Johnson 20hp surging problem

[Abassed]

I have a 1978 Johnson 20hp that has been giving me problems. For starters, the motor wont idle unless i keep the throttle open a little. But the real problem is that it surges bad at WOT. When i first open it up, it will seem to pick up speed just fine, but almost immediately drops down to putting speed. Then it will surge up for about 3 secs, then down 3 secs. That will continue the whole time. already replaced basically the entire fuel system including the fuel pump. This is actually the best I've had this motor running since i got it. I just cant seem to find the problem. I tried pumping the bulb at WOT, and it wasn't very firm, but pumping it made the motor bog down worse. Loosening the cap on the tank did the same thing with alot of white smoke. Any feedback would be much appreciated!

 

[boobie]

Have you had the carb apart and went threw it with a fine tooth comb ??

 

[Abassed]

yeah i cleaned the carb and it actually wasnt that dirty. I didn't have a screw driver small enough to get one orphis out, so i just cleaned it really good with carb cleaner and air. Only thing i could think that would be wrong with the carb is maybe the float? i've heard the advanced timing or idle screw could be a culprit. Not too sure though

 

[racerone]

Check the fuel pump diaphragm.

 

[Abassed]

i could but its a brand new fuel pump

 

[Mohawkmtrs]

New does not always mean good.

I had a Johnson 20 hp in last week with no spark...tested ign. system and found a bad power pack...installed a new pack and still no spark...re-tested everything and still pointed to power pack...put a new, new pack and motor fired up on second pull.

 

[BTL]

I had a similar issue recently and thought for sure it was fuel related, but it ended up being a broken ground tab on one of the coils. I replaced them both with new coils that had a ground wire instead of a tab.

 

[Abassed]

New does not always mean good.

I had a Johnson 20 hp in last week with no spark...tested ign. system and found a bad power pack...installed a new pack and still no spark...re-tested everything and still pointed to power pack...put a new, new pack and motor fired up on second pull.

for some reason i cant find the power pack on mine. I see the 2 coils and the wires lead to something under the flywheel. Guess its in there?

 

[Abassed]

I had a similar issue recently and thought for sure it was fuel related, but it ended up being a broken ground tab on one of the coils. I replaced them both with new coils that had a ground wire instead of a tab.

yeah see i feel like it has to be power related at this point. I went ahead and ordered new coils, plugs and wires since it needed to be done anyway. Hopefully this does some good...

 

[racerone]

If pumping the bulb makes it worse then ------You have issues with the float valve.----Or with the diaphragm in the fuel pump.-----Is the nozzle gasket in place in the carburetor ?

 

[Abassed]

If pumping the bulb makes it worse then ------You have issues with the float valve.----Or with the diaphragm in the fuel pump.-----Is the nozzle gasket in place in the carburetor ?

So what issues might i have with the float valve? its a brand new fuel pump too but i took it apart anyway and the diaphragm looks like it should.

 

[racerone]

Perhaps test run with another tank and hose.-----??---No powerpack ?----What is the model # of this motor?---Or post pictures of this 78 model powerhead !!

 

[dfrants]

According to their website omc did not make a 20hp in 1978. You either have a different year or a different horsepower. Find a model number and double check.

 

[racerone]

Well ---Yes they did market a 20 HP model in some markets.----We wait for the model # or peektures.

 

[Abassed]

it has magneto not a power pack. and i have a title for it saying its a 78 so that's just what i went with unless it could be wrong... Thanks for all the feedback btw

 

[jimmbo]

In 1980 our 79 25Hp was doing the same surging thing, up and down, up and down. I was convinced it was a fuel problem, but nothing seemed wrong. My dad insisted I check the Spark Plugs, I thought he was talking out the Top of his Head. Well it turned out one of the plugs had some kind of deposit bridging the Electrodes. Removed the deposit and the engine was running great again... Just my 2 cents.

 

[racerone]

These motors started using powerpacks and the " magflash " magneto in 77.------Is there a disc on the powerhead with a model #

 

[Abassed]

These motors started using powerpacks and the " magflash " magneto in 77.------Is there a disc on the powerhead with a model #

i know for sure theres a magneto. Im supposed to change the plugs and wires today so ill check the model # on it when i do that and update yall this evening

 

[Abassed]

In 1980 our 79 25Hp was doing the same surging thing, up and down, up and down. I was convinced it was a fuel problem, but nothing seemed wrong. My dad insisted I check the Spark Plugs, I thought he was talking out the Top of his Head. Well it turned out one of the plugs had some kind of deposit bridging the Electrodes. Removed the deposit and the engine was running great again... Just my 2 cents.

i pray changing these plugs is the fix for this! would be the easiest fix. would the motor not keep an idle too?

 

[jimmbo]

i pray changing these plugs is the fix for this! would be the easiest fix. would the motor not keep an idle too?

I just remember the surging(that was 39 yrs ago). With enough throttle, the engine should still run on one cylinder