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J25 starts and idles fine but stalls on adding throttle - what's wrong?

J

J25ELCI

Member
After a rebuild, my 25hp Johnson outboard starts in the button and idles nice and smooth, but as soon as I add any throttle it bogs down and dies. Any pointers as to what needs looking at?

i suspect it's carb or fuel related but where to start? If I add choke, the engine increases revs significantly without stalling.

thanks all
 
Open low speed mixture needle 1/2 turn and try again.-------Perhaps throttle plate opens too soon.---------Any adjustments done on that long rod that goes to the back of the motor ?
 
Thanks for your reply. Not sure what long rod you mean. The one that comes out of the carb, which is the mixture screw I believe, I have tried adjusting to both extremes and various stings in between but made no difference.
 
Model # of your motor ?--------------Is there a long rod that goes from the carburetor to the back of the motor , yes or no ?
 
J25ELCI[FONT=Arial, Verdana, sans-serif] = 25hp Johnson, Electric Start, Long Shaft 81 (1981)

Use Champion QL77JC4 spark plugs, gaped at either .040 (Hot Spark),,, or... .030 (Long Life)

1 - At idle, make sure that the throttle butterfly is completely closed.
2 - In gear, engine NOT running, at full throttle, make sure that the butterfly is absolutely horizontal WOT.
3 - There are two scribe marks on the cam that slides against the throttle roller... the throttle butterfly should just start to open when those scribe marks are dead center with the roller... not before or after.
4 - If you haven't already, clean the brass high speed jet with a piece of single strand wire. This jet is located in the bottom center of the carburetor float chamber... way in back of the drain screw plug.
5 - Make sure that the small round washer is on the vertical brass high speed nozzle, needed to seal with the float chamber.
6 - If there is a offset small brass tube within the high speed nozzle (don't remember if so or not), make sure, with a small diameter wire, that it is clean.
7 - To adjust the slow speed needle valve, see the following:

[/FONT](Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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Oh... One more thing (Columbo!).......

The rod that leads from the vertical throttle arm located on the port side of the powerhead... the one that has a small threaded barrel with a setscrew in it. To adjust that so that full throttle is applied at the proper time......

1 - Back the set screw out slightly so that the barrel will slide on the rod.
2 - Put shift lever in forward gear. Spin the prop to help align the gear and shifter dog lobes.
3 - Move the throttle so that the vertical lever is touching the stop on the powerhead.
4 - Push the rod so that the throttle butterfly is horizontal.... WOT.
5 - Adjust the barrel so that it is up against the nylon block on the vertical throttle arm and tighten the set screw.

Yeah, I know it takes a little doing to push that rod and tighten the set screw at the sane time but you'll figure it out
 
Did you surface the flange on the carb and install a new carb mounting gasket? Sounds like a vacuum leak to me where the carb mounts to the manifold. On a glass table with 220 grit surface the flange on the carb pulling in one direction only until it all shines the same. Install with a new gasket and pull the carb down slow and even to nice and snug, do not overtighten. preadjust the slow needle out 1 1/2 turns from lightly seated. Check adjust engine sync between ignition timing and throttle opening (very important to check every time you pull the carb). Then see if you can dial in the carb after the engine is fully warmed up in the water in forward gear.
 
Sorry, I'm away at the moment. Thanks for your answers. I'll look through them properly when I get home and answer each point.
cheers all
 
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