1973 Johno 115 piston scoring - is it terminal?

**Gilera** · 12-28-2023, 02:56 AM

Have put a fair bit of time into this new to me motor getting it to run and then getting it to run right. Came on a boat and don't feel that i paid anything for it really. Noticed when checking an earth connection that one of the transfer port inspection plate bolts was quite loose. When i checked the others they were all pretty loose. Not super loose but no effort really on the spanner to turn them. Wondering if this might be contributing to my rough idle i decided to change the gaskets. Started on port side. Cylinder two not looking great. Compression fine though. What do you guys reckon? No other choice than to run it and see what the future holds i guess?

The coil on this cylinder was very suspect. What causes an isolated cylinder to score? Lean condition?

Click image for larger version.

Name: 20231228_181928.jpg
Views: 46
Size: 418.3 KB
ID: 528227

Click image for larger version.

Name: 20231228_181727.jpg
Views: 43
Size: 402.9 KB
ID: 528228

**ChrisCarsonMarine** · 12-28-2023, 08:32 AM

Probably got hot once upon a time,don't worry about it...very loose forgiving motors,Chris

**James45157** · 12-28-2023, 09:53 AM

i think chris is correct run it they are very loose motors

**Gilera** · 12-28-2023, 02:09 PM

Originally Posted by ChrisCarsonMarine

Probably got hot once upon a time,don't worry about it...very loose forgiving motors,Chris

Great news thanks!

**H2OPERF** · 12-28-2023, 08:11 PM

If its been bored os they may not have undercut the aluminum at the bottom of the sleeve.

**pcrussell50** · 12-28-2023, 08:25 PM

Originally Posted by H2OPERF

If its been bored os they may not have undercut the aluminum at the bottom of the sleeve.

Is that an engine-destroying oversight? Or just sloppy work that the engine operator can live with?

-Peter

**H2OPERF** · 12-28-2023, 08:34 PM

If its been running and its still running let it rip, But I have seen it wipe out a motor..

**pcrussell50** · 12-28-2023, 11:19 PM

Speaking of which, back in Australia for a few days, Cairns which for the Americans here is a big’ish coastal city, deep in the tropics. Lots of boats. Lots of fishing. Regular boats and regular people. Already seen more carbureted Jhonos and Mercs than I can count. Even a few V4 crossflows dripping wet on the trailer where it had just recovered. Salt brackish and fresh out here.

Heaps of old and new carbie motors. More use than show. Australia allowed new carb motors until 2020, or just before.

-Peter

**skialot2** · 12-29-2023, 01:29 AM

That is not what you want to see. It could run like that for another 5 minutes or another 5 years. It will probably run hot and make some noise.

**FMP** · 12-29-2023, 09:23 AM

Feed it 40:1

**pcrussell50** · 12-29-2023, 05:29 PM

Originally Posted by FMP

Feed it 40:1

my high compression 1975 bubble back, I would mix in a little leaded race gas if I was going to run hard.

-Peter

**James45157** · 12-29-2023, 06:04 PM

if its at the bottom of the piston skirt it may run a long time who knows those had running clearince of .055 to .oo8

**Gilera** · 12-31-2023, 02:01 AM

Could it cause it to have idle and running problems? I have troubleshooted the hell out of it and still running like crap. Cant even get it to idle without a lot of throttle lever advance. Surges when running like bad spark or fuel delivery. Like its missing but i have good spark and plugs are all dry. Could it do that?

I've cleaned carbs 3 times, changed carb gaskets, replaced and sealed core plugs with nail polish, changed coils, power pack, checked compression 130 on all 4 cylinders, timed it used WOT cranking method at 4 deg less than spec, replaced fuel pump, replaced spark plugs, linked and synced, tried different fuel hose, cleaned fuel connection to motor, changed fuel lines on motor, checked for spark over 7/16" gap, checked reeds, fresh 91 octane fuel and synthetic oil. Changed transfer port inspection plate gaskets and checked rings. Checked all stator and timing base specs with multi meter and DVA. Cleaned every earth connection on motor and coils. Replaced rectifier. Only think i havent done is taken off the flywheel to visually inspect flywheel magnets, timing base and stator. The parts i have tried are used but I have been quite careful to keep only working parts from other motors but possible I am replacing a failed part with another one which is the nature of not just throwing endless money at the motor until i know what is wrong with it. Funny thing is that after replacing coils i thought it was fixed. Started and idled ok without missing and jerking. Tried to take it out today with the family and back to unable to idle and kicking about. The water coming from the exhaust relief ports did feel pretty hot but no signs of overheating as such. Because the motor is new to me the scoring could have been done last trip or last decade, I dont know. Its just the only obvious thing i can find wrong with the motor. Whatever the issue is it is somewhat intermittent in nature and severity.

**pcrussell50** · 12-31-2023, 04:37 AM

For completeness, you should probably do a leak down test too. The legends here will tell you tales of motors that look good under compression but reveal issues under leak down.

-Peter

**skialot2** · 12-31-2023, 03:21 PM

Check and test the whole bleed system. a bad bleed system would cause a rough idol and stalling. Kind of like having a vacuum leak on a car. Check for clogged, collapsed or pinched hoses. Test all the check valves for one way operation. Test the reed valves as per the manual. I believe you need a pressure gauge to connect to the bleed system.