Something's sapping my power
 

This is an issue that's been dogging me for a while now, and I'm getting sick of throwing time and money at the wrong things.

I'm getting a notable loss of power throughout the rpm band, but only when I've been driving the car hard at the track - it never crops up on the street. It starts just as my tires are getting warm, so after a lap or so. The car still accelerates and there's no stumbling or unevenness across the power band, but it feels like I'm down to maybe 75hp or something. The engine sounds perfectly normal, and if I pit and let the car cool down for 10 or 15 minutes, it'll drive normally for a lap or 2. The power lack is such that I can barely hold a steady speed on an uphill straightaway (Mosport) with the pedal to the floor, 3rd gear, 5000rpm.

Car is a 72 911T with original Zenith carbs, carbs were rebuilt 2 years ago (by me) and tuned professionally, brand new MSD coil, replaced CD box with blue Permatune used box, new plugs and plug wires summer of 2006, new points last summer, timing, advance, etc. all set by the shop when the carbs were tuned.

I've floated this problem here on Pelican before, but that was before the coil and CD box were changed, and that didn't fix it, even at a track day in cold weather (10C/50F). The only thing that gets rid of it is letting the car sit for 10 or 15 minutes. And again, this problem has never came up on the street, only at the track.

Any thoughts? My next step if I can't fix it will be to just take it into the shop for diagnosis, which I know will hurt my chequebook significantly!

Thanks!
Chris

others smarter than I will be more helpful but I'm betting it is in the fuel system. -that's all I've got.

That came up before, but that doesn't explain the problem only occurring when hot and the first lap or 2 being normal. FWIW, I did go to an empty dead end one day last year, and drove around like I was doing an autocross, to really swish the gas around to see if there was any starvation issue or anything - no change.

I am not an expert, but here is my thought:

I have heard of fuel pumps delivering full pressure and volume at cool temperatures like at startup and for a few minutes, then when the going gets serious, they show their weakness. Maybe heat soak is overwhelming it and you are depleating the reserve of fuel in the carbs. You then run on the reduced fuel flow the pump gives while warm, which is nt enough for tack use.

Hope this helps. I enjoyed your transaxle project.

I've had a similar problem before but on a Fiat with a weber carb. It turned out to be a speck of dirt somewhere in the carb. I would never blame the carb because I had just rebuilt it as well. The problem would only manifest itself when the car was nice and hot. As soon as it cooled down, the problem would dissapear. Problem only went away once I blew every single orifice of the carb with an air compressor after soaking in carb cleaner overnight...

Yes, the transaxle read was good.

(as to the quoted post) Howsabout a fuel pressure check when she's good and toasty? Just for good measure? No Pro here either..sounds like something heat related (way too generic, I know)...starting with the fuel pressure (while toasty) seems easy enough.

I'm sure the pros will grab hold of this thread.....speaking of toasted elements.

...standby for news.

Best,

Do you have any data on engine temps? I say this because there is a remote chance that you're running lean at the top end and the motor is wanting to seize. Have you checked the plugs after such a run? I know you're not supposed to pull them hot, but you need some data on this. A cylinder head temp sensor would be good, too. Oil temps are behind in terms of real time data. If you could somehow see into the exhaust ports, you'd have an idea of the mixture.

Maybe you can find some temperature paint for the headers right at the flanges. At 1200 degrees, you're in a dangerous zone.

I sure wouldn't go back to the track until you know about this and I hope this is not the problem. Maybe it's ignition, like that used Permatune

After reading the title of your thread I've had to resist the urge to quote lines from Dr. Strangelove.

Anyway, I'm with the fuel/lean group; it doesn't sound like spark or air.
Fuse/Relay hot?
Fuse contacts/wires corroded or broken?
Pump electrical contacts corroded or broken?
Plugs look lean?
Fuel pump hot?
Fuel filter clogged?
Crimp in fuel line?
Crud in the tank - is the filter sock in the tank slightly clogged?
Fuel pump flow rate cold vs. hot?

Bet it's in there somewhere.

Is the fuel tank venting ok?

In line with Milt's suggestions, you might also look under the fan shroud to check for mouse nests blocking cooling to one of the cylinders.

This is a long shot, but does the car seem unusually quiet when it loses power? Sometimes loose stuff in the muffler (like dogfood stashed by rats) can occlude the outlets when a car is driven hard.
regards
Phil

911mot also has a good poing; berettafan has a thread on that which you might find interesting:
http://forums.pelicanparts.com/porsche-911-technical-forum/411853-fuel-tank-vent-system-capped-may-source-my-fuel-starvation.html

Okay, I'm absolutely no expert (the only racing I do at my age is to the bathroom after a spicey dinner) but I'm going to offer something completely away from the above posts.

You stated, "It starts just as my tires are getting warm, so after a lap or so. The car still accelerates and there's no stumbling or unevenness across the power band..."

Since the engine seems to be running/sounding correct, could your "lack of power" be the brakes binding--something that might happen after a few laps of pedal action? Then, after a period of rest, the pistons retract and all "power" is returned until more brake action binds them again.

Just another possibility and it might be easy to verify

up the fuel pressure for racing. route a fuel pressure gauge inside the car so you can see what's going on. tap it in just before the carbs.

