(Go to the end of this page to see what was really going on!!)
The Aprilia, like quite a few others, is fitted with an English made Zenith 36VI-2 carburettor instead of the standard Italian made Zenith 32/VIM. Along with slightly opened inlet ports these give a noticeable increase in performance.
Gradually over the last two years the car had developed problems when starting from hot, picking up from idle and slow running. I’d pulled the carburettor off a few times and checked it over as best I could, cleaned all the jets and sealed the gasket surfaces for air leaks. Various advice suggested modern fuels vaporizing as the possible cause, but it wasn’t until recently Don Hume suggested I take the air cleaner off and look down the carburettor choke after the engine had stopped.
Sure enough for 30 – 40 seconds after the engine had stopped fuel was dribbling in from the float bowl down the choke. So it was flooding. One has to ask why I hadn’t done this check years ago, unfortunately I have no answer!
In my copy of “The Modern Motor Engineer”, Jan 1937 there is a complete section describing this Zenith carburettor. The eventual and obvious conclusion I came to was the needle and seat was worn therefore the level of fuel in the float bowl was always too high.
The easiest solution was to send it off to the experts, in this case John Cheal at Wilson Carburettor Service in Airport West. A couple of weeks later back it came with a new needle & seat, and some wise advice that the rest of the thing was all there! Now back on the engine it starts without any problems, picks up perfectly from slow speeds, accelerates more smoothly at all speeds, even the choke works now.
The problem is back!!
Some advise from another expert, Gavin Campbell at Classic Carburettors, suggested the fuel flow rate could be too high. Now I have made a few changes over time. I’d replaced the 1/4″ fuel line from the fuel pump to the carburettor with 5/16″, and made a new banjo inlet on the carby with a bigger internal diameter. I’ve ordered a different needle & seat to the new one, changed the banjo dia to 3mm and will pick up some new 1/4″ fuel line. Will keep you posted on the progress!
So the new N&S (2) didn’t make any difference. Put a bigger washer (A) under it to lower the float level, partial success. Still dribbling fuel but less. Change the fuel line from the pump to the Zenith back to 1/4″, no improvement. Fitted a second washer (C) under the N&S, SUCCESS!! Maybe I shouldn’t be so confident as I’ve only gone for a couple of short runs so far…
Update 10 April 2013:
Well it turns out the problem wasn’t the needle & seat after all. A run yesterday to the local service station to fill her up with petrol and the problem had returned. This time so badly by the time I got home she was running terribly and wouldn’t idle. Off with the air cleaner and again fuel was dribbling down the choke.
After some head scratching I took the carby off. For some unknown reason the only thing I hadn’t check before was the float. Sure enough it was half full of petrol. Putting it some hot water revealed small bubbles coming from around the mounting bracket. Drilled a 1/32″ hole where the manufacturing hole had been soldered to empty it out. Soldered the holes using my electric iron, no naked flames!!! Checked the float level, which was now about a centimetre higher than before, took the larger washer from under the N & S. A test run and so far, again, it’s ok.
So the bottom line ignore most of the above! Can someone tell me why it takes me years to figure these things out? And also why the experts didn’t check this when they had the thing???
October 2013 update
Even after all of the above the engine still wouldn’t idle and was hard to start when hot. Time to check a few other things starting with the ignition timing and would you believe I had it totally up the creek. Instead of setting the timing when the points are just breaking I had them set to when they were closing. Yes I’d been turning the distributor the wrong way which meant the timing was way out. Thinking about it it had been that way since I fitted a new distributor back in 2011. Once that was done everything came together and she ran better but, of course, a new problem emerged, an air leak in the inlet manifold. Using a piece of garden hose as a stethoscope I could find were it was coming from, removed the manifold and found the gasket had broken. A new thicker 0.8mm paper gasket and sealed with Hylomar Universal Blue (the only sealant I have found that is petrol resistant), and all was well.
So finally after seven years the carburettor works as it should. To start from cold I use a small amount of choke, run for 30 seconds and it can be closed and the engine idles perfectly. Starting from hot is instant and now slow running is perfect.
As a final postscript I did the ALR Castlemaine Rally and Alpine Tour, 10 days 2,400 kms and had no problems and a month later it is still fine.
p.s. If anyone ever comes across a Weber 36 DR5 please let me know.