Occasional lack of revs

[Clifford Pope]

This is an intermittant fault, but getting a bit more frequent.

It's a 240 with a basic mixer and open-loop system. Considering the absense of feedback, and the overall lack of sophistication, it has been running really well and giving good fuel economy. The only rare fault has been when sometimes the main solenoid sticks shut after standing for several days, or after refueling, but it always opens after a few minutes of trying and then gives no more trouble for weeks.

The new problem however is what feels like fuel starvation. Sometimes it simply will not rev above about 2500 rpm, although it pulls well below that speed. Foot on the floor slogging it in higher gear has bags of power, but changing down and trying to sensibly use higher revs and it hesitates.

But only rarely. It vanishes the instant I switch to petrol, and if I give it a good blast on petrol it will then run faultlessly on gas for days afterwards. It's as if doing that sucks some blockage clear.

It's all fully serviced, clean filter, no heavy ends, newish Denso plugs, HTleads, rotor arm, etc.

Can this be the solenoid partly closing sometimes? Or an ailing vapouriser?
There's not much else to go wrong.
It was converted 10 years ago, and has done probably about 150,000 miles on gas.

[classicswede]

My first thought would be heavy ends. Have you just drained it or flushed it out with cleaning fluid?

[Clifford Pope]

Whenever I have unscrewed the plug while hot nothing has ever come out, and it all looks clean inside. I'll try giving it a flush with carb cleaner.