Hey all, thanks so far for all the suggestions. Couple things I forgot to mention:

1. new fuel pump and filter 2 years ago, rerouted to the front suspension pan, rather than the original near the rear left wheel. So highly unlikely it's the filter at the pump or the pump itself. Most of the lines have been replaced recently, though not all.

2. New muffler last summer - Bursch sport.

Milt, no, I have no data on engine temps when hot, other than what the guage tells me. I haven't checked the plugs in a while, but I can pull them to at least diagnose them.

Something just occurred to me - could improperly gapped plugs be the cause? When I replaced the plugs 2 years ago the shop said the gap was set right at the factory, but it's possible that could be a mistake, or maybe gapped for MFI, etc.

Can you pull them hot? Wouldn't they be hot enough to melt the rubber in the plug socket?

It's possible that both the Permatune and original Bosch CD box are flakey, in exactly the same way, because swapping them out makes no difference at all, but that doesn't sound too likely, does it?

I have no idea if the fuel tank is venting properly - never checked that out, but I will. Sounds simple enough to check.

initial, something along those lines has crossed my mind before, and if I get a chance, I'll see if I can borrow another set of carbs to test out, and see if that makes any difference. FWIW, I have had someone else press the gas pedal down and I've watched to make sure each throat is fully opening (with the car not running, of course).

LJ - interesting theory. How could I test it?

JW - What if the fuel pump isn't adjustable? I guess I'd have to replace it with an adjustable one? And one easier to access - I'd have to pull the suspension pan to access it as it is now - not something I'd be wanting to do every track day.

Don't know for sure, but if that's the cause you can bet the calipers will be extremely hot--more so than on a car that doesn't have binding brakes.

I had a permatune in my car. My suggestion would be to replace it with an MSD system or stock CDI unit. My old permatune would act erratically when the car got hot (track time).
Maybe richen up the mixture. It may help the car run cooler on the track.

His brakes should not get that hot on two laps, but it's worth a check. I know they have temp paint for rotors.

Lifted from the Net: "Determining how hot your brakes are getting is relatively simple. You can apply special heat-sensitive paint to your brake rotors that will fade when a specific temperature has been reached. Most brake manufacturers sell the paint in sets, with different colors for specific temperatures. Simply apply the special paint to small sections of the outside edge of the disc (not the disc face). After a run, pull the wheel and inspect the rotor. The hottest section that has faded paint shows the range you've exceeded, and the next section in the range that has paint that isn't faded shows you what you are still below."

It's better to wait until it's at least comfortable to do this with your bare hands. Real hot, no. And, no they won't melt the rubber. That's high temp silicone mixed in with some rubber like plastics.

Maybe it's a transaxle bearing going out:eek:

Just kidding;):D:p

You don't think it could be something as simple as vapor lock?

I find it odd that the engine would still spin like that without power. It sure sounds fuel related. Almost sounds like a slipping clutch, but you'd know if that was it, of course.

You don't by any chance have a soft rev-limiting distributor rotor, do you?

Sorry for all the replies. I am just throwing out all of my crazy ideas.

Is there such a thing as partial vapour lock? The engine still runs and has some power, just significantly reduced, which means at least some fuel is getting into the engine.

If the clutch were slipping, the rpms would climb without the engine actually turning faster.

I do have a rev limiting distributor, for 6200rpm. I can easily see redline when the car's cool, or in 2nd gear going downhill, etc. so it's for sure not the distributor, unless its limiter kicks in early when hot.

I'll definitely look at getting some temperature paint for the rotors and do some braking tests on the street. Not sure where or how I'll do the test, but I'm sure it will be fun!

Possible that heat is having some strange effects on the advance mechanism in the distributor? No advance would certainly pull your power way down.

If the brake(s) are dragging the rims will get warm. Pull up to a stop, and put your hand near the rim to check for heat. If you cannot detect heat that way, put your hand on the rim. If it is brakes, I could help since I am just an hour away.

An hour? More like 20 minutes. I'll check that - thanks.

JW is going down the mixture leaning path - his logic is always a good bet.

Paint is a pita and a coarse measurement. If you want to test rotor temps, a hand-held IR thermometer would be a better bet plus you could compare left to right for differences.

Think fuel.

If he has fuel starvation would the engine not run lean? Then it should get very hot. Surely that would also be noticeable. What about a bad CDI or coil?

My volkswagen did the same thing and an airplane will too! high revs , more air+ humidity in air= carb ice. and it goes away in a couple of minutes, wont show up when driving "normal" and it's hard to catch. just my 2 cents worth.

Interesting idea - never heard of it before. How do you cure it permanently?

There is a low pressure generated in the intake to draw in air; the air is accelerated through venturis and around the throttle plates which creates a further pressure drop. Reduced pressure results in reduced intake air temperature. If the air is moist and the temperature drops to at or below the dew point, you can get fog or precipitation in the intakes. If the temp is cold enough that precipitation can freeze and restrict air flow. You occasionally see the phenomenon when a jet engine is at high thrust settings and a cloud forms in the inlet.

Look at your intake air snorkel. On my '71 there (was) a hot-air riser that ducted hot air into the intake when the cabin heat lever was activated. The warmer air was to remove/prevent formation of ice in the intake. I suspect it is the same or similar on your '72. Sometime around '73 the warm air supply was moved to the driver side and made automatic.

Finally I'm able to update this. MSD 6200 CD box to the rescue - problem solved. Had a very happy day at the track this week! :D