Won't rev past 5000 RPM

[bkcbuilder]

Did a DE track day Wednesday....all was good for 4-5 laps then started going flat coming off the corners. Second would just fall off above 4000, wouldn't pull, then 3rd and 4th would have trouble getting past 4500.

Instructor was a ASE certified tech for Audi /VW/ has done Porsche work ...he said check electrical secondarys first(whatever that is) pulled the vac advance line off and have no vac at idle and just a jump when rev'd but fall back to zero when held at 2000-2500 rpm. Line is new. Checks out with a vac pump.

Read a long post about vac advance by Souk, and doesn't seem like it is necessay or my problem but it could be.

Car is a 79' SC ROW...pulled from a barn last summer, replaced fuel pump, filter and accumulator, drained tank cleaned and replaced blown airbox....set the CO and she ran great. First track day and she shows her age after 5 laps.

Any clues? Brian

[tctnd]

A little recognised but surprisingly common cause of higher speed power loss is a blocked muffler. Dislodged rust flakes or loose rusty baffles can occlude the outlet. I've also seen one car with similar symtoms as a result of rodents stashing nuts and dogfood in the muffler. If the exhaust seems unusually quiet at higher speeds I would be suspicious.
regards,
Phil

[Grady Clay]

I’ll speculate there is sediment in the fuel tank and the
track driving disturbed it. It is possible to restrict the
fuel flow so it feels like it “runs out of power.” This
would particularly be true of a 911 left sitting for a long
period.

Check the fuel pressures and flow. Clean the filter in
the bottom of the tank. Drain the tank by siphoning
via the fuel level sender opening. Be VERY careful
around gasoline!!!!!
"
"
(C) Dr. Ing. h.c. F. Porsche A.G.

Best,
Grady

[JFK713]

Do a search,

I believe early SCs had a problem with oil foaming or pickup under continuous high rpm running. If I remember right, there is an oil pump update?

[Grady Clay]

John’s oil question got me thinking about this some more; I’ll offer some more serious recommendations.

Just on the off chance something is mechanically wrong, I would inspect the sump plate and screen and cut apart the oil filter. Additionally I would do cranking compression and cylinder leak measurements. A valve clearance check and cam timing measurement aren’t out of order. What do the sparkplugs look like?

Hopefully it is something simple in the ignition or fuel systems.

Best,
Grady

[bkcbuilder]

Do coils have a tendancy to break down at higher RPM....looks to be original.
Tank was cleaned before I put the new pump on, filter and accumulator also replaced at same time....could be crud from fuel line from pump to filter...seems like a stretch.
The problem is not a miss, just a huge loss of power....feels like a rev limiter without the noise ...just stops pulling above 5000.
Haven't pulled plugs or done compression...plugs new before event.

[MotoSook]

I have a very long thread that documented my wild goose chase..of electrical causes, but in the end, I got bad gas that clogged my filter (filter was relatively new, so I over looked it in my troubleshooting)....

Change your filter. I have a clear hose that I connect to the fuel line before the filter....I than run the pump to drain the tank, then clean the strainer...pump tone chages when it goes dry.

If bad gas you will see it in a clear hose...

Bad gas or bad fuel filter is my guess.

Edit:

Also, the track action may have stirred and dislodge the crud that was left on the tank wall...thus more crud in the fuel and fuel filter. My car started and idle just fine. I could baby it past 4000, but it was powerless. Without fuel...it was not going anywhere...without fuel = no energy for HP...poor combustion. The limited RPM was dictated by the maximum flow rate my clogged filter could pass. It got worst as time passed. I noticed it on the drive home from the track...it got worst the more I drove it.