engine seize mystery riddle

mkmachine · Dec 10, 2015

I've got a bit of a puzzler, and I thought maybe I should post it here and see what people think.
I have a 72 combat (bike #1) I bought last year, and over a few months went over just about everything and was getting it together for a vintage ride in June. I put a few hundred miles on it, and was having some issues, so I went had a shop redo the head, replacing everything. I put the new head on and it fired up first kick and ran perfectly.
I put a few hundred more miles on it to shake it down, and had everything working beautiful. The last shakedown ride before the event, it threw a rod. (Much later, I took it apart and the oil pump is in pretty bad shape.)
I have a second combat engine (bike #2), of unknown, but supposedly good condition. I have the receipt for an engine rebuild, but unknown mileage, if any. Three days before the event, I figure what the hell, I'm going for it. So I pull the blown up engine out of bike 1, and pull the engine out of bike 2.
I take the "new" head off bike 1, and the two valves on the thrown cylinder are slightly bent. The guides look good, so I replace the valves and put head 1 on the bottom of engine 2. Engine 2 looks like it's nearly brand new. Minimal carbon, and the bores still have the cross hatching.
I put engine 2 into bike 1, get everything together, and without even touching the carbs, it fires up literally on the first kick, and idles beautifully.
Now the problem: riding it around the block, if I got it warmed up and rode it for any length of time over 3000rpm it would "seize" and lock up the rear wheel. I could pull in the clutch and coast, and if I let out the clutch to try and bump it, it was still locked up. Shutting it off, then I could kick it over and it would start again.
I tested this by just revving it with the clutch in, and it stalled at about 3000rpm.
(So I ended up riding a different bike on the vintage ride.)
My first thought was that the pistons were soft seizing in the bores.
I finally had a chance to take it apart this past weekend. The oil pump is practically new. And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.
I haven't had a chance to pull the rest of it apart, but now I'm stumped.
If the pistons were seizing shouldn't there be wear on the bores?
The problem has something to do with heat and rpms. What else could make an engine seize?
I'm ruling out drivetrain and primary, because I know all of that to be working.
So, before pull the barrels and split the cases, any suggestions for things to look for?

thanks,
mk

mschmitz57 · Dec 10, 2015

"Shutting it off, then I could kick it over and it would start again."

I would think you shouldn't be able to kick it over if the pistons are seized.

acotrel · Dec 10, 2015

If the motor is actually seizing there should be aluminium left in the bores after you pull it apart. If it is merely tightening and stopping, the piston to bore clearance is probably insufficient. The other factor is that the jetting might be too lean. The second motor sounds as though it has been reconditioned. You should probably check all clearances including the big ends by using Plasti-gauge.

Torontonian · Dec 10, 2015

Stumpy stuff. So you pull in the clutch (which disengages the motor) and this frees up a locking rear wheel ? So maybe a transmission issue ? I will have to read again. Or you could reword things better.

texasSlick · Dec 10, 2015

mschmitz57 said:
"Shutting it off, then I could kick it over and it would start again."

I would think you shouldn't be able to kick it over if the pistons are seized.

I picked up on that also. I wonder if mkmachine meant to say "after cooling it off, then I could kick it over and it would start again."

mkmachine wrote: "I tested this by just revving it with the clutch in, and it stalled at about 3000rpm."

If it is a heat issue ... How long did it take holding revs at 3000 before it stalled? How long does it take before you can kick it over again?

Very strange behavior. Usually if pistons seize, there is remarkable damage. I am wondering if there might be some loose part in the primary case that is fouling and jamming the chain. I would expect, if there were such a part, it would rupture the chain, but it might serve you well and save a lot of work, if you had a look in the primary case before looking elsewhere.

The classic transmission issue, the layshaft bearing, will lock up the rear wheel but disengaging the clutch will NOT free it, so I would not look first in the transmission.

Slick

Rohan · Dec 10, 2015

mkmachine said:
I haven't had a chance to pull the rest of it apart, but now I'm stumped.

