1979 Johnson 70 ?s

[trx450always]

So our family has had a 1979 sand dollar with a 70hp johnson. no VRO. the engine has ran perfectly excapt for the time the water pump failed 3 years ago and we needed a new head.
Over the past 5 years the boat has slowly lost power. now it has trouble pulling a tube up to planing speed.
I am looking to do a compression test and other stuff, but what would be the best way for me to restore the engine and get some power out of it.
Sam

 

[Benny1963]

Re: 1979 Johnson 70 ?s

decarb and run compression check .
bennyb1963

 

[trx450always]

Re: 1979 Johnson 70 ?s

yes, I will be doing that tomorrow. I will be replacing the pistons and rings even if the comp is over 100PSI. just becasue it has been 30 years. lol

any place to get the OEM parts.

 

[rebuilt]

Re: 1979 Johnson 70 ?s

From what little I've found, OEM parts are still available for 40 yrs from the date the motor was built. You could try the BRP site, if you have to use oem. This site has a load of parts. Mostly sierra. If not try ishopmarine. Add a .com to that and you're there. KR

 

[trx450always]

Re: 1979 Johnson 70 ?s

thanks. I will try those sites.
Would you all reccomend doing a completely new top end, or just replacing the rings? I will also be cleaning the carbs and replacing the fuel pump because it appears to be losing power.
If I wanted to make more power what aftermarket stuff should I look at getting? High comp pistons? Better intake or anything else?
thanks a lot,
Sam

 

[rebuilt]

Re: 1979 Johnson 70 ?s

The best thing you can do is replace with stock. If you have to have more umphhh, get more motor. Before ya just go to buying parts, a little detective work would behoove ya. These things don't wear like a 4 banger, the rings don't rotate. They have to be fixed to clear the ports. Soooo. since they don't rotate, the bore wears out of round. Eliptical. Find out how much metal you've lost, and get the right oversize after figuring for a bore job. A decent shop would measure for ya if ya don't have micrometers. Oh, did I mention get a manual?? A full overhaul? Yep, bearings the whole bit. Measure EVERYTHING. If ya go halfway on a worn out motor, chances are it'll leave ya halfway between where you started from and that spot on the water you thought you were going to. Good luck. KR

 

[_brad_]

Re: 1979 Johnson 70 ?s

I say if you have over 100 PSI on all cylinders, why in the world would you want to change out the rings?

If it ain't broke dont fix it. I say if there is good compression, do a decarb and call it good.

Change the rings out when and if the head ever gets toasted.

 

[rebuilt]

Re: 1979 Johnson 70 ?s

Great point. He hasn't run the comp check yet, just stated if 100 would rebuild. Whatever, not my money. I've read guys on this board that had 'em down to 70psi and still running. I've read over and over that 100 is good to go. But, if he wants to rebuild, go for it. KR

 

[trx450always]

Re: 1979 Johnson 70 ?s

ok, well I just ordered some new plugs, and a carb rebuild kit. Tomorrow I will check the comp and see where I am at. I guess if I am over 100 then I will just leave it be. If not, then I will go from there. I have micrometers and the whole bit. So that will not be a problem. We have a new 21 foot sea ray but this is just a project boat. i was actually thinking of milling the heads like .020 and running some 105 race gas. More of just a fun project than a serious boat. Just gonna use it in the bay when I do not feel like spending all day cleaning and prepping the ski boat. The hull in this will probablly crack in half before the engine goes. lol

 

[noelm]

Re: 1979 Johnson 70 ?s

yes fully agree, do not rebuild it just because of it's age, if compression is good, then find out the reason for the low power.

 

[trx450always]

Re: 1979 Johnson 70 ?s

ok, the only thing besides compression IMO would be gummed carbs, bad plugs, bad fuel pump, or bad ignition. I am betting on either the carbs or the compression.
Thanks for the quick help guys. I will be sure to let you know what happens.
Also one more question, the engine itself is rusting pretty bad where the white paint has chipped off. Any technique to clean it up, or just have at it and repaint with a hightemp marine sealer?

 

[Benny1963]

Re: 1979 Johnson 70 ?s

yes if tour comp readings are 100 + AND EQUAL GIVE OR TAKE A LITTLE
you should be good my 70 reads 110 104 after decarb
it pushs 16ft alum bateaux 40 + mph on gps
benny 1963