Low RPM at WOT

[wormboy]

1989 150 Evinrude; the manual says that the WOT rpm should be ~5k, but I am topping out at 4k; i.e., the throttle is all the way to the console. I'm a complete novice with this type of thing-- could it be that the throttle linkage needs to be adjusted to allow for more room at the top, or could this be something more complicated like a synchronization or timing issue?

The engine is on a 18.5' cc, and the top speed reads ~46 on my fish finder-- it's quite a bit of engine for this boat, so I don't really need more speed--- just want to make sure things are as should be.
Thanks......

 

[Silvertip]

Re: Low RPM at WOT

Probably not a throttle or cable issue. But check it anyway. Move the trottle lever full forward. Then check the port side end of the throttle shaft on the carb. The roll pin in the end of each shaft should be perfectly vertical. If not, the throttle is not opening fully. Put the lever in neutral and disconnect the throttle cable. Hold the throttle linkage on the engine against the idle stop and then bring the cable back up to the linkage. If the attachment points do not line up exactly, adjust the trunion (threaded barrel on the cable) until it does line up. Reattach the cable and check full throttle again. If the roll pin is still not vertical, you need to do a link and sync. DO NOT try to adjust for this with the cable. Thats not how its done. If the carbs are opening fully, then you may be severely over propped or its down on compression and hence, power. That engine should spin up to 5500 RPM.

 

[Silvertip]

Re: Low RPM at WOT

One more thing -- 46 MPH at 4000 RPM says you better check the tach as that may be giving you an erroneous reading.

 

[wormboy]

Re: Low RPM at WOT

Thanks Silvertip-- will check the linkage as you recommend; the speed and tach issues are another question; how can I check if the tachometer is working correctly? It's also possible that the fishfinder is giving me an exaggerated mph reading at higher speeds--- it seems to be about right at lower speeds, though.

 

[xtraham]

Re: Low RPM at WOT

i personaly have never seen an accurate speedometer on a fish finder the placement of the transducer, prop wash, and tide all have differant effects on them, try using a gps and check the difference

 

[Silvertip]

Re: Low RPM at WOT

If you look at the back of the tach you'll see a dial with some numbers around it. It should be set to 6P for your engine. If that's correct, the only way to verify the accuracy is with another tach (like an inductive pickup style). For speed accuracy you'll need a GPS. Regardless, if the tach is correct (which I doubt) you adjust engine height (not trim) so the AV plate is flush or an inch or two above the keel. After that, you decrease prop pitch to bring the RPM up. Once you are at 5500 RPM your speed will be whatever that RPM and prop pitch will deliver. I really think however that RPM and speed numbers are way off. But then what the heck -- I've been wrong before (just once or twice though!! d

 

[dsinger]

Re: Low RPM at WOT

Use a timing light to check the tach or I should say an advanced timing light. Set it to eight and take half that for your RPM's

 

[wormboy]

Re: Low RPM at WOT

Thanks for all of the advice-- I will look at the back of the tach tonight; it may never have been set for this engine, and in fact was not properly connected when I first bought the boat (which is 3 yrs younger than the motor, so prob not originally matched).
the fish finder i have has a separate wheel attachment for speed-- and doesn't use the transducer, but i definitely agree that wash, turbulence, etc is going to affect this at different speeds and probably different trim.

I appreciate all of the replies-- hope all of your own troubles are as minor!!
Thanks again!d

 

[wormboy]

Re: Low RPM at WOT

Hey Silvertip-- I checked the tach and the dial on the back had the arrwo pointing to "12 pole" so I switched it over to "6 pole" as you suggested. I won't know if this makes a diff until i run it on the lake this weekend. In the meantime, and when you have a moment-- can you explain what this setting is and does?
I appreciate the patience displayed on this forum-- the reason men don't like to ask directions is because they don't like to look dumb......