1986 Johnson J10RCDB not achieving high RPM--wrong carburator?

[cprodave]

I recently became the proud (at least third) owner of this outboard. It starts and runs OK at low/medium RPM but won't get to full throttle consistently. Previous owner I bought it from mentioned guy he bought it from may have tried to convert it to a 15hp (and has since lost contact with that guy). So I can't ask the obvious questions about history.

I checked the screen on Fuel Pump and found no significant debris. The top of carburetor is stamped (with very shallow stamping-- best as I can read) 397088. But I can't find this p/n Carb for either 9.9hp or 15hp in 1986, 1985, 1984 or 1987 (both Early and Late Production).

So is 397088 a 9.9HP or is it 15HP? or is 397088 not a legitimate p/n?

I understand Internal Throat diameter (.625 or .875) is a way to discern 9.9 vs. 15HP Carburator. Also I am aware Main Jet could have been swapped/wrong. And I am looking to potentially rebuild the Fuel Pump.

Any suggestions would be appreciated. Thanks in advance.

 

[cprodave]

Also what is standard setting for Idle Adjust on the Main Jet? Thanks...

 

[racerone]

The main jet is fixed , no adjustment.-----Idle mixture screw is set out about 2 turns to get the motor running.-----Then you adjust from there.

 

[flyingscott]

Are you in neutral when this is happening?

 

[kbait]

When you say it won’t get to full throttle consistently, do you mean that it has achieved full throttle?

 

[cprodave]

Not in Neutral, this happens when in Forward Gear pushing a 12foot fiberglass skiff with 2 people. Yes it achieved full throttle but won't stay there. The outboard would get to highest RPM's then slowed down even though twistgrip on the tiller remained at full throttle. After reducing the twistgrip to "match" the reduced RPM, then moving twistgrip back up higher, the RPM's will not go higher.

Thanks for advising 2 turns on Idle knob.

 

[tomhath]

Sounds like you might be losing spark on one cylinder. Check plugs to see if one is fouled or shows other signs of not firing.

 

[flyingscott]

Pump the primer ball on the fuel line when that happens.

 

[cprodave]

Thanks for these ideas. I examined both Spark Plugs just now--they have equivalent appearance, although a bit unusual that there is unburnt fuel mixture in both plugs.

I will try to get on the water tomorrow to try the fuel primer bulb test.

I checked Idle Adjust. It was 1 turn out from lightly seated. I reset it to 1 1/2 tu

 

[cprodave]

Oops, reset to 1 1/2 turns out.
​​​​​​
Can anybody shed light on the mystery carburetor p/n? I am curious to know if wrong carb is root cause, but hesitant to remove recoil starter, etc to get the carb off.
​​​​​​thanks all.

 

[flyingscott]

There is no part # for those carbs. The 9.9/15 hp carbs show up as the same part # the model # of the motor determines which carb you get. . Your motor will run just fine with a 15 hp carb on it and have a little more power. Now people will folllow this and say no you can't do that but they will be wrong. Look at the link and sync, make sure the fuel pump is working. The carb may need to be rebuilt but as far as the wrong carb it will run fine.

 

[cprodave]

I went to the boat ramp yesterday, engine started after a couple pulls, ran it for a couple minutes on the trailer before attempting to launch. Darn thing died! Would not restart even though it ran OK (not great) 3 weeks ago when I had it on the water (saltwater) first time since I bought it, and then flushed it before storing (I can't remember if I flushed in a tank or used muffs). Took it home, put it in a tank and it started after a couple pulls while I had Spark Tester on each Spark Plug Wire through each Spark Plug. Tester was set at 5/16 inch and sparks jumped the gap consistently. Plugs look fouled though. PO had changed them although I have no idea how long (i.e. how many hours) ago. So I will go from current Champion L77JC4 to hotter plugs.

Any recommendations (on a specific Brand or p/n of hotter plug, or in general)?

Worse yet (potentially) I am not seeing a telltale pee stream at the boat ramp or in the Tank, although this was OK 3 weeks ago. Does water need to be forced initially into the water pump (either by forward motion of the boat, or by using the flusher muffs)?

One other thing I noticed (tentatively) the twist throttle is not turning mechanically through the number of degrees rotation compared to other small OMC outboards I have. That might be part of the problem too (with feeling like we're not reaching full RPM's). I hope to address the throttle twist problem after I get engine consistently starting and running at lower RPM's.

TIA for any help.

 

[flyingscott]

Why would you go to hotter plugs? You might want to find out why they are fouling. The water in a tub needs to be 6" above the pump to work correctly. Are you seeing water out of the exhaust reliefs or the 6 holes above the cavitiaion plate..

