NEED HELP- Engine miss at high RPM's

[Plavan]

During my last weekends race at Cal Motorspeedway, the engine started to gradually lose power and miss (not running full power) at the higher RPM's (6K-7.3K). At the beginning of the weekend and at the previous races it would start after 5 or so laps. The last race it was doing it on the pace lap. This has been happening over the last few races. It feels almost like a fuel starvation. It progessively got worse. It happened more in the corners at the beginning but was doing it in the straights by the end of the weekend. The last few races I thought my fuel filter was getting clogged but that is not the case.
These are the things we did to try and fix the problem:
1. Changed to a nice big fuel filter
2. Moved the fuel lines away from the hot oil lines
3. Took apart the Webers and cleaned them. Corrected the float levels
4. fuel pressure is set to 5 to 6 pounds (Carter fuel pump)
5. Rechecked timing and set point gap

Possible problems????
1. Distributor needs rebuilding?
2 . MSD box getting too hot and wacking out?
3. Coil gone bad?
4. Fuel pump not keeping float bowls full?

Anyone ever run into this? Any suggestions to help me find this gremlin would be great. This is getting frustrating.

[engken]

You stole my question. I am having the same issue with my 930. No Webers though - CIS. Will start trouble shooting this weekend.

[slakjaw]

your fuel psi looks good.

but it may not be putting out the volume you need at those rpms.

hope that helps
Kyle
71 911

[wevoid]

Chad,

In cases like this, I always revert to one of my father's best ever pieces of advice.

"when you think you have traced it to the carburettors,...... go back and check the ignition system again"

Two other things might be worth checking - contrary to dad's advice.

1. Any debris in the fuel tank that is causing a partial pick-up blockage
2. Do you have any braided fuel line in the fuel delivery system? if so, old lines, with cutter style ends can begin to fail internally and a small flap of material can act as a flap valve across the inside bore of the hose. Normally only a problem when you require higher fuel demand, would not show up in setting the pressure.

Regards

Hayden

[Plavan]

The fuel lines are less than a year old. I may take the top of the fuel cell off to check the pick-up.
What would be the correct Gallon Per Hour for a 2.7 race motor with 40's IDE? My jets are 155's?
I am now leaning towards worn bushings in the distributor or a faulty MSD. I was having problems with the MSD rev limiter. I was never hitting it at 7300. I put a cap with a 7300 limit and did start hitting that. I then tested the MSD with a 3000 pill. it did not shut the engine off until 4200......Hmmmmmm.

[wevoid]

Chad,

If it's timing scatter from a worn or problematic distributor, you might be able to see that when you check full advance - is the timing mark solid and stable in the strobe?
I dont have any experience with the MSD, but I do trust the rotor button limiter implicitly. Is there any reason not to run that all the time?

Have you changed the type of fuel you are running? new or old lines, the more toxic fuels can be really hard on the lines.

Regards

Hayden

[Plavan]

Hayden,
I'm not sure about the full advance. I was the guy with the foot on the throttle when 3 people had the timing light out. The fuel I use is the 110 race gas. Sometimes I mix a little super unleaded in because of my low 9.5/1 compression to save some cash.
I will be taking it to a Porsche shop here in Modesto that has a dyno (You know Jay Jarvis). I'm sure we will be able to replicate the problem now.
BTW I still love the WEVO for the 901. You know excatly what gear you are in (or downshifting to) everytime unlike my old Weltmiester.

[wevoid]

Chad,

I would be interested to know what it is once your finished.
Always good to add another Q&A to the data base for future reference.

Good luck!

Regards

Hayden

[jluetjen]

I cast my vote for electrical. A couple of thoughts...

1) Have you checked your battery for voltage?
2) Have you checked your alternator for the correct output?
3) Are the distributor and plug wires clean and dry, especially the boots?

If it were fuel related you'd be running lean at high load and high RPM according to what you described, in which case I'd expect your motor would have melted down by now.

It'll be interesting to see what they dyno reveals.

[wevoid]

Chad,

Another thought,.... if you are having trouble getting a repeat on the dyno - try to effect a dodgy earth and see if that makes it happen.
Have you done anything in the car that might have changed the quality of the earth to the motor or ignition system?


Regards

Hayden