'84 70 HP Johnson OB Idles but "knocks off" when put in geat

[Harold T. Yates]

Engine has not been run much in the last couple of years. Starts and idles good, but once you put in forward or reverse the motor dies. Before I put the houseboat in the water you could put the motor in gear, forward or reverse and the motor would continue to run. When I put it in the water, started the engine, the motor cut off instantly once the prop had force against it. Any suggestions? Thanks! Harold T. Yates

 

[jtexas]

Re: '84 70 HP Johnson OB Idles but

Re: '84 70 HP Johnson OB Idles but

"not run much in the last couple of years" -- needs a carb rebuild. Soak in carb cleaner, blow out passages with compressed air, reassemble with new carb kits. Drain & clean fuel tanks & fill with fresh gas. Good as new.

 

[mikesea]

Re: '84 70 HP Johnson OB Idles but

Re: '84 70 HP Johnson OB Idles but

do you have a tach,if so what is your idle rpm,in the water it should be about 800,in neutral,650 to 700 in gear,out of water with a flush lit you may see about 1000 rpm.thats how i ran my 70 hp. i dont recall the book specs,but probably not far off. im thinking you may be dropping a cylinder,giving you the impression the eng is running ok,out of water at idle,a tach would show a differance of rpm ,what you can do is,while running on flush kit,remove each plug wire as it is running,if you find one not making adifferance,then indeed you have a mis firing cyl.i would bet the problem is in carburation,probably got foul when sitting,dont run it long if you arent getting fuel to the cyl.no fuel means no lube as well

 

[ManOfwar690]

Re: '84 70 HP Johnson OB Idles but

Re: '84 70 HP Johnson OB Idles but

i had a friend who had the same problems tried power heads and tons of outher stuff turned out it was the timing wheel it was shimed in some manner then ran fine for a year and then replaced

 

[jtexas]

Re: '84 70 HP Johnson OB Idles but

Re: '84 70 HP Johnson OB Idles but

manofwar what is "timing wheel" far as I know this motor has no such part. don't you think it's a bit premature to talk about replacing the whole powerhead?

mikesea is right on this motor will run great on two cylinders till you put a load on it.

 

[DJ]

Re: '84 70 HP Johnson OB Idles but

Re: '84 70 HP Johnson OB Idles but

mikesea is right on this motor will run great on two cylinders till you put a load on it.

I agree. Its' having set for awhile pretty much tells the story.