Carburation Caper
After the first 500 miles or so you tend to get a good sense of any underlying mechanical issues a car has. Having covered nearly twice that in my MK2 Polo Breadvan, it has shown very few signs of any problems. It did however progressively start running a little worse. The symptoms were occasional rough idling and ‘jerkyness’ at low speeds as well as erratic throttle response.
A common problem on the MK2 Polo is the rubber carburettor mount between the carb and the intake manifold. A wobble of the air filter housing and observing it moving a lot when applying the throttle seemed to suggest this might be the issue. A subsequent look at the mount itself confirmed it had split.
Removal and replacement is relatively straightforward - check out the following YouTube video from Ruined Rides to get the general idea:
Basically you remove the air filter housing, taking a note of the vacuum hoses that need disconnecting. Then unbolt the carb from the manifold from underneath. It helps to remove the breather hose behind the carb first (mine was cracked and needed replacing anyway). It’s a 3 ended crankcase breather hose that connects to the block underneath the intake manifold, the air filter housing and the rocker cover. This gives better access for either a small ratchet or a ratcheting spanner (10mm is the nut size).
With the carb mount replaced I took a test drive to get it up to temperature and see how the car was idling. The car drove fine but it was no longer idling without the choke and was still running rough at idle with an occasional misfire.
Generally the starting point for (and most common cause of) running issues is the ignition system and it is advisable attend to this before tinkering with the air / fuel delivery. I uncovered several potential issues including:
A pitted / worn rotor arm and distributor cap
A slightly cracked vacuum hose
A slightly split HT lead to one of the spark plugs
I resolved to replace these, however I also checked for and established good spark to all of the plugs leading me to believe none of these were the cause of the running issues.
I checked for further vacuum leaks around the carb and the tightness of the carb mount. This was all fine so I did some research on the Weber carburettor on my car.
After absorbing lots of information about the model of carburettor I have and reaching out for advice, a ClubPolo forum member suggested I may simply have an incidental fuel blockage. The suggestion turned out to be spot on:
I removed the jets, blew through them and cleaned them with carb cleaner. I also sprayed the carb intake and cleaned the re-usable fuel filter I fitted a few months ago. Having carried out this simple cleaning process the car ran much more evenly and happily without choke once warmed up.
So the problem is solved and who knows whether I dislodged something while replacing the carb mount that clogged the idle jet, or if that was the running problem all along. But I now also have a list of a few other worn parts to replace and once I’m done that should have completed a good bit of preventative maintenance for any future running issues too.
I’ve also learnt a lot about the car’s HZ engine and Weber 32 TLA carburettor (with manual choke conversion) in the process. Anecdotal evidence would suggest most cars with the Weber carburettor seem not to have the ‘vacuum reservoir’ system present on other cars. As a result it is normal for there to be some minor fluctuations of the idle speed. This can occur even when everything is running fine and there are no misfires, fuel or ignition problems. On the 1 litre Polo they should technically idle at 800rpm but typically will oscillate between 700 and 900. There are some potential fixes I may investigate for this too, but that is very much a job for another time.
Hitting the road in my Polo once this problem was sorted, it was pleasing to find a noticeable improvement in power delivery. It’s not a fast car, but that means making sure it’s in good running order is of even more importance. Besides it’s fun to squeeze as much power out of cars with small engines as you can!