Piston has acne

[noah4009]

I have a 1971 55HP Chrysler. As you know I did everything to get this motor running and it does. Tho rough and it has the jitters. Well, fine time to think of this but I need to know before I go any further. The top piston looks like it has a bad case of acne, while the bottom looks fine. Never really paid no mine to it til I read other threads. Starts right up. Runs but jerks, but also noticed air get in line. See thread air in fuel line. I quess I need to check compression. Correct? Any chance all is fine? How hard will it be to do work on this motor if compression is bad? Is it worth it? This is my first
motor and it is just a learning experience for me.

 

[mrwglo]

Re: Piston has acne

I think this could be from either dirt in the fuel or a lean condition on the carb. Frank will know!

 

[Frank Acampora]

Re: Piston has acne

Sounds like you are experiencing the start of detonation. The top cylinder always runs a little leaner than the bottom. Try opening the low speed needle on the carb a little.

With these engines, the lean setting that gives a beautiful idle is too lean for cruising or full throttle. At cruise, you will experience aluminum throw-off from the piston top. At full throttle, it may destroy the piston. Better to have a poor idle with a slightly rich mixture. WAY easier to change plugs frequently than repair engine.

Correct it now before major damage is done and the piston can still be used. Wait and you may score the bore requiring disassembly,overboring, and oversized piston.

 

[kandil]

Re: Piston has acne

And I would give it a seafoam treatment

 

[noah4009]

Re: Piston has acne

Sounds like you are experiencing the start of detonation. The top cylinder always runs a little leaner than the bottom. Try opening the low speed needle on the carb a little.

With these engines, the lean setting that gives a beautiful idle is too lean for cruising or full throttle. At cruise, you will experience aluminum throw-off from the piston top. At full throttle, it may destroy the piston. Better to have a poor idle with a slightly rich mixture. WAY easier to change plugs frequently than repair engine.

Frank you mentioned opening low speed needle. My carb has 1 adjustment at top of carb. It is a tillotson WB carb. Do I screw this out. The piston was like this when I got it. Engine seems to run rough, could this be the cause? I will check compression this weekend.

 

[Frank Acampora]

Re: Piston has acne

Yep! That needle on the front top of the carb controls the low speed mixture. And everybody just feels a COMPULSION to play with it no matter what the problem. It is out there just like a pimple and you just HAVE to squeeze it!

Start by seating it lightly and count the turns in. Next, set it to 1 1/4 turns out and then adjust when the engine is warm. close until the engine runs rough or dies. Count the turns. Set back to 1 1/4 and open until engine "sags." set the needle to the misway point between the two settings. Now accelerate full throttle from a dead stop (hole shot). If the engine barks, loads up then clears itself, it is too rich. Lean (screw in) about 1/8 turn at a time.

If the engine sags and almost dies, then picks up, it is too lean. Richen about 1/8 turn at a time until it picks-up with no hesitation.

UNDER NO CIRCUMSTANCES set the low speed needle to less than 3/4 turns out from lightly seated. 7/8 is better for safety.

 

[noah4009]

Re: Piston has acne

Yep! That needle on the front top of the carb controls the low speed mixture. And everybody just feels a COMPULSION to play with it no matter what the problem. It is out there just like a pimple and you just HAVE to squeeze it!

Start by seating it lightly and count the turns in. Next, set it to 1 1/4 turns out and then adjust when the engine is warm. close until the engine runs rough or dies. Count the turns. Set back to 1 1/4 and open until engine "sags." set the needle to the misway point between the two settings. Now accelerate full throttle from a dead stop (hole shot). If the engine barks, loads up then clears itself, it is too rich. Lean (screw in) about 1/8 turn at a time.

If the engine sags and almost dies, then picks up, it is too lean. Richen about 1/8 turn at a time until it picks-up with no hesitation.

UNDER NO CIRCUMSTANCES set the low speed needle to less than 3/4 turns out from lightly seated. 7/8 is better for safety.

Hi Frank, Thanks, I will probable adjust carb tomorrow. You said accelerated fast after adjusting, I have the LU in a garbage can. do I accelerate in gear? If I do a hole shot and its in neutral the tower won't let me go to fast due to the tab on the tower that stops you from starting the motor in neutral to high. I also noticed before when you turn the screw in the motor revs up. As I turn it in do I do anything else, like adjust tower screw or just turn in till it just sags, barks, die etc. I checked compression today and got 115 top, 120 bottom. As you know my engine is a 1971 55HP, Chrysler. Is that good compression for an old motor? Also I installed the purolator filter you mentioned (F50153). $2.99 at Advanced auto parts store. Noticed air getting in the filter . I have filter between fuel pump and carb as you said. When I accelerated, bubbles go in it fast.

 

[Frank Acampora]

Re: Piston has acne

Final carb adjustment MUST be done on the water! That is unless you have a test prop and test tank. It must be full throttle acceleration from a standing start. It can not be done on muffs or in a bucket of water.

Don't worry about the air bubbles. Sometimes a large bubble will stay in the filter, especially if it is tilted down at the outlet side, but most times when you start cruising all the air will work out. Not enough fuel is being pumped at idle to clear the air. The air will just blow through the carb--it will not cause a lean run.

 

[noah4009]

Re: Piston has acne

Hi Frank, Wanted to let you know the motor is running fine. I did all adjustments I can do. Gear shifts proper. All is good. Now what? I need something to do. (LOL) so use to going out in the garage every weekend with something to do to motor. Now nothing, thanks to you. (LOL) Really just wanted to thank you. THANKS Frank You da man.....................

 

[CharlieB]

Re: Piston has acne

WHOA!!!!!

Lets get back to those 'air bubbles' for a minute.

Granted there will be an amount of air still inside that filter BUT.......

he mentioned SEEING air bubbling into the filter........

Combined with a lean top cylinder (described a melting piston).......

he may have an air leak into the fuel system, causing a low fuel level in the top carb, adding up to a destroyed piston/cylinder/engine if left unattended.

BEFORE this motor/boat gets onto the water this fuel system needs MORE attention.

 

[noah4009]

Re: Piston has acne

Charlie, I did mention air bubbles going thru line. I installed a fuel filter between fuel pump and carb. My motor is a 1971 55HP chrysler. It has only 1 carb on it. Seems to run fine now. but I do see bubbles going into filter then carb. It has an old metal fuel tank and the line with the bulb looks pretty old and worn itself. Guess that could be the air problem.

 

[CharlieB]

Re: Piston has acne

Clear vynal hose is good for testing fuel systems, great tool for visual diagnosis of air leaking into the system, HOWEVER, once the leak is stopped REPLACE the vynal hose with approved marine rated fuel line. your do not want the risk of a fuel line leaking and any possibility of fire while out on the water.

My keyboard sticks, making my speling suck, and I don't remember how to spell vynal. ha!

 

[noah4009]

Re: Piston has acne

Thanks