Soft Seize?

I got my '72 Commando on the road and it has 350 miles on the rebuild. 3 times now under a good load and in hot conditions it has lost power and then completely stopped when I pulled in the clutch. Each time I just let the clutch out and it restarted and I went on at a slower speed. I has never smoked or acted like the little incident did any damage but it is bothersome. The last time I was going up a hill at about 65 mph with a fair bit of throttle dialed in and it started to clatter. I pulled in the clutch and the engine stopped. I coasted for a bit and let the clutch out and it started and I went on my way. Each time it has been quite hot. The first two it was over 100 degrees out and this last time it was only about 80 but I had maintained 70 mph for a while before going up the hill.

Any body else have anything like this happen? Am I just flogging it too hard too soon? The engine is over all very quiet compared to what I am used to on a Norton. The oil pressure is staying up at about 47 at highway speed when hot. Pressure drops to just over 20 psi at idle when hot. Valvoline racing 50wt for break in.
Dan.

The usual 2 questions asked about these sort of things is

- have you checked that the oil is circulating, ie is it going back to the oil tank,
and getting warm.

- what pistons are in it, and what clearance were they bored to ?

Dan,

I think you are going to have to give us a lot more information.

LIKE:

What type of carburation, Amals, Mikuni?

What exactly did you have rebuilt on the engine?

Are you running with points or some kind of electronic ignition?

What kind of shape is your electronic in? original? replace any wiring?

just a quick thought, without knowing more the motor suddenly dying is most often an electrical failure to the plugs, like maybe an intermittent ground wire is bad, are your plugs new and what kind are they, are you plug wires new, how about your coils?

how old is your battery, is it fully charged?

this does not seem not carburation as much as it does electrical, sudden dying of power

Either someone has fitted the pistons with insufficient clearance or you are running too much spark advance or too lean a mixture.

If there is a chance of saving the pistons and bore you will need to tear it down now and remove the burned aluminum from the siezure. The burned aluminum is aluminum oxide -just like sandpaper and will destroy the bore when it is run if it has not already done so. Sorry for the bad news. Jim

sorry,

I did not read anything in your post about you fitting new pistons, having lean carb mixture, etc

is this a continuation from another thread where you gave all those details already?

1up3down said:

sorry,

I did not read anything in your post about you fitting new pistons, having lean carb mixture, etc

is this a continuation from another thread where you gave all those details already?

Click to expand...

Do you have another explanation why a freshly rebuilt engine with 350 miles would tighten up and then come loose after cooling for a few moments? Jim

The other thread was " '72 going back together." Sorry I didn't mention that. Jim would have been familiar with it since I ordered some parts from him etc. The pistons were cast ones that have been mentioned as good on here before. I believe they are manufactured by Emgo. The closest clearance I can find in a Norton manual is .0033 which is arrived at by the smallest bore size and the largest piston size allowable. Quality control allowed pistons to be up to .001 smaller than that and the factory bore was allowed to be larger as well so only the tightest engines would have had the .0033 clearance. I think mine was somewhat tighter than that but the ring gap came out perfect with no filing except a very small amount on one ring.

motorson said:

so only the tightest engines would have had the .0033 clearance. I think mine was somewhat tighter than that .

Click to expand...

Thats pretty tight.
Think we may have our answer.

Think you need to check what it should be.
And have a look for any damage, as already suggested.

Where would this tight bore clearance be expected to grab the piston most, at the top or bottom of travel? Appreciate picturing the soft piston Al tack welding into sapphire hard grains. Ugh. Reading of others disappointments with more delaying heavy duty tasks helps me take my own less personal.

I fit the Emgo pistons at .0045 to .0047. Any less and they will sieze. Jim

In this instance how should the clearance be gained, off the bores or the pistons? Good time to think Bore Tech too. Spend then spend some more.

' Tightening Up ' would be the term ??

Imediately after it stops , will it turn on the kickstart - easilly as usual - warm ? ?

sounds like slide cut outs or main jets - maybe bigger mains for run in . Wots yr altitude .

Do the fuel hoses flow freely ? ? no nasty undersize bore fuel line fittings or suchlike ? ? .

personally prefer to NOT use over half throttle - continuous - before 1000 miles , & never on stop .
only intermittant & not held till 2000 , and then only briefly .

