787 banging at 3000 rpm

[jknightg]

I have just replaced the 787 engine in my Sea-Doo speedster
When I started it up at 3000 rpm it bangs about every half a second
It doesn't sound very healthy
Has anyone got any ideas what the banging could be?
Regards
John
PS there is no jet pump connected

 

[ORION '95 Speedster]

Banging? Like as in a loose motor mount? Detonation (per-ignition)?

 

[jknightg]

Good point
More like pre ignition as the exhaust gasses pop too

 

[ORION '95 Speedster]

Pre-ignition will mess things up pretty quicky. Its like punching a brick wall, sooner or later something is going to give.

 

[jknightg]

How can I stop it ?

 

[ORION '95 Speedster]

Timing
Carbon build up in cylinder
Lean
Low octane
Wrong spark plugs

Any of those could cause pre-ignition.

 

[ORION '95 Speedster]

Make sure its pre-ignition before you attempt to fix the problem. Chasing the wrong problem will get you no where, and may even cause more problems.

 

[jknightg]

I think I might have it !
My fuel pump failed at the same time as swapping the engine
I have not used the Seadoo one
I wonder if it isn't getting enough fuel

 

[jknightg]

Head is fine
Plugs are new and fine
Fuel is good (other engine is fine)
How can I make sure ?

 

[ORION '95 Speedster]

If the carb failed at the same time as the motor swap, then foreign material must have entered the fuel lines during the removal of the fuel hoses. If its running lean, it will run hotter than the other motor. Applying choke part way will make the fuel mixture more rich.

 

[ORION '95 Speedster]

Depending on the "banging", it could be a bad rod bearing, wrist pin, or maybe really bad piston slap. Does it get a lot louder when you rev it up?

 

[jknightg]

Good idea
The carbs are fine
I think I need to swap the fuel pump if the new pump may be starving it ?

 

[ORION '95 Speedster]

If the banging sound is consistent and constant, then its probably a mechanical problem related to the revolution of the motor. If its inconsistent or goes away when you pull the choke, or swap fuel pumps/carbs etc, then its obviously not a mechanical issue and must be related to combustion.

 

[jknightg]

Great thx
I will try and let you know
Assuming it is the fuel pump
Any idea where to get one ?
It is a 98 speedster

 

[ORION '95 Speedster]

eBay is a good resource for cheap good parts. Choose wisely and read the descriptions. Only buy from sellers that offer returns.

 

[jknightg]

Ok thx
I have a few things to try [emoji106]

 

[XP 787]

Is it like a knocking noise from engine? Warranty on motor? FYI that boat hits the rev limiter in neutral around 3,500.

 

[jknightg]

More a banging

 

[jknightg]

No warranty [emoji30]

 

[ORION '95 Speedster]

Can you record a video of the motor running and send us a link?

 

[XP 787]

+1 on the video

 

[Cablebandit]

this isnt the neutral rev limiter kicking in is it?

 

[jknightg]

Here is a video
https://vimeo.com/182873334

 

[ORION '95 Speedster]

Sorry This video will be available for viewing shortly.

 

[XP 787]

That's your rev limiter in neutral. Start it and put it in forward and you will hit 7k RPM's. Break in the motor before you do that though.