Please help! I have an engine misfire that comes and goes, usually happens after a 30 mins at highway speeds then slow to 60kph. Misfires at all revs and throttle settings. Won't idle at under 3,000rpm. If left to cool down starts fine and goes for 30k's or so on highway then misfires again. It backfired once during acceleration in first. Sometimes you can drive through the misfire and then runs fine again for a while. Seems the hotter the day the worse it plays up, but always goes fine after letting it cool for 30 mins. Mechanic had fitted new after market electronic ignition and coil. He reckons he has checked them out, cleaned carby's and it only fixed the problem when he backed off the tappets to have 6" & 8" clearance. The engine has a CU prefex (South African) and he couldn't find it listed so thought it must not have had hydraulic tappets. I told him it sometimes has a noisey tapped cold, and then disappears when hot. This means it has hydraulic tappets.
Anyway he reckons the valves must have been riding. The were adjusted
30,000k's ago to hydraulic specs, ie screw adjuster in until just no clearance, then screw 2 full turns in.I don't see how the valves could be riding, or how they could cause a jerking misfire. If they were riding wouldn't it misfire everytime it got hot? Have checked for vacuum leaks, timing, replaced high tension leads/caps/spark plugs. I havn't been able to test on a hot day, but now I have bloody noisey tappets.
Any advice would be greatfully received!
Neil