Poor running/misfire when warming up

[IHNR]

Hi all,

My rotary has developed some odd symptoms and I wondered if anyone else had seen this.

When I start the bike it runs fine for a minute or two and then seems to intermittently drop the revs down as though one rotor is not running right. Once fully warm it is OK again. I have also noticed that opening out the bike a little harder in the first few miles of riding it causes a bad, clattering misfire. This misfire goes away when the bike has been run a dozen miles or more.

I remember the idling issue a couple of times previously. I fixed it by changing the plug once and on another occasion the bike was put into storage at the end of the year with oil in the rotor housings – there was no problem when I started the bike the following year. As I’ve tried the plugs I suspect a sticking rotor seal. I remember being told at Shenstone back in the day that it was sometimes possible to free stuck seals by leaving the engine soaking with oil in the rotor housings, which may explain why it went away previously…

Bike had been running fine prior to this issue.

Anyone met this before and if so how did you resolve the issue?

Thanks!

[Ian Mc]

When the engine reaches around 60 degrees, the ignition unit retards the ignition below around 2000 rpm to control the tickover so it sounds like the problem is on the fuel side, try balancing the cabs at tickover. A full carb strip down and clean will be the best option. Oil accumulating in the carb is usually the cause of rough running. Look for air leaks on rhe vacuum pipe to the fuel tap, it can soften with age, likewise the oil scavenge pipe which drains the frame. Check the oil pump setting. Use NGK C7E plugs.

[IHNR]

Thanks Ian Mc. I did a full carb strip just over 4K miles ago and replaced parts as per the SU carb rebuild kit - needle valve, jet, seals, etc. This was about 6 years ago and of course modern fuel is not what it used to be. Carbs were balanced when I did the strip and then checked by Richard Negus, who confirmed I'd set them pretty well. I have re-checked the balance about three times since then with no obvious issues, but can check again. I discovered that the oil pump setting was rich a few rides before the problem started showing itself. This has been corrected now. Fuel pipe to the tap seems fine and was also replaced 6 years ago. I will check the scavenge pipe. I guess the oil accumulates on the top half of the carb, presumably causing the piston movement to be a bit sticky, so I'll check that out as well. The misfire under power sounds pretty loud and aggressive - I'm just wondering how carb issues could cause this unless it is the carb piston. I was running with C7E plugs when the problem started. I swapped to new C8E plugs but it made no difference. I also plan to check the transfer ports to see if there is any carbon in them or the frame - if there is a problem with seals carboning up I know you can sometimes see carbon in the ports or frame...

[Ian Mc]

Try unscrewing the plug caps from the HT leads to check if you have a good connection. Check the bolt where all the negatives are fixed to the frame near the regulator. Do you have full voltage output from the alternator and regulator?

[IHNR]

Thanks again Ian Mc. When I took the pistons and piston chambers off the carbs I saw that there was some light surface gum (no surprise) over the whole piston body, but there was also a more serious brown stain on the inlet side. The only thing I could think of is that this was carbon and it certainly looked like it. If so it could only have got there if the seals were not properly sealing the chamber... I therefore got an analogue pressure sensor, put it in the plug hole of one of the rotors, connected it to a cheap digital oscilloscope I have and cranked the engine. The resulting trace showed one rotor had one face reading about 40 psi below the others. I think I've found the smoking gun and will now strip the engine - it looks to me that a seal is sticking, but I won't know for sure until I strip it. Since the engine has been bathed in oil for a month or two, I can't see it freeing itself now... Based on my rather "make do" approach I am impressed with how good a diagnostic tool a digital compression test can be and have now ordered a digital rotary compression tester from someone in Australia and plan to run a check every few months in the future so I minimise the risk of damaging the engine from a problem that is not really showing it's affects yet...