car is a 1978 911 with a 3.0. no A/C, smog pump removed for now.
the car just started running bad. not sure if it is related to the cold but one night i was driving and it started coughing (backfiring while driving when i would step on gas) then when i pulled off highway onto surface streets it ran very rough, idle started going up and down from 950 down to about 700 then back up and so on. also i noticed the tach was jumping a bit as i was driving, so i would be at 3,000 rpms then it would start to jump to 3200 or 3300 then back down it did this for a while. just fast jumps. with a def power loss as well.
one other note, the next day or day after it drove fine but then it went back to running rough. even on a much warmer day. on a cold start it starts but the idle is rough hunts up and down and small backfires when i rev it. i have replaced plugs not too long ago, replaced fuel filter, i have a K&N air filter and a MSD ignition with fairly new wires. any thoughts, suggestions or advice would be great. thanks

 

Hi,

Sorry to hear about your running issues,....its all fixable with some patience and a methodical approach to troubleshooting.

One of your symptoms offers a clue about where to begin: those tach fluctuations. I would kindly suggest spending some time thoroughly checking the electrical system. You want to check the operation of the voltage regulator, alternator, battery, and all the battery cables and grounds. Don't forget about the main ground strap that connects the transmission to the chassis under the car,...

Review your MSD connections and grounds, too. Most people underestimate the amount of current these draw and that's usually the root of ignition troubles and failures using these ignition systems.

Every connector on the car needs to be; bright, shiny & clean,.................quite literally.

Attention to fine detail pays big dividends.

 

thanks, yes seems like a good place to start. i will do an inspection and see if anything seems loose or dirty.

 

Great tip Steve Weiner. Appreciate your shated knowledge gained through years of being one of the top wrench hands on the West Coast.

 

A couple of other simple things you can check ..
does this car have a pop off valve . These things are more trouble than they are worth. make sure iit is seated properly and not allowing unmetered air to leak in. also make sure you dont have any hoses dangling that may cause vacuum leaks. jumping up to 3,000 makes me think it may be a fuel ratio thing

 

My idle oscillation was related to the O2 sensor.

 

Batamn 78 has to O2 sensor ... but good consideration for those with a later SC reading this with similar issues, O2 senor or frequency valve.

 

thanks - will start with the easy checks first then on to the more complex and see if i can't figure this thing out. will update after the weekend.