So we had Wilma back after an extended stay at the VW camper van hospital, but we had been warned she still had one little problem that we needed to get to the bottom of.

She wasn’t running quite right, there was a definite flat-spot.

The engine ran very sweetly, but if you accelerated hard, it bogged down until you backed off the throttle and let it compose itself again.

Accelerate more slowly and the ’van would be happier, but it plainly wasn’t right. I was advised to visit a rolling road, to fine-tune the new carburettor set-up.

Anyone for a VW camper van?!

Actually finding a suitable rolling road wasn’t as easy as I’d hoped.

All the ones local to me specialised in remapping diesel engines, or at the very least, cars with modern petrol engines, ECUs and fuel injection.

A bit more digging around and I found a rolling road in Slough which had pictures on its Facebook page of older vehicles, and even a few older VW camper vans. Plainly this wasn’t a company scared of meddling with carburettors.

We booked an appointment and turned up bright and early at the Novatech HQ. The ’van doesn’t look very promising and I was sure that proprietor Andy Bowden was underwhelmed as we backed Wilma onto the ramps.

I explained what the symptoms were, but in Andy’s line of work, he explained that you check the basics before you do anything. Almost anything, at least. “Tea or coffee?” he asked.

Checking the carbs

Immediately, Andy looked at the brand-new carburettor set-up on Wilma and identified a couple of simple things that needed fixing before we got near the ramps.

He removed the air filters, and right away noticed the throttle spring on the right-hand carburettor was pulling in the wrong direction.

“If the spring isn’t pulling in the right direction, it won’t pull the throttle closed, you won’t be able to set the idle speed and it won’t run properly,” he explained.

He managed to drill a new mounting for the spring, which improved the angle at which it pulled the throttle closed. That was the first improvement.

The next thing Andy checked was that the carbs were balanced, essentially that the carburettors both open and close at the same time, so the two banks of cylinders get equal fuel and produce equal power.

He made adjustments to achieve this balance and to get Wilma idling properly. She sounded pretty good, but I drained my mug as Andy asked me to reverse the ’van onto the ramp.

Onto the rolling road

If you’re not familiar with a rolling road, it allows the engine to be tuned while it is running at speed, and allows the engine’s performance to be measured throughout the rev range, rather than just standing still.

Wilma sounded pretty good, but getting her on the rollers would tell us the full story.

Andy spent 20 minutes strapping my ’van down, putting a huge fan in front to keep her cool, and placing the exhaust gas probe into the tailpipe, readying her to be run on the rollers.

Before any ramp work was done, I was asked to sign a disclaimer: if the ’van broke down on the ramp while being revved, it was at my expense. Oil levels were checked beforehand and Wilma was healthy, so I was happy to sign.

With that, Andy jumped in the cab and took her up to an equivalent road speed of around 60mph, watching the computer through the window, which told him what was going on. Once she hit her maximum engine speed in all gears, Andy slowed her down and let her rest.

As the noise subsided and she cooled down, he told me that she was basically in good shape. There was nothing needed.

She produced 50.7hp, a fraction more than from the factory, which indicated she was healthy. Hurrah! It didn’t, however, explain the flat-spot.

The test drive

Andy suggested a test-drive to see if it was better, so he said we should pop the air filters back on, do one more run on the rollers and then take a drive around the block. So that’s what we did.

Everything was going well to start with, but as the engine speed rose, there was a horrible spitting through the exhaust.

The engine sounded sick and the maximum power generated was down some 20% to 34hp. The only difference between the two runs was the air filters. We seemed to have our answer.

Without the filters, we were back to 50.4hp and the little flat-four 1600cc engine sang happily away right to the rev limit.

The answer!

Whipping the filters off soon revealed the problem.

Although each filter had a 52mm neck, which matched the inlets on our Weber carburettors, the rubber fitting narrowed to around 46mm as it went into the air-filter housing. This restricted the air that could get into Wilma’s engine.

Accelerate hard and the engine needs more air to burn the fuel you’re throwing down its neck. Not enough air, means the fuel can’t burn, and that is where the horrible hiccup was coming from.

These were supplied with the new carburettors, but plainly they should have gone straight in the bin.

So with that, we were done. Andy, slightly reluctantly, sent us on our way without the air filters, as he worried having them on was worse than the unlikely prospect of some rubbish getting into the carbs due to a lack of filters.

A new lease of life

On the road, the difference was remarkable. With 50hp and a two-tonne frame, she’s never going to be fast, but on the way back from Slough, Wilma drove better than at any time in our ownership.

I ordered a pair of K&N RC-1352 tapered air filters as soon as I got home, which were popped on before she was taken on a proper test drive a few days later.

That 40-mile drive took in motorways, A-roads and local streets. There was no pinking, no spluttering and no stalling.

The power delivery was now predictable and she started on the first flick of the key. She was finally running properly!

So now we needed to book a holiday to put her to the test. I felt we finally had a ’van we could use without worrying.

Let’s hope this wasn’t a premature celebration…