Arrrrgggg - it's all in the timing!

Ever since I've had the Porsche, the fuel pressure gauge has registered zero. The car seemed to be running fine, so I figured (in a typical, American, denial-sort-of-way) that the gauge must be broken. No big deal, I'll repair it "sometime".

As I started driving the car more aggressively and learning the feel of a 200 horsepower flat-six engine, I came to the conclusion that something was amiss during hard acceleration - especially around 3500 RPM (feel free to insert cool-guy wrench-head terms like "up on the cam" or "in the power band" here). During these times, the car would hesitate and/or make a gargling noise.

Discussing the issue with some of my newly found Porsche friends, I decided it was a fuel delivery issue - not enough gas was getting into the carbs during these times of accelerating causing the air/fuel mixture to get out of wack (wrench term: "leaning out"). When that happens, the power drops since the detonation in the cylinders is not as powerful and the driver feels this as a hesitation or stuttering. (hey mechanics, feel free to correct me if I'm wrong here, or anywhere else for that matter - remember, I'm an ELECTRICAL engineer - my mechanical skills usually revolve around a soldering iron). Anyway, the first lead to try (also, coincidently the easiest to work with) was the dreaded "Main Jets" within the carburetors. I'm not going to begin to try and explain how a carburetor works - frankly, I think the last guy to really understand the operation of one of these things died in WWII - but, if you feel the need, take a trip over here for a simple explanation; realize, however, that the Weber carbs in the 911 have about as much in common with the carbs they talk about in that web page as my pet cat has with a leopard.

Anyway, where was I? Oh - Main Jets. Yeah, the main jets may be the problem. Sure - just gotta change them, and the problem will go away - no trouble at all. "What, exactly, is a main jet?" you ask? Well, here is my take on the whole jet thing. The carburetor is responsible for delivering a nice fuel/air mixture to the cylinders to support detonation during operation of the engine. It's not as simple as it sounds, so a Rube Goldberg-esq collection of nuts, bolts, screws and scary sounding things like "butterfly valves", "venturis" and whatnot are cobbled together. One function of the carb is to adjust to the demands of the throttle - when Mr. Andretti wants to go fast, we better be able to dump fuel into the engine in volumes. Well, it turns out the engines don't like a constant fuel/air mixture; during idle, for example, the fuel flow needs to be really low - but while under hard throttle, we want to pump that fuel into the engine fast-and-furious like. Cue the Jets. In the Weber carbs on my car there are two different jets for each cylinder - an idle jet and a main jet. During idle, the main jet is blocked so fuel basically trickles through the small orifice in the idle jet into the cylinder. After depressing the throttle a certain distance, the main jets are opened and a much greater amount of fuel flows into the cylinders. The size of the jets (how big the hole is) determines the amount of fuel flowing through it.

Ok, ok Mr. Technical - let me clarify. The size of the jets determines the flow rate ASSUMING THE FUEL PRESSURE IS ADEQUATE.

So anyway - I'm going to try and get some new jets which are just a fraction bigger than the installed ones. Order'm up!

It will take a while - so, I'll just sit tight and drive. Drive! Drive baby, drive!!!

[Meanwhile, a month or two later]

Finally, the new jets arrive, and I manage to get a spare moment to replace them.

Out with old.

In with the .... Damnit! Wrong ones. Shit!

In with the old.

Re-order the new.

*sigh*

Then something clicks in my mind - maybe the problem isn't the jets, but the flow?

On a lark, I pinch off the return line from the fuel manifold - and guess what? The pressure gauge is reading a healthy 3 PSI.

Weird.

Time to surf the net. Learned a lot on good old Pelican.

Wow - this car has been through a lot.

Quick synopses. My car originally came equipped with fuel injection - something that Porsche dubbed "CIS". A common upgrade was to pull the injection system out and put in Weber carburetors. This will add a healthy 15-19 HP to the engine, and sound oh-so-cool to boot! You can't lose!! This is NOT a bolt-on replacement; we're talking about a big job here.

One of the consequence of the Weber conversion is a change in the fuel delivery system. The stock fuel pump is made to deliver a healthy 40 PSI to help force that fuel through those small fuel injection holes - as a consequence of this high pressure, much of the fuel pumped out of the tank (in the front of the car) to the engine (in the back of the car) is not used; so is returned through another line back to the tank. So; fuel tank -> fuel pump -> fuel line -> fuel filter -> injection system -> fuel return line -> fuel tank. Rinse, repeat. Carburetors, on the other hand, need about 3-5 PSI of pressure to function. Quite a difference.

So - a common solution was to have a fuel manifold with one input and four outlet ports - fuel into the inlet, one outlet for each carb (two total), one outlet for a pressure gauge, and the final outlet to the return line. To regulate the pressure, a small clamp is placed on the return line and pinched down until the pressure gauge reads about 4 PSI.

Simple and elegant. Not my style - I like complicated and convoluted.

So anyway, that's what my car is doing with Webers - installed by the previous owner with (I hope) care and prudence.

So - I'm thinking I have something amiss in the fuel pump or line ahead of the filter. Hmmm - time for some more opinions.

Anyway - this entry is getting VERY long winded - I'm going to enclose a snippet of the thread I started on one of my local boards regarding what happened, and call it a night.



Had the car in at work yesterday to discuss my thoughts on the fuel flow issues with some of the wrenches here. I tried pinching off the return line to show them how the pressure on the gauge would build, but no go - the needle moved just a hair. WTF?

Well, I decide that I'll let things be for now (I mean, hey - the car has been running like this for at least 8 months) - 4:45, time to go home. Start'r up, drive a few blocks and...dead. That kind of dead we all know - it's not getting fuel.

FUCH FUCH FUCH FUCH!

I coast into a parking spot, still on site here at the lab. Wait a minute or two. Cross fingers. Crank... Crank... Crank... Sputter.... Crank Crank.

FUCH FUCH FUCH!! *sigh*

Just because my heritage is from the south - I try a few more times. Knowing, deep in my heart, my fuel delivery is nil.

Begin the walk of shame.

I get a ride home - call a flatbed, get the thing up on jackstands in my garage, and begin tearing into the skidplate at around 9:30 last night.

By 10:00 I expose the pump (it's obviously (to me) a low pressure dealie - no markings on it, very small, obviously not OE). Decide to see if the thing is pumping at all, so remove the hose from the outlet side - drain the gas still in the line, put a pan under the outlet, and turn the key to activate the pump. Whirrrrrrr; gas is happily pouring out of the thing.

*sigh*

Ok, I figure I'll reconnect, and check the flow rate out of the filter.

Connect up the outlet again, turn on the pump and wiggle under the car to check for good seating of the hoses. No leaks.

On a lark, I check out the engine compartment while the pump is on.

guess what? 3 PSI!

WTF!?

well - let's see if she starts.

Crank. Crank. Whirrr. Crank. Sputter. Crank Wirrrrr. Wait.... gotta fill up the line. Crank Crank Whirrr Sputter. Wait. Shit, maybe I flooded the carbs. Lessee, one more time. Crank Crank BANG!

Uhoh.

That was the loudest bang I've heard from a motor EVER.

Walk to the engine compartment. Nothing obvious, hmmm - check this, look at that, pretend I know what to look for..WAIT A MINUTE - The dizzy cap is off - just resting on the distributor! Well, that can't be good?

I pull the cap off completely to find a bunch of bakelite (I think that's what it's made out of?) shavings and a broken piece about 1/4" long inside. Well, it turns out that piece is the registration key for the inside of the rotor - I had somehow torqued the rotor enough that the key piece broke, but it still remained on the shaft.

FUCH FUCH FUCH FUCH!

So much for my nice new rev-limiting rotor

Step back from the vehicle. Take a deep breath. What to do, what to do.

Well, I crank the engine by hand via the fan bolt and belt. Things seem to feel OK - compression, no binding, blah blah.

I have my old rotor around here somewhere.... Should I? Do I dare?

Sure.

Pop it in, clean up the cap, button it up. Cross fingers. Crank crank.

Vroooom!

Running. Not even that rough considering I will have to re-time with the new rotor.

WTF!?

Fuel pressure - 3PSI.

Engine idling, 1200 RPM.

All seems well.

Ok - I'll let'r idle for a while, while I button back up the front end skid plate.

15 minutes later, she's still running nice - pressure is 3PSI - I decide I'm going to reset the timing and go for a spin (it's almost midnight by this time - I'm sure my neighbors are loving this). At this point, I'm thinking I might have had a fuel blockage that I inadvertantly cleared when I disconnected the feed line from the pump.

Lower the front end (the only end I had up, so the car was tilted up a bit... foreshadowing

Go grab the timing light, head back to the engine bay - check the fuel pressure. Wait - WTF! 2psi. and falling.

ARRRRRGG

I watch it drop slowly back beneath 1 and the gauge needle hits the stop registering (effectively zero).

FUCHING HELL.

That's it. I'm done.


So I'm thinking that maybe the pump is failing - having the car up in the front was allowing gravity to help pressurize the line. I guess the best plan of action would be to replace this pump.

I'm thinking of buying a new pump, lines, regulator and whanot. QUICKLY. Move the pump into the smugglers box (for easier access - gad, I hate working right underneath fuel lines) and go from there. Does this sound reasonable to you guys?

Alas, this means I will miss the friday DE

But, if the stars align, I WILL be ready for Gingerman in August.