Late 70s to early 80s 49ci 70hp gurus please

I have a 1977 70hp that i have rebuilt and am yet to have running in the water.

I have covered the basics and am now struggling. Carbis rebuilt, 100psi compression. I have spark jumping a 7/16 gap, but looks weak. Video below.
1977 70hp Johnson weak spark - YouTube

The motor is fitted to a later model 1983-1984 midsection with internal trim/tilt. This midsection has a longer exhaust tube runner with seven relief holes in the inner exhaust tube.

The original 1977 70hp had a shorter exhaust tube with only 1 relief hole in the inner exhaust tube, as the pictures show below. The exhaust gas wouldnt have had to pass through water to reach the 2 external reliefs at all.

http://i1200.photobucket.com/albums/bb329/new2boat/IMG_0800.jpg

http://i1200.photobucket.com/albums/bb329/new2boat/IMG_0802.jpg

And this is the longer tube on the 83-84 midsection which is fitted. It doesnt really look like an original tube like in the parts manual so if you have any ideas let me know.
http://i1200.photobucket.com/albums/bb329/new2boat/IMG_0806.jpg

The only way i can see the later midsection affecting idle, is if the water level reaches the bottom of the longer exhaust tube, and creates more backpressure. Meaning the water has to escape through the water to reach the external reliefs.

It runs fine on muffs but dies in the water at idle and when gear is engaged. Carbis have been rebuilt properly and welch plugs removed.

It looks like in 1979 they changed to the longer exhaust tube, and at the same time the carbis changed to the type with the adjustable jet.

Im tossing up wether to change the adapter plate over, or would i be waisting my time.
Or can i fit 1980-1985 carbis? as those motors are still 49ci?

Pulling my hair out on this one!

Re: Late 70s to early 80s 49ci 70hp gurus please

i think by change over adaptors will not solve the problem if you gonna use a 1983\4 gear box cause the 1977 exhuast will not fit the bottom end however the[ plate asembly] wich the exhust bolts to will bolt striaght on to the adator of a 1977 adaptor cause the gaskets are the same , the water tubes for 1984 & 1977 are the same in 1979 i think there were some changes , check it out , i would & fit a plate asembly from 1977 & exhaust to a 1977 gear box on the 1984 leg
as far as it geos cutting out in gear check for weak spark or 1 plug not fireing , the hole on the top on the 1977 exhaust i think its just a breather hole like when you cut the bottom of a bottle off & keep the lid on push it into water the water will not come up the bottle , now make a hole in the top put it back in the water the bottle fills up i think the same principle here , with the exhaust , cheers

Re: Late 70s to early 80s 49ci 70hp gurus please

Yeah i understand what you mean by the hole in the inner exhaust tube being like a breather to allow water to fill the exhaust tube, but im thinking its also a way of letting exhaust escape to the external relief holes at idle?

Why would the later models have changed to 7 holes if it were just to allow water up the exhaust tube? As in this pic?


At the moment i have what i think is a 1977 gearbox fitted to a 1983-84 midsection. I had to fit a slightly longer shift rod to make it work.

My spark is jumping a 7/16 gap easy, but it looks weak when i compare to my other 80's 15hp johnson.
I checked voltages with a dva meter and got 340 out of stator, and the same coming out of the pack to the coils. Have new coils and new plugs. Here are the vids


let me know what you think.

Re: Late 70s to early 80s 49ci 70hp gurus please

Re: Late 70s to early 80s 49ci 70hp gurus please

100 psi seems low for one of those motors. My 75 triple punches my gauge upwards of 145-150 psi. I would try a different gauge and see if things change, but if it doesn't, then it may be tired...

Re: Late 70s to early 80s 49ci 70hp gurus please

Why would the later models have changed to 7 holes if it were just to allow water up the exhaust tube? As in this pic?

Click to expand...

It is different because of the mid-section, as the lower motor mounts changed position for the 3 ram trim swivel bracket. The holes have nothing to do with filling with water as it is filled as soon as you back in in lake, the holes are exhaust/backpressure relief holes as not much exhaust exits thru prop at idle when submerged. Try a 3/8 gap and see if spark brightens up a tad, as for idling/dieing in gear check you sync and link, idle timing, and as suggested get anther gauge and test compression as 100 is low. You posted you rebuilt motor, did you bore it or just hone and rering? If rering the cylinder and piston may be worn and cause low crankcase pressure which will cause idling and dieing in gear problems....

Re: Late 70s to early 80s 49ci 70hp gurus please

i would & fit a plate asembly from 1977 & exhaust to a 1977 gear box on the 1984 leg

Click to expand...

Yeah thats what im currently in the process of doing now. Im fitting the 1977 adapter and inner exhaust over to the 1984 leg.
Im not sure what the adapter i had fitted at the moment was to suit, but when i bought it from a boat parts shop i was told its a straight fitup. So thats what i did.
I was talking to someone during the week who knows these inside out, and he said you have to run the adapter for that engine as they are tuned, and will affect backpressure and idle.

I am also modifying the 1977 adapter to suit the later upper mounting setup, by drill and tapping for the centre bolt.

You posted you rebuilt motor, did you bore it or just hone and rering?

Click to expand...

I had it bored 0.020 and i fitted new piston/rings. My compression gauge is new, so who knows. I have also been checking it cold. It may jump up a little when warmed up.

So i will see how things go once i get it back together, and let you know.