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ContinuousWave: Whaler Performance
posted 05-16-2001 10:14 AM ET (US)
I am posting to sheepishly admit my ignorance as to what WOT is an acronym for. In context on this board, it seems to mean "top speed" or "floored" but I'd like the precise definition.
While I'm at it, I have been warned not to run my 1990 Johnson 88 (pushing 1977 Montauk) over 5,000 rpm - not that I would want to do it very often or for long distances - but is there a real danger to overrevving the engine? Is 5,000 rpm too high, or can it be safely run at higher rpm (and if so, what is WOT for this motor?).
Thanks in advance.
|Tom W Clark||
posted 05-16-2001 10:23 AM ET (US)
Wide Open Throttle
As long as your WOT RPM is within the recommended WOT RPM range, you will be fine. The motor won't actually exlode or otherwise disintegrate the moment you exceed the max RPM.
posted 05-16-2001 11:21 AM ET (US)
Thanks, Tom. Anyone know what the recommended maximum WOT RPM is for a 1990 Johnson 88 SPL?
Mostly just curious.
posted 05-16-2001 11:53 AM ET (US)
My guess would be 5500, pretty standard number, but you should track it down specifically for your motor so you can know for sure.
posted 05-16-2001 01:37 PM ET (US)
While we are on this subject. Wide Open Throttle occurs when the throttle plates in the throats of the carbs are open all the way. This allows the least amount on resistance to airflow; resulting in the most amount of air flowing into the engine on every intake stroke. This also results in the most amount of fuel being sucked in as well.
Checking the carb adjustments so as to ensure that the throttle plates open all the way, but not past, is another trick to getting the maximinum performance.
EFI doesn't work the same but the acronyn still applies because there still is a throttle plate.
posted 05-16-2001 06:01 PM ET (US)
Whaletosh's comment about checking that the
throttle plates are open exactly all the way
is a good one. It's an old car performance
trick. In the late sixties, GM had a
corporate pounds per HP rule. Some
Quadrajets BY DESIGN didn't open the throttle
plates all the way in order to keep from
producing to much power and breaking the
pounds per HP rule. My late wife had a '68
Firebird 400 convertible that was like that.
First time I tuned it up, I checked the
throttles and was quite surprised that they
were 1/8" shy of open all the way. I fixed
the "problem" and after that it would pass
anything but a gas station.
Anyway, I wonder if any of the outboard
posted 05-16-2001 08:27 PM ET (US)
4 stroke outboard mfrs use a device called an "isolator" that is located between the intake ports and the carbs. It restricts the size of the intake port opening, thereby limiting full engine potential.
posted 05-17-2001 01:21 AM ET (US)
I need to check the throttle plates on my carbs, too. I was out with the boat last week for the first time this year, and it just seemed like the engine wanted to run more but I didn't have any throttle left to open.
It is hard to describe, but the engine just keeps accellerating very responsively as the throttle is opened, then you run out of throttle advance. The engine seems like it would be glad to go some more, but you're out of throttle. So maybe there is a bit of throttle plate left to open. I'll have to look closely.
We were turning about 5400 RPM. I think the old Merc is rated at 5500 max.
posted 05-17-2001 08:56 AM ET (US)
I should have also mentioned that you need to be careful when adjusting this. The thing to make sure of is the the throttle plates open up all the way and no more.
The outboard makers don't make their own carburetors. Most of them are Mikunis, a world leader in small engine carburetors. But, regardless all of the carburetors have a stop that is part of the casting or their is some other rotation limit. This stop is fine for when the carb is used on Jet Skis, motorcycles, snowmobiles, ATVs, etc. One doesn't have much torque in a finger, thumb, or wrist turn throttle. But boats use throttle/shift levers, and have a lot more torque. So, there is an extra stop mechanism, which is part of the throttle linkage because the cast in stop is to fragile. Therein lies the needed caution. Make sure that the throttle advance is being stopped because of the throttle stop on the throttle linkage, not the stop on any carburetor. A service manual is a big help here.
Having said all that. This is a crude adjustment. The best way is to "sync" the carbs. This adjustment uses a manometer to precisely adjust the throttle plates so that the air flow into each cylinder is closely matched. Then one adjusts throttle stop to make sure that the carb that has the most advancement is stopped a WOT.
Having said all that I wouldn't sweat it about "syncing" the carbs. We are talking nuances here. Just follow the instructions in the service manual about adjusting the throttle pickup and stop adjustments. Heck, some motors have only one carb so syncing isn't needed.
This all the more reason why EFI is going to replace carbs. It is just much better at matching the fuel requirements to the fuel delivered. Look at how it has benefitted cars.
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