what limits RPM?

**mercpower** · 02-28-2003, 01:16 AM

I am thinking about starting w/ a Merc 200 and building a motor, I know the ecu controls the rev limiter on an efi motor, but on a carb motor what limits the rpm? i'm thinking of buying an old motor and just seeing what I can build and this was my first question. thanks

**JamesE** · 02-28-2003, 07:29 AM

On my engine and most other carb. outboards the power pack/s limit the maximum rpm the engine will obtain. In my case it is 6700 and at this rpm the power pack starts cutting out the fire.

**mercpower** · 02-28-2003, 08:58 AM

I am trying to build my first "performance" motor, is there ways to get around the limiter in the power packs, and would it require different rods and pistons to hold up to the rpm's

**tmerc25** · 02-28-2003, 09:08 AM

There is no such thing as a revlimiting power pack (switch box) on a carb motor.

**Techno** · 02-28-2003, 04:11 PM

I think whats happening is the switch boxes have reached the limit of their design. Doesn't rapair exist for pretty much this reason?
I seem to remember it being said that the switch boxes become saturated- for lack of the right term, and there is a higher end one. Is this the diff between 260 electronics and 280?

Also US-1 doesn't seem to have this limited problem with the carbed engine he builds, at least I don't think so.

**MennoNetherlands** · 02-28-2003, 05:21 PM

only thing that does limit your rpm on a carb engine is the spark advance module on the engine (not all the engines have them )
it is one of the black boxes on top of the engine
and it is limiting the spark advance at high rpms
disconnect the box en set timing at 25 degrees
will help you too rev higher

bye

**Jay R.** · 02-28-2003, 05:54 PM

there are rev limiting power packs. both my OMC's have them. but I'm not shooting for the warp speeds that would require them removed.

**MIKE'S MIRAGE** · 02-28-2003, 08:21 PM

IF THIS A CDM MOTOR, THE REV LIMITER IS IN THE CONTROL MODULE. ONLY THE CARB CONTROL MODULES HAVE THE REV LIMITERS IN THEM, NOT THE EFI. YOU CAN INSTALL THE EFI MODULE ONTO THE CARB MOTOR TO ELIMINATE THE LIMITER. YOU WILL ALSO NEED TO REPLACE YOUR PLUGS. GO FROM THE BU8H'S TO THE BUZH NGK'S. HOPE THIS HELPS, MIKE

**Rickracer** · 02-28-2003, 10:26 PM

Tracy, we've had this discussion before. Just take a look at any Rapair book. OMCs are often limited from the factory.

**us1** · 03-01-2003, 08:19 PM

Merc carb motors don’t have rev limiters. The port timming is what keeps the RPM down.

**Greg Moss** · 03-01-2003, 09:16 PM

the duration of the exhaust port is what determines the Max RPMs of any 2 stroke motor.

**JamesE** · 03-01-2003, 10:28 PM

I guess the "6700rpm" was just painted on the side of my powerpacks for ****s and giggles by some moron at the factory. I was not familiar with the Mercury, but that is the way things work in the world of White and Blue.

**Greg Moss** · 03-01-2003, 10:37 PM

they have rev limiters in the power packs Rapair sells packs with the limiters removed I would think Rickracer should step in here and tell you how he can save you money on them. OMC's are long rod long stroke motors so they tend to make more torque and horse power at lower rpms. nto to mention the pistons weight a ton compared to a Merc. I think the old cross flows had pistons that wiegh 650 grams, that is a lot of metal to be slinging around.