Consistent missfire from 3.3k on up under heavy load

nabilious · 05-15-2010, 02:55 PM

I didn't want to bother everyone but I'm at wits end.

Just fully rebuilt my carb'd 2.7L after 2.5 years of being down. It was pretty extensive. just got it up and running again, but seems kinda lopity lopity lopity just off idle and as long as you are giving it light throttle it seems to be mostly o.k. (intermittent bucking aside). the points are new and just set (several times) the timing seems to be about 7 degrees at 1200rpm and about 34-35 degrees at 6k rpm. The plugs and wires are new (but that didn't help the problem) Cap and rotor are new. The vacuum on the dizzy is plugged (as it had been back when it use to rip.) I swapped out the coil with a known runner, to no effect. doesn't seem to matter if its hot or cold, seems to be up to no good from the first start up in the morning. If you baby it around town, it won't give to much bucking but some for sure and you can make it bad (sounds like 4 cyl's running if you just leave your foot into it.). The carbs are spotless with all the same jettiing that it had before and ran great then. The only spec. change was to go to a Mod S cam from a straight S cam...cant imagine that is the issue though. It has a Permatune box on it, sounds lilke some people don't like them but I'm not sure if thats the issue. The Float bowls are set correctly, I smell some raw fuel coming out of the exhaust, maybe weak spark?? Cant handle the extra gas on the heavier throttle settings? Otherwise the engine seems mecanically sound...its leading me to thing its ignition, my weakest porsche subject. Did I cross some wires when I put it back to gether?? any hints would be greatly appreciated.
Nabil

Steve@Rennsport · 05-15-2010, 03:05 PM

Nabil,

To troubleshoot this, one needs a lot more details to offer anything constructive and useful,....

I see its a carbureted 2.7 with Mod S cams, however I'd like to know:

Carburetor type and ALL details of jetting package
Plug heat range
Fuel pump type and pressure at carbs
Fuel filter type and age
Float levels checked and perfect?
Floats checked to ensure they are not partially filled with gas?

I'm no fan of PermaDoom's and have had enough bad experiences with those over the past 30 years to replace them on any car I "meet".

Carbureted cars run FAR better with an MSD setup,.....

Eagledriver · 05-15-2010, 03:09 PM

You can figure out which cylinders aren't firing by checking the spark plugs or checking for a cold(er) exhuast pipe. Then you can check for things like spark plug wires that are broken or on the wrong cylinder. A spark plug on a non firing cylinder will be wet with gas and have no carbon on it (if it was new).

-Andy

stlrj · 05-16-2010, 06:57 AM

Quote:

Originally Posted by nabilious

Did I cross some wires when I put it back to gether?? any hints would be greatly appreciated.
Nabil

I'd say it's a good place to start.

nabilious · 05-17-2010, 05:34 AM

Thanks guyz,
A little update. All plugs have carbon on them, and appear to have spark. I checked by pulling the wire and poping in a different plug. The carbs are Weber 40 IDA's, Idle jet 52, Main 140's and while I didn't have a float bowl gauge, I let the pump fill them several times and then carefully took them apart, the level was just below the WEBER script on the bodies, about 3/4 to 1 inch below the top. I know thats not very scientific and these are sensitive, but its all I had at the time. I changed the fuel filter (its a big Bosch one) as the car sat for a couple years. That made the fuel pressure go from 3.5 lbs. to 5.5lbs. so there was some restriction there, but it didn't effect the running issues. My original case was cracked so I replaced it with an early aluminum one. I had to make a small block spacer (out of aluminum) to use my dizzy (from a '72 or '73) as the dizzy shaft was too long for the early aluminum case. Cap, Wire, Rotor,plugs and points are all new. If memory serves me, I set the point gap to .013''? whatever the books say for a '73 2.7 RS. The Dizzy has the vacuum port plugged. It seemed to be running fairly well yesterday on all cylinders in the 1200 rpm to 3500rpm range as long as I used very light thottle settings....aka, if I drove like a normal humaniod on the street, I wouldn't be affected TOO much...but the minute you roll on a bit more it goes from intermittant missfire to sounding like you just pulled 2 plug wires off. Another note, I noticed my tack acting a little erradicly yesterday...getting stuck, or dropping 1000 rpms ...but not really broken...just not telling the normal story. I'll get a temp gun and see if its consistently one cylinder or another. I will also try and check to make sure all my Dizzy and Permatune wiring is solid. Would a bad ground make some of these symptoms?
Thanks a million...as always.
Cheers,
Nabil

nabilious · 05-17-2010, 05:38 AM

...and yes, the plug wires were on correctly...I would have felt foolish, but been happy at the same time if that was it!!

nabilious · 05-17-2010, 09:05 AM

the plugs were Bosch platinum's wd5dp or something like that, I changed them with some AC delco's of the same heat range (5) and gapped to .033 or .038 (I kinda forget now)
N

Eagledriver · 05-17-2010, 06:14 PM

With what you describe, I'd suspect carb problems. The idle circuit seems to be fine (you are running on this at low throttle settings). I've heard of people installing the venturi atomizers 180 degrees out (don't know the proper name but they are the things that cut accross the throat with the small tube part of the venturi). These parts only have holes in one end and if you install them backwards they will not flow fuel. Other than that I'd look for other leaks/blockages on the main circuit.

-Andy

nabilious · 05-18-2010, 05:18 AM

Thanks Andy!
I guess I haven't checked that yet, maybe I didn't even think about it truthfully. Makes sense, I've had repeated suspicions about fuel starvation, but the carbs are spotless, jets are clean, and I think the floats are set correctly....also I have run it as hard as it will let me, missfire and all, then shut it down and took the tops of them off and the levels were good. So is that where the main curcuit dumps into the throats? through the pre atomizer venturi things? (small tube of them sticks up and into the airhorns)
Thanks,
Nabil

Eagledriver · 05-18-2010, 05:03 PM

Yes,

Those things are hollow and the fuel gets sucked out the little fingers in the airstream. Only one side has an opening.

-Andy

nabilious · 05-19-2010, 05:28 AM

The verdict is in......

Andy (Eagle Driver) Nailed it. Thanks a million Andy, its a mistake I'm a little embarrassed to mention, but I did have 2 of the Pre-atomizer venturi's in 180 degrees off. When the engine first crapped the bed 2 years ago, I rebuilt them then, then mothballed the whole setup when I realized that this was gonna be more extensive than I first hoped. I've removed and partially disassembled them 6 times in the last couple of days but never pulled those back out. I'm no stranger to the Zenith 40TIN carbs, but this is my first set of webers. What a great forum and I will continue helping others when I might have a good suggestion. Believe it or not, I do have a good pointer periodically......
Thanks again everyone, I was at wits end and almost ready to start throwing money at it and it would have been a while before even a vetran porsche guy would have discovered that. Thanks again all.
Cheers,
Nabil

P.S. ....and man....it rips!!! like a scalded cat!!

Eagledriver · 05-19-2010, 05:43 PM

Glad that was it. One of the cars I race against had one in backward. Made it easy for me to pass him, I was bummed when he found the problem!

-Andy