Valve float?

I have put a couple thousand miles on my 71 750 commando since the rebuild. new pistons 20 over new valves, guides, and springs. I just wanted to test top RPM, so in 2nd gear ran it up. At 6000 RPM it is smooth and pulling strong, at 6400 it cuts out and sounds like it is missing rapidly loosing power. As soon as it drops back to 6000 it starts to run strong again.

I suspect that the valves are floating, not sure as I have never experienced it before. How do you check or know.

And I know like the doctor said " if it hurts when you do that don't do it" After all it did pull the ton, and I don't normally ride that fast. and at 80+ it is great.

If the valves are floating I am not sure what to do to fix it. For now I will just keep it below 6300 RPM

Help or thoughts

Dennis

Be careful, when my bike was new (and I was young) I was going for top speed and a piston hit a valve and made a mess. Evidently the valve stuck open for a microsecond, just long enough for interference.

Jaydee

Dennis C said:

I have put a couple thousand miles on my 71 750 commando since the rebuild. new pistons 20 over new valves, guides, and springs. I just wanted to test top RPM, so in 2nd gear ran it up. At 6000 RPM it is smooth and pulling strong, at 6400 it cuts out and sounds like it is missing rapidly loosing power. As soon as it drops back to 6000 it starts to run strong again.

I suspect that the valves are floating, not sure as I have never experienced it before. How do you check or know.

And I know like the doctor said " if it hurts when you do that don't do it" After all it did pull the ton, and I don't normally ride that fast. and at 80+ it is great.

If the valves are floating I am not sure what to do to fix it. For now I will just keep it below 6300 RPM

Help or thoughts

Dennis

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I doubt that it is valve float. What are you using for springs and cam?
It's more likely fuel or ignition problem. Jim

That sounds/feels/horrifies similar to my 1st Combat 1st week out learn curveballs excitement expecting power band to hold to pass a semi with oncoming to hit bad backfiring noise and power cut about cab rear wheels area till letting off to snick 3rd to get power back and just slip in front of truck in nic of time. My tack was working proper back then and was near on in redzone when valve or point float onset as my 2S Combat really woke up like 3rd piston kicking in ~6800 rpm so expected it to keep on pulling WAY better another 2000 rpm like my ancient backward old school piece of shit only norton parts P!!. On tear down 6 mo later ONLY to fix a dozen+ leaks found thumb nail carb slide missing and chamber peckered up and valve lips and seat bent and chipped but ran fine as I could tell so do not know if was point bonce mis fire, valve float clashing or carb slide processing on almost fatal truck pass but did not see valve impression in piston just the ice pick stabs on every surface. Point being somethng a miss with your valve train not to function properly up to 7000, as my pre-Peel did many times after the over rev valve float event I learned to leave a bit more space for passing. Cam lobes infamous for wearing off so maybe that is a factor in your case, so part of Commando manhood hobby is finding out how lucky do ya feel to continue on before finding out how much an equipped Norton mechanic you are or are not.

Cam is stock that came with the bike. 71 commando roadster.

Springs are new, Not positive that they are Andover, I purchased from the Bonneville shop and they are genuine UK made

GENUINE, U.K. MADE, NORTON COMMANDO ATLAS 88SS ETC. 650 750 850 TWINS VALVE SPRING SET PN# 06-7070 067070


ONE OF THE MANY GOOD THINGS ABOUT NORTON IS THAT THEIR PARTS OVERLAP MANY YEARS !! THIS VALVE SPRING SET FITS ALL THEIR TWINS FROM 1963-75 AND POSSIBLY MORE. CONSISTS OF FOUR (4) INNER SPRINGS (PN#'s 22839 ,NM22839 or 06-2726) AND FOUR (4) OUTER SPRINGS (PN#'s 22838, NM22838 or 06-7822

Ignition is Power ARC from Old Britts

220 main jet

Dennis

If your power loss did not have horrendous noise associated with it, might just be running out of fuel flow from tank or Amal float bowl filling which BSA has a solution for. Wes on his 71 once saved my life but catching and passing Ms Peel going 90s so he held over 110 mph over a few miles to pass and flag me over just before my broke axle tipping tire into mud guard edge leaving THick white smoke clouds behind popped to toss me on head. Point being his old all factory 750 did not lose power holding over 6000 WOT nor should anyone elses.

no horrific noise

Dennis

Dennis C said:

no horrific noise Dennis

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So, good to go......I'd check fuel flow, since you already have a new ignition system in there.

Severe valve float sounds like 8 stroking and feels like a rev limiter kicking in. Well it did to me, I used to set my valves to float at a given RPM specifically to act as a rev limiter (not good practice on a road bike cos it wears out the cam, but on a race bike where the cam is replaced each season, why not!)

But stock Norton valves with stock springs shouldn't float at that rpm. you sure you've not left out the heat insulating washers or similar?

I am sure the washers are in

Like I posted I am not positive it is valve float, but it comes on at exactly the same RPM and it is a sudden not gradual event.

Dennis

Uuuuu a good mystery plot a foot. Abrupt implies electrical but also assure us valve lash holding steady as well as cam chain. Does creeping up on 6400 slowly or quickly feel different? If voltage dropped on an electo ignition that can make their timing go too advanced and feel like hitting end of power band still runs but roughly struggling till backing off some. just guessing for ya.

