Troubleshooting
Diagnosing carburetor and idle issues on the Porsche 356.
General Troubleshooting
TROUBLESHOOTING
We've been working on this section for years: dealing with the most often asked questions as people are trying to determine whether or not they need carburetor work. This page is very general information. This link will take you to a page that's just a little bit more specific, while this link will take you to an entire discourse on IDLE issues, starting with the distributor.
Please note, I'm told I use too many words when just a few will do. John Jenkins, who wrote the IDLE section, does too. Have patience and read through this for the MOST information (probably more than you want along with some redundancy!)
Unfortunately, diagnosing problems, particularly remotely, is like calling your doctor and saying you have a cough and expecting them to tell you why and what to do about it. So many issues that can be caused by a number of different problems. First the obvious — Do you have power? Do you have fuel? Is your petcock open? Forgetting that happens more often than you might think! Lets start with hesitation. This can be a distributor issue, clogged fuel line, lean condition, rich condition, spark plug wires or actual plug problem, fuel pump problem, valve problem, worn butterflies, or worn carburetor throat, to name just a few. So before we diagnose why hesitation, more questions. When do you have hesitation? Up shift, down shift, starting off, when attempting fast acceleration? Some other condition? Note: both conditions can have the same symptoms — do not assume. Regardless of the problem, check for running rich or lean. Rich conditions mean cool engine, but it also means dirty tail pipe, popping through the muffler, dark black sooty spark plugs (one or all - different problems), heavy gas smell, maybe even gas dripping from carb or down over engine somehow (BAD!!!). The last two could be bad float, bad needle valve, or too much fuel pressure! check your fuel pump for proper function, proper pressure. Lean conditions are hot engine, light color spark plugs (should be a dry chocolate brown) -one or all -, popping through carbs (distributor or valve problems will do the same!). jets clogged? fuel pump working? ALWAYS check the distributor to be sure its working properly. Distributor work is less expensive than carb work. Distributor maintenance is often overlooked, just like carb maintenance is. And since 6v timing lights can sometimes be difficult to come by, gets bypassed all together
We will also ask if your volume control screws are working. some issues involve them not responding. One not responding, plus only that spark plug being a different color, are the kind of things that help identify and resolve issues. SPARK PLUGS, taken for granted and overlooked. Recently discussing a rich running condition in a client's car, realized that I hadn't emphasized spark plugs enough. Am not in the position to comment on hot and cold plugs, but can talk about DIRTY plugs. While we use plug condition as a diagnostic tool, they go beyond that. It is the proper spark, at the proper time, to the proper balance of air/fuel mixture that makes the engine work. Distributor controls the time. Carburetors control the air/fuel mix. DIRT affects the proper spark. Pretty much every engine runs rich at idle. Idling at the stop light is one thing, but are you idling the car to let it warm up? Idling while waiting for your spouse to come out of the rest room? idling while you are tuning your carbs? checking your timing? driving in a parade sloooowwwwllllyyyyy at low rpm's. All of these things will load up your plugs. When your plugs are dirty spark is affected. If you sit at idle for awhile and then get out there and get your rpm's up, heat up the engine, that little bit of dirt will burn off. But for a few moments you may have some hesitation, some uneven idling. If that scares you off your drive, you have just left your plugs dirty and the next time you start up the car, it will be worse. Poor spark will not fully ignite the fuel mix in the cylinder and you are left with excess fuel and low power. Then you chase your carbs. To check if you are really running rich, start with clean spark plugs, get the car going and out for a good warm up drive, keeping over 3500rpm. Plan this out carefully because you will want a place where you can just turn off the engine and coast to a stop. Then check your plugs. If your plugs are dark, you are running rich. If they are light, you are running lean. If you are just right, that nice chocolate brown, but you have problems transitioning from idle or when running on low RPM's, then a rich condition at Idle is your problem.
