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Merc 1980 90hp inline 6 question

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shipwreck

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"Hi all, new to this forum wit

"Hi all, new to this forum with a couple questions. I have a 1980 90hp 6 cylinder Merc, I have been told that the only differece between this and the 115hp powerhead is the carbs. Can someone verify this? Also,this engine seems hard to start when cold... compression tests good, spark on all 6 plugs is good. Any ideas? One more thing...it tends to grind when put in reverse. Could this just be a shifter cable out of adjustment or is there something these gearcases are noted for? Thanks for any help you can give.
Chris"
 
First you have to shift quickl

First you have to shift quickly------ no " granny shifting " is allowed. In order to make this motor start properly the best advice I can give is to install an OMC type injection priming start system. Not hard to do. Also check the bleed inserts usually 3 or 4 of the 5 are missing.
 
"Chris, They are hard to start

"Chris, They are hard to start when cold. It is the nature of the beast. Make sure they battery cables are clean and tight so it spins as fast as possible when starting.

The 90HP and 115HP blocks are likely different, as are the carbs or carb jets. You can check part numbers as see what is common and different. It is likely the crank, bearings rods and pistons are identical.

The gearcase should not grinde in reverse. make sure the motor is idleing at 700-800RPM, and put the motor in gear sharply."
 
"Thanks guys. The grinding is

"Thanks guys. The grinding is nothing major, just a light grind as it shifts into gear, fine once it is in. It does not do it if I shift it quickly... maybe it is nothing to worry about. I guess I will scrap the idea of putting on 115 carbs, probably not worth the headache. Could someone please explain the bleed inserts, I did some searching around here and heard of them a couple times, but I still don't know what they do or where they go. I also read something about a primer setup, I will definately look into that. Thanks again."
 
"Sure can, the bleed inserts a

"Sure can, the bleed inserts are wee little nylon parts. The manufacturing procees call for a big hole to be drilled ( breaks fewer drill bits ) and then the insert is pushed in.You remove the bypass covers to find them.Now the tower of power suffers from a poor choke set-up , combined with wear and tear on the motor makes for a hard to start condition.So a friend of mine believed me when I said " lets install the OMC set-up " did the job and hard to start was history.I have 40 years of motor fixing , but you don't have to take my word."
 
"Thanks guys. I will definate

"Thanks guys. I will definately install an OMC primer on it, alot easier on the starter and the motor instead of dry cranking the crap out of it. Compression tested at 127-130 on all 6 cylinders so I think the engine is in pretty good condition. Once it runs, it runs well. I still don't understand the purpose of the bleed inserts. What are they there for and what are the symtoms when they fall out? Thanks for the info guys, I've learned alot from this forum in the last couple days."
 
The bleed inserts go from # 1

The bleed inserts go from # 1 crankcase to the # 2 cylinder( and #2 to #3 etc ) and apparently excess oil and such is blown onto the #2 cylinder from #1 etc. If the insert is not there it is a big hole relatively speaking and you could loose crankcase compression. ( poor starting ) is the result.I install an #8-32 set screw with a small 0.040" hole when I put one of these together.
 
"Thanks, that makes sense. I

"Thanks, that makes sense. I guess since they're such an issue on these I should probably check them before I launch it this year. So these are under the cover on the left side of the powerhead? You'll have to excuse me, I'm an auto tech but have little outboard knowledge. Learning as I go I guess, lol."
 
Just remove the top bypass cov

Just remove the top bypass cover and you will see the hole( 3/16" dia )Use a toothpick to see if you can reach down to the piston below( rotate the flywheel to feel if you are touching the piston.New inserts are cheap.Issue or not they will help the motor start and idle better.
 
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