4hp Johnson - too rich at high throttle

[hydroscott]

Hi All, <br /><br />I have a 1970 4hp Johnson. Had a problem this spring, wouldn't run at high speed unless the choke was pulled about 1/2 way on. I replaced the fuel pump and replaced the needle, seat and float in the carb. Now it runs great at idle but when I pull the throttle past about 1/4, it runs rough and very rich. Plugs are soaking wet and tons of smoke. I have done the following: Swapped all carb parts back to original, swapped fuel pump back to original, checked spark with timing light, checked advance with respect to throttle opening per manual, checked fuel pressure, set float level, changed gas, changed plugs, set high and low speed needle valves per manual. None of this had any effect. I read a previous post about a "bad or missing main nozzle gasket" but I don't know what that is. Adjusting the high speed needle valve all the way in until it bottoms out has no effect - still runs way rich. Any ideas? I'm about to send this thing to the bottom of the river. Thanks.

 

[Xcusme]

Re: 4hp Johnson - too rich at high throttle

OK, it sounds like you've been into the carb a lot! The main nozzle gasket is a donut shaped gasket that fits over the main jet stem and seals the main jet to the lower half of the carb body when you re-mount the bottom half of the carb.<br /><br />To see if you have it in place:<br />Remove carb.<br />Remove bottom of carb.<br />Remove Float assembly (hinge pin,needle, float.)<br /><br />The hole in the center of the float straddles the main high speed nozzle jet. You should have unscrewed this jet for cleaning. The 'stub' of this jet should have a cork or rubber washer around the outside. This washer is about 1/4" thick. This washer seals the nozzle to the lower half of the carb when you re-assemble the carb. Too often, this washer is compressed and doesn't seal properly. If you don't have this washer or it's compressed and not doing it's job, raw gas from the bowl will go unmetered into the throat of the carb at higher rpm's dumping fuel to the intake. The High speed needle adjustment will make little or no change.<br /><br />You posted, "Had a problem this spring, wouldn't run at high speed unless the choke was pulled about 1/2 way on."<br /><br />This sounds like the main high speed jet may be partially blocked, which is conflicting symptoms.<br /><br />Best to look and see if you have the gasket and clean the high speed jet while you're in there....

 

[alcan]

Re: 4hp Johnson - too rich at high throttle

Well you hit the nail on the head. The main nozzle gasket. When you remove the bowl from the carb the main nozzle is what sticks down in the center of the bowl. There needs to be a small gasket between nozzle and bowl. If this gasket is missing or leaking fuel will be sucked directly from the bowl to the nozzle with out being metered by the needle valve. Another problem may be that the needle valves are not truely seated. Remove the knobs or make sure there are no knob stops keeping you from lightly seating the needle valves. Remember the begining settings for the needles are just that, begining . You will have to adjust once you get running.

 

[hydroscott]

Re: 4hp Johnson - too rich at high throttle

That did the trick. Ran great in the tank, just have to fine tune the high speed on the boat. Thanks much for the help