Hi everyone, newbie and then some, to the site and sport bikes. I just got a hold of a 1989 750, I'm guessing it's a ZXR750, since it's green and has one exhaust pipe off the passenger (right) side. If anyone can give me a quick howto to tell which model it is, it'd be appreciated. Anyway, it doesn't run, and that's why I got it. Here's what I know. The carbs are clean, fuel pump works, new gas, new plugs, it's got blue spark and it turns over fine. I checked compression and my gauge said roughly 30 on all 4 cylinders. I was surprised to see all of them so bad, so I thought my gauge was bad, checked it on a nearby lawn mower and it said 125. I don't have a manual and I'm hoping to hold off on buying one until I know it's worth fixing, so i checked timing with my best guess. Removed the crankshaft cover on the passenger's side and turned the number 1 plug to t.d.c., the mark on the indicator pointed straight up, removed the valve cover and the IN- and -EX on the camshaft gears pointed towards eachother. That seemed to be correct. All the valves open and close and no wear is showing on either cam or the head. Anyone have any ideas on where to go next? I'm lost and am far past my motorcycle knowledge. Sorry for the long post.

 

no worries on a long post.. its better to be detailed for idiots like myself that can't think off only a few hints..

try a wet test on that.. could be a blown gasket or stuck/bent valve.

a wet test will only register a (significant) difference with rings. if theres no difference, this will indicate a head gasket or valve leak.

your timing is correct. the little lines inscribed on the sprockets should be level with the top of the head.

you certainly sound like you know your way around an internal combustion engine.. if you're used to cars, throubleshooting bike motors is no different..

i'd be inclined to remove the head and inspect the valve seats for contamination and scoring, as well as the cylinder bore. (after that wet test)
feeler gauge and mic the bore to check runout and locate a tolerance if possible. ring gap should equal about .004" for every 1" of bore diameter (so a 3" bore should have about a 12 thousandths gap)

radial and axial play on tha valves should only be a fraction of a thou i think..maybe pushing a few tenths.. MM increments are way smaller than standard .001" measurements..

withthe head removed (if you elect to do this) i'd gauge out the deck and head woth a straightedge and feeler gauges as well.. i think it's probably rings, but it might be an easy fix like a gasket..
i hope so, man!! riding seasons here, and we gotta get you rollin!

keep us posted, and welcome to KF!!

 

I'll do the wet test this afternoon when I get home. If I decided to take off the head, do you just removed the gears from the camshafts and then slide the cams out and let the chain hang on the lower gear? Since I still have no book I want to make sure I don't have to guess and guess wrong. Thanks for the advice, hopefully it won't be too long before I'm typing with my eyes in the rears of their sockets. What's the easiest way to tell which model I have? All the plastic is over at the former owner's house, all I have is the bike with green wheels. Thanks again. Jared

 

Here's the update: Sprayed Deep Creep (penetrating oil) in the cylinders at noon, at 5pm pumped a few good sized pumps of oil in each cylinder and rechecked the compression. Cylinder 1 - 70psi, cylinder 2 - 65, cylinder 3 - 60 and cylinder 4 -120. The dry numbers were 35, 30, 15 and 35. Does that sound like a head gasked/warped head? Keep in mind the bike hasn't run in 4 or 5 years. Thanks, jared

 

Haven't had any time recently, finally bought a manual on ebay today, should be tearing into her soon. Hopefully the fix will be relatively cheap.

 

Keep us in the loop!

 

Well, I finally got a minute to lend towards the bike. Pistons are scratched up a little on the exhaust side and you can also see scratching the cylinder walls, but I've seen much worse run ok in a car. Head gasket looked fine and the valves all look fine and none of the rings were stuck. I have zero experience in motorcycle engines, so I'm looking for advice. I'm wondering it anyone thinks that scoring would explain the lack of compression and, if so, can this engine be bored andhow much, and which replacement pistons does anyone recommend. Thanks, Jared

 

If your looking at a 89 Ninja 750, then its a ZX-7 H1 just like mine, there was no R model for that year to my knowledge. The #4 cylinder on the wet test is your tell, a jump from 35 to 120 shows that the low compression in that cylinder was due only to dry cylinder walls from not being run, and that mechanically the cylinder is good, or at least better than the others. Since the timing is good, I would look for something common in the 1-3 cylinders for the compression issue. Usually if its just a gasket it will only effect 1 or 2 cylinders, but effecting three may mean a warp. Get a straight edge and a feeler gauge and check the head corner to corner and end to end, and make sure its within tolerances. keep us updated.

 

Nicely done Prom

 

The student has become the master... lol