76' 911S runs on 5 cyl. but only at low RPM's???

vonsmog · 07-16-2010, 05:05 PM

Ok, help me out here guys. I have a 76' 911S Targa with only 45k miles on it. Has backdated exhaust 11 blade fan, pop off valve, chain tensioner guards. Just put in 6 new fuel injectors, new spark plugs, checked plug wires, all fine. Checked and replaced CDI box, rotor & cap. Still I get cyl. # 4 plug is black while the rest are nice off-white. Car seems to skip like it runs on only 5 cyl. from starting off to about 3k RPM's then it feels like the other cyl. kicks in and the car will pull right up to 6500 RPM's without a skip or backfire. I just can't figure out what the problem is. Could it be running too rich in the one cyl., and when the throttle is full open it is getting enough air for a complete burn? When I bleed the injectors they all look like they are putting out the same. But I did not measure the output. from cyl. to cyl. I know I am getting spark, so I don't think it is a spark issue. What should I look for? and how can I test to find its problem?
thanks.

ossiblue · 07-16-2010, 06:20 PM

Have you adjusted the valves and done a compression check? Have you checked the resistance and continuity of your plug wires?

You know #4 is bad. Try replacing #4 plug wire with a known good one (from another cylinder) and put #4 wire on the good cylinder (naturally, so the swap at the dizzy too so the firing order is the same.) Does the miss go away? Does the miss now appear at a new cylinder?

This is at least a start. Let us know what you find.

vonsmog · 07-16-2010, 06:44 PM

I have replaced the plug wire with a new one, and it is getting spark. I also did a compression check and all cyl's are within specs. I did not adjust the valves, would that cause what I am experiencing?

rsscotty · 07-16-2010, 07:19 PM

You didn't mention what is in the distributor...points or Pertronix?

I have seen an engine with Pertronix not run on 6 cylinders at low RPM with an improper gap setup. Too wide in this case.

vonsmog · 07-17-2010, 04:40 AM

Quote:

Originally Posted by rsscotty

You didn't mention what is in the distributor...points or Pertronix?

I have seen an engine with Pertronix not run on 6 cylinders at low RPM with an improper gap setup. Too wide in this case.

Dizzy has points, which are good and set right, as is the timing. Still can't figure out what is wrong with this thing. I think I will shove my little flex camera down the intake an see if something is blocking air flow to the one cyl. I am at a lose as to why this is happening.

DanielDudley · 07-17-2010, 01:31 PM

The only time I ever had this problem, the spark plug was unable to go more than halfway down the the threads, and the spark was shrouded. Any way the injector could be jammed ?

E Sully · E Sully's Garage

Why don't you try switching injectors between #4 and one of the other cylinders to see if you have an injector problem. Even though you installed new injectors, maybe #4 is opening at too low a pressure and ceating too rich a mixtureat low rpm.