rotax 787 will not crank with plugs in

[Chr1sfrench]

Ok here is my problem im rebuilding a 787 from the ground up and i can not get it to turn over with the plugs in. It cranks easy by hand and easily with no plugs in. I know its not hydro locked bc i just put a new crank in. the motor is currently sitting in the hull not connected to the drive shaft so that can not be binding. i know its not the starter selonoid bc i already swapped it with another one. no change. I have also tried 3 different starters that all work on my other seadoos. No change. also tried 3 different batteries. I thought maybe my head was milled so i swapped with another head and still no change. I eliminated it being and bad wired like a bad ground or bad power cable by trying to jump the starter directing to a NON running car battery and it still wont crank. Cylinders are good. Pistons are good. Piston rings are good. lower end is good.

The only thing i can possibly think of is too much squish and i have too much compression. i have a number 5 gasket on now. When i check my compression im getting 160 in both cylinders. ill add some pictures of my set up.

 

[1of500]

Bad cables or starter

 

[Coastiejoe]

Is the starter an OEM or aftermarket?

 

[Chr1sfrench]

I tried two oem and 1 aftermarket.

 

[Chr1sfrench]

And I'm using jumper cables from my truck to a car battery that is good. Not using cables from the seadoo at all.

 

[Chr1sfrench]

Motor is currently like this and I'm jumping it on the bench and it won't turnover with plugs in.

 

[ski-d00]

If you are using a known good car battery, you have a current draw or loss somewhere. It's either in the starter, the connections, the contacts, or the cables. What gets hot while trying to crank the motor over?

Your compression is fine and not too high to turn it over at 160.

Take apart the starter and clean the armature contact surface where the brushes rest making sure the spring force is good on the brushes. Don't use jumper cable squeeze clamp connectors, use clean bolt on connections with good clean heavy gauge cables.

 

[arrow54t]

Could the plugs be too long and contacting the pistons? You said the heads were milled. We know it turns without the plugs installed. Can you manually turn the crank by hand with plugs partially installed or completely installed? Try shorter plugs.
Mike

 

[Chr1sfrench]

I thought maybe the head was milled so I tried another head. It still would not crank over. Spark plugs are correct. I can turn the motor over by hand easily. Nothing seams to get hot when jumping it. I will pull apart the starter.

 

[Coastiejoe]

Everything comes back to a bad starter, poor quality jumper cables, or bad battery. Since the battery came from your vehicle, odds are it is good. Triple check the connection of the cables. They can be excellent, but they really only contact the block at minimal points of the jaws..

 

[Chr1sfrench]

Ok. Just hard to believe that all three starters are weak or bad. They start my other 3 seadoo 787s fine.

 

[racerxxx]

Where is the ground cable grounded on the engine?

 

[Chr1sfrench]

The normal spot on the bottom of the starter.

 

[Chr1sfrench]

I just tried to jump it with plugs in and it gets so hot at the positive side on the battery it starts to melt it

 

[CRei]

Try one spark plug in it??

 

[Chr1sfrench]

One spark plug and it still won't turn over. I can put my thumb over the hole hear it slow down

 

[Coastiejoe]

If it's getting hot, you have a bad cable, bad connection or bad starter.

 

[racerxxx]

A few more questions..............

Did you de-funk the face of the starter and mag housing where they mount to each other?

Did you sand/file the ear on the starter where the ground cable gets mounted?

Did you sand the cable ends where they connect to the battery and the starter ear?

I also don't see a gasket between the mag housing and the case? Did you reuse the original one? If you didn't put one on there the counter balance bearing will rub the housing.


Also, looking at the color of that oil, I'll assume it's TCW3 oil and not API-TC rated oil. If so you might want to swap that out before you get it all buttoned up.

 

[Chr1sfrench]

I've used three different starters same result so how can it be bad starter when all three are good?
I even. Took one a part and cleaned it. Looked like new. The cables can't be bad bc I've tried other cables including jumper cables.
There is a gasket between the mag and case. And I've used two different bottoms ends and two different top ends in different combinations with the same results on multiple starters and batteries. And the oil is just cheap two stroke oil from previous owner.

 

[jjsinaz]

If you've checked all of the other stuff that's been suggested, my only other thought would be that something in the engine is binding enough that it will turn over without the plugs in but once you put them in, the added load of the compression will not let it turn over. How easy is it to turn the crankshaft BY HAND without the plugs in it? I know on the GTS I had before my GTX, it turned over fairly easy by hand with the plugs out, but it was very difficult to turn it with the plugs in.