Air leaks? With the engine running, being VERY CAREFUL, spray a little carb cleaner or air lightly around the edges of the carbs. If there is a sudden increase in engine revs, there is an air leak in that area. IF you decide to try this test, stand back. Matter of fact STAND BACK from your carburetor when ever working on them. — especially w/ carb cleaner — Solex, particularly, spit back with a little flame! Or don't blame me when things get out of hand and you don't have a fire extinguisher at the ready! Worn butterflies on Solex mean air leaks, so hesitation because of air flow when the butterflies should be closed. There might be a ridge worn in the throat of the carburetor if the shaft is loose. With split shafts, the bracket that keeps the butterflies in sync could be worn causing them to come out of sync. or maybe the little block has been lost or worn out. Has your pump rod worn out? the pump cam broken? the pump lever worn out? Is your check valve still working? On Zenith, is your accelerator pump working? what is the condition of the pump link? is the shaft twisted? How is your needle valve? Dirty? Stuck? worn out? Are your floats okay? So the bottom line to troubleshooting engine problems (now you know why your mechanic charges so much) is gathering as much information as possible, and eliminating all the different possibilities that could be causing the problem. Read your books. Ask questions on the 356 and 912 forums. Talk to mechanics. Give us a call. And have as much information as you can, and eliminate the easy ones, one at a time.
You will also find copies of shop manual pages for dual barrel Solex and Zenith within this web site.
(and I know that is not a Porsche engine!)
Idle Issues
To eliminate the distributor as the cause of the problem, take a timing light and note when the distributor begins to advance. You can easily tell by watching the pulley TDC (top dead center) mark which should be at the 5 degree advance point at idle. That will be about 1/4" ahead of the top dead center point if you are working with a light that doesn't give you the angle. If it starts moving at a little over 1000 rpm, between 980 and 1020, you are okay. You might have someone watch your tach for you. Do this when engine cold and hot. Take into consideration that your tach might be slightly off, but look for the distributor to begin to advance in that general 1000 rpm range.
Mechanics seem to have their own favorite way of tuning for idle. Some set the total advance first by moving the distributor to set the total advance and let the static timing be whatever it turns out to be, then adjust the mixture and idle screws. This is the 911 approach to timing and incorrect for a 356. Set the static timing at 5 degrees before top dead center, and then go after the idle mixture and throttle stops, for proper distributor / carburetor set up.
You should be able to get an idle by setting the static timing to 5 degrees BTD, opening the idle mixture screws 1 1/4 turns, and opening the butterflies by about.015".
BEFORE YOU START take out the idle mixture screws and see if any of them have the "ham fisted" groove on the tapered needle. If so, it is worthless and needs replacement before going any further.
Check the idle jets, too, and make sure they are not plugged.
If not, then proceed as follows. With the throttle rods disconnected adjust the throttle stops so that each carb is pulling the same amount of air. Now adjust the throttle stops so that the engine is idling below 1000 rpm and they are still pulling the same amount of air. A synchrometer or Unisyn is needed, the Synchrometer is a little easier to use. Starting with #1 adjust the idle mixture screw so the highest idle is reached with that screw. Lower the throttle stop if necessary to keep the distributor from advancing. Do this for #2 through #4. Now do it again. If the idle mixture screws wind up to be less than one turn out then either your float level is too high or your idle jet is too large. On a Zenith, suspect the float level. If the idle mixture screw is over two turns out then your idle jet is too small. Again, on a Zenith, suspect the float level. After all this is done, your float level is correct, and you still have the problem then worn butterlies are the likely cause.
The idle will pick up after the engine warms up because the fuel mixture is better vaporized. This is why you do it when the engine is at operating temperature.
But with worn butterflies even more gas mixture is introduced. Next thing to look for is an air leak. Cans of compressed air used for lens cleaning are a safe way of detecting leaks. A lot of people use carb cleaner, which can cause a flame to backfire through the carbs. Always keep a fire extinguisher close by.
You should be able to lift the throttle rods about 3/16" after they are disconnected from the throttle arms. This allows for the expansion of the whole throttle linkage after it warms up. The engine expands, too.
Still chasing a problem?
Contact us before changing too many settings; the original baseline matters.