Gsxr 750 1990 compression test results advice?

[Jpich85]

I’m getting the bike ready for powder coat and thought I’d check the compression of the cylinders before the strip down incase I would need to sort the engine out too before reinstalling it, 

i warmed to the engine and check the compression they were pretty low so added a little oil to tech cylinder and redone the tests, I got..

cylinder 1 72psi and 110 after the oil

Cylinder 2 80psi and 120 after the oil

 cylinder 3 80psi and 120 after the oil

 cylinder 4 75psi and 125 after the oil

 I’m not good with engines but from what I’ve seen in the manual that’s pretty low so I’m guessing I’ll need to do a top end rebuild?

i just wanted to check with guys and see what you thought I’d need to do and what else is worth doing while I have the top end stripped for a potential rebuild?

 I enjoy welding and machining but engines scare me but I’m keen to give it a try if I need to?
 

The bike starts fine and it had I think 28k on the clock, I’ve been building this bike over two years now so if it does need a rebuild then that’s fine as I’m in no rush lol

 Cheers

[Jpich85]

I’ve been having a look through the searches and found that due to lack of use the rings can get gummed up and maybe try soaking the cylinders may help, I just wanted to ask what’s best to use to soak the cylinders? Would just petrol be ok? Would I have to drop the oil first?

[Gixer1460]

Its really better if barrels are off and pistons are removed so they can be stood in a solvent / lubricant (Diesel usually works) and the rings spun in their grooves before re-aligning and rebuilding. If done insitu, no guarantee the diesel will get to all of the rings / grooves (unless you used LOADS) so you are back to square one! Of course your rings could be shot, in which case they'll have to be replaced regardless?

[Jpich85]

Thanks mate, I appreciate your advice, I might just give it a try and see if it helps then if it does improve I’ll have to put the rings on the to do list lol, would you leave the oil in or drain it out before I tried adding the oil with the pistons in situ?

[Gixer1460]

Any diesel / petrol / solvent will eventually get past the rings due to the gaps so will contaminate the oil - i'd drop it personally and if fairly new it can be re-used after you do the ring thing! Also try and get the front of the engine tilted up so the piston crowns will be parallel to the floor - this will ensure the solvent / lubricant will flow 'round the edges' If you use a lot of fluid - don't spin the engine over with the starter - it could hydro lock and bend a rod! Just rock the crank back and forth using the nut on the ign. rotor -you are trying to get the rings to move up and down in their grooves!

Edited August 17, 2020 by Gixer1460

[Swiss Toni]

If you have to strip the motor, because of seized rings, as 1460 says, steep the crowns in solvent/diesel. If still stuck, put meths in the crowns, and ignite. Usually frees them off. When free, remove rings carefully (one piston at a time, to save mixing up), soak some string in petrol and using the ‘Diablo’ method, clean the ring lands out. Refit if serviceable. 

Edited August 17, 2020 by Swiss Toni

[Jpich85]

Cheers guys I appreciate your advice, I think I’ll give that a try first and see if it makes any difference, I would like to try and rebuild the top end as it’ll be something new to learn and hopefully I don’t get it wrong lol, if I do rebuild the top end could you tell me what bits will need replacing so I can roughly price it up,

 

You’d need gaskets and piston rings but would I need to buy need head studs and nuts? or can I leave them in the engine if I’m just removing the head? 
any advice would be great

 cheers

[Gixer1460]

Top end includes barrels off (to do piston rings) so you'd need, base gasket and seals / o rings, drain tube o rings (not strictly necessary but guaranteed to leak if you don't), Head gasket and exhaust gaskets. That's the bare minimum - studs and nuts are fine for re-use. A top end gasket set may be cheaper and will give you additional stuff for a full refurb ie. valve stem seals etc.

All can be done with engine in frame but is a damn sight easier if motor is out and on a bench at comfortable height. Once barrels off - stuff rags ' towels into crankcase bores before attempting piston gudgeon pin clip removal - notorious for pinging off into the distance or dropping into open crankcases!

[Jpich85]

Thanks 1460 that’s great info there, I thought I’d have to replace studs and have heard there hard to remove and are a little expensive, I think I will do this as it’ll be good to give it a try and learn another part of the bike plus it’ll feel nice hopefully when I ride it that I built the top end and it actually works, hopefully lol.

 You mentioned about hydro lock? How would I know if I’ve bent a rod?

 When I was compression testing i put the plugs back in on cylinders that I’d tested and I’m worried now that when I wet tested them I could of caused some damage by adding the oil then putting the plug back in lol

[Swiss Toni]

I had a certain 6 cylinder bike from Kansas. Notorious for hydraulic lock on #1 pot. 45deg. bend in rod. Which I wasn’t told about! It still turned over. Worse case scenario is the bent rod could break the bottom of the liner. More money! I doubt you will have put enough oil in the cylinder to cause this problem. We’re talking about a substantial amount of fuel/oil to hydraulic lock. Stop worrying! 

[Jpich85]

Thanks Swiss I am a bit of a worrier lol but I guess it’s all a learning curve but some lessons are more expensive than others. Thanks for your advice