webber carb on it. And most of the emission things are gone....
Whatever computer controls that were in place are reduced to simplicity.
Maybe I need to unplug the tach and run a 12v source to the coil to eliminate the possibility of the tach wire or hot lead going to the coil grounding somewhere....
I just dont know....
~Rich

I think you need to spend a little time discovering the nature of the '87 YJ ignition system. AS Matthew Currie suggested, it appears whatever controls when the coil is fired doesn't like RPM's. If the distributor doesn't have points (not likely that it does), there has to be an electronic equivalent controlling ignition. Since your asking for help on an XJ/MJ forum, you may not find some one with direct experience with the YJ. So study your system and then ask us to draw analogies.

dadstoys2 said:

Thanks but as I stated:
I replaced wires, cap, rotor, coil, and distributor.....
And:
Also this appears to be an aftermartket Webber carb on it. And most of the emission things are gone....
Whatever computer controls that were in place are reduced to simplicity.
Maybe I need to unplug the tach and run a 12v source to the coil to eliminate the possibility of the tach wire or hot lead going to the coil grounding somewhere....
I just dont know....
~Rich

Click to expand...

I would definitely try unplugging the tach, especially if you're not sure whether or not it is the correct tach for the current ignition system. It might be loading the ignition control module in some odd way.

is the ignition module still connected? (driver side fender, below washer bottle, on a CJ, probably a YJ too). if it is, bypass it (nutter bypass), and re-time it.

Well that was it.....ignition mod.
$19.99 at the local parts store.
So I guess on the bright side....all the ignition parts are new
Thanks for the help.
~Rich