1992 Johnson 120 prop question

[z86roc28]

I have a 1993 four winns 180 Freedom with Johnson 120 on it. The prop on there right now is a 17 pitch. I had it out on the lake and the tach was reading max rpms at 5000. I thought it should be closer to 5500, am I mistaken? What pitch prop should be on here to get max rpm. What was on there from the factory? I will be using the boat for skiing and cruising.

 

[Dhadley]

Re: 1992 Johnson 120 prop question

Your max rpm's should be between 5800 and 6000 with that motor. I wouldn't suggest changing props until the set up is maxed out. To gain rpm by simply changing props you'd have to drop pitch. A 120 on that boat should be spinning a 17 (depending an condition and style) higher than that. In fact I might guess that it could possibly have a 19.

To gain rpm by dropping pitch you'll lose efficiency. Tell us more about your set up. Is the motor mounted all the way down now? We assume it's mounted directly to the transom - no set back or jackplate. What brand and style prop is it?

 

[Dhadley]

Re: 1992 Johnson 120 prop question

Now, this is all assuming the motor is healthy and is running correctly.

 

[z86roc28]

Re: 1992 Johnson 120 prop question

I don't know offhand what brand prop it is. I just bought the boat. The engine seems to be running fine, the powerhead was rebuilt 2004 and it starts right up, once its warm of course. We did notice it was idling kind of high in neutral, but other than that it was quiet and smooth! Could it be that the linkage isnt pulling the butterflies on the carbs all the way open? how would I check that?
The motor is mounted directly to the transom, as it came from the factory.
The boat weighs about 1800lbs dry.
Is there a way to look at the prop and find out what pitch it is?

 

[Dhadley]

Re: 1992 Johnson 120 prop question

There should be some numbers somewhere on the prop. Is it aluminum or stainless steel? You may have to take the nut off to see the numbers on the hub.

The motor will idle high when you first start it (we assume it's a 93 also) until it comes out of "Quick Start". Once it warms up and idles down it should idle about 750 in gear. Maybe a bit lower, if possible.

You'll need a manual for the "Sync-n-Link" proceedure. I'd suggest an OE manual. Real simple but if you move the wrong screw it'll be hard to figure it out without the book. The book will also show you how to check the wide open throttle (WOT) positions.

If it was rebuilt you may want to check the timing. Sometimes a rebuilder will retard the full advance timing. That will hurt the top rpms too.

 

[z86roc28]

Re: 1992 Johnson 120 prop question

Thanks, it is an alluminum prop, I know that, is there a way to find out what prop it should have come with from the factory?

 

[Dhadley]

Re: 1992 Johnson 120 prop question

No prop would have come with that motor from Johnson (OMC). Prop selection is left up to the rigger. I'd start by raising the motor 1 hole and see how the hull responds, rpm-wise.

 

[z86roc28]

Re: 1992 Johnson 120 prop question

I guess what I meant was the four winns factory, not the omc factory....

 

[Dhadley]

Re: 1992 Johnson 120 prop question

It wouldn't surprise me at all if it wasn't rigged as good as it could be from Four Winns. I have a Hydra Sports and the trim tabs are mounted wrong and they came that way from HS. And OMC owned HS at the time.