20 hp coughing

[creeksidelc]

Hi, I recently purchased a 20hp Johnson 1989 model. The motor has good compression and it runs well wide open. It also runs well wide open under a load. The problem I have is intermittent coughing out the exhaust. It coughs most of the time at a low speed or idle but occasionally it idles smoothly. I replaced spark plugs and thoroughly cleaned the carb 2 times. Low speed jet is clean and there is no clog. I've tried playing with the needle but it doesn't seem to make much a difference where it is so I just have it 1.5 turns out. All fuel lines are good and there is no leaks all the way back to tank and tank vent is open. I have a fuel pump kit on the way but it's a couple days out. I cannot figure out what is the problem with this motor. I did some research today and I believe that I may have a carb on it thas actually for a 25-30 hp motor. 432698 is the number stamped on the carb. When I looked it up it appears it's been replaced by 439374 which is found on 1992 30 hp johnso as well as others. Could this carb be the problem or should that carb work on this motor? Any other suggestions as to what I could to to try and fix it? Thanks

 

[oldboat1]

Did you remove the core plug up at the top during your carb cleaning?

 

[creeksidelc]

No I did not remove the plug. I took jets out and needle and all the other stuff then I soaked the carb for 48 hrs then blew it out and ran copper wire through it in case any blockages. Then did that again in case I didn't get it first time. Should I take the plugs out?

 

[ondarvr]

If the adjustment screw didn't have any effect then the carbs still not right.

 

[creeksidelc]

Thanks. I guess I will order a rebuild kit and then I can take the plugs out and clean it again. Does anyone know if that carb is compatible with my engine before I rebuild it? The part number shows it to be for several yrs in the 1990's but isn't listed for a replacement for 1989.

 

[ondarvr]

Thanks. I guess I will order a rebuild kit and then I can take the plugs out and clean it again. Does anyone know if that carb is compatible with my engine before I rebuild it? The part number shows it to be for several yrs in the 1990's but isn't listed for a replacement for 1989.

I don't know.

 

[AlTn]

try setting you idle mixture adjustment screw 4 turns out

 

[oldboat1]

I usually find the best final setting ends up about 1 turn out, or thereabouts (i.e., 1 1/2 turn initial setting is usually a little rich if the idle adjustment is working right). Sometimes a PO breaks off the tip of a needle in the carb housing, making it impossible to adjust (even with a replacement needle, which a PO might have tried to use). You would be able to see a broken tip if the domed plug is removed, and you look down in there. Try the setting AITn suggests and see if it makes any difference at all. I think your idle mix screw isn't doing anything, like you suggest.

(ed. part numbers can be for carb parts, bowl etc. Think I would get a kit for the year and horsepower and go with that -- kits fit models from a number of years.)

 

[racerone]

Where is the stop on that throttle rod that goes to the towershaft ?--That is often adjusted because when it is correct it " looks wrong " to a novice.----That could mean that throttle plate opens too early.---That makes it cough.---Post picture of that stop if you can.--At idle there usually is a 3/16" ( 4 mm ) gap there.

 

[creeksidelc]

I'll look more into that needle issue. The old needle appeared to low spots like it had been crammed in so I've already replaced it. I hadn't thought about the office being bad though. Will look into that when I get hone

 

[creeksidelc]

Also I just did verify that the carburetor that is on it is not a carb that the motor calls for. The carb should be on a 1990-1997 model 25 hp. I guess that carb might be able to be tuned to work on that motor but I'm thinking the best bet might be to buy a carb intended for it and then go from there.

 

[creeksidelc]

The fuel pump rebuild kit came in today and I installed it. Still no change. It is running fine it I constantly prime it at least but still isn't getting enough fuel

 

[oldboat1]

usually a good source for checking numbers -- worth a try: http://epc.brp.com/Index.aspx?lang=E&s1=14490b87-f665-45c5-82df-a039a2e9b5e8

 

[jakedaawg]

If you had a carb that is working properly, and it was intended for a larger engine, then a lean condition doesn't make sense. Unless the smaller displacement motor does not allow enough volume of air to be moved through the carb to get it to dispense the intended amount of fuel. If that makes any sense you are smarter than me

What I am saying is confusing even to me. If interested I'll expand on my thinking.

 

[racerone]

??---But these motors all have the same displacement ---Yes the same blocks / pistons etc !!

 

[jakedaawg]

??---But these motors all have the same displacement ---Yes the same blocks / pistons etc !!

My mistake, I was referring to how well they breath actually. I could be wrong but I thought that those twety's had a different manifold and reed assembly which makes for a smaller crankcase chamber. Hence they move less air.

 

[racerone]

Crankcase chamber is also the same size !

 

[creeksidelc]

There are a lot of pages that claim you can interchange these parts and get more horsepower out of these motors. That's why I was hopeful that I could get this carb to work on this boat. But it just doesn't aeem compatible. I looked up the numbers again from the link given above and there is a change in 1990. This carb is intended for 1990-1999. Maybe they designed them to run leaner or suck better in those years. It's beyond me though. I'm no mechanic. I ordered a replacement carb meant for my motor so when it was here I will update and hopefully it works right

 

[creeksidelc]

It turns out that it was the carburetor. The new one arrived in the mail and now it runs like a champ! It idles well and smooth and it runs 4 mph faster on the water than it did before.

 

[AlTn]

good to hear