TOPIC:

I was hoping someone could clarify what could be wrong with my timing. I have a 83 GPZ 750 which I finally got going last summer. It ran great but at the end of the summer, I was cruising along nice and easy then a sudden loss of power and rattling/ but running engine. Checked compression and none in #2. I recently removed the head and a valve broke at #2 cylinder and really beat up the top of the head there then the broken piece of valve lodged into the top of it.. So that is shot. And I am currently trying to find a head for this bike....
But my question has to do with this: I have the crankshaft T mark (the T next to the F1.4 as in the clymer book) lined up with the indicator mark as illustrated in posts here and in the book, but 1 and 4 pistons are definitely not at the top of their stroke, nor are 2-3 either. 1 and 4 are at the top at about a 1/4 turn of the shaft prior to T lining up with the mark. I am assuming that 1 and 4 should be at the top when the T is lined up with the mark? - The head is off now so I can clearly see where the pistons are. And I sure don't want to attempt to put a head back on until I know I will have the engine where it should be to reinstall the cams...
Could anyone tell me what would throw this off? I don't want to proceed with anything else until I know what I am dealing with.
Thanks!
Brett

There is a roll pin that holds the ignition rotor onto the end of the crank shaft. Maybe that roll pin has sheared off. That is very common in a crash situation, but not a normal failure under most circumstances.

I believe the 750 is a one-piece crank like the 650 and 550, so I don't think there is a twist issue that you occasionally find on the big motors.

Another possibility is that the timing rotor was replaced at some point with one from another model. If your bike is a Zx750, then it came with electronic-advance. The rotor will be a one-piece rotor and might have the number 1069 stamped on it somewhere. It's possible to retrofit an older ignition onto the motor, even from a different model size. A 550 advancer should fit and most/all advancers from Kz650's and Kz750's would fit. This could lead to the timing marks being in the wrong positions. If you can post photos of the ignition rotor and pickup area, maybe someone can identify if you have the correct one.

Looks more like a connecting rod is bend or twisted.
You have to pull the cylinder for a closer look for more damages.

Will a bent connecting rod alter the timing of TDC? I wouldn't think so. A bent CR is just a shorter CR, no?

You a right loudhvx but everything is possible without a deeper look into the internal engine parts.
Some pics of the damaged engine parts are very helpfull

The fact that the 1 and 4 pistons are both off the mark makes it unlikely to be a rod. If only one of them was off the mark that would make sense, but for both to be off that would mean two rods bent by the same amount.

I would inspect the ignition rotor before pulling the block.

Irish Yobbo wrote: The fact that the 1 and 4 pistons are both off the mark makes it unlikely to be a rod. If only one of them was off the mark that would make sense, but for both to be off that would mean two rods bent by the same amount.

I would inspect the ignition rotor before pulling the block.

Any time the head is removed the cylinder block should be too or the probability is high that a base gasket leak may occur.

Thanks for all the responses. It helps clarify things as I could not find in the book what keeps that rotor on in one particular orientation. I will take loudhvx's advice and try to get the rotor off and see what I can find out. It appears to be electronic ignition. I just didn't want to mess anything up more but it doesn't sound like I will. - Except for getting that bolt loose that holds the rotor on - Any recommendations or tricks given the head is off and trying to loosen that bolt just turns the timing gear at the bottom while the chain wants to get pinched in there. I know I took one of these off years ago on a kz550 but at the time the engine was all together and providing resistance. But as I have the engine now, it seems like I would have to pull up hard on the time chain to make it act like a chain whip while I try to get the rotor bolt loose. Or is that a bad idea?
Thanks again!

There should be a big nut-shaped fixture on the rotor in addition to a slightly smaller bolt head.
The larger nut-shape is what you use to turn the crank or hold the crank.

You hold the nut with a big wrench while turning the bolt. The bolt is just a normal bolt.

Here's the rotor you should be looking at.

You were right Loudhvx. That pin was sheered off so the rotor was spinning at times. And thanks for the explanation of the rotor- mine was so corroded that it looked like the bigger nut was separate from the rotor which was what made it seem difficult. I got the rotor off then lined it up as it should where the pin was and now TDC does jive with where the pistons are for TDC. Next is to decide how far to go with this engine rebuild- As you can see the head got torn up, as did the top of the piston. I am not sure if any metal got down low.
I've notice it is hard to find a used head for one of these. But there are a few bikes around craigslist that are beat up but "running" while mine looks great, but is not running.... Maybe it would be smarter to swap out the whole engine?

That is some pretty bad looking damage.

Maybe someone here can chime in on the possibility of welding up the gouges and re-grinding the dome in the head.
But I suppose if you were going to do all that, you could use a standard Kz head and up the compression on that. That way you won't have to worry about a hidden crack somewhere.

I forget if the Zx550 motor had higher compression than the standard Kz. In the 550 world, the Gpz's had 10:1 where all the other Kz550's had 9.5:1.

loudhvx wrote: That is some pretty bad looking damage.

Hold my coffee.

A XS1100 yamaha I bought for parts. The valve head broke off somewhere out of state, and they kept riding it home. Beats walking:


There wasn't much to lose at this point. Beats walking: