1978 Johnson Seahorse 70HP Qestions?

capt littlelegs · Jun 16, 2010

dustycrockett said: ↑

traxxas, the telltale hose outlet is a nipple on the lower cowl, starboard side.

You can access online schematic diagrams and parts catalogues at http://shop.evinrude.com click on "electronic parts catalogues", select "johnson" in the dropdown box and drill down to your hp/year/model#

Water pump service seems intimidating at first but you can do it:
1. remove prop
2. to disconnect the shift linkage remove the 7/16" hex head screw under the bottom carb accessible from the port side of the engine. I find it's easier to get to if I remove the front of the fuel pump and move it aside. It's held in place with a thumbscrew. There's a screen filter there you need to inspect anyway.
3. remove the trim tab (scribe a mark across the edge so you can get it back like it was).
4. remove the retaining bolt up in the trim tab cavity.
5. remove the other retaining bolt from the bottom side of the antiventilation plate.
6. remove the four retaining bolts from the perimeter, just above the antiventilation plate.
7. pull the lower unit off.

the shift rod height is adjusted by screwing it in or out, so don't twist it or else it won't fit when you put it back.

8. There's a grommet around the driveshaft, don't forget it.
9. Four hex-head screws hold the water pump housing down.
10. Slide the old impeller off the drive shaft.
11. Slide the new one on. Slip the woodruff key in place just before the impeller covers the keyway.
12. Put a new housing gasket in place.
13. Grease the inside of the housing (trailer axle grease is fine), and shove it down onto the impeller while turning the driveshaft clockwise with your hand. Don't use any kind of tool on the driveshaft splines.
14. Fasten the housing back on.

On reassembly:
* put molly lube on the drive shaft splines (but not on top, there's not enough clearance).
* the rubber o-ring just below the drive shaft splines -- you need that.
* put omc gasket sealant (or some kind of anti-sieze) on the retaining bolts -- or else corrosion will set in & it'll be a ***** next time.
* use 3m #847 or permatex #2 on the joint between midsection & lower unit. www.ishopmarine.com has 847 -- permatex is way harder to clean off there next time around.

I find it's easier to get it back together if I disconnect the shift lever from the cable (starboard side of the powerhead). Just pull the small cotter pin.

Your motor will run best on Champion QL77JC4 spark plugs gapped at .030.
Use only fresh 87 octane gasoline mixed 50:1 with TCW3 certified oil. I add 1 oz. of Seafoam per gallon -- BRP Engine Tuner, Yamaha Ring-Free, or similar. Highly recommended.
Use a "Marine" lower unit gear lube if you can get your hands on it...it will tolerate water a little better than automotive without breaking down.
Click to expand...

He's done this before!

Traxxasmaxx · Jun 19, 2010

Good Stuff

Hey thanks dustycrockett. I am taking her out tomorrow and will be buying som plugs in the morning. So that number really helps. Just out of curiosity last time I took it out for some dumb reason I didnt let it warm up. At full throttle it just wouldnt go, but then out of no where it would take off. I was thinking I might have fouled a plug and when it got fire it would run like normal. Maybe its a coil though. The guy I bought it off used it everyday to check his crab traps and never had this problem but he always let it warm up first. Today I put it in a dunk tank and when in neutral it runs and revs like a champ. No delays in throttle response. When I take it out tomorrow if it has the same problem even after I let it warm up any ideas as to what it could be?

dustycrockett · Jun 21, 2010

yeah it'll do that cold -- did it get better after it warmed up?

If not, bogging like that is a classic symptom of dirty carbs but it could be a number of things. I always do a compression and spark test first. Compression should be about the same on all 3 (within 5 or 7%), anything upwards of 100 would be ok, I'd expect around 120. Spark should jump a 7/16" gap on an inline tester clipped to the block. Inspect the fuel tank for water & debris. Make sure the fuel line holds pressure.

If it bogs, but you can nurse it up to WOT, check to make sure the throttle cam hits the roller right on the mark, and the timer base moves smoothly to its stop without sticking when you advance the throttle (see attached -- that's a mid-eighties model, but all the parts are there). Those plastic fittings holding the throttle/spark rods in place, they only last 29 years or so before they start breaking up, and you'll get some weird symptoms.

Running in a tank isn't a good test of how it'll perform under load. In neutral, it can sound surprisingly good on 2 cylinders.

I have one of those but its a '79 vintage evinrude -- virtually the same motor. In spite of having no formal mechanic training I've never put it in the shop. Get yourself a shop manual. There's really not much can go wrong with this motor that you can't fix yourself. I highly recommend the factory manual, www.iboats.com or www.kencook.com or www.marineengine.com all have it. Will pay for itself many times over.

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1978 Johnson Seahorse 70HP Qestions?

capt littlelegs New Member

Traxxasmaxx 1978 Johnson Seahorse

dustycrockett Junior Member

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