Johnson 2-stroke fuel/air mixture adjustment

[brendantownsend]

I have a Johnson Seahorse 4hp 2-stroke twin (1985 - Mod. no. BJ4BRC0B, s/no. B039078) and recently had the engine (ahem) "fully" serviced, though I took it out on the water and it died shortly after every time I increased the throttle. However, it would then pull-start straight up again, and then die on higher throttle, and so on.<br /><br />Suspecting fuel starvation I closely inspected inside the tank and found that the internal mesh filter had both corroded and broke away at the base, with other debris in the tank collecting around the only exit point left. I also found the elbow (linking tank to fuel line) so gummed up that it was incredible that fuel could pass at-all (I had to FILE this away with a needle file and repeatedly jet out the resulting plugs with carb cleaner!). I also replaced the perished and kinked fuel line, put in a proprietary in-line fuel filter, and all started up and runs OK without leaks. The morale of this story being that if you want it done "right" then do it yourself.<br /><br />However, continuing on this basis, I now want to set the mixture on the carb as it appears to be running rich (sooty/oily plug deposits and always runs best from cold without choke). Naturally I'll re-clean any residual debris from the carb beforehand.<br /><br />Incidentally, I am running with the recommended 100/1 fuel-mix ratio. I have a Seloc manual (#1302) but very confusing with the way it's presented as it tries to cover too much, and the carbs vary considerably between models and years.<br /><br />Does anyone have any experience with the carb fitted to my particular engine, that can give guidance to the adjustment procedure and location of the adjustment screws? I have ordered another manual though we're holidaying shortly and I'd like to sort it out as soon as possible.<br /><br />Many thanks in advance.

 

[BoatBuoy]

Re: Johnson 2-stroke fuel/air mixture adjustment

Don't know anything about that carb, but I'd like to comment on the 100:1 fuel mix. Others here could tell you the years, but that ratio was recommended at one point and then changed back to 50:1 with a service bulletin. If anyone has conflicting info, pls respond.<br /><br />EDIT: See "C. Fuel/Oil mix- Most Motors Covered Here... " in the FAQ

 

[brendantownsend]

Re: Johnson 2-stroke fuel/air mixture adjustment

Originally posted by BoatBuoy:<br />[QB] Don't know anything about that carb, but I'd like to comment on the 100:1 fuel mix. Others here could tell you the years, but that ratio was recommended at one point and then changed back to 50:1 with a service bulletin.<br />

!!!<br />Many thanks for this heads-up.<br /><br />It may not be running so sweetly as it might after a good mixture tweak, but at-least it will stay lubricated with the appropriate fuel/oil mix. If anyone knows anything further about the year(s) applicable to the service bulletin then that would be very welcome (thank you).<br /><br />Incidentally, regarding those self-professed "service" agents, I checked against the recommended plug rating and they had fitted new COLDER plugs (i.e. B7HS), whereas the hotter B6HS are equivalents to the original Champion L86 listed for the engine. Just as-well should someone confirm I need to maintain an increased oil mix!<br /><br />Once again, many thanks

 

[Xcusme]

Re: Johnson 2-stroke fuel/air mixture adjustment

Brendan,<br />You'll do best to run at a 50:1 mixture. As for the carb. If you havn't already opened it up, nows the time. Clean with carb cleaner and blow out all passages with compressed air. You may notice a small round gasket on the down tube in the center of the fuel bowl. This is called the Boss gasket. If this gasket is not making a good seal, it will be almost impossible to get the low speed idle adjustment set right. When this gasket leaks, fuel is drawn directly into the motor, bypassing the high speed idle adjustment needle, and causing an overly rich condition at idle (thus-no choke needed to start). A carb rebuild kit is available which contains the necessary gaskets and parts to put things right.<br /><br />Our good friend Joe Reeves has this text to offer regarding the carb needle adjustments.<br /><br />Depending on which carb you have, pick one of the following.<br /><br />(Carburetor Adjustment - Single S/S Adjustable Needle Valve)<br /><br />Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.<br /><br />Start engine and set the rpms to where it just stays running. In segments of 1/8<br />turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to<br />respond. As you turn the valve in, the rpms will increase. Lower the rpms again to<br />where the engine will just stay running.<br /><br />Eventually you'll hit the point where the engine wants to die out or it will spit back<br />(sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4<br />turn, you'll find the smoothest slow speed setting.<br /><br />When you have finished the above adjustment, you will have no reason to move<br />them again unless the carburetor fouls/gums up from sitting, in which case you<br />would be required to remove, clean, and rebuild the carburetor anyway.<br />--------------------------------------------------<br />(Carburetor Adjustments - Two Adjustable N/Vs)<br /><br />Initial settings are: Bottom high speed = seat gently, then open 1 turn out. Top slow<br />speed = seat gently, then open 1-1/2 turns.<br /><br />Setting the high and low needle valves properly:<br /><br />NOTE: For engines that DO NOT have a shift selection, obviously there is no<br />NEUTRAL position. Simply lower the rpms to the lowest setting to obtain the low<br />speed needle valve adjustment.<br /><br />(High Speed) Start engine (it will run pretty rough), shift into forward gear, take up to<br />full throttle. In segments of 1/8 turn, waiting for the engine to respond between turns,<br />start turning in the bottom high speed needle valve. You'll reach a point whereas the<br />engine will either start to die out or spit back (sounds like a mild backfire). At that<br />point, back out the needle valve 1/4 turn. Within that 1/4 turn, you'll find the<br />smoothest setting.<br /><br />(Low Speed) Slow the engine down to where it just stays running. Shift into neutral.<br />Again in segments of 1/8 turns, start to turn the top needle valve in. Wait a few<br />seconds for the engine to respond. As you turn the valve in, the rpms will increase.<br />Lower the rpms again to where the engine will just stay running. Eventually you'll hit<br />the point where the engine wants to die out or it will spit back. Again, at that point,<br />back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed<br />setting.<br /><br />When you have finished the above adjustments, you will have no reason to move<br />them again unless the carburetor fouls/gums up from sitting, in which case you<br />would be required to remove, clean, and rebuild the carburetor anyway.<br /><br />--------------------<br />Joe (30+ Years With OMC)

 

[brendantownsend]

Re: Johnson 2-stroke fuel/air mixture adjustment

Thank you BoatBuoy, Xcuseme.<br /><br />Simple sound advice, comprehensive though succinctly put.<br /><br />This is excellent and precisely what I believed this forum was about. As a new member I can't thank you enough for your time and trouble to reply, and for your cherised advice.<br /><br />Armed with this information, I know what I'll be doing this coming weekend <br /><br />Once again - many thanks!