96 GTX Drops one cylinder at high speed

[bz1mcr]

My '96 GTX has been running great, but recently it started dropping a cylinder at high speed. Starting and idles are normal, part loads to about 4500 RPM are normal, even acceleration is normal, until it starts to top out then it suddenly is running on just one cylinder. If you hold the throttle it will eventually suddenly start firing again, taking speed to 7000 Rpm, but then droping again to one cylinder and maybe 3-4000 RPM. This will repeat in random fasion over and over. If you release the throttle while operating on one cylinder it will always immeadiately start firing on two at idle or low speeds. There is no random missing as if fuel starved etc. It runs steady and strong, but sometime on one cylinder and sometimes on two when holding heavy throttle. Transitions are as if someone flipped a switch. One instant it is running on two then one then back to two. I installed new plugs and it changed nothing. I am guessing it is ignition related because it happens so quick, but it could be something else that acts fast.

What else I can check or do my self?
Don

 

[derek]

cut the plug wires back about 1/4 to 1/2 inch and screw the ends back into the wires. i guess you have already changed the plugs.

 

[bz1mcr]

bad rectifier- causing ignition problem

I have not tried this but it has been suggested that it could be a failed rectifier. If so there is an easy test. Just disconnect the red wire, which will disable the battery charging circuit, if the miss goes away the rectifier needs to be replaced. The machine will start and run ok with the lead off, but the battery will slowly discharge. It should be fine for a test ride of up to 1/2 hr. But, don't leave it disconnected, eventually the battery will go low and ignition and cranking will be disabled.

It's an easy test and fix if that's the problem.

It will be several days befor I can try this, I'll let you know what happens.
Don

 

[timmyboy76]

Sounds like, a good idea...

 

[dghal]

fuel issue,, have the old grey fuel lines been changed?

 

[bz1mcr]

Sounds like, a good idea...

I should. It was your suggestion.
I was very hopeful because it was easy to test and an easy fix if I just needed a new rectifier. Unfortunately, I just tried it and ---no change. I still cannot get it to sustain full power/speed. However, instead of one cylinder dropping off instantly, today sometimes it went off more slowly as if starving for fuel.

I do have the infamous grey fuel lines and perhaps they are at the heart of my trouble. I need to investigate more as at fist I thought it was some kind of ignition issue, now it seems it could be fuel.

All suggestions are welcome!