Below is an exhaustive list of things that could be causing your problems
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* Valve Clearance: The Haynes Manual indicates that incorrect valve clearance may cause hesitation.
* Intake Manifold: Two considerations regarding the intake manifold: Pre-heat and air inleakage.
o Pre-Heat - If the manifold pre-heat tubes (also called heat risers) don't get hot, fuel from the accelerator pump just "lays" in the manifold and doesn't vaporize. Drive the car around the block and then feel the pre-heat tubes. They should be hot over their entire length (use caution -- they should be VERY hot). Several things may prevent the pre-heat tubes from being heated -
+ The tubes may be clogged with carbon deposits. See our procedure for Clearing a Blocked Heat Riser.
+ The muffler is not creating enough pressure difference.
+ The heat riser flanges on the exhaust header may not have been drilled through into the header to allow the flow of exhaust gases (either direct from one side to the other or pulsing back and forth). If this is the case, you will have to remove the exhaust header and drill through the flanges.
o Air Inleakage - If air is somehow leaking into the intake manifold downstream of the carburetor, the fuel mixture will be too lean and the car will not run properly. The system is easily checked for leaks by 1) choking the carburetor with the palm of your hand over the throat (if the engine immediately dies, there is no leak), and 2) spraying starter fluid (ether) on every possible leak point and listening for changes in the engine speed as the ether is sucked into the manifold. For more information, please see our Air Inleakage Discussion.
One critical area that is not very accessible is where the manifold attaches to the engine itself. Spraying with the straw applicator that comes with the starter fluid usually does the trick, as you can then get the spray closer without it dispersing too much. (On single port engines, this connection has a large copper 'washer' as a gasket, and this gets compressed slightly when tightened. On dual-port engines, this gasket is a flat paper type, which is easily damaged.)
On later models the intake manifold is in two pieces, joined by rubber connectors with hose clamps. Loose clamps at these points can cause air inleakage.
* Vacuum Advance Distributor: There are two types of vacuum distributor -- single vacuum and double vacuum. Make sure that the vacuum system is operating correctly by sucking on the vacuum lead(s) and looking at the movable points-plate inside the distributor to see if it moves to the relevant stop. If you now pinch off the hose, the plate should stay near the stop. If it drifts back to the centre, the vacuum chamber has a leak in it.
* Incorrect Ignition Timing: There has been a great deal of discussion regarding ignition timing with different distributor/engine combinations. With the 009 distributor on the 1600cc engine, the proper timing is to set the MAXIMUM advance at 28-30 degrees at 3500rpm, and then check the static timing, which should be between 5-8 degrees BTDC (the 009 distributors vary a bit in total advance). The single-vacuum distributors use 7.5 degrees BTDC - static timing on 1500/1600s, and 10 degrees BTDC for 1200s. The proper timing setting for the vacuum advance distributor with two vacuum hoses is 5 degrees retarded (ATDC) - set with the engine running at idle revs. Timing changes as the point gap (dwell angle) is changed, so make sure the points are properly set (50 degrees plus/minus 2 degrees) before setting the timing.
* Spark Plugs: The Haynes Manual indicates that fouled spark plugs may cause hesitation.
Rob Boardman suggested the use of NGK plugs to help with the hesitation problem - NGK is the world's biggest manufacturer of spark plugs. In the VW they are reported to have a greater tolerance to the changes in cylinder head temperature changes - that is, they have a good "heat range."
I used to use Champions (L88s, then L86YC I think from memory) but changed to Bosch many years ago when I was told that Champions had a rougher thread on them, which tended to cut the aluminum heads out after a while, and were easy to cross thread. The Champions did work just as well though, and that was years ago - they may have changed the threads by now.
I now use NGKs with great success, after they were recommended to me by a VW mechanic with well over 20 years experience. The correct NGK plug is a B5HS for the 1/2-inch heads, and B5ES for the 3/4-inch heads (Type 4s, and replacement new factory heads from Brazil/Mexico for beetles).
If you can find the 'grooved electrode' type, the numbers are B5HY, and B5EY. The grooved center electrode is supposed to generate the spark at the sides of the electrode, rather than burying it in the gap - better at lighting lean mixtures, and better at propagating the flame. The grooved plugs could conceivably reduce the 009 distributor flat spot a bit.
* Carburetor: Spitting/sputtering/cracking is an indication of the fuel/air mixture being too lean. The engine speed (RPM) at which the problem occurs tells which jet needs to be changed. Test the engine's performance through the range of 1000-4000 RPM, paying attention to steady throttle position through this range.
o If the engine runs good at 3000-4000 rpm but stumbles elsewhere, the correct main jet is being used, and the problem lies somewhere else. If the hesitation problem occurs at higher rpm (2500-4000), a larger main jet needs to be installed. If the main jet doesn't solve it, try the accelerator pump (see below).
o If the stumbling occurs at 2000 RPMs and lower, a larger idle jet may need to be installed to enrichen the mixture. (Don't go much beyond 65, however.)
o If you have an 009 centrifugal-advance distributor, a larger main jet may sometimes help to compensate for the flat spot inherent with this distributor. The standard jet is 127.5 (in a 34PICT/3 carburetor); try a 130 or even larger.
