1975 johnson 25 75r25 fuel problem

[meatloaf]

Hi folks, great forum and thanks ahead for any thoughts you might have on my problem. My outboard runs pretty good overall but at full throttle I only get about half my speed with choke open. When I pull the choke all the way closed, the speed increases to what I consider full speed. I talked to a local marine tech here and he said the high speed jet probably was clogged, so I did a rebuild on the carb and actually found nothing in the jet at all. I have all new parts on the engine, coils, plugs, points, condenser, head gasket, etc. I bought this engine a year or so ago and it has always done this, compression is also good on both cylinders. The engine is functional this way but I would like to get it running proper if you guys have any idea what might be the problem. I have thoughts of a weak fuel pump but they are costly and don't won't to buy one if not needed. Also, when running I have tried pumping the primer bulb to see if perhaps the fuel pump was weak but the speed did not change when I did this.

Thanks
Meatloaf

 

[jbjennings]

Re: 1975 johnson 25 75r25 fuel problem

Meatloaf,
You probably don't want to hear this but it still sounds like your carb is dirty.
If not that, you might check for vacuum leaks around your carb or intake manifold. Did you replace the fuel lines? How's your fuel tank pickup tube? Any indication the fuel pickup tube might be leaking air? (That's far-fetched)
Is your float set properly so the fuel bowl is filling properly?

All indications are that your carb is still dirty. It only takes a little bit of varnish or whatever in that fixed high-speed jet to cause what you describe.

Good luck,
JBJ

 

[meatloaf]

Re: 1975 johnson 25 75r25 fuel problem

Thanks for the reply, yes I actually did recheck the jet in the carb, I took it out and visually looked through it and no obstruction was visible. As for the fuel lines they are also new. If I do have a leak somewhere what do you think is the best way to find it?
Thanks,
Meatloaf

 

[jbjennings]

Re: 1975 johnson 25 75r25 fuel problem

You might try spraying a little propane gas from a cheapo propane torch around the carb base and intake manifold and fuel lines. If it smooths out, that's an indication it's sucking in the propane and richening the mixture. I'd think if you had a vacuum leak you could tell it on idle, though. I'm wondering if there's any chance it's the wrong size jet? I don't have the knowledge to tell you how to figure that one out, though. You should be able to spray a little wd-40 around the carb and stuff and possibly find a vacuum leak.
How about taking the fuel line from the fuel pump off the carb and turning the motor over with the rope and seeing what the fuel pulse looks like? (Be careful you don't catch the motor on fire)

For it to pick up with the choke closed, there has to either be not enough fuel or too much air. (as I'm sure you already know). I'm thinking too much air. Air leak in lines somewhere?
Oh, one last thing, are you using a crappy walmart primer bulb? You could have a bad check valve in that thing--they're notorious for it!

Wish I could be more helpful,:redface:
JBJ

 

[tashasdaddy]

Re: 1975 johnson 25 75r25 fuel problem

did you remove the jet, and run stranded wire thru it.

http://forums.iboats.com/showthread.php?t=244013

 

[F_R]

Re: 1975 johnson 25 75r25 fuel problem

Are you actually looking at the fixed jet or are you looking at the main nozzle? The jet is in the very bottom of the carb bowl, way deep in the hole behind the drain plug. It has to be absolutely positively CLEAN! The orifice is precision sized to provide exactly the right flow of fuel. Just a thin film of crud will make it run lean, which is what you are doing. I'm putting my money on that jet being gummed up. OR, somebody has replaced it with one of the wrong size.

 

[meatloaf]

Re: 1975 johnson 25 75r25 fuel problem

Morning guys, yes I did take the jet completely out (had to file down one of my best screw drivers to do it...lol), but it was clear and I ran some small copper wire through it to be sure it was clean. But I do have one of those cheap Walmart tanks with the bulb that came with it so maybe you are on to something. I checked the gas flow with the line disconnected but can't remember what type of flow I had, should it be constant? Or will it pulse? Anyway, my tank is plastic so maybe getting a metal tank would be better anyway. Thanks for your thoughts guys.
Meatloaf
Crappies are biting here in the south and I'm getting the itch......lol

 

[tashasdaddy]

Re: 1975 johnson 25 75r25 fuel problem

plastic tank is fine. those hose have an arrow on the bulb that should point to the motor, also, when pumping, point the arrow to the ski, the check valve works better in that position.