Mysterious (to me) power drop

[RichardL]

The low compression and oil on plug, a result of the rings now sticking in the grooves after nipping up?

Ralph,

Nothing apart yet, but I've been thinking about this. I think cylinder wall scoring more likely to be a source of leaks than rings sticking. Unfortunately, this seems all too likely for my situation.

Richard L.

 

[Klaus]

I had this phenomen with the engine in my Scrambler. The engine run fine but with a little more work against wind, or climbing long hills my enginge begun to nipp. I have done tousands of miles with this engine and have checked all. Ignition, valve clearence, carb set up, but everthing was fine. A failed plug thread oblige to lift the cylinderhead, and the result was a splited piston.
This was not the first time I had a splited piston with the engine like to nipp at harder work, so the piston heating up.
So have a look at your pistons.


cheers Klaus

[JulianS]

My A10 suffered a similar power drop last year, feeling like a partial seizure.

Found no marks on piston or bore but did find a partial obstruction in the oil pipe from tank to engine. New pipe problem gone.

Worth checking the oil flows freely.

[ralph]

With no slap that I could hear, I've just accepted that burning some oil is something to live with.   The  looseness of the pistons may be why I'm seeing oil and having nothing to do with the present mystery


Richard, based off your comment above, I'm now thinking if your pistons are loose, and poss allowing oil to get up top to burn, they are so much less likely to seize. One that caught me out was after id adjusted the rear brake and hadn't really allowed for the way my foot touches on the brake pedal - blistered the paint on the hub. However,given you are burning oil, you prob want/need to investigate, but I'd look all the easier options first.

[RichardL]

Started bike this morning and ran another compression test right after. Compression on both sides even and very good (me thinks). Yes, John, I did forget to open the throttle for the first test. 

So, I'm back to hoping for a fuel issue, though I know nipping is not fully ruled out.

Here's a video of how it ran this morning. https://youtu.be/D4qYNJldQfA

I think recording in the garage accentuates clatter (due to acoustical reflections). It sounded better in person.

Richard L.

[trevinoz]

I still think it was nipping, Richard.
Probably due to leanness at that throttle position causing excess heat.
Why not lift the needle by one notch and go for another 70 MPH run?

[RichardL]

Trev,

If I run the test of lifting the needle one notch,  and it nips up a second time, do I risk doing damage I didn't do before?  Another question, if it's actually inadequate supply to the bowl (or even both) how would I know based on the test ride?

Just want to say that I'm very appreciative of your comments.

Richard L.

[trevinoz]

Richard, I understand your caution because of the expensive consequences if you make the wrong decision.
You could try a plug "chop" which may give you some idea.
Maybe look at all of your carby settings, jets etc.
Make friends with someone with a dynamometer. 
My bike years ago had a propensity for seizing due to a tight bore so it was a case of riding with the left hand poised around the clutch lever.
This was due to me believing the piston clearance given in the service sheets for the RGS.

[orabanda]

Richard,
Check for kinked (flattened) fuel line (restriction in the line).
Was it a hot day? Maybe go up one size in the main jet. At worst, it will be a bit rich; at best it won't seize!.

Richard

[RichardL]

Richard,

It was just under 80F.

I skipped using the main jet that came with the concentric kit because it was smaller than the existing. That said, I've dumbly forgotten what size that was.

I just went and bought some fittings  to make an external-tube float level viewer. I figure that's another thing that needs checking in the fuel realm.

Really tempted to try the plug chop as a new trick, and I might get to that, but I figure if I start by running rich then get to fine tuning.(I think.)

Richard L.

[Clive54bsa]

Richard, if you have an alloy head, before the "plug chop" run, pull both plugs and liberally apply anti-seize on the plug threads. Years ago I did a plug chop, and stripped the threads right out of the head, by not letting the plug cool down before I removed it.
Good luck with it.

[RichardL]

I hope I'm not underestimating the value of the plug chop, but I'm putting it after determining if I've got a starvation problem. If no starvation, I'll run real rich for a test, THEN, if no nipping, I can use the plug chop for fine tuning the mixture. I think I've got the sequence in a decent order. Feel free to chime in if I've put any carts before the horses idiot.

Richard L. 

[ralph]

So, my guesses are: fuel starvation at speed (concentic); valve float; mag windings sketchy at speed. I'm really leaning (no.pun intended) fuel starvation. Any thoughts or other ideas?


auto advance unit malfunction?
 Just had a look at your video - engine sounds ok with no blue smoke. I'm bearing in mind Mr muskrats tight tappet idea would make sense then see your oil return. Maybe I have a crap oil pump but to say you should have a relatively "dry" sump, it seems to be returning a lot of oil. Could oil for the rockers not be getting there and a valve stem nips up? wouldn't take a lot to check with slack banjo bolts.

nice oil fiter by the way

[muskrat]

G'day Richard.
I wish you had pulled the lead on the left to compare. I know you have to increase the idle speed to run the one plug test but to hear it at proper idle (1/2 the speed it was) would help.
As a side, running a concentric on an alloy head it's hard to get anywhere near the recommended fuel height in the bowl. 0.170 to 0.240" below top of bowl. With the downdraft angle I could only get 0.260", any higher it would flood.
So in saying all that, yours must be on the lean side if it doesn't flood. At 70mph cruise on a flat road you wouldn't be on the main jet. Lift the needle!
Cheers

[jachenbach]

Richard, I'm voting fuel starvation. I re-read the initial post and can't be sure....Is the concentric carb a recent change? If so, my rule of thumb is always go back to the last thing I did and confirm all is well. If not, I was going to suggest checking fuel level when it dies. If you rig a level tube, remember to close petcock immediately when it slows down or it could refill by the time you stop. Another option is to just pull the plug on the bottom of the float bowl and measure the amount of gas when idle and again by the side of the road when it starts acting up. A little messy, but easier than making up a level tube for a concentric.