You need to go right through it, look at EVERY bearing and wear surface,
and find the one that is grabbing or too tight.

seattle##gs · Dec 10, 2015

are the pistons scuffed front and rear?

mkmachine · Dec 10, 2015

It could be kicked over right away.
It's not the transmission or drivetrain.

Engine 2 was obviously recently rebuilt, and it may not have been broken in correctly.
my thinking was along the lines of what acotrel mentions. That the tolerances on the bores to pistons are too tight. But wouldn't that show up on the cylinder walls? They look really nice. Clean with crosshatching.
Is it possible the valve guides were damaged, and the valves on one cylinder are locking up, and the wheel locking up is due to engine compression? or is that crazy talk?

Rohan · Dec 10, 2015

Valves can be installed with not enough clearance, and grab in their guides.
And then get walloped by the piston....

Likewise the gudgeon pin can be too tight.

Folks who build car engines love to build aircooled engines with too tight tolerances - this always needs to be watched for....

mschmitz57 · Dec 10, 2015

Could improperly aligned rotor be rubbing the stator, heating-up and expanding and locking-up?
After it cools it could release and allow the motor to turn again.
I've seen this happen to a friends MK3 and it locked the motor solid. It didn't loosen however.
I'd pull the primary cover before going any further and inspect everything.

concours · Dec 10, 2015

mkmachine said:
I've got a bit of a puzzler, and I thought maybe I should post it here and see what people think.
I have a 72 combat (bike #1) I bought last year, and over a few months went over just about everything and was getting it together for a vintage ride in June. I put a few hundred miles on it, and was having some issues, so I went had a shop redo the head, replacing everything. I put the new head on and it fired up first kick and ran perfectly.
I put a few hundred more miles on it to shake it down, and had everything working beautiful. The last shakedown ride before the event, it threw a rod. (Much later, I took it apart and the oil pump is in pretty bad shape.)
I have a second combat engine (bike #2), of unknown, but supposedly good condition. I have the receipt for an engine rebuild, but unknown mileage, if any. Three days before the event, I figure what the hell, I'm going for it. So I pull the blown up engine out of bike 1, and pull the engine out of bike 2.
I take the "new" head off bike 1, and the two valves on the thrown cylinder are slightly bent. The guides look good, so I replace the valves and put head 1 on the bottom of engine 2. Engine 2 looks like it's nearly brand new. Minimal carbon, and the bores still have the cross hatching.
I put engine 2 into bike 1, get everything together, and without even touching the carbs, it fires up literally on the first kick, and idles beautifully.
Now the problem: riding it around the block, if I got it warmed up and rode it for any length of time over 3000rpm it would "seize" and lock up the rear wheel. I could pull in the clutch and coast, and if I let out the clutch to try and bump it, it was still locked up. Shutting it off, then I could kick it over and it would start again.
I tested this by just revving it with the clutch in, and it stalled at about 3000rpm.
(So I ended up riding a different bike on the vintage ride.)
My first thought was that the pistons were soft seizing in the bores.
I finally had a chance to take it apart this past weekend. The oil pump is practically new. And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.
I haven't had a chance to pull the rest of it apart, but now I'm stumped.
If the pistons were seizing shouldn't there be wear on the bores?
The problem has something to do with heat and rpms. What else could make an engine seize?
I'm ruling out drivetrain and primary, because I know all of that to be working.
So, before pull the barrels and split the cases, any suggestions for things to look for?

thanks,
mk

I read that description as it quits running at 3000 rpm, an attempt to bump start, it erroneously seems "seized", from using too low a gear. Starts with the kicker fine.
I offer that it's not seizing at all. Sounds like a carburetion issue, or electrical.

And yes, if it had seized, there would be marks.

o0norton0o · Dec 10, 2015

concours said:
I read that description as it quits running at 3000 rpm, an attempt to bump start, it erroneously seems "seized", from using too low a gear. Starts with the kicker fine.
I offer that it's not seizing at all. Sounds like a carburetion issue, or electrical.

And yes, if it had seized, there would be marks.

