An all too common problem is the necessity to maintain the idle speed at an abnormally high level just so the car will idle in the driveway without dying, and being forced to constantly keep you foot on the gas pedal, even in neutral, just to keep the car running. Many people decide that the only possibility is that their 27-year old carburetor might be on it's last legs and needs replacing.
Dave expressed the problem as follows -
I need to figure out why the engine won't idle at less than 1200rpm. And every now and then it will die when it coasts to a stop -- I suspect the two problems are related. It doesn't want to time at 5 degrees ATDC -- if I retard it much below top dead center it starts to run real rough and then dies. I've got it set at about 3 degrees ATDC, and it purrs beautifully. I know, it's the top end that is the most important should I just rev it up to 3000 rpm and set it at 30 degrees BTDC?
Rob's suggestions -
Dave condensed this to -
Dave wrote -
I tore into the idle and timing. First thing I discovered was that the reason the car was running so crappy was that the accelerator cable connection under the pedal was all screwed up. That connection is a very poor design, in my opinion. I bent the metal piece on the end of the cable around so that there's no way it can come loose from the pedal or get twisted around scroogy. Then around to the back and re-adjusted the accelerator cable on the throttle lever for the umpteenth time.
Finally, I got to your most "timely" advice. Went back and forth -- max advance, back to idle, back and forth. Finally settled on 32 degrees BDTC at 3500 rpm, which gives 5 degrees ADTC at 800rpm. (Purr, purr!)
On the bright side -- I slapped on the dwell/tach and the timing light. The timing was a little bit retarded, and the car was running (!) at about 450 rpm). So I advanced the timing up to the 5 degrees ATDC mark, and the idle increased to a little over 800 rpm. Took it for a spin runs like a top! If it weren't for the nagging little problems!
Rob responded -
Interesting. Your experience points out the close relationship between timing and smooth running very well.
Dave later wrote -
I need to have a timing discussion with you one more time. I absolutely could not time my Bug to spec. When I tried it simply would not run. I ended up setting it at about 5 degrees BTDC (10 degrees advanced from spec!), and it ran wonderfully, with no hesitation and lots of pep. I fiddled and fussed with the timing, as you recall, and finally gave up and just timed it where it ran the best.
Rob responded -
That makes me wonder if the retard vacuum line was failing to do it's job, because if it was and you were setting it 10 degrees BTDC, the max advance would be about 45 degrees! No VW would run well with that. But with the retard vacuum line inoperative, 10BTDC would be about 32 degrees max, which is at the top end of OK for the Beetle (28-32 degrees is the usual range quoted).
Dave wrote six months later -
Following our son as he drove the car home from college, we noticed that the car backfired a number of times, a sign of lean fuel/air mixture. So -- I think we're right back to where we were in our discussion six months ago -- the car seems to want to run 7.5 degrees advanced (BTDC) at idle.
Rob wrote -
The idle speed and the spark advance are closely tied together, obviously. Playing too much with idle speeds ruins the idle airflow and spoils the timing adjustments too.
There seems to be little real progress there. It will all come down to ONE faulty part you know -- PROBABLY the carby, from all the other things you've tried. Sometimes, Beetles being able to run when everything is wrong is a pain -- makes it harder to find the problem than if it didn't work at all.
Before spending any money on carburetors, let's 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 carburetor to run TOO RICH rather than too lean.
Assuming no smoke or other symptoms of rich-running, and since I know you've already checked your timing, plugs and so forth... (you HAVE, haven't you? :-) -- failure to idle is usually the result of improper timing or too lean a mixture -- let's take a look at what might produce a lean burn.
If you mention 'lean,' the first thing people think of is that the carburetor 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 carburetor, 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 (or starter fluid) on the suspect areas. If they are leaking, the sound of the engine will change (i.e., increase) 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. To check for wear, hold the throttle full open (i.e., with the throttle-plate vertical) and see if there is any play where the shaft passes through the casting of the carburetor body.)
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. Re-bushing a carburetor 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 carburetor, 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 carburetor, 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 carburetor. 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 (i.e., rust in the float-bowl -- which you can drain and inspect without dismounting or dismantling the carburetor), 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. If the problem indeed had to do with rust or dust, and if you have not dealt with the root problem, you'll need to clean the carburetor 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 and orifices (pull it out, see for yourself). But like any carburetor, the Solex requires its fair share of maintenance. And carburetors do wear out, like any other part of your vehicle. But when repairs were needed, Solex carburetors were meant to be overhauled, not thrown away.
To sum up, the symptoms you describe (cutting out at idle, necessity for high idle) 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.
- By Robert S. Hoover -- 4 November 1998
Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.
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Last revised 9 July 2004.
