weird misfires

[svandamme]

Hi,

i'm running the MSD 6a ,PN6200 with a Bosch blue coil
on my 924

recently the car develloped some weird misfiring
usually starts off with the tach needle going wild, blipping to 6K rpm
when the engine is probably only doing between 2K and 4K

note , the tach needle starts blipping before the actual misfires
and it's attached to the tach output on the MSD box

once it starts acting up for real , it seems to be only onder load
lot's of backfires in the exhaust, and the whole car bucks

coast a bit ,while i rev the engine with the clutch in
or i try revving under load
it seems to happen less often if i keep the revs low ( 2000-2500 rpm) , which ain't that practical as it means i'm almost lugging the 4 banger constantly

i'm not sure if either makes one bit of difference
the problem sometimes goes away by itself only to return minutes later
at any time when it does happen , the car will idle if left to idle
if it goes away , the tach needle behaves normal again

i first suspected the plug wires ,as these were the old stock wires, Beru's
had those replaced with Magnecors, but the problem persists
dizzy cap, rotor, plugs are all less then 4000 miles and they look fine
tried cleaning all of them , didn't help one bit

the tach blipping makes me think the MSD might be the problem
as the tach is fed directly from the box

has anyone seen this before?? anything else i could try or check?

does anyone know if i can wire up the blue coil , and not use the MSD box if only as a test, or do i need a normal coil for that

if so , how do i wire things up? don't have a factory manual or haynes around...


thanks in advance,
Stijn

[helowrench]

when I had this problem on my stock 924 the culprit was always a carbon track inside the cap. I later (after 6 or 7 caps ) found that my timing was set about 3 degrees or so too far advanced.
Worth a check.

Good luck
Rob

[svandamme]

same exact symptoms? tach going mental and so on ??

i checked the cap , there was some carbon , but very little
scraped off the contacts , didn't really help

i'll see if i can score a fresh one for a try...

[helowrench]

the carbon tracking will look kind of like a really thin pencil line. Hard to find. I was having this happen every 250miles or so. The carbon will run down the inside of the cap to the step where it meets the distributor.
put a new cap on it and poof my symptoms dissappeared..... for a while.
I got to the point that i kept a couple of new caps in the car.

Rob

[helowrench]

Yes tach would freak out

[svandamme]

i'll definately give this a go, got a day off on thursday to work on my cars , the both of them...

[svandamme]

just gave the dizzy a wipe with my hands. bit to early to call it full on success. but it didn't act up as much on my short drive home
i'll slap a new one on thursday... that'll probably it

won't be bored thursday though, got a new gremlin to sort out
headlights are stuck in high beam mode, and the headlights are out at the same time, only had the lower fog lights to work with...
something funky with the relay panel , if i push it while messing with the relay , the engine kills..
always leave something unfixed they say...

big thanks for the tip about the carbon thing
i thought that the only thing to check on hte cap was the contacts...

[helowrench]

the fusebox is a bastard child. It is made up of copper or brass traces that are riveted together with steel rivets. any moisture and dissimilar corrosion sets in aggravated by the current passing through the juncture. I priced a NEW fusebox here at the Porsche dealer in Dallas for around $80 last summer.
You can either remove yours crack it open drill out the rivets clean the connections then solder/braze the bars back in place, or buy a new one (then fix the old one for a spare) .

Rob

[Razorback1980]

Hey Rob, That's a good tip to remember. Any particular year that tends to happen to? What year is yours?

Thanks

[onZedge]

You should have read the MSD 6A manual before you installed it. You CANNOT use the stock BERU wires for several reasons. Most importantly, they are definitely not rated for the voltage the 6A can produce. MSD requires that wound resistive wires are to be used. The stock BERU wires are not resistive and actually look like CATV coax cable if you were to cut one open. The spark plug wire contacts, as BERU calls them, DO have a resistor in each end but it's not the same as wound resistive wires. The stock wires will also emit excessive EMI, when used with the 6A, that can wreak havoc with your DME (which your '77 924 does not have) and cause radio static.
If you do bypass the 6A for a test MAKE SURE you use an additional current limiting resistor (0.8 ohms) in the + side of the coil circuit or you will eventually cook the resistor embedded in the rotor. (AMHIK)
I have a MSD 6A, MSD Blaster II coil and Magnecor wires in my '87 946 and am completely satisfied with the performance of this combination.
On another note, you can widen the spark plug gap to take full advantage of the of the 6A's potential. Wider plug gap means higher voltage (hotter spark) before the coil voltage jumps that gap, but not too wide of a gap that could cause cabon tracking problems and running problems. Experiment.

[svandamme]

Originally posted by helowrench
the fusebox is a bastard child.

i'll second that
and since my car is long over due for new cabin seals , with bit to much wetness inside... i'm sure that'll be it

it's my own fault really , refused to do any preemptive work on nthe 924 untill the 911 was running reasonable again...

[helowrench]

mine was a 78 924 na
yes was
I do not own it any more, but sold it to a friend, so he calls me any time there is a problem for advice.
I still lurk here regularly because I will most likely own another.
I do miss that car, but had to sell it because I had too many cars at the time and Wifey tolerated it for quite some time before it became a problem.
When you move and it takes longer to move the cars and car stuff and tools, than it takes to move an entire 3300sqft house full of her crap (and 4 kid's stuff) there might be a problem.

Rob

[Razorback1980]

What would that problem be?

[helowrench]

Well, it isn't a problem for me, I now have a detached 16X25 shop,
but you see when I sold my 280zx because parts were starting to add up quick, I kind of slipped into the aircooled addiction, next thing you know I have a convertible bug (wife claimed that one), bought a ghia (my DD) then the phone call came in, guy down the street that I knew had a bug wanted to get rid of some stuff......................
I wandered into his yard and behind his shop was
66 bug
68 bug
69 ghia
68 notchback
so I
BOUGHT THEM ALL
(but honey the were on sale-buy three get one free)
good deal and came with even more crap I mean "spare parts"
anyway then someone called with a free car (good title-runs like crap come get it 1988 buck park avenue)
so I took the Suburban and towed that one home too.
then local code enforcement came around so
sold the buick
sold the 68 bug
sold the 924
am in the middle of selling the 66 bug
keep the driveables in the driveway
still got more crap "spare parts" for more VW's than I know what to do with I have fenders and bumpers and decklids in the attic and literally 5 gallon buckets of carbs and oil coolers and engines and (honey, some of this stuff is very hard to find now)
so I try to be good
then I get the call today, come get this buimper out of my garage, I know the chrome is still good, but I have no room.
So I hope the attic is not near the weight limit yet, it will have more tonight

Rob

[svandamme]

sounds like you got a good thing going there
i need to park my 911 60 km's away cause housing is so expensive in R'dam, and none have proper garages in em
hence the slow progress.. i need to plan going to the garage , making sure somebody is there, only saturdays on weekends, or days taken off during the week...

[Razorback1980]

I have a friend that might be looking for a running engine for a bug if you have an extra. His budget is very limited so it would have to be an inexpensive engine.

[helowrench]

have him go to the dallasaircoolers.com website and post in the houston aircoolers and aircooled parts wanted section, that will be a whole lot closer to him.
Rob

[Razorback1980]

I'll do that, thanks Rob