Home › Forum › Ask A Member › 9.9 Johnson Carb Question
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outbdnut2.
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June 21, 2017 at 11:07 pm #60097quote outbdnut2:I saw that bearing in the carb kit photo but with it sitting on the gasket, I wasn’t sure if that’s what it was – Thanks for clarifying. I’ll order the kit now – as a minimum, I know I need the welch plug I pulled out, and will now pull the bearing too now that I understand what/where/how, etc. of it. Meanwhile, I’ll borrow a carb from a 15 and try it.
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A couple of you recently mentioned the NGK plugs. I’ve got BZ7HS plugs in this one already, and have tried BZ7HS-10s I borrowed form a well-idling 15. I fixed several issues on another 9.9 a couple years ago and replacing the Champions with NGK plugs was like magic making it idle perfect, so I’m a believer.
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Yes – I always put a nut on the bottom of those recoils when I pull them – learned that fast first time I pulled one!
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An observation while sticking a wire through the holes below the welch plug: When the throttle plate is all the way shut, only one of the three idle holes into the side of the carb throat is downstream of the throttle plate, so the other two are not in use. Is this enough? or is something worn on the throttle plate where at least two holes should be feeding at closed throttle plate? The center hole is almost directly under the closed throttle plate actually being partially blocked by the edge of the plate.
DaveWell, those holes are being used, but air is pulled through those two holes closest to the needle valve (due to low pressure in the mixing pocket) to help meter/emulsify the fuel properly….So, let’s just say you plugged the most forward hole, the engine would run rich at idle/throttle plate closed because you are blocking the air supply and forcing the carb to pull more fuel up through the low speed plumbing/passages to the mixing pocket…
But, now let’s think of what happens when you crack that butterfly just open a bit….Now, the two holes farthest from the needle valve are under low pressure and will be delivering fuel/air mix to the venturi.
Now let’s crack the throttle plate just a bit more, and all three holes are subject to low pressure and will be drawing fuel/air mix into the venturi.
So, let’s just say the hole closest to the needle valve was plugged. At dead idle, with the throttle plate closed, you would have a rich condition. Open the throttle plate a bit for off idle and you would have a lean condition…June 22, 2017 at 2:06 pm #60127Good explanation! I didn’t know the function of those three holes was so complex.
Thanks!
DaveJune 23, 2017 at 1:37 am #60154Many times the number on the carb defines the layout of those holes, emulsifying tube and so forth even if the carbs look identical.
June 23, 2017 at 5:31 am #60166quote rudderless:Many times the number on the carb defines the layout of those holes, emulsifying tube and so forth even if the carbs look identical.The # on the carb that is printed in black ink on the carb mounting flange? Where would i get the info on what that # relates to?
"Some people want to know how a watch works, others just want to know what time it is"
Robbie RobertsonJune 23, 2017 at 5:57 am #60167quote rudderless:The 74-75-76 head have issues if low speed is desirable. The later heads are lower compression, have a charge deflector to protect the plug from fuel spray. I just bought a 89 head for my 76 to improve low speed. Those heads do not have that cooling jacket around the plug like past years. I will have to mod the cowl to clear the thermostat though..I just looked at a 1975 15 hp head i recently acquired . It doesn’t have the raised ridges(charge deflector) on the inside of the head dome,like my ’76 later upgraded head. It also has the early water cover with the straight center divider in the water passage/T stat cover . You say it is a higher compression head?
Have you thought about milling the later head to raise the compression to the level the early head had ? Would there be a benefit to that?
Thanks,Jim PSB"Some people want to know how a watch works, others just want to know what time it is"
Robbie RobertsonJune 23, 2017 at 5:57 am #60168I’ll bet that’s the number you’ll find on the parts list…
June 23, 2017 at 7:58 pm #60195My 76 head has the number 15 stamped on the top coil mount. If one cc’s the 2 different heads the older one will have less volume.
I just got my 89 head in the mail. It looks as though the thermostat pops off at high speed or lifts off the seat. I will be installing the head as soon as the new thermo arrives.
I might putty the head to check for clearance but I probably will not increase compression.
I will report on how the 89 head works..
June 24, 2017 at 4:18 am #60217quote rudderless:My 76 head has the number 15 stamped on the top coil mount. If one cc’s the 2 different heads the older one will have less volume.I just got my 89 head in the mail. It looks as though the thermostat pops off at high speed or lifts off the seat. I will be installing the head as soon as the new thermo arrives.
I might putty the head to check for clearance but I probably will not increase compression.
I will report on how the 89 head works..
That "15" means it is off a 15 hp. My 9.9 has a "10" . My friends 1976 15 hp also has a "15" on the top coil mount.so i surmise that the number signifies the Hp ,if the powerhead gets separated from the rest of the motor?
I have a ’89 9.9 and the t stat is sure different than the early ones."Some people want to know how a watch works, others just want to know what time it is"
Robbie RobertsonJune 24, 2017 at 4:41 pm #60248Huh, didn’t realize the early models had different heads for the 9.9 and 15hps….I’m guessing that they eventually superceded to the same part number….
The easiest way to determine if you have the old vs new style head is to look at the thermostat water cover. Early models will have a water cover that is straight on its stb/right side. Later model covers were scalloped out around the plugs. These early models had several repair/update kits to combat plug fouling and ignition woes. There is no need to update the older style heads unless you are having plug fouling problems….June 28, 2017 at 1:00 am #60500OK – that stuff on heads was interesting and I learned a bit there – Thanks!
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Now back to my original 9.9 idle problem. I got psyched up to work on it again and I’m finally getting somewhere. I swapped a carb from a good idling 1989 15 HP and it idled super-good. So looking further into the 1977 9.9’s carb, I found there was no bearing on the needle; and remember I got a piece of glop out of the needle-end of the housing the other day, so that was some of the problem too, as it was up by the needle. The carb bowl , needle/seat. etc when I first took it apart was very dirty and the float needle was jammed with crystalized stuff, so I got to looking at the passageways from the bowl to the low speed needle again, figuring there was probably dirt everywhere, even though I had previously run some carb cleaner through all the openings I could find. I’m trying one last time to avoid pulling out those lead shots. I have the welch plug off the mixing chamber and have verified the three holes to the throat are clear, and hole to the needle is clear.
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I determined that part way up the cast tube that hangs through the center of the float, downstream of the high speed jet, there are passageways on each side just above where the brass venturi tube hangs down and ends. One goes up to the bowl vent that is on the carb throat opening rim. I backflushed that with carb cleaner, and passageways are well open. The other passageway next to the brass tube goes through the carb casting up to the low speed needle. I bent the carb cleaner plastic tube slightly on the end about 60 degrees and put it up in the needle housing trying to get it to find the opening where the gas comes up. After moving it around some while spraying cleaner, I hit the pasageway dead on and was able to backflush the gas idle passages back down from the needle housing into the bowl big time – I didn’t see if any dirt came out because I was standing outside holding it away from me, but now I’m reasonably confident that the idle gas passageways are open and clean. So tomorrow, I will put it back together with the new bearing in the carb kit and see how it does. If that fails, I will pull those lead shots to make darn sure all the idle gas passages are clean.
Dave -
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