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1985 johnson 100 hp commercial question

C

Capnstern

Member
I need a little advice. I have recently purchased a 1985 100 hp johnson comercial outboard. Compression close to 115 psi across the board. I have also purchased a 1986 johnson 90 with a cylinder down, mainly for the power trim. The 90 has a lower unit with a crack in it that has been epoxied and doesnt appear to leak. I've read that the commercial outboards have a lower compression ratio. I have given some thought to pulling the cylinder heads to inspect the deflectors in the water passages and am wondering if i should put the heads off the 90 on the 100? also, my motor has the bigger v6 style case in great condition. Should I run it or would I be better off if i repaired the 90 lower unit and ran it instead? I have access to a TIG welder and am fairly skilled at welding aluminum. Any thoughts?
 
Your 100 does not have low compression heads. If the crack is on the bullet anywhere, no it cannot be welded. Reason being the minute you start welding it will start to warp and It will throw off the propshaft alignment. If it is in the skeg yes it can be welded.
 
Ok. Thank you for the info flyingscott. If I got my hands on a standard small diameter gearcase like what is currently on the 90, would I be better off to run it, or better off with the large gearcase unit? In your opinion, even though i dont know if i would want to run it,would it hurt anything to run the cracked lower unit sealed with epoxy? Of course, it would need to be checked often for leakage. My concern with the large diameter gearcase is that the gear ratio is different vs the small diameter. Some say you can compensate with a different pitch prop and some say you cant. I do not know the reasoning behind either argument. I just remember reading something several years ago while i was trying to find a replacement lower unit for a 65 hp mercury. Thanks in advance.
 
Thanks racerone. I thought so, but really wanted someone else's opinion. I had it in my head that prop selection would be an issue as most engines with the v6 case are much more hp, pushing bigger boats. I'm content to run the bigger lower unit as barring anything idiotic on my end will not be as prone to wear and tear with my lower hp engine. Thanks!
 
Its a large V-6 housing running the high alitude gears(2:1).. All depends on what type of hull its going on. If a toon or ski use the big'en
 
faztbullet, Its going on a 1990 bumblebee 254, 16' shallow vee bass boat.The internet says hull weight is 575 lbs. I assume that it is not running V6 gears as its painted the same dull grey primer color as the rest of the motor. Haven't had a chance to check the ratio due to work though.I have not gotten a really good look at the damage on the 90hp lower unit as its laying on its side in the garage. I'm going to hoist it in a couple of weekends so I can get a good look at the lower unit. It didn't have any water in it when I drained the lube.
 
Slap it on, put at 3rd mounting hole, get a 13 thru 13-1/2 x 17-19 pitch prop..speed be in mid 40ish... Was built right down the road from me. Quick lil hull and will need to learn to drive as it will chine walk when on pad.
 
Faztbullet, it was built down the road from me too. Two of the three boats I've owned were! So just to make sure I understand, Should I run the big lower unit or the one off the 90hp and I assume you mean use the third mounting hole down from the top? Also, I do have a manual 6" jack plate that was given to me a while back. Should I mount it? As far as Chinewalking, For comparison on this hull, I had a late 70's merc 85 hp on it. 37 mph with a 3 blade 12x19 pitch aluminum prop. It was quick to jump up on plane seemed to handle great, and felt rock solid steady. I didn't ever run the jack plate on it as when I mounted it on the boat with the merc, I my inexperienced opinion,I could never get it low enough to keep the water intakes from sucking air . Of course, that motor used a transom clamp, which is probably why I couldn't adjust it enough. I Never could get the tach to work right so I don't know how many RPM's, although it didn't sound excessive.
 
Ok. I have pulled the lower unit off of the 90hp parts motor. It seems to have a slight rumble sound when you turn the driveshaft, so i ain't to sure about the driveshaft bearings being usable . While we are on the subject of driveshafts, i doubt this is usable. See the pic. My question is, what could have possibly caused this? IMG_20191202_182901_compress53.jpg
 
There are still folks who will refuse to do maintenance.----That spline should be greased on a regular basis.----Once a season or every 2 years with light use.-----Rust in the spline joint is abrasive.
 
Yeah i agree.I always grease the splines when I replace a water pump Between the rumbling pinion bearing, the melted impeller/housing and the crack in the gearcase, i'm going to keep this one for parts and use the larger type O gearcase... although I haven't dropped it yet. I am hopeful i don't find any issues with it. The lube was clean, it seems to turn smooth (in neutral) but the water intake screen isn't there and I can see its full of crap thru the holes and there appears to be some surface corrosion on the bearing carrier. I'll drop that one tonight and have a look. Do y'all see any issues using the larger lower unit?
 
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