It's a carrier heater about 20 years old pilot is on when I turn up thermostat, just the fan goes on filters look clean

When was the last time it was cleaned and the burners adjusted? Have you killed power to it, waited a few minutes then turned the power back on in case it needed reset?

It was cleaned last year have not tried turning off power to reset it

Adam, if the pilot is on and the burners don't light, I would change the thermocoupling. That's cheap and easy. Good luck

Furnace has electric start - after 4 minutes fan comes on but burners don`t light.

Sounds like the pilot flame is not being sensed. If the flame senser doesn't sense the pilot flame, it will not allow the main burners to kick on. So, either the pilot flame is not hitting the flame senser directly, or the flame senser needs cleaning. How many wires lead to the pilot assembly? If only the sparker wire, then the flame is sensed through the sparker.

parttime, actually, if the pilot is lit (and this unit had a standing pilot with a thermocouple, which I don't believe it does) then the thermocouple would be fine. On thermocouple systems, if the thermocouple is bad, the pilot will not stay lit.