Low RPM on 86' 50hsp?

[outhere99]

So far I've replaced the ignition wires, plugs, fuel pump diaphram and water pump. Compression is 147-150 in all three cylinders, and each plug is getting a good spark.

It starts up quickly, but under load on the water it has trouble running up. Yesterday It would 'try' to speed up, but would eventually die (sometimes stalling).

Before I pull the two carbs for a cleaning, any ideas? The fuel line looks pretty new, but I did notice if I gave the bulb a few squeezes the RPM's would pick up (on the muffs).

Thanks,
Jeff

 

[outhere99]

Re: Low RPM on 86' 50hsp?

Any ideas?

Another thing I noticed was the starter has a mind of it's own. When the engine stalled, I'd pull the throttle to nuetral, and it would start cranking on it's own until I played with the key. The starter wasn't engauged while it was running, though.

 

[tredragon]

Re: Low RPM on 86' 50hsp?

turn up your idle mixture. take wraparound cover off, idle mix screws should be near the carb air intakes (check www.mercruiserparts.com for exact placement of idle mix screw). Turn about 1/4 counter clockwise each, start. If it idle's rough, turn idle speed up (should be labeled pretty clear once cover is off). Under load it should take off better now. Might have to play with mix and idle speed, but it worked for me!

 

[This_lil_fishy]

Re: Low RPM on 86' 50hsp?

I have an '86 3 cyl as well. The idle mixture screws are at the front, if you take the front cover off you should see two rubber plugs, if you pull those out you look straight in and see the idle mixture screw (makes for super easy adjustment on the water). You won't need to remove the entire surround.

I had a similar problem and found I was running on two cylinders. A definate carb rebuild is in order, and very simple. Replace the floats and needles if you have foam floats. The new floats are plastic, and the new needles do not have rubber on them, but work very well. Replace the plugs, you may have a crappy one.

Also, man, I wish I had your compression. You sure that's accurate. I have 120psi across the board. You have quite a spread but all high numbers. You should turn over about 4 complete revs if I remember right by the manual.

Ian

 

[outhere99]

Re: Low RPM on 86' 50hsp?

After cleaning the carbs (only using a spray can of carb cleaner), and replacing the fuel line (tank to engine) She runs a lot better. A bit of surging at higher throttle settings, but at least I know where to fix. I'm going to pull the carbs and do a better cleaning. Plus check the floats mentioned.

BTW.. I was surprised to get readings that high as well. I checked with two guages, and spun the motor at least 6 revolutions. I have new plugs, wires also. No fouling seen.

 

[outhere99]

Re: Low RPM on 86' 50hsp?

I replaced the floats, needle/seats and cleaned everything. Runs like a brand new engine! Not really sure if I've adjusted the idle mixture correctly, though.

Kind'a hard to tell on an engine that has two carbs for three cylinders. I'll leave them on the rich side, though.

BTW... shouldn't the mixture be set with the boat in gear (under a slight load)? The book doesn't mention that, but I seem to remember that from years ago.