1972 20hp johnson not running properly

jwilkey84 · Apr 1, 2008

I have posted about this problem last winter, but now that spring is here I want to bring it up again. 1972, johnson 20hp, electric start, long shaft. when I got is last fall it would start but not run for more than a few seconds and it would immediately die if you would give it any throttle, and make a gasping sound as it died. So here is what I have done. Compression is good, spark is good, COMPLETE carb kit installed, new low speed needle valve, new wiring, new impeller, coils look brand new, and I also cleaned the points with emery cloth. The motor ran really crappy until I cleaned the points with emery cloth, and then it seemed to run good in the tank for about 20 minutes. It was responsive at the throttle and idled pretty good. So I thought it was fixed and I put away until the ice came off of the lakes here in iowa a couple of weeks ago, and I put it on the boat to try it out and it runs just like it did last fall. I think about the only thing I have not done is replace the fuel pump. Someone told me they think it could be the fuel pump or the float sticking, just wanting more thoughts on this before I go buying more parts.

F_R · Apr 1, 2008

Re: 1972 20hp johnson not running properly

jwilkey84 said:
I have posted about this problem last winter, but now that spring is here I want to bring it up again. 1972, johnson 20hp, electric start, long shaft. when I got is last fall it would start but not run for more than a few seconds and it would immediately die if you would give it any throttle, and make a gasping sound as it died. So here is what I have done. Compression is good, spark is good, COMPLETE carb kit installed, new low speed needle valve, new wiring, new impeller, coils look brand new, and I also cleaned the points with emery cloth. The motor ran really crappy until I cleaned the points with emery cloth, and then it seemed to run good in the tank for about 20 minutes. It was responsive at the throttle and idled pretty good. So I thought it was fixed and I put away until the ice came off of the lakes here in iowa a couple of weeks ago, and I put it on the boat to try it out and it runs just like it did last fall. I think about the only thing I have not done is replace the fuel pump. Someone told me they think it could be the fuel pump or the float sticking, just wanting more thoughts on this before I go buying more parts.

I suppose you did check the high speed jet orfice plug in the bottom of the carburetor when you were in there, right?? 90% of the time that's why they conk out when you open the throttle. Being clean isn't good enough. It has to be absolutely clean.

jay_merrill · Apr 1, 2008

Re: 1972 20hp johnson not running properly

I would replace the points also. While I know that filing points down to get rid of pits, etc. is common, I usually just replace them unless they are in very good condition. Frankly, they are so inexpensive that it doesn't make much sense to do otherwise.

PS:If you saw this before my last edit, nevermind on the high speed jet - I just doublechecked the parts diagram and see that it is a fixed jet.

jwilkey84 · Apr 1, 2008

Re: 1972 20hp johnson not running properly

I am sure it is absolutely clean, I have had it apart once to clean it before I installed the carb kit, twice when I installed the carb kit, and I have taken in offf a third time to clean again because this thing is frustrating me. The high speed jet wouldn't explain why it won't idle for more that a few seconde without dying, would it? I just don't understand why it ran so good for 20 minutes, after I cleaned the points, and now I am practically back to the exact same place I was before I had added any new parts, done any cleaning, or cleaned the points. I know there are many skilled minds on this forum and I would ask that if you have time and the desire, to read my post through a few times and thoroughly think through this with me. I feel like I am working on a car and I don't know what's wrong with it, so I just throw parts at it till it is fixed. There has to be a logical next thing to do besides throw parts at this thing. Thank you so much for your time and thoughtfullness.

jay_merrill · Apr 1, 2008

Re: 1972 20hp johnson not running properly

I agree that if it dies at idle after a few minutes, it probably isn't the HS jet. Assuming that the LS needle valve is adjusted properly, try choking the motor a little bit to see if it keeps running. If it does, you are getting too much air and worn areas in the carb body, where the throttle butterfly shaft runs through it, are a common cause. If this seems to be a possibility, smear some bearing grease on the outside of the carb at that point and see what happens. This will tend to create a temporary seal at the wear point and may be revealing as to what is going on.

Another possibility is a disintigrating diaphram in the fuel pump. Even though you cleaned the carb, bits of the diaphram might be getting into it. I recently installed an inline fuel filter between my fuel pump and carbs for this very reason. I bought a Marpac with a replaceable filter and clear glass casing, so I can inspect it periodically for signs of debris.

samo_ott · Apr 1, 2008

Re: 1972 20hp johnson not running properly

That's a pretty spiffy looking filter!

jay_merrill · Apr 1, 2008

Re: 1972 20hp johnson not running properly

LOL, thanks! It's modified - I put the elbows in it so I could mount it to the side of the air silencer on my 65hp Johnson. The black nipples usually go directly into the ends of the filter.

