Opinions wanted about engine glitch

[swcarroll]

Hello, I have a 67 911 and a stupid question. (I am a novice with these things, so bear with me.) It had run perfectly until last night, when it suddenly experienced a loss of power. There seemed to be brief periods when full power returned as I stroked the accelerator pedal. My sense was that it was due to a fuel problem. This morning, I took the car out again with the same result.

I removed the air cleaner and confirmed that all carb throats were getting fuel when depressing the accelerator. While idling the engine (it idled smoothly) I pulled the plug wires one at a time, and each one showed spark and would slow the idle. Then, I covered the velocity stacks with my hand, one at a time. A couple of them showed significant suction and would nearly kill the engine when covered. The others showed modest suction and barely slowed the idle. I put everything back together and test drove the car again, and it runs just fine now.

Any ideas about what's going on?

[chiroracer]

When you placed your hand over the venturi it used the engines suction to dislodge the offending particle or blockage. It is odd that the suction you felt would be less though. My 71 with webers runs better and better the more frequently and harder I drive it .fuel will go bad very quickly if it sits.The jets are very easy to pull and check too . Also start at the source,it may be rust or other chunks in the fuel tank, time to check it out too. how are your filters,how old are your fuel lines ect. Cheers, Steve

[swcarroll]

I've only owned this car for a few months, so I'm unsure of the age of fuel lines, etc. There's an in-line filter on the discharge side of the fuel pump which looks reasonably clean, no particles that I can see. The soft fuel lines downstream of that filter appear to be newer than originals. I assume they can shed pieces as they age, so those are candidates for replacing?

[304065]

Can you post a photo of the filter, pump and lines, we can tell you whether they are original.

Modern Ethanol-containing fuel destroys the fuel system of old cars.

Is there rust in your air cleaner? A particle of rust could have gotten into one of the air bleeds.

What does a synchrometer tell you about whether all six are pulling the same? Your experiment with the hand suggests that possibly the weber shaft couplers have failed, allowing some throats to be more open than others. Hard to say without more test data.

Anyway, sparks first, most fuel problems are really ignition problems. Then fuel, check the carbs thoroughly and fuel system, then air-- do the correlation test to see if all the throats are open the same. That is where I would begin.

[swcarroll]

Update: After the engine ran great for about ten miles, it started to do the same thing, but worse. Power sapped down to where the car would barely move. I stopped the car but kept it idling (which it did well) and tapped on the fuel pump, thinking there may be gremlins inside that were gleefully plugging the fuel flow. What I got was an almost immediate leak from one of the connections to the fuel filter. "Holy cow," I said, or something to that effect. Game over, the car then hitched a ride on the back of a tow truck to the repair shop. I did make an observation that may help you experts determine the cause: As the power was declining, stabbing the accelerator clearly reduced the power output. Power would slightly increase as I lifted. Any other ideas?

[Jim Breazeale]

Update: After the engine ran great for about ten miles, it started to do the same thing, but worse. Power sapped down to where the car would barely move. I stopped the car but kept it idling (which it did well) and tapped on the fuel pump, thinking there may be gremlins inside that were gleefully plugging the fuel flow. What I got was an almost immediate leak from one of the connections to the fuel filter. "Holy cow," I said, or something to that effect. Game over, the car then hitched a ride on the back of a tow truck to the repair shop. I did make an observation that may help you experts determine the cause: As the power was declining, stabbing the accelerator clearly reduced the power output. Power would slightly increase as I lifted. Any other ideas?

Fuel starvation. Could be caused by many different things.

Regards

[M_deJong]

Lets not overlook our friend the vacuum leak. Could be a bad intake gasket or warped manifold.

[swcarroll]

Thanks for the comments. The repair shop should have made some progress on it early next week. I'll report their findings. Since the problem temporarily cleared up and then reappeared, I tend to agree with suggestions of fuel starvation caused by random debris in the lines.

[JimVillers]

swcarroll ..... Try checking the fuel lines between the tank and the fuel pump. A small crack in an old line can allow air into the fuel stream and reduce the fuel pressure and fuel quantity from the fuel pump.

[swcarroll]

Good suggestion, will do, thanks!