70hp gearcases, midsections, timing

[dcr42t]

I have a 1977 70hp Johnson which i have upgraded to a newer midsection, i think its 1984, with has the internal trim.

When fitting the gearcase and adjusting the shift rod, i noticed something wasnt right. It was about 10mm too short, and then i realised the adapter plate between the block and midsection was around 10mm taller than the older 77 midsection. The driveshaft fits only 15mm into the crankshaft spline, when usually it would be in about 25-30mm.

I really dont want to have to change the adapter plate, as bolts are rusted etc.

Can i just lengthen or change the shift rod and leave the drive shaft as is?

Anyone else had this drama?

 

[Chinewalker]

Re: 70hp gearcases and midsections

Re: 70hp gearcases and midsections

Swapping the shift rod shouldn't be an issue, but you're knocking out almost half the engagement of the driveshaft splines. I can't say for sure if less engagement of the splines would result in shortened lifespan, but there's a good chance it would...

 

[dcr42t]

Re: 70hp gearcases and midsections

Re: 70hp gearcases and midsections

Yeah, im thinking it may want to chew out the splines a little quicker or shear, but i will see how it goes.

I got another shift rod from a wrecker, from an 84 gearcase. It is 10mm longer than the 77 shift rod. I adjusted the shift rod height to the later model height and all good.

Just have to sort out the idle, and timing.
I have set the wot advance to 17deg.
I know you dont really have to set the idle timing, but is there a base value i should be seeing at idle rpm? Like 4-8deg btdc?


I have a manual on order, but havent recieved it yet.

 

[WernerF]

Re: 70hp gearcases and midsections

Re: 70hp gearcases and midsections

I would expect the idle timing to be a few degrees after TDC. On the newer 70hp models (19? WOT timing) the idle timing is 1? BTDC when the throttle cam just touches the cam follower to open the carb butterflies. But then the rpm is higher then normal since it's the spark timing actually setting the idle rpm and not the carbs.

 

[dcr42t]

Re: 70hp gearcases and midsections

Re: 70hp gearcases and midsections

Ok, thanks mate. I will check what mine is doing when the cam touches the follower.
The main trouble im having is getting it to start straight away as soon as i turn the key, and idle smooth. Just want to make sure timing is right while cranking, before pulling the carbies again. But it could be fuel related though. I have cleaned the carbies, but didnt remove the plugs or jets as i could see through the mains, and blew through the idle circuits, and they looked spotless. I changed the 3 seals on the main jet tube. If it runs without touching the butterflies, but just using timing, thats saying the idle circuit is ok anyway isnt it?

 

[WernerF]

Re: 70hp gearcases and midsections

Re: 70hp gearcases and midsections

Yes, I would say the idle circuits deliver enough fuel, if the engine is idling on all three cylinders without coughing. This engine idles nearly the same with (any) one cylinder missing, so it might be overlooked.
On these carbs the idle circuits don't seem to be prone to clogging. Being located in the carb's top and fed via the brass tube makes the circuit drain empty when the engine is stopped.