Need help...RPM way too high!

[saumon]

Hi,

On my slightly underpowered boat (17'6" aluminum side console rated for 90hp max.), the RPM of my engine (1993 Merc 60hp 2-stroke 3 cyl. #0D074327) is way too high. At idle it's running a bit over 1000 on my console tach but at WOT i had to stop the throttle lever at 3/4 to be at 5500. I've tried once (for a split second) to push it at the end and it reach 7000! If i set the idle speed slower it dies.

As for the engine history, it had two overheating episodes but in both cases was stopped immediately when the alarm goes off (impeller self-destruct on a nicked w/p baseplate). Since then, compression whent down to 90-90-115 but it's still running fine. I've done pretty much everything on that engine (waterpump, coils, carbs overhaul) exept removing the powerhead, that i'm planning to do next winter. Maybe i'll be lucky and only have to deglaze and re-rings! As for the prop, i put a 10.13" x 15"pitch on it.

I'm lost, what could be the cause of this high RPM:

- the fact that the boat is underpowered? I think it should do the opposite...
- the prop? it's already the higher pith available for this engine...
- a bad tach? are they reliable? is this the kind of device that either work or not, or could they give a bad reading. And can i test it by hooking one of those generic tach found on auto-parts stores?
- do I need to do a link & synch (i have a Factory manual, seems pretty easy)
- or can it be caused by the low compression? and is it a waste of time to try to adjust the throttle linkage and timing with such an uneven compression?

Many thanks in advance for you inputs!

Steve

 

[jmburock]

Re: Need help...RPM way too high!

Your idle speed should be 650-700 rpm in forward idle. Max RPM at WOT should be around 5200 rpm. Most likely problem is your timing + prop. Are you sure the tach is set at the right setting. Sounds like another overheating is in your near future unless this problem gets fixed.

 

[saumon]

Re: Need help...RPM way too high!

Your idle speed should be 650-700 rpm in forward idle. Max RPM at WOT should be around 5200 rpm. Most likely problem is your timing + prop. Are you sure the tach is set at the right setting. Sounds like another overheating is in your near future unless this problem gets fixed.

I already know the recommended RPM, so while running i stay under 5500, and for these engine, prop pitch goes from 10 to 15.

 

[j_martin]

Re: Need help...RPM way too high!

At over 1000 idle, when you put it in gear it would, or should give you a strong desire to kick yerself in the az. If it just lightly clunks in normall like, your tach is wrong.

At 7000 rpm, that engine would be screaming in a way that should give you similar self destructive feelings. Again, could be your tach.

If it's going slow, and spinning fast, and it is really idling high and not clunking too bad going into gear, you have a spun torque hub in the prop.

hope it helps
John

 

[saumon]

Re: Need help...RPM way too high!

Thanks you all guys,

i've never played on tach before but after a couple hours of reading on previous post to have an idea of where am i going, i remove the on-dash tach and found that it has 4 settings (4 poles, 8 poles, 10 poles and 12 poles). Mine has "slipped" to the 10 poles settings! Maybe due to 14 years worrth of vibrations...Put it back to 12 and idle RPM went to 800, give a bit of throttle and went to 1400. Just to be sure I place the setting on 10 and RPM reading switch to over 2000 without any difference in the sound of the engine, so I put it back to 12 poles and everything was now fine.

 

[snappernz]

Re: Need help...RPM way too high!

just wondering what's the wot now as i have a 60 hp and can only get 5100rpm out of mine

 

[saumon]

Re: Need help...RPM way too high!

Haven't tested it on the lake yet. Yours can also be a prop issue: a 1" decrease in pitch will give you around 200 RPM more...or simply a slightly misajusted throttle linkage: check that the carbs butterfly were fully open when the lever was pushed all the way in.