75 Johnson 70hp

This motor is the inline 3 cylinder. It idles and will run good until it gets to about half throttle, then its as if it floods out, then dies.If I let it sit for a short time, it will start up and run fine as long as I don't go over half throttle. If anyone knows what the problem is,or can advise me on troubleshooting the problem I would be greatful.

Re: 75 Johnson 70hp

Welcome fellow Floridian - Next time out when it starts fussin squeeze the primer bulb. If it smooths out that indicates a possible fuel delivery problem. I suspect that it is time to rebuild the carbs & check all other components of the fuel system.

Re: 75 Johnson 70hp

Yes, what SoLittle said.<br /><br />Start with the fuel filter, then the fuel pump. If that doesn't fix it, look into the tank pickup. <br /><br />With three carbs it is unlikely all three would go bad at once unless they have been contaminated.<br /><br />Good luck.

Re: 75 Johnson 70hp

I have replaced the fuel filter and fuel pump but I will try the primer next time I take it out. Someone had told me that the motor idles off of one carb and as you trhottle up it opens the other two. But all three flow flaps open evenly, so I don't think he was right. I have concitard compression in one cylinder might be bad enough to cause it to die out once its under enough load.I will check that this weekend. I have also concidard an ignition problem, but I'm not sure how to check that. Nay

Re: 75 Johnson 70hp

Nay...the engine idles off of all three carbs. Take a look at the float bowl gaskets. If they are black, your in need of a carb overhaul. The new gaskets are made of Viton, a material that's more resistant to the new gas and not so apt to dry out. Vitons color is a redish brown.<br /><br />Do you have any idea when the last time the carbs where overhauled?<br /><br />Good idea to get a fuel compression test and do a spark check also. If your considering keeping the rig, make an investment into a service manual.

Re: 75 Johnson 70hp

Regardless of the engine's problem, the first two things to be considered and checked are the compression and ignition (spark).<br /><br />The spark plugs should be Champion QL77JC4 plugs, gapped at .040.<br /><br />Compression should be 100+ psi and even on all cylinders. Rig up a spark tester whereas you can set a 7/16" gap for test purposes. Spark (with the s/plugs removed) should jump that 7/16" gap with a strong blue lightning like spark.... a real SNAP!<br /><br />If the above two areas check out okay, in all probability the carburetors are fouled, in which case they will require removing, cleaning, and rebuilding.<br /><br />Note that the engine runs on all three carburetors regardless of the rpm. The engine derives its lubrication from the fuel/oil mixture. If one carb was out of operation as your friend suggest, the associated cylinder/piston would not be lubricated at that rpm. Guess what that would lead to?

Re: 75 Johnson 70hp

I have found one of the three coils was not firing. I'm sure that the motor running on only two of the three cylinders was my problem. Now I have the task of finding a new coil. Thanks to everyone for your replies.<br /> <br /> Nay

Re: 75 Johnson 70hp

WadeNay. i have a 1974 and some extry parts . the coils are avaible at N A P A auto parts i have several that will work . all the older modles have that coil . the ones that mount on the outside . the onley thing that the 1974&1975 has different is the pistons . they arenot avaible .i had the same probablem . it would run great cold when it warmed up it would break down . let it sit a few minutes and it would plane and then break down . e mail me at papasage10@yahoo.com. i can help you with the coils . i have several of them

Re: 75 Johnson 70hp

I have the same model motor only a 91 had the same problem. Used OMC engine tuner twice it seemed to help but the biggest help was gapping the plugs to .040 not .030 this made a huge difference plugs do not appear wet any more .. Keep us posted

Re: 75 Johnson 70hp

Papasage -- the 1976 pistons can be used in the older motors. You just need a wrist pin bearing kit or the 1976 rods. No big deal, dont let that stop you from putting one together.

Re: 75 Johnson 70hp

yes Dhadley i am aware of that . i am looking for some used pistons . i have some for the newer one . i am just looking for some more . didnt want to buy new ones . i have 2 of the original ones but i would like to have the newer ones . they are for the no led gass . i have 3 from a 1979 onley 1 is still good . i might could use 2 of them . i havent cleaned them up and inspected them . i am looking for matching set . papasage