My guess is concours got it right...

if your engine cuts out in 1st gear while you are rolling along, you will not get a bump start by simply releasing the clutch with the key on, unless you bump the gearbox selector to 3rd gear where your rear wheel will have more of a mechanical advantage over your engine. If you just let out the clutch in 1st, your back tire will just skid until you come to a stop...

Dances with Shrapnel · Dec 10, 2015

Accepting mkmachine's seizure descriptions at face value, my first thought is the ring gaps are too tight. Ring seizure is very quick with a very quick recovery time....provided nothing gets messed upwiththe rings and ring lands.

Since this is the second engine you are having trouble with in this rolling chassis, I would also confirm that the oil feed from the oil tank is not obstructed or constricted.

mkmachine · Dec 10, 2015

thanks for all the suggestions.
I guess I'm not very good at bump starting it, I didn't realize I needed to be in a higher gear.
I will offer that if it was a carb issue, the rear wheel wouldn't lock up. As an Amal user, I've had plenty of carb problems. I'm very familiar with the carbs cutting out.

It could possibly be a stator issue. I'll have to check. But I still don't think the stator would lock up the rear wheel in this manner.
Rings are a good place to check too.

worntorn · Dec 10, 2015

I had something similar to this occurring with my 650ss when I first brought it home. That problem turned out to be an intermittent problem due to a bad or corroded internal contact in the ignition switch. First time it happened I thought the engine had seized as it locked the wheel while riding and skidded to a halt. It was also a tight new engine with lots of compression. The bike was in second gear at perhaps 30 miles an hour when the ignition cut out and the wheel locked up.
Once stopped it was clear that the engine wasn't seized as it could be kicked over freely. It even started, then did the same routine again when I drove off.
New ignition switch fixed it.
Glen

rx7171 · Dec 10, 2015

>>And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.<<

If the bores are stock diameter then the pistons if new will not be oversized and should not create a possible piston seize due to too tight a clearance.

Triton Thrasher · Dec 11, 2015

rx7171 said:
>>And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.<<

If the bores are stock diameter then the pistons if new will not be oversized and should not create a possible piston seize due to too tight a clearance.

Eliminate the obvious, before looking for interesting mysterious causes.

A slow speed event that stops a new built engine in such a way as to make the tyre yelp, then lets the engine turn over normally afterwards, is usually a piston seizing in the bore. And it's often caused by a too-tight rebore.

How big is the clearance between lower piston skirt and bore?

I've never seen a piston in bore seizure that didn't leave obvious marks on the front or back side of the guilty piston.

concours · Dec 11, 2015

Another clue to the mystery...
"I tested this by just revving it with the clutch in, and it stalled at about 3000rpm."

From that, I extract:

OP is using "stalled" & "seized" interchangably.

3000 rpm being a cliff to fall off, rather than running it hard, etc.

I wonder if it was immediately re-started with the kicker?

Dances with Shrapnel · Dec 11, 2015

mkmachine said:
Now the problem: riding it around the block, if I got it warmed up and rode it for any length of time over 3000rpm it would "seize" and lock up the rear wheel.

"Lock up the rear wheel" at over 3,000 rpm

Sounds like a seizure to me.

Piston seizure will usually leave a tell tale on the skirt or sometimes the barrel surface whereas ring seizure (if it does not destroy something) will leave no trace or mark and the engine can almost immediately recover as the ring(s) cool.

This is an easy one to check. Check all the rings for appropriate gap.

concours · Dec 11, 2015

Dances with Shrapnel said:
mkmachine said:

Now the problem: riding it around the block, if I got it warmed up and rode it for any length of time over 3000rpm it would "seize" and lock up the rear wheel.

Click to expand...

"Lock up the rear wheel" at over 3,000 rpm

Sounds like a seizure to me.

Piston seizure will usually leave a tell tale on the skirt or sometimes the barrel surface whereas ring seizure (if it does not destroy something) will leave no trace or mark and the engine can almost immediately recover as the ring(s) cool.

This is an easy one to check. Check all the rings for appropriate gap.

http://m.harborfreight.com/digital-insp ... 61839.html

If your time is worth anything, ninety bucks will show the scuff/score from jammed rings.

engine seize mystery riddle

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