 

[racerone]

What do you understand about " hotter " plugs?----Do you think there will be hotter spark to make starting easier ??-----Motor should NOT go to full throttle in neutral.----There is an interlock that prevents that.

 

[Rick.]

Regarding carb you can go to leeroys ramblings and have a look. He shows the throat size of the two carbs. It's a good site with lots of great information. Best of luck. Rick.

 

[cprodave]

FlyingScott, from my 2cycle dirt bike experience 40plus years ago we often resolved fouled spark plugs by going to hotter plugs, but this may have been wrong approach! I will go down a different path now. Thanks for advising about 6inch water above pump, I may not have had that at the boat ramp as boat sat on trailer. I could not tell about the exhaust reliefs or 6 holes, I will look next time.
Racerone, my thinking on hotter plugs is probably wrong (as above). Yesterday I bought same as original (also as specified on OMC/BRP diagram) Champion L77JC4 Plugs. Should I leave at factory gap (seems to be about .045) or regap to OMC/BRP-specified .030? LeeRoy’s Ramblings indicate .040 may reduce fouling especially if trolling a lot (which I will not be doing). Old plugs were about .045 . Thanks for advising about the Neutral Throttle Over-rev Interlock. My other/smaller outboards don’t have this feature (and I believe Leeroy doesn’t mention this—I need to get an Owner’s Manual).
Rick, yes I have read much of LeeRoy’s website for several years. I would like to avoid removing Recoil Starter, Carb etc to measure inside of Carb Throat, but eventually I may need to do that (despite inputs indicating 9.9 and 15 hp have same Carb body).
Thanks to all for inputs.

 

[flyingscott]

.030 is the gap. You cannot compare an outboard to a dirt bike they are very different applications only thing in common is they are 2 strks.The exhaust reliefs are right under the hood on the back so easy to see. To see what carb you have is easy. Take off the airbox both pieces ,and pull the mix knob out. Take the grommet off that goes around the mix knob and look straight down the throat of the carb. Externally the carbs are identical so the venturi tells the difference. If you have the older 9.9 carb a brass tube goes right through the center on the 15 it does not. Otherwise the Venturi size will tell you the difference. You were not told they had the same body what you were told is OMC/BRP does not list seperate par #s for the carbs. Reason being is they dont want people converting them.

 

[Joe Reeves]

The fact that the engine hits its full throttle rpms, even temporarily, indicates that the carburetor is functional. Carburetors do not come and go... if they're bad or the wrong one... they stay that way. In your case, the carburetor is okay. If you measure the rear venturi hole on the carburetor, I'm sure some member can tell you if it's a 9.9hp or a 15hp carburetor.

I have it written down somewhere..... some-where! .

Do the test as "flyingscott" states with the fuel primer bulb while underway (post #8). With that, you'll be acting as a manual fuel pump... and if that cures the problem, I'm sure I don't need to explain that.
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On some of those small hp engines, having the slow speed adjustable needle valve a little too lean... that'll have an effect on one cylinder. Do the following as a check of that setting.
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(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.
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[racerone]

If you ran the motor with a dry impeller you might want to inspect it.

 

[cprodave]

Flyingscott, thanks for confirming proper Gap is .030 and for clarifying location of exhaust reliefs. New plugs have been regapped to 0.030 for my next test. I removed the SlowSpeed Knob and Grommet. There is only a short height brass tube (i.e. not full height bottom-to-top) just beyond the Choke Plate inside the Carb Throat. I also checked Carb Throat Width using heavy paper/cardboard templates/gauges I made (one 0.625 inch wide, other 0.875 wide). Unfortunately the gauges confirmed that Throat width is 0.875 inches, so this is a 15hp Carburetor. Thanks for clarifying what I was told about identical p/n Carbs and rationale for OMC doing this. In your opinion will this outboard ever function properly without modifying the exhaust (or any other parts) to 15hp?
JoeReeves, as above it appears I have the wrong Carburetor. If I can get this outboard running on the water I will do the Primer Bulb test. If Primer Bulb fixes the too-low RPM’s problem then next step is Fuel Pump rebuild, correct? Or because of wrong Carburetor would Primer Bulb Test (while underway) be irrelevant? I already checked the Fuel Pump screen, it had almost no debris. I hope I can get this running well enough to do the S/S Needle Valve Adjustment.
Racerone, I am keeping fingers crossed I didn’t screw up the Impeller, although as Preventive Maintenance Impeller should be changed. I hope the Crankshaft isn’t frozen to the DriveShaft…
Thanks to All for the input…