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73 6hp engine surge under load

TheMagicWrench

New member
I have a 1973 6 hp Johnson 2-stroke engine (model 6R73S) that surges under load. I bought it second hand about 8 months ago in decent condition. For a little while I have had more difficulty starting it and it does not idle well at low idle. Then last week I took it out and while under way it started to bog down and slow. With the throttle kept wide open, it will spend 10 seconds to up to a couple minutes moving slowly and then surge up to a higher speed and stay there for 10 seconds to a couple minutes. I haven't noticed any pattern. I tried pumping the primer bulb with no effect. I also put it in neutral a few times and had no problem reving it and maintaining a constant RPM. I supposed that there was a fuel delivery problem, and so after that trip, I rebuild the fuel pump, and it started up easier than before and also idled much better. But when I took it out today, I had the very same problem with the engine surge/bog. The gas in the external tank is fresh and has oil and stabil in it. Any suggestions for a next step? I am thinking to rebuild the carb, but I wonder if there are something else I should check first.
 
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Sounds like it is running on one cylinder at times.-----Simple test.-----Does spark jump a gap of 1/4" on both leads with a snap you can hear, yes or no?
 
Hello, Magic. Racer is on to it. Sometimes the main spark coil (one of them) will arc internally while underway. Other tests may show poor point continuity, or occasionally a bad condenser. I would focus on spark first. You will see a weak spark with the aforementioned test. A yellow or orange spark is not desirable. You want a nice crackling white or blue color, having no trouble to make that 1/4" jump.
 
If you haven't stripped and cleaned the carb before,it will definately help.
My 6hp evinrude fisherman has the pedestal type pull start,it can be a pain to remove and refit,which you may need to do if you take the carb off.
 
VB, what I do is remove the 2 cap screws (bolts) on the retaining sleeve, remove the verticle rope guide post, then just swing the top of the recoil unit to the right and forward, keeping the bottom of the unit intact. You can then access the carburetor cam follower and mounting nut.
 
VB, what I do is remove the 2 cap screws (bolts) on the retaining sleeve, remove the verticle rope guide post, then just swing the top of the recoil unit to the right and forward, keeping the bottom of the unit intact. You can then access the carburetor cam follower and mounting nut.

Wish I knew that before I did mine TG,Haha.I put new coils in mine and an impeller,ran it on my rowing boat for an hour,

then put it in the shed. (2yrs ago)
 
I checked for spark and there is none whatsoever in the second cylinder. I found a point of wear on the #2 high tension wire and am going to replace it, but I don't think that is the problem (since there wasn't any arcing there either), but I also noticed that the #2 coil has a glossy sheen on it while the #1 is dull and matte. Is that a sign of over heating or increased resistance? In any case, I'm not sure what the next step would be.
 
No surprize to me.-----Replace the coil on the no spark cylinder.----Replace condensers.----Clean set and points to 0.020"----Test for spark again.
 
There are quite a few companies making these coils. The original coil is probably on the defective cylinder. Closely examine the points surfaces for pitting and/or previous filing. If they have been dressed once already, I might suggest you replace them with a new points/condenser set (both cylinders). Once this is done, you should have nice hot, and reliable spark.
 
If your points are melted on the face,your condenser is defunct.
If your ht lead wire is bared,It will be earthing on the block.
Sounds like a coil has gone down.
 
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