While you're into the carburetor, it would be a good time to overhaul the carburetor and install a new kit. (See our Carburetor Overhaul Procedure.) Check the float; if there is a dimple where the needle valve impinges on it, replace it. After reinstalling the carburetor, make sure to tune it correctly. Idle speed may cause hesitation if it is improperly set. (Please see our articles on tuning One-Screw Carburetors and Two-Screw Carburetors.
* Automatic Choke: Make sure the automatic choke is working properly (wire to the coil attached, choke properly adjusted and opens the butterfly valve in the top of the carburetor as the engine warms up). See our Automatic Choke Adjustment Procedure.
* Idle Cut-off Valve: Make sure the idle cut-off valve is properly seated, electrically connected and working. Test the valve by removing the wire, turning on the key, and touching the wire back to the connector on the valve. You should hear a distinct "clicking" sound; no "click" means "replace."
* Accelerator Pump Faulty or Out of Adjustment: If the engine stumbles when you open the throttle, they problem could lie with the accelerator pump, or it could mean that either the main jet or the idle just is too small. Check to make sure that the pump delivery tube is pointed directly down the carburetor throat, and check to make sure that it discharges fuel when the throttle are is pulled down sharply. If it doesn't, it is likely that the accelerator pump needs to be overhauled (i.e., diaphragm replaced). If the pump is discharging fuel down the carburetor throat, check the discharge volume (see our Accelerator Pump Adjustment Procedure). For the 34-PICT/3 carburetor, the squirt volume should be 1.45-1.75 mL. Adjust it per the procedure. If you are running an 009 distributor, adjusting the accelerator pump to full stroke helps to minimize the flat spot that is inherent to this distributor.
* Clogged Fuel Filter: Most air-cooled VWs are fairly old, and there is a good possibility that the fuel tank may be rusty on the inside. This rust can clog the fuel filter(s) and contribute to the hesitation problem. Some VW fuel pumps have a small fuel filter under a brass bolt head in the rear (rear is rear of car) side of the pump - near the fan belt. If your car has one of these pumps, check this filter too. (See our Fuel Filter Service Procedure.)
Someone recommended that we remove and clean the gas tank. We have since done this, and our experience is documented at Fuel Tank Removal and Refurbishment.
* Fuel: According to Rob Boardman, the hesitation problem is almost certainly a lean-burn thing. VWs like a fractionally rich mix -- they don't like lean burn conditions. A larger main jet in the carburetor will provide a richer mix and may help the problem. Also, Rob advises the use of a brand-name fuel with at least 91 octane (more aromatics and less methyl tertiary-butyl ether - MTBE - to achieve the higher octane. Only the 1200cc engines and the newest low-compression factory (Mex/Brazilian) engines are happy on 87 octane gasoline.
Dave's son accidentally made an interesting discovery that may well have contributed to their hesitation problem. His VW was almost out of gas, so he went to the gas station to fill up. The last time he gassed up the brand-name station was closed, so he filled the tank with 92 octane at the local convenience store. On this occasion as he was on his way to the gas station (brand-name) with a near-empty tank the car died at every corner; once the tank was full with the good stuff the car ran fine.
Some more data points would be helpful, but the apparent lesson is that the cheap stuff is just that. The more expensive brand-name gas is higher quality and the Bug seems to like it a lot more. The VW seems to be sensitive to the quality of the gas.
Someone wrote regarding possible contamination of the fuel - When you run the tank down near empty you are also allowing any sediments and general crap to enter your fuel lines. Almost any gas tank in any car has depression areas in the bottom of the gas tank. The crap and water that has condensed is supposed to settle into these low areas and not enter the system.
Regarding octane levels, someone reported that his VW has the most pep when he uses 94 octane gas. He tries to avoid convenience store gas stations, having experienced poor running when he filled up there. He also reports success with the use of a fuel dryer in the gasoline. This may help remove the water and some of the crap that might be fouling the fuel lines, carburetor, etc. Isopropyl alcohol is said to be superior to methyl alcohol for this purpose.
Rob advised me that leaving the car out under damp conditions overnight may result in condensation in the fuel tank, resulting in water droplets in the fuel. He says that the only sure way to avoid condensation is the aircraft way - fill the tank up to the brim every night so there is no airspace above the fuel and thus no opportunity for water vapor in the air to condense in the tank overnight.
We came to the conclusion that the hesitation problem may be caused by the fuel in one of several ways: 1) the gas is poor quality or low octane; 2) the tank is rusty and is depositing corrosion products in the gas, and/or 3) there is water in the gas, most likely as a result of condensation. All of these would make the hesitation problem worse when the tank is near empty.
