Low compression on my fave bike. :( Dammit.

bonester · Post by **bonester** » Sat Dec 23, 2006 5:52 pm

Had a bit of a miss in my GT since my Melbourne trip. Put a new set of plugs in it today and it is running better. I did a compression test however while I was changing plugs and I wish I hadn't. Got 1000kpa across three of the cylinders and 600kpa on number three. Put oil in and the compression jumped to equal the others. This points to ring sealing I guess.

I put rings in it about 15000-20000km ago and hoped that would be the last of it. I do flog the little bastard but don't abuse it much.
I'm wondering if the rings in #3 have stuck or frozen in the lands? I cleaned the lands out with a broken ring which at the time seemed a rough way around the problem. It scratched and gouged the flat surface of the lands- would this cause blowby or the rings to seize? #3 is the piston that previously gummed up its rings and the piston skirt touched the bore- wonder if it is doing that again. Can't hear it this time though.
I really don't want to spend any more $$$$$$$ on this engine. Got a spare but I'm not sure of it's condition. I read somewhere on the net of stuff called Marvel Mystery oil, which is supposed to free stuck rings. Is there an Aussie equivalent or some other bodgy method of trying to unseize my (suspected) frozen rings?
I had a ride of MattBs GT550 when I was in Melbourne and his seemed much nicer off idle- maybe his has more even compression.
Any ideas? Thanks!

Felix · Post by **Felix** » Sat Dec 23, 2006 5:58 pm

If they're stuck because the metal is scratched or something like that then I doubt any oil would free them...

Ummm...to be devils advocate...is it really that big a deal?

bonester · Post by **bonester** » Sat Dec 23, 2006 6:07 pm

Only a big deal coz I'm fussy, and like riding long distance on the old shitter. 4000km in a week, week before last. Maybe I should get some more of your 'SBR', Felix!

Gosling1 · Post by **Gosling1** » Sat Dec 23, 2006 6:16 pm

mmmm, sounds like a quick hone and another set of o/s rings might be needed....

its a pain, but no other way to get the compression back without the expense of new pistons.

Cleaning the ring-lands with a broken ring is OK, as long as you don't scratch the shit out of them.....

it pays to soak the piston-crown in something like 'Penetrene' overnight before digging out the old carbon deposits........even if you just take out the #3 piston, give it a good soaking, and bung it back in, this may be enough to free the rings up (at least its a quick and cheap*ish* solution ?

I would go for a hone and o/s rings.....this will fix it for another 50k at least...

bonester · Post by **bonester** » Sat Dec 23, 2006 6:29 pm

Do you reckon I could wear out a set of rings(or one piston's rings) in 15000-20000km, Gos? I did a ring and hone job then. I guess I'm only hugely concerned if I have BROKEN a ring in #3. Otherwise I'd be keen to bodgy it for the next few months. Got lotsa higher priority work bills for the next few months.

Whaddabout glazed bore in #3?

Gosling1 · Post by **Gosling1** » Sat Dec 23, 2006 6:40 pm

bonester wrote:....Whaddabout glazed bore in #3?

thats definitely possible.........I reckon the ring-lands have just carboned up again, especially in the scratched spots .....this can cause ring-sticking.....

15-20k ?

thats not much on its own, but I know how quickly you have racked up those k's, and what was behind your bike for most of them !!!!

I still can't believe you have that sort of set-up on a 550 !!!

I think if you had a broken ring in #3, then the comp. wouldn't have risen so quickly with just a bit of oil poured down the plughole.....it wouldn't have risen at all.....

time to pop the head off and have a Captain Cook down the bore mate, its the only way of knowing *for sure*...

bonester · Post by **bonester** » Sat Dec 23, 2006 6:48 pm

Cheers mate!

One thing I should mention too is that the rough running seems intermittent and at lowish revs- 3k-4k.

Johnnie5 · Post by **Johnnie5** » Sat Dec 23, 2006 7:04 pm

its either a stuck or cracked ring for the drop like that , the others have decent compression

lets face it ,do it in a day , and a bottle of aviation cement

pull the heads and barrels off, check it out and go from there

worst case drop the spare motor in or maybe worth putting it in anyway and seeing what its like

do i hear a bbq happening at your place ?

Felix · Post by **Felix** » Sat Dec 23, 2006 7:11 pm

The rough running may indicate carbs more than compression...hard to say without stripping everything down. If you are going to go that far you may as well just build up engine #2 and plonk it in one weekend...

My old mechanic used to have SBR in 44Gal Drums

He wouldn't think much of your engine being low on one cyl...he'd only really do anything if most of the pistons were below 75% of their rating...or vastly uneven. He had plenty of stories of old holdens effectively running on 4 or 5 cyls for years...only finally succumbing to body rust to get them off the road...

It is sorta hard to recommend doing anything...you either put up with it and keep an eye on things, or you strip down and get ready to spend time and $$$.