new piston in 76 135hp johnson comp not equal

[delsol]

i have a 76 johnson 135 i installed new piston last season the comp is 155 155 155 125 in new cly boat idles good till i put it in water it runs good sometimes then wont go anywhere when i spay 2+2 or either in carbs it will take off somethimes if i let deler rebuild carbs should the motor still run ok with diff compression 155 in 3 4th 125 maybe rings did not seat yet please advise because i live in pa and would like to figure it out soon<br />bob

 

[Bradster941]

Re: new piston in 76 135hp johnson comp not equal

Geez, I would be happy with a compression of 120 through out. The one (low) cylinder isn't the problem.<br /><br />I would be looking at going through the carbs on a 1976 in PA. if you didn't winterize it correctly.<br /><br />Also, be careful on using Either. It washes away the lubricant and burns Hot and Long on the stroke. You could easily be replacing another piston soon...<br />

 

[delsol]

Re: new piston in 76 135hp johnson comp not equal

ya i think its carbs and needs timing set

 

[JB]

Re: new piston in 76 135hp johnson comp not equal

Hi, Bobby.<br /><br />I agree with Bradster. Your comnpression on the new cyl is not your problem.<br /><br />2+2 and ether are not the right stuff to spray into the carbs. If you are going to do that, use fuel mix.<br /><br />If it seems to idle okay on the hose, try pulling each plug wire and checking for a hot, blue spark to the block (1/2" to 3/4") and a drop in the idle speed. If idle speed does not drop, that cylinder is not making power. If there is no spark for any dead cylinders troubleshoot your ignition.<br /><br />A cylinder that makes no power but has good spark is lacking either fuel/air mix or scavenging pressure. Try the fuel mix. If there is no change look for pressure leakage from that crancase. <br /><br />Good luck and let us know what you find.

 

[Joe Reeves]

Re: new piston in 76 135hp johnson comp not equal

Bobby..... Pistons do not fail for without reason. Something had to cause it. If you or someone else rebuilt the engine without tearing the carburetors down and rebuilding them, you are inviting a repeat of the overhaul scenario.<br /><br />A clogged carburetor will cause a cylinder to run lean, which in turn will destroy a piston, cause cylinder wall scoring etc.<br /><br />If you have installed the same type piston as what exists in the other three cylinders, and you did correct any defects in the damaged cylinder (O/S Boring, Honing etc), I can not imagine why you would have a 30lb difference in compression. A compression difference of that range is unheard of.<br /><br />However, if you did not correct any damage that usually exists on a cylinder wall, in a cylinder that contained the damaged piston, the scoring etc would account for the compression difference, and unfortunately that would lead to having the new piston fail in a short amount of time.<br /><br />It would help to have some history on exactly what was performed in that overhaul.

 

[dkondelik]

Re: new piston in 76 135hp johnson comp not equal

I'm not reading too clearly this morning so I appologize if I missed something.<br /><br />Is the LOW cylinder the one with the new piston?<br />Di you bore or hone the cylinder before replacing the piston?<br /><br />Good Luck<br />HAVE FUN!

 

[delsol]

Re: new piston in 76 135hp johnson comp not equal

low cly has new piston......i honed it out and crossed hatched it and mike it it had a litte scoring so i was able to hone it clean but compression is only 125 vrs 155 in 3 others is it possible the rings are not set there is like 2 hours on power head that does now run rt

 

[Bradster941]

Re: new piston in 76 135hp johnson comp not equal

So your not done with the Break in?<br /><br />And, are you double oiling?<br /><br />I generally pack a lunch and make a day of it when doing a break in.

 

[Dhadley]

Re: new piston in 76 135hp johnson comp not equal

Lets see....we took a cylinder that was most likely worn close to the max tolerance and made it bigger (honing) until the scratches went away. Then put a standard piston in. You mentioned you measured the cylinder. What was the finished bore size? 3.504" is about max before it needs boring. Was it indeed a NEW piston or a DIFFERENT piston? Were the rings new? What brand piston?<br /><br />Good luck!!

 

[delsol]

Re: new piston in 76 135hp johnson comp not equal

measured 4.0 honed and crossed hatched ..new piston new stock rings and new crank bearings i know i should have did all 4 cyl but i did not could the rings not be seated yet the motor has like 2 hours on it but its not running rt... think carbs are bad need rebuilding idles fine on trailer but in water wont idle right,...has some power then wont get out of its own way....when i pull engine cover off and spray gas it will somethimes take off with a delay then do 4k then run at 1500 rpms??/

 

[delsol]

Re: new piston in 76 135hp johnson comp not equal

honed 3.5 not 4

 

[digimortal777]

Re: new piston in 76 135hp johnson comp not equal

you need to do at least 15-20 hours of low revs and about half throttle running, and make sure you double your mix too<br /><br />then try short bursts of full throttle and see how she goes

 

[delsol]

Re: new piston in 76 135hp johnson comp not equal

ok i will try that

 

[Dhadley]

Re: new piston in 76 135hp johnson comp not equal

If it runs when you spray fuel in that particular carb then most likely the carbs needs cleaning. Since this is the same cylinder that tore up, the dirty carb could have caused the first problem.<br /><br />Carb cleaning is a standard part of engine rebuilding as is a water pump and thermostat service.<br /><br />Good luck!