Engine running rough above 2000 RPM

[Michael J.]

Hi, my '68 has developed a problem over the last week months. It runs rough and stutters and surges when I am cruising above 2000 RPM. It starts and idles great, and under acceleration it is fine, but at steady highway speeds over 2000 RPM or so it behaves badly. It runs, just rough with the stutters and surges. So far I have changed out the carb (new Holley 4150, BB engine ), new coil, checked for vacuum leaks and electrical leaks off the plug wires and fit of all plug wires at distributor. I am wondering what else could be causing it. It has a Pertronix ignition, plugs and wires (all this was installed/replaced in 2003), as was the fuel pump replaced in 2003 (not over 8000 miles since then). Would any of these things cause this problem if starting to go bad? Thanks for any help.

[Tom H.]

Re: Engine running rough above 2000 RPM

I think I'd start with the timing. Where do you have it set ?? Have you adjusted or replaced anything else since it last ran correctly ??

[Rich W.]

Re: Engine running rough above 2000 RPM

The coil.. Is it a Petronics also..maybe try a kv output test when hot..maybe it is getting a random misfire from the unit itself..was the distibutor rebuilt in 03 also..did you replace the bushings inside..I know people like the convenience of electronic ignition but I really prefer points..they last the test of time which most electronics don't..8k in 8 years..and the problems that they may give us.

[Timothy B.]

Re: Engine running rough above 2000 RPM

Michael,

Are you saying the engine ran the same with the new carburetor as the old carburetor/ How about the Petronics, did it ever run OK with the ignition.

[Michael J.]

Re: Engine running rough above 2000 RPM

Thanks for the feedback. I have tried various timings, started at 8 degrees BTDC, went down to 4, and up to 14 but still the same problem. The new carb is a newly restored/rebuilt original type Holley, the old carb was a 4779, it ran fine before I replaced it. After I started having this trouble I put the old one back on (changing the filter each time) but the same problem either way. The old coil was a stock OEM Delco type unit, the new one I tried was a NAPA, but the problem remained either way. I bought the car last year, I also prefer points, but it came with this Pertronix setup. I am really thinking it must be the Pertronix module, as I have tried most everything else, unless plugs and/or fuel pump could cause this. I don't know how long the Petronix things last, but 8 years may be it.

[Rich W.]

Re: Engine running rough above 2000 RPM

Mike I can't say for sure the Petronics is the problem..but a points setup will cost about 15 $..I put the accel in mine..the 46oz if I remember right..they were around 50$ though..do you have a way to check total timing? Might be something to look at also.

[Michael J.]

Re: Engine running rough above 2000 RPM

Yes, I guess putting points back in may be something to try too. I did find out the problem gets worse the more you advance the timing. I think the Pertronix conversion allows simple, easy points reinstall. But then again you have to set dwell and gaps too.

[Rich W.]

Re: Engine running rough above 2000 RPM

Points are not something to fear..all that's needed is a basic dvom or dwell meter..set the dwell to 30 and your done with them! I have the snap on advanced timing light..something I bought many years ago for to much money..but it does everything..look on ebay for one..not much anymore since they are useless on new cars.

[Paul O.]

Re: Engine running rough above 2000 RPM

Mike

This maybe obvious but have you checked the spark plugs for condition. I have seen a build up of contaminants that will cause a problem at a steady state RPM but does not cause problems at idle or under acceleration. Also is the problem also there when your stopped in neutral and raise the RPM to 2000 or more in other words no load condition.

One other check of your plug wires is at night hood up minimal lighting or none visual check the wires for a spark from wire to ground at the higher RPM.

Paul 18046

[Michael J.]

Re: Engine running rough above 2000 RPM

OK, I will check the plugs. The problem happens when you sit in neutral and rev the engine, it gets rough with the surges and shutters at about 1500 RPM actually. I did the dark garage test last night, didn't see any arcing around the distributor or plug boots area. I did see some faint, blue flashes in the wires between those areas, but those disappeared when revved. Thanks.

[Michael J.]

Re: Engine running rough above 2000 RPM

OK, I have now checked some plugs. They look OK, no melted, burned, or cracked electrodes, no buildups at all on them. I also checked the gaps and they are factory spec. So, on to replacing the Pertronix.

[Timothy B.]

Re: Engine running rough above 2000 RPM

Michael,

Pull the vacuum advance hose, plug it and go for a ride around the block to see if the engine is smooth at cruise rpm. This test may tell you if timing is advanced to far.

[Michael J.]

Re: Engine running rough above 2000 RPM

Thanks, yes I tried that early on, as I was playing with the timing thinking that was the problem. The funny thing is that the car was running great 1 month ago, it just seemed the problem happened overnight and got progressively worse.

[Paul O.]

Re: Engine running rough above 2000 RPM

Mike

One other thing check the lead wire going into the distributor when the base plate moves for the mechanical and vacuum advance that could cause a short to ground or the wire is damaged internal causing you to open the circuit. It happens often in TI systems.

Paul 18046

[Michael J.]

Re: Engine running rough above 2000 RPM

OK, I'll try that too. In Googling "Pertronix failure", I get some very interesting info from various cars. After reading it I think I may know what I did wrong to make this problem start so suddenly. It says the #1 cause of Pertronix failure is leaving the key "on" for more than a few minutes. I think I did a no-no there when I was testing circuits and lights a few weeks ago