Low compression high leak down test

[rsuarez]

Hello everyone, I am new to the forum... here is my intro for those interested viewtopic.php?f=108&t=9373.

This bike is new to me, it seems to be in good shape and most importantly the engine has been barely touched.

It feels strong considering the age of the bike ant its only apparent problems are strong rattling from clutch and several oil leaks from studs and base gasket.

Before tearing down the top end for gasket replacement I did a compression test to check if anything else would be needed. To my surprise found compressions ranging from 105 to 80 psi. Consider that the test was done at 8,850 ft of altitude which represents a 25% compression loss. Therefore the targeted compression was 125 psi.

Confused by the discrepancy from the reading versus the engine performance I decided to do a leak down test on cylinder 6 which was the one with lower compression. 80 was its reading. Surprisingly the test showed only a 5% loss through the rings.

Someone knows how this is can be?

Thanks in advance.

Ramon

[whatscooking]

I have a tractor that I bought used. Looked good ran good but smoked. Ran a compression test on it and came in low. Tore it down to find the oil rings gummed up. Re rung it and honed it went back together. Rechecked the compression only to find little improvement. Got another compression tester. Compression was where it needed to be all along.

[daves79x]

Good advice, test with a couple of different testers. Are you testing the engine warm or cold? Those reading are not uncommon, but since you are tearing apart anyway, not a bad idea to hone and re-ring and lap the valves and install new valve stem seals. Not a huge job or expense.

Dave

[rsuarez]

Thanks.

I am on it already. Pulled the head and cylinder and found all reasonably well. A little bit of blow by on 2,4 and 5th piston and this on the cylinder wall. It is barely noticeable with the finger. I hope it can be removed with the hone.



Surprisingly, I found the head to be loose, being this the main cause for the leaks.


Best.

[daves79x]

You'll not likely get all the rust marks out by honing, at least all the more you should go with stock bore. However, I'd hone all six and re-ring - you'll have good compression, just a little blow-by in that cylinder that may be noticed as a wee bit of smoke. This is assuming you touch up the valves and install new stem seals as well.

Dave

[daves79x]

I've used these ring sets with good results, if you're interested: http://www.ebay.com/itm/6x-CBX-1000-STD ... a2&vxp=mtr

Dave

[patzx12]

A car guy I know always hones the standard bores and replaces the pistons and rings rather than just the rings. He reckons the ring grooves wear on high mileage engines.

[rsuarez]

Hi guys,

Well I did all the stuff, rings, honing, and when I was about to start assembly I found that one conrod was very tight on the big end. Damn! Take the engine out, turn it over and this is what I found.



Its an original bearing which I think its never been changed. The only thing particular about this cylinder is that the conrod has an "1" measurement while all the other are "3".

Any ideas what caused it? BTW, The crankshaft is in perfect condition.

Thanks.

[daves79x]

I'm not sure what you are showing us there in the photo. The bearing half-shell should be tight in the rod and it looks not to be in the photo. Main thing is now you should get the correct bearings using the shop manual chart. Mixing 1 and 3 rods in an engine is OK, but not the letter codes (A,B,C). Letter codes are weight codes and you don't mix them. Numbers are just the exact big-end bore size for the rod and you use that to calculate what color bearing to use. You can probably still see the color splash on the edge of the bearings you have there and see if that matches with what should be there. I'd Plastigauge all main and rod bearings now that you have it apart.

Dave

[rsuarez]

Hi Dave. Yes, thats the point, this bearing came out of the bike and it seems to be wrapped. And I say wrapped because if I extend it over the conrod bed it covers perfectly one half. Any idea of what could have caused it?

Best,

[daves79x]

OK - something's wrong here. If the bearing fits perfectly on the crank journal without forcing, and it sits in the rod as you show - then that's the wrong rod or the wrong bearing . Measure the ID of the big end of the rod and compare it to the manual's specs. A '1' code CBX rod big end should be right at 39.00mm ID. If it is not - then it's the wrong rod and I would have no clue as to how it got there. If the rod measures 39.00mm, then measure the crank journal. Depending on the code for that journal (found on the crank end) it should be between 39.98 and 39.99mm. If that also measures correctly, then the bearing is way wrong and is from something else. That's all it could be. Depending on the bearing color (can you see that on the edge?) the bearings range in thickness from 1.49-1.50mm. Take another rod off and compare if you question what you measure. Keep us posted.

Dave

[rsuarez]

The thing is that it doesn't fit the crank neither. The way this came assembled was that by torquing the conrod the bearing opened to fit the crank, therefore the conrod is tight while pivoting. I will measure again but it's a 1 with a B for a green bearing.

[daves79x]

Ok - so the bearing does have to be forced down onto the crank journal? The bearing must be tweaked somehow. Just verify all measurements and that a green bearing is correct for that journal (cross-referencing the crank and rod codes) and get a new set of bearings for that one.

Dave

[EMS]

It looks like the bearing has the correct circumferential length for the conrod. If it had the correct radius and would be for some other crank/rod, it would have to be shorter. It is most likely the correct bearing and has been bent.
That's my guess.

[Jeff Bennetts]

The thing is that it doesn't fit the crank neither. The way this came assembled was that by torquing the conrod the bearing opened to fit the crank, therefore the conrod is tight while pivoting. I will measure again but it's a 1 with a B for a green bearing.

Let us know what you find out, I'm very curious how this could happen.