1977 Johnson 70HP

D

deofc

Seaman
Joined
Nov 8, 2003
Messages
61
Motor has been running fine for 3 years, I recently let it sit for a month and now it cranks up fine and runs fine up to about 1/2 throttle and then the motor just bogs down. I can put it in neutral and manually advance the throtte linkage and it will rev up past 1//2 throttle with no apparent problem, but when I am underway the rpm's just drop off after 1/2 throttle. Any ideas. Thanks.
 
JB

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: 1977 Johnson 70HP

Hi, Deofc.<br /><br />First: Never rev a 2 stroke with no load on it. They often go into thermal runaway and scare the ****ens out of you.<br /><br />They sometimes grenade parts all over the yard.<br /><br />You may be making power with only one or two cylinders, or you may have a failing fuel supply.<br /><br />Confirm that all 3 are making power by disabling each spark plug in turn (at idle). If there is no drop in rpm, that cylinder is making no power.<br /><br />Check for spark. If you have spark, overhaul the carb for that cyl.<br /><br />If all 3 make power at idle try pumping your primer bulb when you try for WOT. If it picks up, check your tank vent, check for restrictions in the tank pickup and hoses, check for air leaks in the gas lines and, finally, test your fuel pump for a strong output.<br /><br />Good luck. :)
 
D

deofc

Seaman
Joined
Nov 8, 2003
Messages
61
Re: 1977 Johnson 70HP

JB --- Thanks for the reply. I will check all of that this weekend. I appreciate the help AND the quick response.
 
S

SpamPutney

Seaman Apprentice
Joined
May 3, 2001
Messages
30
Re: 1977 Johnson 70HP

JB,<br /><br />Correct me if I'm wrong, and I shall bow to your superior knowledge. :) <br /><br />I recently had the same symptoms on my 1980 Johnson 70. I checked all things related to fuel, and all was well. I identified the top cylinder as the problem, by disabling the plugs one at a time. But, I HAD spark at the top plug. After scratching my head for a few days, I decided since I was POSITIVE my fuel was okay, it had to be spark related, even though my eyes told me the spark was fine. I replaced the coil for that cylinder ($20) and that fixed the problem.<br /><br />Hope this helps.
 
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