I put my engine back together today and leak tested it with the following results;
The left/upper cylinder holds 8psi for at least 10 minutes with no noticeable drop in pressure shown on the gauge.
The right/lower cylinder however, shows an immediate drop as soon as pressure is put in. It settles at about 2psi but after 15 minutes reads virtually zero on the gauge.
I kept topping the pressure up and spraying my special Rocol leak detector spray around possible leak points. If there's even the slightest gas leak, this stuff will find it. No external leaks were found though.
I next plugged both inlets and pressurized the right/lower cylinder again to see if cross leakage would eventually fill the other cylinder and hold pressure once the whole engine was on 8psi.
It did indeed. After topping up a few times the other side of the engine eventually filled to 8psi after which the gauge held pressure for 15 minutes with no significant drop.
So air seems to be able to pass from the right hand side of the engine to the left through the center crank seal. BUT it cannot pass from left to right which i don't understand at all.
I found a comment in an old post from a member called jeffco which read;
"Whenever I do a primary leak check on a twostroke, I do a vacuum check as well as pressure. I have had crank seals that would seal positively (100%) on the pressure check, but would leak terribly when a vacuum was applied. (-5lbs) Just something to keep in mind."
This sounds like the same sort of thing.
Does anyone know what's involved in fixing the problem?
Is it a guaranteed seizure if i were to ignore it?
Thanks for any help gentlemen. _________________ After years of moaning about immigrants now i am one...
Well, i have the same problem thats why it seizured again and again....
If air leaks through halves take that crank out!
I sprayed some carb spray in the one inlet and then blanked it.After that i filled with air the other inlet.finally i unblanked the first inlet and saw lots of bubbles....
NSR-LOVER, thankyou for the reply. I found a post of yours from a few months ago asking for information on crank rebuilds - did you have any success in getting your crank repaired? _________________ After years of moaning about immigrants now i am one...
I collected some infos from dexter and michalisen from greece cause here seems everyone is busy telling their opinion...i havent rebuilted yet i am waiting for the bearings to start.
The crank can be rebuilted with new bearings but im not sure about the rods since honda doesnt sell new.
if your broblem is only the bearings it seems that it can be fixed with new ones and of course good job from the person who will fit the bearings and succesfully assemble the crank.
Nah the bearings on my crank are all virtually brand new. The engine has only done 115km since the last full rebuild.
The bike ended up being sat in storage for a year and a half during my move to New Zealand. I found a leak on the crank case joint where presumably fuel vapour in the crank cases had (over time) eaten through the Blue Hylomar i used to join them. So i split the engine and rebuilt with new left and right side crank seals but this time i used Threebond 1207B to join to crank case halves.
I re-tested the re-built engine for leaks only to find (what looks to be) cross leakage as described above. But i really don't know enough about the subject to be sure and i've not found any answers in any of the posts i've looked back through using the forum search function. I've been searching 'crank seals'.
NSR-LOVER, do you have a new center crank seal to fit when you rebuild your crank?
I've no idea what the center seal looks like or how it fits - if anyone has a picture or could describe it that would be helpful. _________________ After years of moaning about immigrants now i am one...
The outer is just a thin o-ring sitting in a groove on the outside of the bearing.
People have successfully replaced it, by carefully expanding it over the crank.
Where they got it, I don't know
Considering your previous "case joining" history, are you sure the cases seal in the middle _________________ Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom
Last edited by pbekkerh on Tue Oct 06, 2009 1:56 pm; edited 1 time in total
Center crank seal is very similar to a normal deep groove ball bearing rubber contact seal. The difference is that it has a spring like an oil seal on it's inner lip and this actually seals on the crank web, not the inner race of the bearing.
I do know that people here in Bodge-land have actually fixed these things. I won't go into their methods as the last bloke I spoke to who was bleeting on about how easy the fix was then went and seized it
The o-ring on the outer race is a service part and is available. This seals the outside of the bearing, not the inside.
Yes, care needed when fitting.
**Top Tip**
Don't try to fit in on a frosty day in the shed or you'll snap it for sure! Roll it between your hands to warm it up, then it should go on without any problems. Also a bit of grease on it before fitting the crank can stop it from getting nipped by the case halve during assembly.
pbekkerh wrote:Considering your previous "case joining" history, are you sure the cases seal in the middle
The left side of the crank case holds 8psi 100% for atleast 10 minutes with no noticeable drop. Its definately not a problem with the crankcase joint since i re-made it with Threebond 1207B.
The crank case joint was good when the engine was first put back together in 2006 and the engine ran very strongly. Its only recently that i've been trying to get the bike back out on track since moving to NZ and problems have arisen. After a couple of little test rides i found the front of the engine covered in sticky dirt and then noticed a trail of fuel leaking from behind the flywheel. I decided to do a leak down test and that's when i found the leak on the crank case joint. The flywheel side crank seal was also letting air past.
It seems that over time fuel has dissolved its way through the Blue Hylomar in the crankcase joint and the crank seals have been damaged by the weight of the crank during the bike's storage.
Since the rebuild a couple of weeks ago all external leaks have been cured. I'm just left with this curious cross leakage which only lets air pass from right to left through the center seal (presumably). _________________ After years of moaning about immigrants now i am one...
Found this old thread in a search, along with another thread where Hermit mentioned 40mm PVC waste pipe fits into the inlet manifold. Cheers Hermit, fits like a glove.
So I made a leak tester. First I tried removing the mufflers and plugging the exhaust there. It was a little optimistic I know. The exhaust flange didn't leak a drop of oil, but wouldn't hold any pressure.
Then I removed the spannys and plugged the exhaust port/manifold and yay, air tight seal in the engine.
First test:
Cylinder A=9psi
Cylinder B=0psi
1 hour later:
Cylinder A=9psi
Cylinder B=0psi
Nice.
Test two:
Cylinder A=0psi
Cylinder B=8psi
1 hour later:
Cylinder A=2psi
Cylinder B=6psi
Another hour passes:
Cylinder A=2psi
Cylinder B=6psi
So I have a tiny leak one way between cylinders.
Not cool what so ever, but not life threatening if it doesn't get any worse.
I have decided to do another 50 odd kms and check it again.
I have a Koso dual EGT attached now which already paid for itself the other day.
I changed main jets, when reattaching the air box I accidentally folded over one of the carb to air box rubbers and it was leaking air. Didn't notice until I rode it and saw the EGT doing weird stuff. Throttled right back, nursed her home and found the problem.
Last edited by {.bLanK}GoD on Mon Dec 06, 2010 1:39 pm; edited 1 time in total
Worth getting in there is the question? On the one hand, a leak is a leak, on the other, piston rings (for example) will leak a little past, and that's not a problem.
I vaguely remember writing something about how the problem was resolved but just in case:
I ended up just running it (after a mechanic's suggestion) and having a ride around the valley for like 20 minutes, came back in, put a test on and the leak was completely gone. Everything sealed 100%.
All i can conclude is that something seated itself properly and thus the problem solved itself. Happily. LOLZ.
This same scenario has happened again since. The very same things happened: crank went back in, i put a leak test on, found the one way cross leakage (in the same direction), fully assembled and ran the bike for a short time, re-tested for leaks and found none.
Luckily the issue is consistent and fixes itself! _________________ After years of moaning about immigrants now i am one...
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