Needle jet behaviour has changed over time

[Swarfcut]

Pete.. You got me thinking tight valves and this is a definite possibility. Mosin had a problem back in February with pushrods which turned out to be tight in the guide inlet valves. Scott contributed to this thread.

 The drawback with giving a diagnosis of all these problems is that the omission of just a single relevant fact sends us off in the wrong direction. The initial post would indicate hot running, and I don't know if the motor is running on straight unleaded or a lead/octane booster mix.

 Scott. Before making a start on the magneto, have a check on the forum, looking at valve guides and sticking valves. Some scenarios are very similar to your experience. Blocking the tank oil return with a finger for a few  moments will give the rockers and valve gear a bit more oil..see if that helps next time it acts up. You could also try some oil in the fuel to act as upper cylinder a lubricant.

 Swarfy.

[Scott and Jay]

Thanks for your further input, Swarf, Peter and Iron (earlier Julian, about petrol taps too). So, more info:  It runs just 95 unleaded, but I do add upper engine lubricant. It had K-liners put in the inlet and exhaust valve guides. This was about 4 years ago. You are meant to be able to run these a bit tighter. The clearance we made them was: 2.5 thou " exhaust; 1.5 thou " inlet. The valve springs were all tested. This has been fine ever since. The rocker feed bolts are clear and there is a good oil feed. I checked them a couple of months ago when I renewed my head gasket. The tappets are at 14 thou " exhaust and 12 thou " inlet. I looked at some "sticky valve" threads. They said you might be able to see a sticky valve - by looking at it as you turn the engine over with the tappet cover off.
 The trouble with my symptom is the bike runs fine - then it loses power at 60 mph - if you slow down to 50 it's ok again. The other thread I looked at was a long one about "650..problem child.." He had eliminated electrics (by trying a new mag and electronic ignitions). The sticky valve was causing it to cut out altogether, particularly after a lot of throttle and heating up. I think mine would have this behavior, still, when cold. I can't get to any high-speed road, though - without riding a bit of distance. It's not as if my problem is being caused by overheating. The "hot running" comment was because of light coloring in the cylinder head, and plugs. It's as if the hot running is a by-product of what is causing the loss of power at 60. As you can see above, I think I've eliminated the fuel supply/carburation explanation.Those other threads were also about bikes that had just been rebuild or had been sitting idle for a long time. Mine has been running fine. This loss of power at 60 started, in March - before I noticed the leaky head gasket and replaced that. If you can accept this, I would rather eliminate the mag first - before the sticky valve, because that's easier. To eliminate the valve I would really have to pull the head again, which I've just done...

[muskrat]

G'day Scott.
If your going to look at the sparkles side i'd check the auto advance first. The only way to check when the motor is running is to use a timing light and mark the cush nut and wire pointer from a cover screw hole. Bringing the revs up slowly may show a sticky point.
Cheers

[Scott and Jay]

Oh yeah, ok - thanks, indeed, Muskrat. I don't suppose you can do that from the timing side, then?

[duTch]

 Without going back for a look, have you tried changing the plugs ?

[Scott and Jay]

Yes, new plugs

[muskrat]

G'day Scott.
You could but there will be oil going everywhere and only for a very short time.
Both mine have permanent timing marks on a disc behind the cush nut. Only loose a cup full of oil in the primary and can let it run for 10 minutes (chain lube will take it longer) if you need to.
Just look for a nice even increase in advance with no stops or steps by increasing revs slowly. Get Jay to work the throttle while you look at the strobe on the mark.
Cheers

[RDfella]

As I understand it, and if I have remembered the posts correctly, this only happens at 60mph, regardless of whether you approach it from below or dropping down from 70.
To my mind this would rule out electrics. Sounds to me like overheating, but what’s causing it? Are the plugs the right heat range? Maybe they could be overheating but cooling again with a different mixture at a different throttle opening.
Then there’s this ticking noise. Sounds suspiciously like a wide tappet. That wouldn’t cause the symptoms, unless it was a valve sticking as it got hot (though that should surely be worse at higher speeds). Those new valve guide inserts are bronze if my memory serves correctly (though it may not at my age). Bronze guides require more lubrication than cast iron – is lube ‘upstairs’ sufficient?
So we’re down to possibly three possibilities! Engine overheating caused by timing out at those revs (although advance should be ‘all in’ by then), valve sticking when engine warms up or a weak mixture at that particular throttle opening. Presumably the petrol tank air vent is clear, though that would be more time-related than speed-related. I think the mag itself is an unlikely culprit, unless something is loose and bobbing about at a certain frequency. Talking of which, 60mph is about the speed these engines start vibrating. Could the fuel in the carb be upset at that speed? More questions than answers I’m afraid.

[chaterlea25]

Hi All,
I have seen similar symptoms on my brothers SR
It eventually  turned out to be the mag slipring breaking down  at 60 odd mph

John

[muskrat]

G'day RD.
You mention harmonics (frequency). That is also feasible. It may happen at the same rev range in the lower gears.
Scott, can you try it at about 50 in 3rd? Have you done a plug chop at the trouble spot?
Cheers

[Scott and Jay]

Thanks for these further thoughts, everyone. I actually took the mag and auto-advance unit out to our engineer, John. The auto-advance looks fine from a visual inspection. He can test the  mag, and he knows of similar cases to what you mentioned, Chater.. Failing that we'll be back to the other suggestions. It's easier for me to work from the timing side, that's why i just went ahead and took the whole mag out. It did happen once in 3rd, Muskrat. I haven't done a plug chop straight after, yet. There's upper engine lubricant in the petrol, RD - and good oil feed. It may take a while to report back, further, thanks...

[Scott and Jay]

I’d like to keep you helpful people informed. I have put the mag back in but it will take a while now, to get round to timing and eventually test ride. John ran the mag for a few hours at all
Revs - it’s fine. But that was after he fixed something wrong with the points cover that he observed. The nut around the pickup for the cutout looked to have been rubbing on the points retaining spring. He put an O ring at thr bottom of the points cover thread. What i might have had (if anything) might have dropped out when we were drying it before the National Rally. There was also too big a washer under the said pickup
 Nut / which he replaced. This fault, supposedly, “could have cut in at a certain rev range”. I needed to eliminate all
The more likely sounding petrol/air possibilities, with your help, though - before the deeper dive, into magneto. Hope this proves to be the answer...

[mikeb]

keep posting the updates - i hope you get it sorted.
this weekend my solution to fixing my spluttering a10 was to push it to the back of the shed and take the b33.

[RDfella]

S&J – the sub-optimal findings of your engineer may just be the secret. Let’s hope so. Amazing how relatively minor issues can create so much grief.

[Scott and Jay]

Thanks for your replies, guys. But, unfortunately, I have to report a negative result - after correcting this apparent mag fault, and thoroughly testing it. It went brilliantly (after I changed the HT leads over) - for 3k, at a steady 60 mph. Then it just died and spluttered, again. It doesn't happen when I'm accelerating and it is on the change from slide to needle - range (1/4 throttle). Then it's fine when I go down to 50. The bike runs beautifully in every other sense.  I'm inclined to try your suggestion, Klaus - about slide choice. By a "smaller cutaway" do you mean I need to try a #3 slide, or a #4 (mine is currently 3 1/2)? John and I had ruled out such carb settings, because they hadn't changed since it was new, after which it ran perfectly for 4k miles. But could that change over time?  Otherwise, the options are more work. The auto-advance looks fine, it's got marks to show it's opening up, and snaps back fine. John said I could wire it up and ride it. The testing with a timing light might have to happen, but maybe, later. As for "sticky valve" - I mentioned the K-liners and John said they would either run fine, or seize up. Pulling the head again is even harder