Morris Minor Owners Club

hello everyone
got a strange one here,i cant work it out never came across this before,right,driving around town on and off the accelerator no problem,runs fine,now on the way to work theres a straight peace of road about half a mile long,if i keep it at 50 mph its fine ,soon as i accelerate its running on 3 cylinders,then it clears and runs on all 4,theres no jerking nothing,if i stay at 30 in 3rd for a short while it does the same,runs on 3 cylinders then clears up fine,so if im driving about normaly it ok,soon as im on a straight and just touching the throttle and keeping it at a comfortable speed it plays up??i wish it would miss all the time,because soon as i pull over to check it out its runing on all 4,no smoke,new leads plugs,points coil,fuel filter,i hope this makes some sort of sence,i was wondering if a tappets got tight?as there is no jerkyness just very smoothly starts to run on 3,when it comes back to running on 4 you have a job to notice it sort off comes back gradually.

Top up the oil in the carburettor dash pot. Its there to damp the movement of the carb piston, maybe its momentarily flooding the engine when you accelerate.

Sounds as though it's running very weak - but could be several other things - check if the vacuum advance is working ok ? Your 'light throttle' running should be pulling very high vacuum and maximum vacuum advance. If the diaphragm is bust - it will be a) not advancing as it should and b) drawing in a small amount of air (= weak mixture) - as you open the throttle the amount of air will become insignificant and so it runs 'ok'. just a possible theory!

I had one do this once. Compression checked out OK. After much faffing with ignition & carb eventually took the head off. One exceedingly worn valve guide. The valve rocked from side to side but at certain RPM started to stick. Fine at other speeds. Must be something to do with sympathetic vibration / harmonics / or just sod's law. New guide & valve and all was well. Check other avenues first though.