sc miss under load between 1 - 2 k rpm
 

hi we have finally got the sc up and running on 6 cylinders and we have arrived at the the following problem:

the car has a missfire at between 1-2k revs, only when the car is in 2nd 3rd 4th and 5th gear, it is fine in first and when not under a mechanical load, ie you can rev the nuts off it when it is just parked up. we can ease the problem out when in 2nd gear by running loads of dizzie advance (10 deg plus at tickover) this also reduces the problem in third gear as well but doesnt solve it totally in 3rd or fourth.

hc's are low
co is as low as we want it to be

ie everthing is adjustable on the mixture front.

good compression
good leakdown and therefore is sealing well (rings etc)

fuel system

new injectors all producing good flow rate
new fuel pump
fuel pressures are all within correct range
fresh fuel

all vacuum hoses are correct and tight mostly new. good vacuum being produced measures at dizzy connection.

ignition electrical:

leads

new coil
newly recon alternator
dizzy cap good
rotor arm good

cdi working correct



anyone have any ideas?????

any thoughts??

What year SC? Is it the later SC with lambda computer under the passenger seat?

:)

1981 euro spec it has no lambda runs with ssi's and a 2 in ione out back box

i would check the wire that plugs into the distributor the green shielded one check carfully if theres any cracks in the insulation, have you checked the resistance of the plug wire ends? sounds elecrical to me. gary

What makes you think its missing? Is the car bucking in that range? Is it difficult to drive?

Agree. Also, have you pulled the plugs and checked their color? I have had bad "new" injectors out of the box, can never be sure about anything these days. Also, be sure that the wire between the distributor cap and coil is tight (properly inserted) at both ends.

brand new plugs and leads

redid the distributor as per the gunter thread on here today.



plugs are good

new plugs and leads plugs are on the right side of grey on the electrode

in that range the car is popping and unwilling to pick up

today i did a vac test and in that rev range the otherrwise good vac drops to around 10hg

gordon

You should not be operating any 911 motor such low rpm's, even just cruising at a very light load, much less under any sort of a load. It will be very much happier if you keep the rev's up to 3,000 or so, with maybe 2,500 as a bare minimum for low load cruising. You are simply lugging it down too much - I doubt there is anything wrong with it.

so you are telling me that an sc wont pootle along at 1.5 - 2 k revs???

oddly it used to quite happily, i am afraid i have to disagree

What plugs and plug connectors?

beru oem plug leads the briaded sort that they scrub the porsche logo off if they are not putting them in porsche boxes, and bosch xxx5's which is hotter than spec but wont hurt.

new info

i have measured tthe vacuum at rising revs and i note at tthe point of throttle application from tickover 850-900 rpm up to 2000 rpm the vacuum at the dizzy falls from 18hg to around 10 hg, this is the area in which the problem occours.

does that ring any bells? float any boats?

Gordon

Sounds good. You're sure that all the plugs are gapped right, 0.8mm?

That it might, but it certainly isn't very good for it. Lugging it at such relatively low rpm is hard on the rod bearings and wrist pins particularly, and somewhat less so the mains. That, and the fan really isn't moving much air, leading to higher than necessary temperatures in the cylinders and heads.

It may very well be that all of your work on the ignition and injection have actually unmasked its true low rpm habits. Prior to all of you work, in its sub-optimal state of tune, it may very well have been masking these symptoms with its lower cylinder pressures.

plugs are gapped spot on,

as for the low tick along every 911 i have known along with every 944 is as usable as my alfa gtv, they may not be happy about rumbling along in low gear and it is not something i intend to do long term but they will and it certainly doesnt provoke a set of missfires and exhaust/intake popping when you open the throttle and ask them to work harder. sorry i cant see that the car shouldnt run like every other motor.

Gordon, while this is a longshot, it appears that the misfire is under initial load; so try another coil. Perhaps the new coil has a secondary winding short. Also, even though new, check for any carbon tracking in the dizzy cap - might want to look at #6 first which in your previous post, wasn't firing. Check for coil wire or plug leads arcing. Lastly, recheck initial timing.

Richard

Is it possible that you got a bad tankful of gasoline? CIS is pretty picky about fuel quality, and I've seen some pretty strange symptoms over the years. Intake popping often occurs with incorrect mixture (lean), and can also be caused by a sensor plate that's out of adjustment (especially following a backfire, etc.).

gas is good 95 ron.

no 6 wasnt firing but that was the plug leads that prob is totally gone now mixture spot on all cylinders same temp etc. this is just the oddest problem i have seen, i am going mad with it. the car ticks over like a dream, increasse the revs slightly and it goes all pete tong, get past that and it is sweet again !!!!

Is there any change if you retard or advance initial timing?

Richard

i can ease some of it out if i advance the timing, but not totally

...but not totally. Rats!!
Gordon, I'm useless to you at this point. It appears that elec is good. It stutters initially when your initially ask for more fuel,but then cleans out & runs like a champ. Almost sounding like fuel or small airleak. I don't remember if you replaced the fuel filter or not. I'm guessing you did.

I hope someone with far more expertise chimes in. Hang in there!

Richard

How confident are you that the valves are correctly adjusted? A too-tight valve can sound like a miss or an exhaust leak under load.

Brian

brian,

we have adopted the back to basic approach the methodolgy is now leading us to go back to the valves and see where we are with them

My appologies, I misunderstood your initial description of the symptoms. There are as many forms of "misfire" as there are folks trying to describe one. Exhaust and intake popping are far different from a stumble, for instance, yet many will call both a "misfire".

Anyway, what would be different in a no load vs. under load state at 1,000-2,000 rpm? Throttle position would need to be different (more open when loaded). What does this affect? Air box vacuum, which affects initial advance and the mass airflow sensor. I would focus my efforts in those two areas. Things like valve adjustment won't have such a narrow range of affect.

Have you checked the vaccum advance/retard mechanism?

also my apologies for the description of the issue not being so good.

i am veering toward the vac advance on the dizzie. when i cleaned out the dizzie i removed the vac unit which didnt hold a vacuum well and if shaken made a noise like a childs rattle!!

holding my breath for monday when the new one arrives.

Gordon