This has stock twin amals, aircleaner and tweety mufflers. ?
And choke system. ?

Try pulling on a bit of choke.
If it goes better or revs higher, its fuel.
If it cuts out lower rpms, it may be fuel too.
But too much choke it may not like.

That checking the fuel flow - both taps open ? - would be the 1st step.
And checking your valve clearances and spark timing.
hth.

cjandme said:

Dennis C said:

no horrific noise Dennis

Click to expand...

So, good to go......I'd check fuel flow, since you already have a new ignition system in there.

Click to expand...

Tank breather ?

Rohan said:

This has stock twin amals, aircleaner and tweety mufflers. ?
And choke system. ?

Try pulling on a bit of choke.
If it goes better or revs higher, its fuel.
If it cuts out lower rpms, it may be fuel too.
But too much choke it may not like.

That checking the fuel flow - both taps open ? - would be the 1st step.
And checking your valve clearances and spark timing.
hth.

Click to expand...

Stock Amals and box air cleaner with K&N filter

The chokes have been removed

I did try it with both petcocks open

Have not tried sneaking up the attempts have all been full throttle and watch.

Only tried it in 2nd gear nowhere to try third that would be 100 mph and I would rather spend the money on the Norton than the ticket.

Going to call Fred at Old Britts, the power Arc is supposed to work well at low voltage but I had though that the mapping may be off .

Could also be the main jet, but I had thought that that would be a more gradual onset.

Dennis

Dennis C said:

Could also be the main jet, but I had thought that that would be a more gradual onset.

Dennis

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Typically. If it's fuel-flow (or lack thereof), it usually manifests itself as a "flattening out" at wide open, rather than an abrupt, obvious dropoff. That, and you generally feel a surge forward as the throttle is rolled off of WOT, allowing the fuel to catch up with the air flow.

Mine did this same-sounding power drop off when dodging a car while in first gear. It suddenly fell flat on its face as if I'd pulled one of the plug wires. A quick glance down showed close to 7 grand (I didn't take the time to scrutinize the true reading), but a shift to second launched me out of danger. I haven't run it much past 6 grand since. No need to in my case.

Dennis C said:

Could also be the main jet, but I had thought that that would be a more gradual onset.

Dennis

Click to expand...

Not necessarily so. An extreme example from years back when I was ice racing a 2 stroke and the first time out on the ice with a new expansion chamber, when I opened the throttle the bike pulled strong until I went from 3/4 throttle to W/O and it was like the ignition was switched off, total silence. Rolling the throttle back instantly started pulling again. I ended up going up many sizes on the main jet to fix it.

Hm Powerarc has some quirks but next step is put time light on to see if over-under advancing which would stiffle similar to your report. For some reason the designer claims he worked with Norton drag racers that said 36' full advance was their reccomendation which may not apply to any other Nortons I know of. I doubt Old Brits ever sold one with that crazy curve installed but stranger things happen. In one case a fella found bad fit on cam so made a cone adapter but i ain't installed my P-Arc so don't know how'd you'd tell but may mess with curve at some rpm threshold. Don't know how you'd search this fella's part offering up but think I saved address in adapter package.

I have had Peel over suck new Amals feeding but it took a handful of seconds at WOT before power stumbled and just a few seconds let off could go WOT a few more seconds before pooing down again, Both taps open and BSA flow mod allowed endless WOT, which is oxymoron in public. Pop tank cap on the fly just in case to double ceck venting. If piolt screws over 1.5 turn out for best idle fuel level may be a bit low too. Times of water in fuel worked ok till given throttle would lift some the water off bottom to stumble and sometime would suck in all out by momentum other times had to drain bowls and tank. Rare but possible. One time a big winged water bug type monster got in tank and caused intermitent stumble power lost which would slow fuel draw so bug drifted off drain but later would cover it on low feul which is how I saw it flushing around on fuel up. Old trick based on this, drop a small rag in someones tank, quits regularly but starts right up again to tease the snot out of someone.

Does it do it at 6400 in all gears, if so it's not carburetion.

Try hitting that same rpm on a long downhill going into an up hill. If it does it downhill, its not carburetion.

If you want to feel what too lean feels like suddenly yank open the throttle to the stop while rolling slowly in first or second gear and feel the way the power goes away.

A CV carb opens the throttle for you at just the right rate. With a conventional carb you're doing it subconsciously, like counter steering.

My hunch would be a bad connection between the pickup and the black box. A bad points wire, bullet connection, something like that.

I had opposite reaction when a carb-manifold fastener loose or float bowl hanging loose, ran dam good on high throttle but stumbled on let offs but still ran ok with some easy blips and did not discover the reason till gasing up with fuel spilling out or at home poking around and gasket swung down. Hope we learn something soon. Once in a while with power fall offs I reach under and jiggle wires often enough hitting good again, for a while, so at least knew where to look next.

mistry is solved. I spoke with Fred and he sets the map up to start to cut power at 6500 rpm and will not let it go over 6800. I never tried to see what would happen if I stayed on the throttle, rolled it off as soon as it started to miss.

For what it worth he does it to keep it easy on the motor. He can re map if you insist against his advice, and infact sets up the reinforced and built up race bikes to redline at 7500, Long story short it is doing what it is supposed to do

An important point about the Power Arc is that you CAN NOT use a timeing light, it fires 3 times on each strock, so no way to know which light to check.

Dennis