PLEASE HELP- 787 no start after engine rebuild

[Srdan1413]

Hey guys,i need some help from more experienced than i am.
I have 1996 Challenger(single engine 787,no oil pump- running premix)
Engine was rebuilt,carbs were rebuilt, brand new battery,brand new oil in tank, rotary valve adjusted to 147 degrees, compression 153 psi on both cylinders, ski is cranking but will not start.. after cranking for more than 8-9 seconds will back fire crazy loud (my neighbors must love me with that noice )
I have nice spark also. Black fuel lines.. PLEASE HELP ,I DO NOT KNOW WHAT TO DO ANYMORE, I LOST WHOLE SUMMER TRYING TO FIGURE OUT ISSUE

 

[Hunterb]

Hey guys,i need some help from more experienced than i am.
I have 1996 Challenger(single engine 787,no oil pump- running premix)
Engine was rebuilt,carbs were rebuilt, brand new battery,brand new oil in tank, rotary valve adjusted to 147 degrees, compression 153 psi on both cylinders, ski is cranking but will not start.. after cranking for more than 8-9 seconds will back fire crazy loud (my neighbors must love me with that noice )
I have nice spark also. Black fuel lines.. PLEASE HELP ,I DO NOT KNOW WHAT TO DO ANYMORE, I LOST WHOLE SUMMER TRYING TO FIGURE OUT ISSUE

Have you tried squirting some fuel into the carbs or even putting a little fuel in the spark plug holes and then cranking it. If you have spark and compression and the rotary valve timing is correct it must be a fuel problem.

 

[Srdan1413]

Have you tried squirting some fuel into the carbs or even putting a little fuel in the spark plug holes and then cranking it. If you have spark and compression and the rotary valve timing is correct it must be a fuel problem.

Yes,we tried that multiple times- nothing happens

 

[etemplet]

Did you replace the flywheel? Timing is likely off. Do some research. I say this because I just finished a 657X engine. I told my wife I'd haave it running in 15 minutes. Two hours later still nothing. LOL , I pulled the rotary valve cover, checked the timing which I knew was good but you gotta check. I had fire up the azz. I turned the engine over and over. Every now and then it would umph but not pop. I pulled the flywheel and replaced it. Ski fired right up. I dunno what happened or why that flywheel was on there but... it was the wrong one for the application. In my defence I purchased a used bottom end and the flywheel might have been on there. It's been a while. LOL

Get a timing light. mark the PTO for top dead center on the number one mag cylinder. Note the position where it is firing the plugs. If you have compresssion and fuel... timing is the only other issue.

Did you snip the plug wires back?

Now then... how are you checking the spark? I use the spark checker that plugs into the wires. You can easily see the spark. Here is a wrinkle. I had a ski that would not start as long as I held the starter engaged. I could see the spark checker and the coil was firing but... the COIL was bad and could only deliver enough spark when the stater was disengaged. It was indeed weird. The engine would crank and crank and never pop. As soon as I let of the starter the engine would fire right up.

So there is my contribution on Weird stuff. Hope it helps you think out of the box. Good Luck !!