Mysterious (to me) power drop

[RichardL]

Just putting this in the carburetion topic because I'm not really sure yet what the problem is. Haven't checked it out yet because I'm thinking about it over a pint at the local (sorta local).

My intention was to post a report of what a great ride I had today, and it was, right up to and after the mysterious incident. I had been riding surface streets for about 25 miles, frequently at 60 MPH and occasionally touching 70. Then I got on the interstate and tried doing 70 for about a half mile. That's when I started noticing a serious dropout of power and I thought she was going to die. I pulled over to ride on the shoulder to nurse her to the next exit. Riding on the shoulder and the rest of the way here (the local) smooth running with no bad noises or vibrations. Back to cruising at 60.

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?

Richard L.

[metalflake11]

I'd go down the fuel starvation road too, you have described symptoms I had a few years ago to perfection.

[Bsareg]

Fuel tank breather hole?

[trevinoz]

Was it starting to nip up?
Those are the symptoms, loss of power and tightening of the engine.
If it happens after prolonged high speed running, you may be a bit lean in the carby at that position.

[RichardL]

Thanks for the toughts, guys. I just pulled the plugs and found left plug and cylinder wet with oil. Compression test on that cylinder reads about 25 lb. lower than right, but I'm sure it was firing all the way home because running on just one would have been obvious.

Under normal circumstances I could see where nipping up would be suspected, but the last time I fixed a nip-up, piston clearance was over spec and I considered not even going with it (being already at +60s). 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, or maybe it's just now gotten bad enough to matter. On the other hand, maybe a guide has loosened up.

Thoughts, again?

Richard L.

[trevinoz]

Hi Richard, why I suspect nipping is recently I had the same problem with my Norton. I was on a new motorway and sat on 70 mph for some time when the engine started to tighten.
I immediately pulled the clutch and the engine cut out but I was rolling at a good pace and quickly let the clutch out and the engine started with no further problems but I have since raised the needles by one notch. I haven't repeated the high speed run yet.
By the way, this engine was rebuilt in 1994 and had never tightened before this.

[RichardL]

I'll check compression  again tomorrow. If the results are the same I think I must, at least, pull the head to get a better look. I try to avoid too many compression tests in  a row in order to.preseve the knee, which seems like it might  have finite time left.

Richard L.

[ralph]

possible seizing was my first thought while reading your post.

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

Hopefully you discover a fuel issue

[Triton Thrasher]

Sounds a bit like a partial seizure, doesn't it?

Overgeared?

Too-weak needle position?

Mixture bias?

Ignition timing difference between cylinders?

[chaterlea25]

HI Richard,
Have you checked for tight valve clearance?
Remember to hold the throttle full open when doing a compression test

John

[worntorn]

Hi Richard
A leakdown tester will give you more info than a compression tester without hurting your knee. Leakdown testers can be had for $50-80.
I have a good compression tester here but haven't used it since getting the leakdown tester. Same as you, my right knee is showing signs of wear.

Glen

[Clive54bsa]

Richard, try the easiest thing first, open both pet-cocks.
I had a similar experience after treating my Super Rocket to a new MonoBloc, no misfiring, no back firing, just got slower and slower on the flat and I had to shift in 3rd on any slight hill. Plug color was good, so I opened the reserve side and it went away, or just coincidence.

[RichardL]

Unfortunately, you seizure enthusiasts are very convincing. I'm not the type to log every task and numerical detail, so I've been trying to remember which of the two bores was less oversized as of the last assembly. I can't.

Looking at TT's list, regarding overgearing, not much choice but to be in 4th at 60 or 70, but it was level ground. I suppose the equation changes with the weight of the rider, if so,  I would be an equation changer. As for the other things on TT's list, all I can say is "why not" and "maybe so". Not having tried 70 at length before, I can't rule those out (presuming seizing is actually the problem).

Clive, yes, while crying in my beer, I did consider petcocks as a possible cause of fuel limitation. If I had thought about it at the moment of the incident I still wouldn't have experimented for fear of making things worse and/or being stuck on the interstate.

Thanks, yet again, for all your input. I won't be getting to findings for a couple of days, due to holiday events.

Richsrd L.

[Triton Thrasher]

Yes, 4th gear is normal at over 60 mph, if not on a significant uphill section, but sometimes people alter their sprockets, to create a higher than normal 4th.

[muskrat]

G'day Richard.
Does sound like a partial nip up to me as well.
My other thought is tappet clearances closing up and propping a valve open. Usually only a problem with alloy heads but it is surprising how much more heat is generated at 70 as opposed to 60.

A few years ago my 51 A7 nipped up so I gave it a quick hone, new rings and put the bad piston in the good bore and the good piston in the bad bore. That's a few thousand miles ago and did 130 miles on Friday at 60-70mph.
Cheers