very low WOT rpms

[monach5.0]

HI, I've got a Johnson 1984, 90hp V4, crossflow. 13 3/8 x 17prop. I just got it. The max rpms I am getting are 4000-4200(WOT). I thought that they like the high revs, 5500. I checked the spark plugs and line to the plugs there's spark. My last o/b died by rod through caseing. Also the tacho goes haywire for a few seconds then ok for tens of secs then haywire again, only at full throttle, as I remember. I am very reluctant to take on the water again due to damage that I may be doing. Any ideas guys.

 

[moterboat]

Re: very low WOT rpms

What Type Of Boat,what Size,how Much Weight? Whats The Speed At Wot?

 

[Gary H NC]

Re: very low WOT rpms

You need to do a compression test,then check the spark with a spark tester,It should jump a 7/16th gap with a good blue spark.
Dirty high speed orifices in the carbs could do the same problem also.
If the Tach is acting strange i doubt its accurate anyway.Take a volt meter and check voltage at battery with engine off then check while on the water running about 3000-3500 rpms see if its charging.

 

[72SideWinderSS]

Re: very low WOT rpms

When was the last time you de-carbed the motor?

 

[monach5.0]

Re: very low WOT rpms

What Type Of Boat,what Size,how Much Weight? Whats The Speed At Wot?

The boat is a 1981 f/glass Monark 17ft 1/2 cab. The boat weighs approx 3/4 tonne, 750kgs. At 4000rpms WOT, speedo showed 30 mph, 55 klmph. I have just bought the o/board. Prop is 13 3/8 x17p.

 

[monach5.0]

Re: very low WOT rpms

When was the last time you de-carbed the motor?

I have not had the motor for long, 3 weeks. The seller said it had not long been reco'd, only 15 hours use since. Will attempt decarb. Don't use sea foam in Australia, will try tune-up spray.

 

[monach5.0]

Re: very low WOT rpms

You need to do a compression test,then check the spark with a spark tester,It should jump a 7/16th gap with a good blue spark.
Dirty high speed orifices in the carbs could do the same problem also.
If the Tach is acting strange i doubt its accurate anyway.Take a volt meter and check voltage at battery with engine off then check while on the water running about 3000-3500 rpms see if its charging.

Spark good. Comps are 1.120, 2.110, 3.120, 4.110. Not had o/board long 3 weeks. Sell told me it had a recon, only used 20 hours. sat for a few months not used that was why uneven comps.

 

[72SideWinderSS]

Re: very low WOT rpms

If you don't have an OEM manual, I would suggest that you get one, well worth the cost.

What I would do:

1) Take the carb's off and give them a thorough cleaning, removing the discs that are above the the idle passages and replacing, install new OMC/BRP carb kits.

2) Perform a link and sync, including setting the WOT.

3) Check the setup of the motor, measure the distance from the bottom of the boat to the A/V plate on the motor

 

[steelespike]

Re: very low WOT rpms

Tach fix may be as simple as checking connections at the rectifier.May look better with proper operating tach.