Mercruiser 120hp - Couple Questions

[sidebojj]

I have a '77 Mercruiser 2.5L 120hp that has a couple issues. First issue is running hot / borderline overheating. This problem seems to be inconsistant, I was camping all last week and used the boat every day. Some of the time I would take the boat out and the water temp would climb to over 200degF at no wake speed just getting to the lake, then once I get to the lake if I open it up and raise the rpms it usually drops down to 180 and stays there. Long wide open runs seem to get the water temp pretty hot as well as slow idling. We decided to pull the themostat out and realized it was already removed. I ran the motor with some of the hoses off to make sure it was pumping water, and it was. After I put it back together it seemed better, cruising at 160 instead of 180 and crawling up the canal at 180 instead of 200+. Then the last day at camp it started to run hot again. Any suggestions? Water pump? Guy said he had just replaced the pump last year.

Next issue (possibly related) are a couple of ignition questions. First what is the correct gap to set the points to? Next, I noticed some detonation (pinging) at WOT and was going to set the timing. Where should It be set? I was told 1deg advanced?

 

[ziggy]

Re: Mercruiser 120hp - Couple Questions

i'd put the t-stat back where it belongs. it should be a 140 for rwc or 160 for fwc. i'm assumin ya got rwc. so 140.
might try it then and see if it overheats. but 1st suspision for over heat conditions is the impeller. the one in the lower unit. not the circulation pump on the ft of the engine.

if service manual 2 is the right book fer yer 77 and i think it is.
point gap .022, dwell 31-34, timeing 8* btdc, idle 500-600, wot 3900-4300. plug gap .035...

 

[cheburashka]

Re: Mercruiser 120hp - Couple Questions

I'd check the exhaust manifold and riser for corrosion. My 140 had similar symptoms due to rust blockages in the manifold water passages. I don't know whether there's a way to check water flow through these passages while the manifold is connected to the engine, but it does sound like you have a problem with water flow--especially if it's running hot with no thermostat installed. Was it run in salt water? Does it still have the original manifold and riser?

 

[sidebojj]

Re: Mercruiser 120hp - Couple Questions

Original Manifold, run only in Fresh water as far as I know. I am new to boats but doesnt the water temp read from engine water tempature? Unless I have this mixed up doesn't the water get pumped through the outdrive, to the water pump and from there it circulates through the engine before it cools the exhaust and out the exhaust? Or does the Exhaust get its own fresh water and the water exiting the engine is seperate?

ziggy - thanks for the ignition specs

 

[cheburashka]

Re: Mercruiser 120hp - Couple Questions

You're right about the circulation. The thing is, if there's a blockage in the system, the water can't flow like it should. You may have rust doing the same job as a partially closed thermostat, keeping the water in the block until it gets too hot.

You might be able to blow out the system by disconnecting the hose that exits the thermostat housing and hooking it up to a garden hose. See what the water looks like when it comes out the back. It should flow very freely and be clear. Backflushing is ideal, but you need to install a special fitting.

 

[Haut Medoc]

Re: Mercruiser 120hp - Couple Questions

I'm going to go along with the restiction at the water dumps at the riser.....
You need to pull the riser off & see if the dumps are clogged with scale.....
Those two little holes are the only place for the heated water to exit......
Put the thermostat back in....
Frankly, if you have a thirty year old manifold & riser, they are way overdue for replacement......​

 

[sidebojj]

Re: Mercruiser 120hp - Couple Questions

Update: Pulled the riser and was suprised to see a couple pieces of water pump impeller in it. (guy I bought it from said he put one on last season) Replaced the impeller and upper housing and so far seemed to stay at or below 160 on the muffs.

Next question.... I have a slight miss that I am trying to track down, I have replaced the points, and put in new plugs. picking up plug wires, dist. cap tonight after work.

I plan to put on the new parts and reset the timing. This is the first motor I have really had to mess with that has points so I was looking for some tips / procedures on setting the point gap and dwell. When I set the point gap I bumped the motor until the distributor was in the "points open" position, I loosened the screw, slipped in the feeler gages and tighted the screw, It just seemed as though the points came back closer then the .020" I set them at when the feeler was removed. Is there a better (correct) way to do it? As for dwell I have no idea how to check or set it so any help there would be great. Thanks in advance.