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1968 Johnson 20 hp stumbles

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lakegastonpack1A

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I have a 1968 Johnson 20 hp motor that has been in the family since new. Usually starts good, and runs ok. The last couple of times I have had it in the water, it will be running along fine, then suddenly start cutting back, and losing power. After a minute, it picks back up, and runs ok, but it may do it again. It has never cut completely off when this happens.
I have replaced plugs, and gas is fresh.
Any ideas ? Thanks.
 
Check for strong spark !-Not just spark that will jump the gap on a sparkplug laying on the block.---A sudden / instant change in operation is usually due to ignition problems.----Rig up something that will force the ignition system to jump a gap of 1/4" or more.-----------Have the coils ever been replaced ?---If not it is likely due for a new set.
 
kimcrwbr1 said:
Have you tried leaving the gas cap loose or is the vent fully open? It could be a restriction on the strainer in the tank. Look at the primer bulb and see if it is collapsed. Ethanol has a habit of messing with our outboards put some seafoam in the tank that may dissolve the varnish that builds up. Has the carb and fuel pump ever been rebuilt? Give the engine a thorough diagnostic, check compression, check spark gap with a spark gap tester, check engine sync and link, service the fuel system and tank. How old is the impeller? When running after about ten minutes put your hand on the head and count slow to six. It should be hot enough you want to pull your hand away but dont have to.
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Fuel line and bulb are not old and hard. Bulb not collapsed. Vent on tank cap fully open. Always run "green" Stabil, and occasionally, regularly run a little Seafoam. Plugs have several hours on them, but are not really old. Water Pump impeller old, but moves good water, and head not terribly hot, even after a long run.
I kind of suspect that I need points and condensers, but I really hate to mess with that, because the motor cranks so good, but I have to wonder if there is corrosion or a bad connection of some kind under the flywheel ?
I guess I should do a compression test. What should it be on this motor ? Or is it really ok as long as both cylinders are fairly even?
The motor is old, obviously, but it doesn't have that many hours on it. It has run more in the last 8-10 years than when it was new.
Thanks for any help !
 
If the impeller is " old " you need to replace it !!!----It can suddenly fail and you will ruin a fine motor !!!!---------Maintaining the stuff under the flywheel is easy.
 
Thanks for all the info. I have bought points, condensers, and plugs, and will get them installed this weekend.
One more quick question-- what is the point gap setting. I saw a reference to .020, and need to confirm that.
Thanks again....I will probably be back !
 
Follow up on this motor--been running ok after a good tune up last year, until a few weeks ago, when it started being hard to start, and seemed to be flooding. Carburetor had not been cleaned in many years, so I got a kit, and went to it. Had trouble getting it going again, but eventually got it started, and I thought running strong. After that first run I had not run it for about 3 weeks; tried to run it today, but it was very hard to start, and then keep running, but eventually ran ok. I assume I need to fine tune the carb adjustment, but would like to hear if anyone has any other ideas.
Thanks !
 
There has been no response to a spark test----Does spark jump a gap of 1/4" , yes or no ?----------Poor spark is the main cause of a hard to start condition !
 
Yes, the spark is strong. Points, condensers, and coils changed recently as part of the tune up, and the spark looks good. The motor runs good....once it ever starts.
 
Thanks for the suggestion to adjust it in forward gear. I put it in again today, got it started, and did just that, and it seems to do good. Shut it down for a while a couple of times, and it cranked back up the way it should.
I think the old girl really needed the carb cleaning/kit, as it ran better today than it has in a long while. Just hope it lasts !
Thanks again !
 
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