1975 25 johnson carb question

[meatloaf]

I bought a 1975 johnson 25 back in the summer. The engine ran rough so I installed a new carb kit and it ran much better. The problem I'm having now is that the engine will only run about half speed on the lake unless I pull the choke all the way out. When I choked it fully it ran wide open and great. I don't mind running it this way but I would like to have it running properly. Thanks for the help.....great forum!

 

[fireman57]

Re: 1975 25 johnson carb question

Welcome to iboats. When you rebuilt the carb did you soak it good in carb cleaner and blow out all the orafices with compressed air? Next time out instead of using your choke try pumping your primer bulb. If it picks up and runs good when you do this then it is probably a weak fuel pump.

 

[steelespike]

Re: 1975 25 johnson carb question

If you did the carb and didn't check the fuel system for dirt and gunk and deterioration it may have contaminated the carb all over again.

 

[F_R]

Re: 1975 25 johnson carb question

Bet you didn't take out and clean the fixed high speed orfice jet did you? If there is even the least little bit of gum in the jet it will result in what you are describing. Get it squeeky clean and it should run.

It's down deep in the hole behind the plug in the front of the carb bowl. It takes a special screwdriver to get it out. But you can make your own from a screwdriver with a 3/16" shank. Cut off the business end and grind/file a new tip as shown. The secret is the parallel sides to the blade. "Normal" tapered sides will tend to spread the jet and the harder you try to turn it, the tighter it gets.

 

[meatloaf]

Re: 1975 25 johnson carb question

I bought a 1975 johnson 25 back in the summer. The engine ran rough so I installed a new carb kit and it ran much better. The problem I'm having now is that the engine will only run about half speed on the lake unless I pull the choke all the way out. When I choked it fully it ran wide open and great. I don't mind running it this way but I would like to have it running properly. Thanks for the help.....great forum!

 

[meatloaf]

Re: 1975 25 johnson carb question

Thanks for the fast replies guys.....When I rebuilt the carb I did not soak it in anything the bowl was very clean, I did blow everything out and it appeared the be free flowing through the high speed jet. When I was running it on the lake I did squeeze the primer bulb but this had no effect on speed. I guess with the choked pulled out the engine is pulling a vacuum on the fuel system so it runs at high speed? Also I did replace the fuel lines and used fresh fuel. I was affraid to remove the high speed jet because of potentially damaging it...I do have a manual and it said the jet could be damaged if not careful, I made a small attempt at removing it with a small screw driver but it was very tight so I decided not to continue

 

[F_R]

Re: 1975 25 johnson carb question

The jet is in the very bottom of the carburetor where the last few drops of fuel remain and turn to gum. Any gum reduces the diameter of the orfice and makes the motor run lean. By pulling the choke out, you make it richer and overcome the lean condition caused by the restriction in the orfice.

You can remove the orfice without removing or disassembling the carburetor if you make the screwdriver. Or, you can remove and disassemble the carburetor and soak the bowl and orfice with carb cleaner without further disassembly. Your choice, just so it gets absolutly clean. Nothing else will do.

 

[meatloaf]

Re: 1975 25 johnson carb question

Thanks for the advice I'll re-examine and clean the high speed jet.
Do you know where the screw driver you described can be purchased for removing the jet? I prefer using the proper tool if one can be purchased.

 

[F_R]

Re: 1975 25 johnson carb question

Any Johnson or Evinrude dealer. Part number is on the picture.

 

[meatloaf]

Re: 1975 25 johnson carb question

I pulled the carb this evening and made the tool needed to remove the high speed jet, I soaked the bowl in some cleaner for about 2 hours before I unscrewed the jet. When I viewed the orifice it was completely open and was clean. It's possible it could have had some thing in it but nothing showed up at inspection. Could anything else cause the problem of having to choke engine for high speed? I don't think my fuel pump is causing this since I pumped the bulb several times while running engine on the lake with no effect.

 

[ezeke]

Re: 1975 25 johnson carb question

When the motor is running, the fuel is sucked from the float bowl by the action of the pistons. The fuel pump just keeps the float bowl full. When you close the choke the pistons continue to suck but get less air and therefore more gasoline.

If air is getting into your fuel system from other than the carburetor throat, it could cause a similar problem. Also, you might want to look down the throat of your carburetor while moving the throttle to assure that the throttle plate is working properly.

 

[fireman57]

Re: 1975 25 johnson carb question

Like ezeke said you have air coming in from somewhere. Check your new lines for possible overtightening of a clamp that cut the fuel line. If you used zip ties make sure that they are tight. Did you replace the gasket between the carb and block? When you are running you can take a light oil like sewing machine oil and spray it on the connections when the problem occurs. If it picks up speed then you have found your leak.

 

[meatloaf]

Re: 1975 25 johnson carb question

Thanks guys I do use plastic ties on both ends of the fuel line so i'll check these again.

 

[perkdp]

Re: 1975 25 johnson carb question

At wide open the butterfly should be straight across the carb throat. If it isn't ,, adjust the linkage.

Do you ever get to a point where it seems wide open, without the choke then slows back down when you give it more throttle ?

Could the vaccum release on the tank be partially clogged and the choke out increase the pump vaccum to pull enough fuel ?