Hi all,

Could someone please tell me the symptoms of having a failed centre seal.

My bike will start fine and has had a crankshaft OH and top end rebuild less than 1000 kms ago.

My idle is sometimes erratic and will increase from1250 to 1800 rpm. Also can’t seem to gets the exhaust notes in sync.

Any info appreciated.

A bad centre seal normally makes it much harder to start, and sometimes difficult to idle. The revs will also often "hang" if you blip the throttle, or when riding and slowing down to a stop. You'll usually see a significant decrease in bottom-end performance too.

It's very difficult to diagnose without seeing the bike, but if the crank was rebuilt by reputable agent, it sounds more like it could be an air leak. Maybe the inlet manifold, or the crank seal behind the flywheel. You can spray a little carb cleaner in both areas and see if the revs rise, which would indicate a leak, but a proper leak down test is the only real way to be sure. That will also [usually] show up a failing centre seal.

However, you want to check the simple things first: carb balance and air screw settings.

To potentially reduce the number of more generic replies, or maybe prompt replies from members with specific experience, t would help if you put the model in your profile, or stated it in the opening post.
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

Check us out on Facebook! Please "like" our page!

Thanks for your informative reply Andy.

Carried out a leak down test and it wouldn’t hold any pressure at all. Switched the pump from left to right and still the same. Now suspecting my seal has gone.

Watched some Japanese videos on you tube where they pour a mixture of fuel and 2T oil into the left reed inlet or down the left crankcase with the cylinder off to check if it seeps to the other side?

Any issues with doing that?

Pouring a 2T mix down one side isn't a cast iron test, but it will often show a failed seal.

Be aware though that sometimes the seal may leak one way more than the other, may leak under a little pressure, or won't leak at all with a little pressure! Hence why it's not a great test. Just to confuse matters, even a serviceable seal can leak using this test!
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

Check us out on Facebook! Please "like" our page!

Thanks Andy,

Do you mean a used functioning bearing/seal by serviceable?

Do the 88 year separate seals tend to last longer than the integral seals?

As Andy stated, sounds like an air leak, i would certainly be having a close look at the inlet manifold to make sure it's not perished or cracked, i have also seen this sympton on a newly rebuilt engine where the flywheel side seal hadn't seated properly, good luck with it.
_________________
Proud Father of , 05 ktm 400exc supermoto 2018 honda crf rx supermoto

Serviceable = still working.

The 87 & 88 style centre seals certainly seem more robust than the 87-96 integrated seal, in my experience. We've rebuilt 1988 and NF5 cranks (before the T2 pattern seal was available) and reused the original seal many times, including NF5s that have gone on to do the Manx GP and Classic TT without issue. Not saying they are all always good, but most are.

As a note, for anyone maybe reading this later down the line, the 1987 (MC16) centre seal is a different ID to the 1988 MC18RJ, and there's currently no replacement. Again, they tend to be fine to reuse when rebuilding most cranks, but if the seal is damaged/worn, the only [current] solution is to find another crank with a good seal.

*Obviously there's no guarantee when re-using a seal. The client must accept all risk.
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

Check us out on Facebook! Please "like" our page!

My cases leaked on the l/h side and one of my seals didn't. I found the T2 seal worked better than the Honda one. Could be a shelf life issue. My friend Phil used a dabbing method of building up the sealer. We ended up using 2 inlet manifold gaskets and sealant.
_________________
Why do things simply when you can complicate them

Poured petrol/2T mix into the left reed inlet and straight away the right side crankcase filled up to the same level.

Poured the same mix into my spare 88 year engine which wasn’t properly stored/air tight and not a single drop of liquid passed to the other side.

89 year engine down and 88 year back in and motorbike is idling fine with no hanging or increase in idle. Both cylinders warm up while reaching operating temperature and same exhaust notes from both silencers.

Very well done for a maybe 34 year old centre seal.