1978 75 HP Chrysler outboard will run on muffs great but not in water

[charakel]

I have a 1978 Chrysler 75 HP outboard Model # 757H8D That runs great on muffs, full throttle and idles well, but on the lake is another story. On the water it will start but not stay running. It will however idle wonderfully in warm up mode with the button pulled out. but as soon as I lay back the throttle and push the button in it dies. I need some help. What are the symptoms of the water impeller going out. Could that be the problem? Thanks!!!

 

[Frank Acampora]

A bad impeller or one going bad will not affect starting or running until the engine overheats so badly that it stalls and seizes. Change or clean the spark plugs. Go to the top of the forum and watch the video on synchronizing carbs and timing. Be certain the engine is set correctly then adjust idle speed to 700-750 RPM in the water.

 

[charakel]

No Title

Well I have a not so good update. Did a compression test and 2 of the 3 tested close to 150 and the 3rd the bottom cylinder tested at 0... Not happy. Why can people just be honest when you buy used stuff. Oh it runs great!! Again I get shafted. Any suggestions on repair? Take it in or do myself? Attached are pics.

 

[Frank Acampora]

That is damage from a lean run. Put the piston at bottom dead center and examine the bore. You may have gotten lucky and the cylinder is not badly scored. If that is the case the piston can be replaced through the reed opening behind the carb. If the cylinder is scored then you will need to disassemble the engine and have that cylinder over-bored. A new piston in the correct oversize will cost between 70 and 125 bucks.

If you do decide to rebuild or even if you can simply replace that piston, remove all three carbs and disassemble them Clean well paying special attention to the last one. Put a cheap in-line filter BETWEEN the fuel pump and carbs.

 

[chrome dome]

if l read it correctly, you've been running it at WOT on muffs?
if so, you alone "may" have caused the compression issue,
should use some sort of tank... like a modified 44 gal drum..when running at WOT at home etc

muffs are only good for low revs

 

[charakel]

I ordered my parts and I am going to attempt this repair myself. To replace the piston, Frank Acampora, you mentioned replacing it through the reed opening. Sadly I too am a fairly newbie to outboard ownership and repair. How do I gain access to the piston via reed opening?? Thank you for helping me.

 

[Frank Acampora]

You first remove all three carbs. Then you remove the manifold. DO NOT remove the six angled bolts. They hold the reed Vee blocks.The gaskets will break so you will need to make new ones out of 1/32 gasket paper sold in all auto stores.The reason you will not buy three new gaskets is because the stock gaskets are a single piece that seals not only the reed opening, but also the reed block to the manifold. There is no need to remove the reed blocks just to replace a gasket.

Now the crankcase is open and when you position the crankshaft you can access the two 12 point 1/4 inch bolts holding the rod cap. Remove them and push the piston out the back. Be sure to collect all 16 roller bearings. If you do not count 16 then there is one or two still in the crankcase.

While the piston is out, examine and mark the rod and cap. they only go together one way

Reassembling the rod on the crankshaft is a tedious job. Use an assembly lube or a good quality grease to hold the rollers and cages in place. Finger tighten the cap bolts and check alignment with a pencil point or other sharp object. Then turn the flywheel a couple of timed to be certain there is no binding. Torque the cap bolts to 190 inch pounds.

Clymers will show this but on an open engine. Your job will require more patience because you are working in a confined space.

 

[oldboat1]

Another option is to look for a donor powerhead, shopping with your compression tester. Depends on what you are most comfortable with, but may be worth a try.