Here's the installed view:

jwilkey84 · Apr 2, 2008

Re: 1972 20hp johnson not running properly

any other ideas, fellas?

samo_ott · Apr 2, 2008

Re: 1972 20hp johnson not running properly

If the compression is good, which you say it is, btw, what is it? Then it's fuel starvation or spark failing under pressure/load. Are the coils new?

HighTrim · Apr 2, 2008

Re: 1972 20hp johnson not running properly

Have you tried spraying pre mix into the carb throat? Choking it to see if it picks up? Pumping the primer bulb?

I also assume your vent is open on the fuel tank right?

What is the condition of the fuel lines? Age of fuel?

jwilkey84 · Apr 2, 2008

Re: 1972 20hp johnson not running properly

gas is new, compression if I remember right was equal in both cylinders and around 123. I have not tried spraying premix in to the carb, though choking usually helps it if you can get it choked in time before it dies. The coils look brand new, but I am sure they are not. They show no signs of cracking or excessive heat. I think I'll try the premix thing and see how that affects it. If that works, would you assume a faulty gas pump?

HighTrim · Apr 2, 2008

Re: 1972 20hp johnson not running properly

If spraying the pre mix helps, there is still a fuel delivery/carb related problem.

Pumping the primer bulb and having it pick up would make you lean towards a faulty fuel pump.

If choking it helps, re clean the jets.

jay_merrill · Apr 2, 2008

Re: 1972 20hp johnson not running properly

If choking the carb helps, I would say you are either not getting enough fuel via the SS jet or you are sucking air into the carb from somewhere other than the intake. I would try opening the jet a little to enrich the mixture - do this is small increments and try the motor again each time. Or, if you can keep the motor going by "playing the choke," try to adjust the SS needle as you do to see if you can find the right mixture. You should end up at a point where the motor idles without "huffing" or "coughing."

If this doesn't work, try the grease trick on the throttle butterfly shaft. If there is improvement, you have been sucking air between the shaft and the body of the carb. In such a case, you may as well go on a hunt for another carb.

If neither of these things works, I would say that it is time to open the carb up again to see if there is junk in it once more. If so, you will need to figure out where it is coming from.

PS: If you manage to get the motor running by adjusting the SS needle valve, don't forget to set the idle per the service manual instructions.

jwilkey84 · Apr 12, 2008

Re: 1972 20hp johnson not running properly

I took the fuel pump clear apart today and everything looked great. I also replaced every inch on fuel line on the motor, including the tank line. I also put new hose clamps on all fuel line. I will try starting the motor probably tomorrow. Hopefully it runs good! But if not I will try spraying in some premix into the carb. BTW how often do the butterfly shafts actually get worn to the point to where they suck extra air in there? Is it a common thing?

jay_merrill · Apr 12, 2008

Re: 1972 20hp johnson not running properly

On older motors, yes this can be a frequently found problem and it is just plain old wear between two parts. I have not experienced it personally but my OMC/BRP dealer tells me that he sees it a lot in the "oldie but goodie" motors. If I am not mistaken, it is the carb body that wears more than the butterfly shaft.

jwilkey84 · Apr 13, 2008

Re: 1972 20hp johnson not running properly

so the idea is to put some grease around any possible air leaks on the outside of the carb,right?

tmcalavy · Apr 13, 2008

Re: 1972 20hp johnson not running properly

If it just runs crappy in the barrel, it may be sucking in exhaust laden air if any at all. Have you tried running it on the boat?

jwilkey84 · Apr 13, 2008

Re: 1972 20hp johnson not running properly

yes, I had it running good in the barrel immediately after I cleaned the points. So it sat for a week or two, and then I put it on the boat and tried it and I was back to the same old problem. I really was expecting it to run good on the boat since it did in the tank, but I was mistaken.

tmcalavy · Apr 13, 2008

Re: 1972 20hp johnson not running properly

Did you synch the throttle/timing advance to the carb? At the Start position, the back butterfly on the carb needs to be just beginning to open, and then open smoothly as you advance the throttle. I had to link-n-synch my old Merc 500 everytime I pulled/cleaned/rebuilt the carbs...maybe yours is out of synch?

jwilkey84 · Apr 13, 2008

Re: 1972 20hp johnson not running properly

that still doesn't explain why it ran good for 20 minutes after I cleaned the points, though. At this point I am just running the throttle with my finger while the cover is off of the motor. No matter if you open the throttle fast or slow, it immediately makes a noise like is is sucking air, and then dies. While at idle, when it starts to die, if you choke it in time it will try to stay running. It is almost ready to go to the "real" mechanic, instead of me.

1972 20hp johnson not running properly

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