Cutting out at Idle

by Robert S. Hoover

In a message dated 98-11-04 01:12:15 EST, you write:

"I have a problem with my 1200 Bug (as always). You might remember that I had my inlet manifold changed due to an air leak in it, well after that, my bug was running fine, and then after about 6 weeks starting cutting out at idle speed. The Carburettor was removed and the jets cleaned, and other new parts installed from a "carburettor rebuild kit". Once again she was fine for a few weeks, but now is again cutting out at idle thus forcing me to constantly keep my foot on the gas pedal even in Neutral. The only possibility I can think of is that my 27 year old Carb might be on it's last legs and need replacing. Is there anyway to tell whether it is this, as a new solex carb is quite expensive and I don't want to replace mine only to continue having the problem. Also is the stock Solex 30Pict3 the best Carb for my stock engine?"

Before spending any money on carburetors lets take another look at the problem.

Your carburetor is a fuel METERING device. (Many think its only function is to MIX fuel and air, a task largely accomplished in the manifold.) Barring damage due to corrosion or excessive wear of the throttle-shaft bearing, the typical symptom associated with advanced age is for the carb to run TOO RICH rather than too lean.

Since you've not mentioned smoke or other symptoms of rich-running, and since failure to idle is usually the result of improper timing or too lean a mixture -- and since I know you've already checked your timing, plugs and so forth... (you HAVE, haven't you? :-) -- let's take a look at what might produce a lean burn.

Mention 'lean' and the first thing people think of is that the carb must be providing too little fuel -- especially when the problem goes away when you hold the throttle open a bit wider. But the other half of the equation is air. Rather than too LITTLE fuel, you may be getting to MUCH air. Indeed, this is the common cause of this complaint and for a very simple reason.

At idle-speeds the throttle-plate is almost fully closed. This reduces the flow of air thru the carburetor (and activates the low-speed fuel-delivery circuit) but it also lowers the vacuum in the intake manifold, far lower than when the engine is running at maximum rpm. Under those condition it is quite common for the engine to draw in an excess amount of air -- so-called 'vacuum leaks' -- at the gasketed flange under the carb, through leaky hoses, or where the ends of the manifold attach to the cylinder heads.

AND around the shaft of the throttle plate, if the bore is excessively worn.

You can check for such vacuum leaks by squirting a bit of WD-40 on the suspect areas. If they are leaking, the sound of the engine will change when it sucks in some of the WD-40 (which is mostly kerosene). You'll get a bit of white smoke if the leak is very large.

After 27 years of service I would expect your carburetor's throttle-shaft bore to show considerable wear -- and to be sucking excess air at low rpm. This should be the first thing to check. If the throttle-shaft bore is worn, the proper repair is to install a 'bushing-kit', once standard stuff when there was a VW dealer in every town, now so rare most people have never even heard of such kits. (Hold the throttle full open [ie, with the throttle-plate vertical] and see if there is any play where the shaft passes through the casting of the carburetor body.)

Re-bushing a carb isn't difficult -- nowadays they use metal-filled epoxies to hold the new bushings in place. But if you've never done the job and can't find a bushing kit, you're facing quite a task. For a quick fix, build an external 'seal' around the leaky bore using RTV. When the rubber compound cures, you can still move the throttle (albeit stiffly) but the RTV will greatly reduce -- and often stop -- air being sucked in around the throttle- shaft bore (be sure to do both sides).

The other areas where an air leak might occur are easier to repair, the answer being the proper installation of a new gasket, replacement of a hose and so forth.

But as to age effecting your carb, other than internal corrosion or ovaling out the throttle-shaft bore, carbs are relatively 'ageless'. When metering gasoline, wear of a brass orifice is a function of pressure and flow-rate. On a carb, both are quite low. I think you'd need to run about 25,000 gallons of gasoline thru the main-jet before you'd begin to notice erosion of the orifice... and any erosion would tend to ENRICHEN the mixture rather than cause it to run lean (the low-speed circuit draws its fuel thru the main-jet). But the truth is, when running at highway speeds, a good percentage of the fuel burned does not flow thru the main jet but is discharged directly into the carburetor's throat from the spray-bars in the high-power circuit... and from the accelerator-pump outlet. Erosion in these areas would have no effect on low-speed operation.

In any case, the main-jet, pilot-jet (ie, low-speed jet) and emulsion tube are replaceable. IF you suspect erosion, replace them. The cost of a set of jets is a pittance compared to a new carb. But the odds are, they don't need to be replaced.

Contaminants such as dust and rust could cause blockage of the smaller passages in the low-speed circuit and might give rise to a lean idle (usually, they block the circuit completely -- it does not run at all) but the cause of such problems is clearly evident on inspection (ie, rust in the float-bowl [which you can drain & inspect without dismounting or dismantling the carb], rust in the fuel-pump filter, or dust around the upper orifice of the emulsion tube) and responds to proper maintenance -- repair of the fuel tank in the case of rust and of the air-filter with regard to dust. I know you've just cleaned your carb but if the problem had to do with rust or dust, and if you have not dealt with the root problem, you are back to where you were a few weeks ago -- you must clean the carb again. But this time, also take care of the root problem.

The Solex carburetor is more robust than most realize. (And more resistant to blockage. By design, the mesh of the fuel-pump filter is smaller than any of the jets & orifices [pull it out, see for yourself].) But like any carb, your Solex requires its fair share of maintenance. And carbs do wear out, like any other part of your vehicle. But when repairs were needed, Solex carbs were meant to be overhauled, not thrown away.

As to a 30-PICT-3, I think this would be a poor choice for a 1200cc engine. The 28-PCI (ie, manual choke) or -PICT (electric choke) would be a better choice, followed by any of the other 'round-bowl' carbs. The 30-PICT-3 (I think this was the first of the 'square-bowl' carbs) incorporates a special 'low-emission' low-speed circuit. For this new low-speed circuit to function properly there must be a certain minumim rate-of-flow thru the carburetor. At idle, the flow-rate provided by a 1200cc engine is probably too low to insure stable operation of the low-speed circuit found on these 'low- emission' carbs. (The 'square-bowl' carbs were used on the 1500 & 1600cc engines.)

To sum up, your symptoms are most likely being caused by an excess of air rather than a lack of fuel. Before spending any money on repairs, make sure you have accurately diagnosed the root problem. With rare exception, age alone is not grounds for condemning a part, especially one designed to be repaired.

-Bob Hoover

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