Snap, crackle, pop! Where should I start problem solving?

[Eric P]

My '71 MFI 911E is running rough. It runs. It idles fine. It revs up great (no problem with the MFI transition from low to higher rpm). It just isn't smooth at any RPM. It snaps, crackles, and pops randomly at any RPM when I hold it there. It is like it "just needs to clear its throat" but never does.

It NEVER did this before. "Before?" you say. Well, before I made a couple major changes: totally rebuilt dizzy (including optical ignition) and new, aftermarket fuel pump in front with the fuel cell.

I'm suspicious of the ignition but don't know...where should I start the process of smoothing it out?

(Note: I did have the dizzy 180 off at first, the engine barely ran, well didn't really. So I thought I just fouled the plugs. Took those out, and cleaned them. No change.)

[javadog]

I don't know how you cleaned the plugs but sometimes cleaning them is not effective. You may have to replace them.

I assume you have checked the ignition timing, right? Did you install the correct fuel pump? Have you checked the delivery pressure?

JR

[Eric P]

A mechanic once told me, "Once plugs are fouled, you can't really fix them." It sounds like this is what you are suggesting.

The ignition timing is correct as far as Z1, advance, etc. I don't know if it matters where the optical trigger is aligned inside or not (given timing is correct). Should type of plus or gaps be changed with the optical trigger (stock CD unit)?

As for the fuel pump, I believe the specs are correct but I haven't actually checked pressures. I'm going under the assumption that problems would be worse under high RPM (more fuel demand). This is NOT the case. The problem is consistent whether I hold it at 900RPM, 3000RPM or 7000RPM.

[javadog]

How did you clean the plugs?

I'd start with a new set of the correct plugs, correctly gapped, no anti-seize. I'd then check the fuel pressure. Porsche wanted the pressure to be .8 atm, plus or minus .2. The assumption that more fuel is needed at high load and rpm is more related to the delivery volume that the pump can handle. Just make sure you don't have a pump designed for a carburetted or CIS car.

JR

[Eric P]

Wire brush and points file. I basically cleaned the carbon off with the brush and used the file to lightly "square up" the curved electrode. I have a set of new NGK BP8ES I guess I should try.

You are right, the stock pump for my car is rated at 14.2 PSI, 0.8±0.2 Bar, 125 liters/hour. The pump I installed is rated at 15 PSI and 155 LPH. I understand the MFI injection pump needs to have 11.9PSI. I'm ASSuming that my replacement pump is definitely up to that task. Would that be wrong?

[javadog]

Try the plugs first. Make sure you check the gaps first. The new fuel pump seems close enough, it's right at the high side of the spec for pressure.

JR

[Eric P]

Yeah, I'm gonna do the plugs. They probably got gunked up pretty bad when I had the dizzy wrong the first time. There was actually a brief fire in the left side stacks too...

Can the pump be too high (assuming it isn't some crazy EFI 100PSI spec)?

[javadog]

Porsche's spec for the fuel pressure works out to 8.8 to 14.7 psi. Your 15 psi pump is close enough to the high side of their range that I wouldn't worry about it yet.

The pressure can be too high but I don't know of any pumps that would have a much higher pressure that aren't intended for CIS or Motronic cars, in which case the pressure isn't even in the ball park. Change the plugs and report back.

JR