Some turkey wecked his 3rd nissan turbo the other day - rattle rattle rattle - young tyke all keyed up
and likely bootfulls & boost on New Motor - somethig'd ' picked up ' .

The ancient ' run in ' proceedure was to ensure nothing ' PICKED UP ' , heat cycle - light load - first 50 miles .
and progressively proceeding henceforth .

With Good oil , it can be possable to ' thermally get interfearance ' without ' pick up ' which'll produce particulates ,
of metal fileings , in english - if spelt wright . Have had a pommy car sans water in rush hour highway use -
imediately after stopping the thing almost turned - then wouldnt . 45 minutes before you could put a hand on it .
( Did Know , someone alongside had said ' theres steam coming out of your bonnet ' " I know , its steam powered "
The Point Being , with the moreys in the valvoline - It DID NOT ' pick up ' . Cooled down & refreshed - it wasa fine .
apart from the ignition which was still shot - the stupid electric cut out in the carb - and the rings , which were de tempered .

It takes a fair bit of heat to detemper modern piston rings .If anything has ' picked up ' it will rattle & / or produce metal filings ( not powder ) .
Actually sounds to me like you may have to much advance , check its 28 deg. and maybe bigger mains - wotteryr slide cut outs ??

oopsie , theyre ermgos . :?

that apart , heat tends to expand the crown , so the top expands , though remember the wire bound anti expansion pistons . Sounds like you need to hone to manufacturers clearance , unless its winter in Alaska .

Never bore under factory set oversize, unless you know you are using different pistons,always add the oversize and check what the clearance would be, it should be within factory specs, if not find out why. Unfortunately pistons normally shrink when they pick up, but on the bright side you should have enough to hone out and clean the bores up, give it max clearance if its got sleeves as the heat transfer will be even less. If your skirts haven't collapsed hit the seize marks with a coarse file, hone the bores to get the correct clearance, check your circlips are tight, some are to small, new rings and have another go.

Have recently had exactly the same thing happen with my 350 Matchless, with the exception that it did not 'clatter' and so might not be the same cause. It cut out after about a mile at wide throttle opening, or sooner if pulling up a hill. Felt like a seized engine, but no tightness when kickstarted again. Would sometimes start if coasted for a short while and clutch let out again.
Turned out to be fuel starvation because of an airlock in the pipework to the monobloc carb. despite the fact that I had both taps open. Fuel supply could keep pace with use at moderate speeds, but not when pulling harder when float bowl got gradually depleted. Same thing would happen with restriction in supply from blocked taps, filter etc..
Of course this did not cause a clattering from the engine, so above might all be irrelevant to your problem!

Did you do the engine assembly yourself? If so what piston ring end gap did you set/check?

Or someone else rebuild, and you're the operator only?

Not so sure its seizing. He says it stops then he lets the clutch out and it turns right over again. He describes clatter then stopping. I'm thinking running out of fuel, pinging, the just plain stopping. If it was seizing it would not clatter just before stopping, quite the opposite.

With the tight piston clearance it is most likely nipping up in the bores, however one other possibility is that one or more valves are sticking in the guides. Years ago I had a bike that did this after a rebuild. The clatter from the valve train was quite loud, perhaps that is what you are hearing?

Glen

Hm, as mentioned already, my 2 Combats will run out of fuel in about 30 sec of Hi throttle with factory fuel taps unless I open both of them. Tank air vent blocked is famous for intervals of shut down then fine a few sec later, so is a big ole bug or a rag dropped in to entertain you. Not that big a deal to pop the top to know what to do next. I've also had boyah trigger wire insulation get soft with heat and fully part the already fractured conductors then cool down and let start again, in cycles. Too bad can just order up x-ray and lab tests to get a clue w/o surgery.

Hobot brings up a good point, prior to disassembly, perform a compression test, then again with spoonfull of oil added. You may need this peice of the puzzle later.

A borescope (now cheap at HF) would also help you decide what to do next. http://m.harborfreight.com/catalogsearc ... gle.com%2F

Rohan said:

motorson said:

so only the tightest engines would have had the .0033 clearance. I think mine was somewhat tighter than that .

Click to expand...

Thats pretty tight.
Think we may have our answer.

Think you need to check what it should be.
And have a look for any damage, as already suggested.

Click to expand...

Pay attention to Mr Rohan. Tight clearance and seizures are not a coincidence.