STUMPED - misfire at high RPMs

[Cory M]

We've been trying to fix this for a long time but nothing has worked so I'm throwing out a lifeline here..

We have an 88 911 racecar with a carburated engine that misfires intermitently above approximately 5500 rpm. We have replaced damn near everything and the problem persists. We can't replicate the problem in the garage or on the street - only on the track or a chassis dyno. Has been to a couple of different competent and experienced porsche mechanics and is with one of them now.

So far we have replaced or rebuilt the following parts at least once, many 2 or 3 times:

plugs
plug wires
ignition box (one pertronix and 2 MSDs, currently has MSD)
ignition system wiring
cap and rotor
distributor
timing reset several times
rebuilt alternator
valve train & springs checked for damage
carb inspected
carb rejetted
compression checked
fuel filter
fuel pump
fresh fuel
coil

The car used to run fine with the old motronic box but has had problems since it was replaced with the aftermarket ignition. All of the wiring was removed so it is not easy to return to the old set up. The new set up is nothing exotic and should work fine.

We have run out of ideas.

Any thoughts?

HELP!!

[RSTarga]

coil?

[Cory M]

Originally Posted by RSTarga

coil?

replaced that a few times already with new and good used parts

[RSTarga]

loose ground somewhere? what ignition are you running? something computerized?

[cameron.arnott]

Cleaned out your fuel tank?

[kidrock]

Could be something as simple as a vacuum leak...maybe a very small one or in a very inconspicuous place, because your problem only seems to be occurring at a pretty high RPM. If it was a big leak, you would notice misfire throughout the range.

[BURN-BROS]

What kind of wires? Where is the ignition box mounted?

Keep in mind that inductive voltage can play havok on the MSD. Typically we mount the box underneath the drivers seat to protect it.


Also, what gap are you running?

Alternator ground strap and good ground at the transmission to the body are places to check.

[Cory M]

The wiring has been checked many times by many experienced people so I'm pretty sure it isn't a loose ground although it seems like something electrical. It's running an MSD box, don't recall which one, but it is commonly used on 911's.

The tank was drained and we've replaced the pump and filter. We did find debris in one of the carb jet but it was cleaned up and the problem persists. There is a fuel pressure gage mounted in the engine compartment so we can verify the pump is working while testing it on the dyno.

I just got an idea, I'll have to find out if the relay and fuse for the fuel pump have been checked or replaced. If the fuse is bad the pump wouldn't work, maybe a faulty relay? Still not sure why it would only stumble at high rpms but it's one more thing to check..

[Cory M]

The box is mounted in the engine compartment on the left side, I'll mention moving it to the mechanic. The wires I have referred to are all of the wires going from the box to the coil, etc. The first guy was pretty sloppy with the wiring but everything has been rewired since then and the box itself has been changed. We've run a couple different kinds of plugs, I don't recall the gap used but they were checked and we have the right kind of plugs in there now.

I think the ground straps have been checked but I'll make sure. One of the things we found was that the alternator was putting out too much voltage, we think this fried the first Pertronix box, the alternator was rebuilt and checked but the miss didn't go away.

Sorry I'm short on all the little details, the car is at the mechanics now and he has been trying a bunch of different things..

[Shawn 357]

Originally Posted by Cory M

The wiring has been checked many times by many experienced people so I'm pretty sure it isn't a loose ground although it seems like something electrical. It's running an MSD box, don't recall which one, but it is commonly used on 911's.

The tank was drained and we've replaced the pump and filter. We did find debris in one of the carb jet but it was cleaned up and the problem persists. There is a fuel pressure gage mounted in the engine compartment so we can verify the pump is working while testing it on the dyno.

I just got an idea, I'll have to find out if the relay and fuse for the fuel pump have been checked or replaced. If the fuse is bad the pump wouldn't work, maybe a faulty relay? Still not sure why it would only stumble at high rpms but it's one more thing to check..

If the fuel pump relay or fuse were bad it would not run at any RPM so we can cross that off the list.

[BURN-BROS]

Sorry, spark plug wires.


Solid core or spiro wound...If it solid core then definitley relocate the MSD. I would do it regardless wire type.

[Shawn 357]

Originally Posted by Cory M

The box is mounted in the engine compartment on the left side, I'll mention moving it to the mechanic. The wires I have referred to are all of the wires going from the box to the coil, etc. The first guy was pretty sloppy with the wiring but everything has been rewired since then and the box itself has been changed. We've run a couple different kinds of plugs, I don't recall the gap used but they were checked and we have the right kind of plugs in there now.

I think the ground straps have been checked but I'll make sure. One of the things we found was that the alternator was putting out too much voltage, we think this fried the first Pertronix box, the alternator was rebuilt and checked but the miss didn't go away.

Sorry I'm short on all the little details, the car is at the mechanics now and he has been trying a bunch of different things..

The grounds have been cleaned and tightened even added some extra grounds at one time (looked pretty rice) so we can cross this off the list.

I guess I should mention that I am Cory M.'s Brother and have been helping with this car but I couldn't figure it out and after a couple race weekends wasted gave up and that's when we took it to the shop.

About the only thing I can think of that we have not done is pulled off the valve springs and checked them for how much pressure it takes to compress them.

[Shawn 357]

Originally Posted by BURN-BROS

Sorry, spark plug wires.


Solid core or spiro wound...If it solid core then definitley relocate the MSD. I would do it regardless wire type.

We have used both solid core and spiro. I am not sure what are on it right now because the shop just changed them again but I know I have run Magnecors, Clewetts, grounded, not grounded, etc.

I don't remember what the plugs are gapped at but I have checked the gaps on previous sets of plugs and we have also tried several different heat ranges.

Please do not take this like I'm shooting down your suggestions, they are not only welcome but appreciated.

Thanks,
Shawn

[BURN-BROS]

Originally Posted by Shawn 357

We have used both solid core and spiro. I am not sure what are on it right now because the shop just changed them again but I know I have run Magnecors, Clewetts, grounded, not grounded, etc.

I don't remember what the plugs are gapped at but I have checked the gaps on previous sets of plugs and we have also tried several different heat ranges.

Please do not take this like I'm shooting down your suggestions, they are not only welcome but appreciated.

Thanks,
Shawn

No worries on my end. Relocating the box was the solution to symptoms similar to what has been posted. FWIW the inductive voltage problem usually burns out the CD box at some point.

[Steve@Rennsport]

Cory,

Call me tomorrow later in the afternoon as I need to do some Q&A in order to help and this is a poor medium for that type of exchange.

[s_morrison57]

The solution is easy, get a new battery, that miss fire will disapear, your battery will hold a charge and start the car but if you check it it will probably be down. Saw this lots racing Formula Fords with no alt. and a bad , old battery would usually be the problem but guys would chase plugs and wires and caps and miss the true problem. I had trouble with the same car one time and it was an in carb sceen filter that was semi - blocked. It had the same high speed miss , so check your fuel delivery and get a new racing battery and the high rev miss will be gone. It's the only thing you haven't checked/changed and should be one of the first places to look.
Finn

[rsscotty]

I've seen a high rpm misfire problem on a 3.4 twin ignition/twin MSD box go away by removing the rev limit chips. They must have been coming on early.

[techman1]

Never been to a high end dyno, or any come to think of it, but do any have a sort of data logger that hooks up to the engine that can tell if it is the same cylinder misfiring?

It may help if you could tell if it was always the same cylinder.

[rick-l]

When you replaced the DME how do you get ignition timing? Did you put an old distributor in also?

[mede8er]

Copuld be the rev limiter, but my rule of thumb is top end misfires are normally fuel related....check your jetting....