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1971 Larson GM 250 Inline 6 165 HP

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uspoet10

New member
Hi All,

New to forum and relatively new to boating.

Boat:1971 Larson w/ GM 250 inline 6 165 HP. Raw water cooled.

Drive: Pre Alpha

I picked up the boat a couple of months ago and 1st time out overheated. I put in a new impeller kit...including base and upper housing. New thermostat. Still overheated next time out.

I pulled the exhaust manifold and riser. Several passages were completely clogged with rust scale. Cleaned with muriatic acid and flushed. Replaced all hoses. Installed new water shutter...was missing.
Boat can run all day at 3300 RPM but will overheat...gauge in red at 3500.

I was thinking the next step would be to change the circulating water pump. Is that what you guys would do? Any other suggestions? Timing issues? I haven't replaced any ignition parts yet. ANY suggestions are welcome!!!

I'm getting ready to go on vacation (family reunion) and will be gone for 3 weeks so I will have a lot of time to dwell on this problem.

Thanks!
 
Welcome aboard !

You checked for the flapper / shutter in the exhaust pipe ? Can and will block the exhaust.

You checked to see if you have good water at the t-stat ?

Submerge the drive, remove the long hose coming in from the transom at the t-stat housing, hold it straight up and start the engine if you have a 8-10" your good quick shut off the engine you need to do this for a few seconds only.

Say you don't have good water to the t-stat then back flush the long hose see what comes out the drive water pickup.
 
Boat can run all day at 3300 RPM but will overheat...gauge in red at 3500.
Click to expand...
That is a water flow issue. Did you install the thermostat pellet down? Since the exhaust manifold was full of rust you can bet the rest of the engine is in the same condition. Pull the water circulating pump off and have a look. You can flush the engine thru the two openings. Look at the pass thru coupler (remove the long hose) on the gimbal at the inside of the transom where the water inlet hose attaches...may be plugged w/corrosion.
 
Thanks for the reply MM.

The flapper was missing. I put a new one in. I had to cut one of the brackets to get it in. Hopefully it didn't break loose:(.

Are you saying 8-10" at idle? Cooling is fine up to 3300 RPM. Gauge in the green below halfway.

I back flushed after I pulled the exhaust manifold. One of my concerns was that the missing flapper was not intentionally removed and was lodge somewhere. I bought an inspection camera and s***** it in from both directions and did not find anything.

Is is safe to run it above 3500 RPM to check water flow?
 
Thanks for the reply guyjg.

The Thermostat housing only allows me to install the T-stat in 1 direction... spring side down.

The rust problem was evident in the engine. After pulling the T-stat housing and cleaning, I look into the cavity and could see a lot of rust...some scaling at the opening. Plucked out as much as I could. Would this rusting cause insufficient heat transfer and contribute to the overheating at high rpm?

The water circulating pump is on the list for replacement but I won't be able to get to it until the end of the month.

I did notice a slight missing at low RPM's. Like I said in OP I haven't touched any of the ignition components. I haven't even looked at the spark plugs yet. Could timing cause overheating at higher rpms? I'm not a motor guy just your basic DIY'er so please excuse the dumb questions.
 
No question is dumb just have a open mind and like Archie allow us to learn you something.

You have to make sure there is no flapper stuck in the exhaust pipe.

You have to make sure the elbow is open the transfer ports between the mani and elbow are clear.

You have to make sure the impeller in the drive is good and or new with a new housing.

The engine circulation pump can be a auto one but must have ss or bronze blades.

Saw one once that were like razors.

Backwash the hose that feeds the t-stat see what comes out.

Understand grass hopper ?
 
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The engine has had 42 years of rust development. Give all what MM says a shot and see what happens. When you pull off the circulating pump flush out the block w/a garden hose...it may help.
 
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