Troubleshoot an Overheat Condition

[cj8mule]

I'm working on a 1978 120hp 2.5L that is overheating while under load.

In the driveway:
I can run the engine on muffs all day long and temp will stay around 160 degrees.

In the lake:
Idleling in neutral, it will overheat.
Idleling while underway, it does not overheat. I can run it up to around 2000 rmps and it will not overheat. Above that, it will start to overheat.

The engine feels underpowered and will not run above 3100 rmps. It takes about 100 yards to get the boat to plane out. It also backfired a couple of times through the carb while at full throttle as I was trying to plane out.


Here's a little parts background:
New manifold, new waterpump, carb rebuild, and a fresh tuneup. I am pretty confident the boat is propped corectly.

I want to check the flow from the water intake hose. I thought I would recheck the timing. The distributer has a Pertronix ignitor conversion and I am not sure the weights are working.

If the weights are working, I should see the timing advance while rpms go up?
I need to check the engines re-circulating water pump?


ARRGGG.... my head hurts! I think there's more than one problem going on here. What would you check and in what order?

 

[Don S]

Re: Troubleshoot an Overheat Condition

For the overheat, this may help.

View attachment 82_04 water pump output test.pdf

For the advance timing, you should see 32? ? 2? around 2500 rpm. Make sure your point dwell is set before you set your base idle timing, then check your advance timing.

 

[cj8mule]

Re: Troubleshoot an Overheat Condition

For the overheat, this may help.

View attachment 176269

For the advance timing, you should see 32? ? 2? around 2500 rpm. Make sure your point dwell is set before you set your base idle timing, then check your advance timing.

Thanks Don.

I haven't been able to take the boat to the lake for the water flow test. I did find the weights in the Pertronix distributer stuck... there was no advancing at all. I have an extra points style distributer that I installed.

It all makes sense now why the previous owner had the distributer advanced so much!

I won't know for sure until a water flow test if it's all fixed up. Will retarded timing make the engine run hot? I know the motor is going to run a lot better and get on plane now.

 

[achris]

Re: Troubleshoot an Overheat Condition

I'm working on a 1978 120hp 2.5L that is overheating while under load.

In the driveway:
I can run the engine on muffs all day long and temp will stay around 160 degrees.

Good, we can do some basic set ups without have to worry about getting too hot.

In the lake:
Idleling in neutral, it will overheat.
Idleling while underway, it does not overheat. I can run it up to around 2000 rmps and it will not overheat. Above that, it will start to overheat.

Overheats idling in neutral but not in gear???? Or is this a typo?

The engine feels underpowered and will not run above 3100 rmps. It takes about 100 yards to get the boat to plane out. It also backfired a couple of times through the carb while at full throttle as I was trying to plane out.
Here's a little parts background:
New manifold, new waterpump, carb rebuild, and a fresh tuneup. I am pretty confident the boat is propped corectly.

If it feels underpowered, it probably is. Not running over 3100 revs? Check the tacho for accuracy (use a 'shop tacho')... If it is accurate, then you are most definitely over propped, given the engine is doing it's things properly. But based on a few things in your last post, sounds like the engine needs some attention.

I want to check the flow from the water intake hose.

The most usual starting point for overheat problems of this nature is the seawater impeller (inside the drive).... Have you checked/replaced that? If it's any more than a couple of years old, replace it anyway. With all new gaskets, and inspect the housing and base very closely. Any doubts, replace them. Look for slight melting of the housing on the side, where is contacts the stainless steel cup, and melting of the base very close to the drive-shaft. You might see the plastic melted very slightly under the base plate.

I thought I would recheck the timing. The distributer has a Pertronix ignitor conversion and I am not sure the weights are working.

If the weights are working, I should see the timing advance while rpms go up?
I need to check the engines re-circulating water pump?


ARRGGG.... my head hurts! I think there's more than one problem going on here. What would you check and in what order?

My order of investigation would be...
1. drive impeller (and while you're in there, test the water pocket cover gasket for leaks)
2. drive pump output (also use a length of clear hose to make sure you have a solid water stream coming from the drive)
3. exhaust elbow The old 120 elbows have very narrow passages and they tend to block up rather easily.
4. exhaust manifold
5. timing (both initial and advance curve)
6. thermostat, (for restriction)
7. engine mounted circulating pump

What size boat, and pitch of prop are you running? If you can give me the figures (and the drive ratio if it's not 1.98:1) and the speed and revs too, I check them in my prop calculator program...

Chris..........

 

[cj8mule]

Re: Troubleshoot an Overheat Condition

Thanks for the reply Chris.

The engine did get to 200 degree while the chokes fast idle was on... so I guess it didn't really overheat at idle. Sorry, for not stating that corectly.

The boat is an aluminum 18' SS Starcraft. It is a new to me boat that I've restored. It has a new manifold, bellows, seawater pump, fresh carb rebuild, and great compression. I've checked the tach with my meter and it is as acurate as they get. The drive ratio is 1.98 and is propped with an aluminum 3 blade 48-78118-17. I don't have a gps so sorry, I can't give a speed/rpm ratio.

I did check the water pickup for debree and it was clean. I've got a chance to get the water flow test performed this morning and will post up the results.

Thanks
--dave

 

[Don S]

Re: Troubleshoot an Overheat Condition

The engine did get to 200 degree while the chokes fast idle was on... so I guess it didn't really overheat at idle.

Marine carbs do not have fast idle cams on them. If you have one, you need to get it off the boat and get a marine carb.

Also, 200? is way overheated for a marine engine.
At idle in neutral, either in the water or on muffs, you shouldn't be seeing over 140 to 150?
That engine should be running a 143? thermostat.

 

[cj8mule]

Re: Troubleshoot an Overheat Condition

Marine carbs do not have fast idle cams on them. If you have one, you need to get it off the boat and get a marine carb.

Also, 200? is way overheated for a marine engine.
At idle in neutral, either in the water or on muffs, you shouldn't be seeing over 140 to 150?
That engine should be running a 143? thermostat.

The carb is a marine rochester 2GC. It has the fuel overflow connection on the horn. Maybe my terms are messed up or someone added it, but there is a fast idle cam on this carb.

I have a new 140 degree thermostat from the merc dealer. I also got a chance to do the sterndrive water pump flow test and averaged 3.9 liters which is above the minimum required. I still have an overheating issue though, but have not gone through all the stuff AChris suggested.

Since the motor still slowly overheats while idleling in gear, I have some more work to do :grumpy: However, the boat runs great now since the weights in the distributer are working. It will run up to 4300 rpm now quickly.

I'm going to check the exhaust elbow next and will probably just replace it.

I appreciate all the feedback guys!