Possible centre seal failure?

[MCN.]

Took the X5 for a thrash today, ended up getting a lift home sponsored by the RAC

Was accelarating down a slip road when the engine died and wouldn't restart.

Roadside fiddling revealed the following -

Fuel flowing and float chambers full.

Signs of spark.

Examining in more detail when i got home I discovered the following -

Reasonable looking sparK, however plugs are not getting soaked in fuel despite lots of cranking over with the electric boot. The carbs are full of fuel.

Inducing fuel manually, via plug hole / airbox, produces some chuntering but it won't start.

The nature of the failure being so sudden made me think ignition but now i'm not so sure, i'm starting to think it's eaten the centre crank seal hence why it's not pulling fuel through.

Any other silly old bastards with 2t experience care to share their thoughts?

Ta

[Pete.]

Possible but more likely that you holed a piston.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[stirlinggaz]

Hi,
Seeing as you've been through the basics of spark & fuel, what about compression?
Takes 5mins to test.
then I'd be whipping the head off to see what damage I had done while thrashing it

cheers,
GAZ

[MCN.]

[MCN.]

[stirlinggaz]

[quote="NKVD fan]
It didn't seize, and there is enough blast out the plug holes to lead me to think the pistons are functional enough, but a compression test wouldn't show a leaking crank seal anyway.[/quote]

Hi,
yes im aware compression test wouldnt rule out a failed crank seal but why would you presume it is the crank seal thats at fault, before you have even ran through the basics?

Like I said it takes 5mins to test compression, properly. Fuel, Spark then compression.
if you have them all THEN its time to dig a bit further.

thats the normal sequence for me. After fuel, spark & compression I would start at the top end & work down.

cheers,
GAZ

[MCN.]

Quelle surprise. Compression is 110 psi, 115 other, i.e. spot on.

It's eaten a crank seal, theres no pull through the carbs when it's cranked over.

[stirlinggaz]

[MCN.]

I've never checked compression, although being that it's a new block and pistons i assume it's ok, remember it's in a fairly soft state of tune.

It is the oddest 2t breakdown ever, and i've managed to break them in most ways over the years!

[bypass2]

float heights or floats damaged or floats sticking letting to much fuel in

[MCN.]

[bypass2]

[quote="NKVD fan"]

[ZebraDriver]

Snapped woodruf key - sparks happening at wrong time

[MCN.]

[Pete.]

Block the inlet, block the exhaust, run a leakdown. You'll soon know if that seal is gone.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[stevo as b4]

Would you do the leak down on one pot though and listen for air escaping out of the other pipe or plug hole?

The only other thing I can think of if Blue isn't 100% certain it's a crank seal failure on the centre main, would be a possible weak spark/ or coil breaking down under load, (i.e it plugs spark out of the bike but not under the compression pressure?

It's funny for a coil to suddenly break down though is it not?

How many miles have you done on it now, and was the crank re-built when restored?

Unless the bike had been dry stored for a year or more after the re-build with no oil used during assembly, I can't see how a new or sound crank seal would suddenly go like that?

The usual story with stored dried out strokers is that they often start and run ok when first re-commissioned, but after a few miles the crank starts tearing up the dry/perished seals and you lose crank compression or get an air leak and runaway revs.

As you said though if it wasn't a blocked carb/fuel supply fault then there are few other things that would cause a twin to stop running on both cylinders?

[Pete.]

[MCN.]

The engine was on the bench for a few years after it was rebuilt before i looked at the rest of the bike. One thing that stands out is that we split the engine again after the bike was finished to investigate the noisy gearbox, and my mate had a suspicion the centre seal was skewed in the cases slightly (there is no positive location, cheap suzuki shit design). The bike has done 700 miles since it was MOT'd last year.

[Pete.]

No surprise then if the seal is cocked the rotation of the crank will tend to pull the lip sideways. Add a bit of case pressure and it might have turned the lip over.

It wouldn't be that hard to add some kind of stop for the seal to rest against.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[MCN.]

Quick update as I've stripped it this morning.

https://i5.photobucket.com/albums/y164/BLUEX5/Mobile%20Uploads/image_zpsfnyxoi50.jpg~original

Crank seal is floating around in four bits.

Looks like my diagnosis was bang on as per usual. Now time to rebuild with new crank and casings.

[Vincent]

[Major Doss]

I reckon OP's main seal is shagged. Well done on tearing it down/identifying.

[MCN.]

[stinkwheel]

I didn't see this at the time it was first posted but yeah, a really frustrating and difficult to diagnose fault you only get on multi-cylinder 2-strokes.

I've had it before on Jawas. They have a bolt-together mechanical centre seal around the centre main that has to be heat-fitted on reassembly.

Usually comes on more gradually. A reluctance to start, sometimes starting by introducing fuel or bumping for a faster turnover then ok once they're running. Gradually gets worse and worse until they won't start at all.

You check the engine and everything seems ok with the fuelling, spark, air supply and compression.

As the OP found, the main signs are a) Dry plugs despite monumentally long periods kicking over and even after introducing fuel directly through the inlet port. and b) Almost total lack of vacuum through the carb(s).

The latter being the clincher, you would normally hear a distinct sucking induction noise with the airbox rubbers removed and it should such the palm of your hand against the choke of the carb. Fit balancer guages and they'll read nil or next to nil. Engine just swapping the same charge of air back and forth between the two sides.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stevo as b4]

Well done and well diagnosed to Blue anyway!

I suppose this is one of the few engine faults on a twin that could cause both cylinder's not to run too?

What a bummer though on a 700mile old engine, having to strip a nicely re-built bike down for one rubber seal right the in the deepest depths of the motor.