Suggestions please...

[bmcecosse]

Yes - could have been partial fuel starvation, thanks for the suggestion (which I had considered as a possibility myself) - and it's certainly possible the last person to top up the oil (few weeks ago) is to blame for the open oil cap. I'll be having a word with him about it............

[Roni]

Is this a "lean misfire"? At the particular conditions the engine was operating in at the time the needle, at its position, might have been providing a leaner mixture than needed. Knowing if this was true would require the temporary fitting of an air/fuel ratio gauge ( http://www.nightrider.com/biketech/diy_ ... _probe.htm ) and recreating the conditions. Another guide might be to pull the choke out a bit and if it gets better it was too lean.

[bmcecosse]

Good point there - I never thought to try pulling the choke. I haven't even had a chance to look at the plugs so far (which are 'hard' N6Y so shouldn't have been the problem). But I do run the engine weak for best economy......

[moggiethouable]

The saga continues apace [frame][/frame]

[IslipMinor]

Excellent!!

[bmcecosse]

Excellent indeed.......I obviously knew this thread would attract some 'interest' - but you have all surpassed my wildest expectations...

[plastic_orange]

Is the tank venting properly? - as a blocked vent can cause a partial vacuum resulting in a lean mixture - or an interuption in fuel supply.


PO

[bmcecosse]

Aye - wondered about that too - but it ran fine once it got 'over the top' and I was able to ease back and let it settle into the cruise at 60 mph.

[moggiethouable]

[frame][/frame]

[bmcecosse]

Fantastic........how do you do it????

[moggiethouable]

Fantastic........how do you do it????

Very kind................... alcohol.

[bmcecosse]

Finally got a few minutes of dry to have a look at it. Plugs looked fine, 3 and 4 slightly large gap. Valve gaps - #2 exhaust was a bit tight at 15 thou (ok at 12 thou) , the others ok at 15. I hadn't checked these for a long time - so it's good to know there is no real sign of any recession - despite no additives etc. Inlets all fine too - decided to open exhausts to 18 thou and inlets to 15 thou. No - it doesn't sound 'tappety' Also retarded the ignition timing very slightly - just to be sure. Took it out for a thrash - seems to be going well enough, but not able to replicate the long/hard/fully laden slog on the uphill Mway! Couldn't spot the compression tester anywhere handy - and a large G &T was beckoning......and anyway it had started to drizzle again......
I doubt it was a sticking valve - the (940) head is far from new and the valve/guide fit not exactly minimal..... Possibly it was a valve gap closed up with the heat - but I think best bet is probably the weak running at high load -either from needle shape or fuel pump insufficiency. If it happens again I will try easing out the 'choke' as suggested earlier. Thanks for all the interest folks !