June 17, 2017 at 1:43 am #59805
Out of the 3 I worked on the 1991 carb was the easiest for me to dial in. It has the adjustment screw to allow it the butterly to be tweaked while running at idle speed and dialing it in. It has like a SS hex head screw on it to make adjustments even while running tells me that was done for a reason. I will post the diagram of 1991 carb. It also had a totally different look and build to it. You can look for yourself but out of 3 I own I got the best results with this particular carb. I was working on like a 75,85,91 . The 75,85 had a carb like yours not much difference in them. The 91 best I could tell did not have the bearing needle valve it was eliminated (?). If at all possible try a 1991 style carb. I did notice that the low speed needle had to be turned out more on the older style carbs like 2 1/2 + turns to get a good idle. I did swap carbs while working on engines a 15hp carb compared to a 9.9hp carb in my humble opinion are almost identical except high speed jet size and yours is running well at high speed that would be the size you need if you tried a 15hp and it wouldn’t run high speed well. I measured carb throat sizes on 9.9 and 15s and they were very close.
Like Fleetwin said you can tweak when the butterfly opens or not to see if that helps and make sure throttle butterly is closing all the way not a weak spring. Check your reed valves also to see if their closing properly no light coming thru when they are closed.
Hope that helps,
http://www.marineengine.com/parts/johns … CarburetorJune 17, 2017 at 1:55 am #59807
After spending 3 years on my 76 here are my thoughts on these older carbs..I have had to set the float level slightly higher with the aftermarket plastic floats for the idle to be right on these first carbs. It would lean stall when throttling back. I wonder if the cork and plastic floats are actually equivalent. I got this tip from a member here I think.. The following story is for my 15hp but might be of some help..
I took out some cam lift from the beginning to about 3/4 or just where the cam starts to curve up. Not sure what cam you have on that 9.9..I can’t see how there can be a smooth transition from fully closed to just opened. So I set the roller at the vary beginning of the cam to just have the carb barely opened. I soldered the 2 holes closed on the butterfly and filed a flat on the top of the butterfly as the later ones have. It now runs better than ever. Even the wife commented about how smooth the motor runs when docking and such.
When I first got this motor 3 years ago all apart in a box it had a piss poor carb setup with it. I have spent hours on that carb..looking at ebay pics of carbs and looking at every carb exploded view I could for these motors. I have had it apart I betcha 20 times in 3 years and have made many changes that were a step back..June 17, 2017 at 2:10 am #59808
I am not sure if the 1991 would bolt up to a 76 that would need checking. Sounds like you have a great deal of experience with these motors. I will say I was challenged with getting a good consistent idle with these motors. I sold every one of them this winter in the Keys. I wasn’t a fan of them. I am a fan of the 9.5s .
Hope you get it fixed. I like the mods you did to your other carb. Very interesting. My buddy purchased my motors he is a live aboard in key west. I will keep that in mind if he has a problem he uses one of them daily.
JoeJune 17, 2017 at 2:31 am #59809
I believe the motor your working on doesn’t have points so you can’t use a timing fixture on my buddies 1961 50hp Mercury when the throttle started to open the points needed to open or it would stall not really your problem but interesting about timing of ignition verses carb. I was thinking what if you took off the cam roller on carb and adjusted timing to see if you could get a good idle if you find it then you would know when butterfly needs to open. I think one of mine I actually had the low speed screw turned out like 4.5 turns letting more fuel in at idle. Just wonder if that might help.
Also when I slow down an outboard I do it slowly and let everything catch-up since its all mechanical. I am sure you do also but I have had to show folks that when operating an outboard.
JoeJune 17, 2017 at 2:46 am #59810
The later 93 and up intake has a passage between the intake runners just before the reeds. And only a half baffle at the carb. Look at intakes on ebay or images and you will see the difference. The op has a 77 and has electro ignition. The position and size of the holes under the welsh plug are another tuning tool.
The unequal intake runners with a single carb I suspect is causing resonances at low rpms with that motor. The changes to the intake in later years might be an attempt to clean up the lower rpm.June 17, 2017 at 2:59 am #59811
When I was young I didn’t even know an outboard could run well at low speeds. I thought we were flying if it would go well at high speeds lol. As I get older I appreciate knowing that they also run well at low speeds when everything is working together in harmony. A lot more goes into that and I think its a great challenge but can be very frustrating to get there. But when you do its an awesome feeling. Nothing like it.
Not many reed valves worth checking to see if they might not be sealing correctly to rule it out or rule it in.
JoeJune 17, 2017 at 3:15 am #59812
A lot like getting an over camed 4 stroke to idle well enough..
These motors really needed 2 small carbs though ;)..I still haven’t ruled out trying a single Mikuni slide valve carb just to see how that works.
Hope all this off topic helps the op..
I have used cc reeds last year but they burnt quickly. This year I have Boyesens and I will see how long they last.June 17, 2017 at 3:32 am #59814
Someone here might elaborate on this more about spark plugs.
One thing I did do to the 1975 this past winter was a trick I learned in a Scott Atwater manual to the NGK spark plugs I had in it. It was called old timers tricks in a manual written in 1950. 1 trick that I applied was this Champion would call their plugs like J6J or J6C the J at the end meant the side electrode was ground half-way back to where it was only covering half way over the center electrode making the fire jump in a sideways motion. I filed my NGK plugs to that patern and it seemed to help at idle making plug less likely to foul.
The other old timer trick was to mark the spark plug where the insulator curved tip is on outside of plug and when you install plug make sure the mark is in the down position even if you have to put in a second washer this helps a cylinder that is fouling. It also mentioned widening the plug gap on that cylinder also a tad.
Just something to ponder,
JoeJune 17, 2017 at 3:54 am #59817
Yup..I index the ground electrode towards the intake as I use the older, higher comp head. I side gap the plugs at .030. The half ground electrode as I understand is to help keep carbon chunks from getting caught in between during the days of 40wt auto oil. Since point to point is ideal for easy spark I side gap and point the ground electrode. The older head showers the plug with intake juice so I use a b6 on top and a b5 on the bottom. ngk of course..
US Member - 2 Years
Topics: 135June 17, 2017 at 5:01 am #59822
I have learned a lot on this thread especially page 4 . I have tried to index the spark plugs also on my ’76 9.9. So i should have the ground electrode at the bottom,with its point facing up to the center electrode?
"Some people want to know how a watch works, others just want to know what time it is"
Robbie RobertsonJune 17, 2017 at 11:27 am #59825
The side electrode has a curve shape to it where it is joined to spark plug side it the curves and goes over the center electrode. As I understand it the curved part needs to be positioned where it is at the bottom of cylinder when screwed into the engine with the ground part facing up toward top of cylinder. Putting a mark on outside of plug with like a sharpie lets you know where the bottom curve is while your screwing it in because you can’t see it. I think that is what you and I are saying if so your correct
If you look at the pic of spark plug you see the curved electrode and it curves out over the entire center electrode. When you grind the curved side electrode you grind it to where it doesn’t go over the entire center electrode it only goes half-way over it. I hope that helps explain it. It makes the spark jump from the center electrode sideways to the curved side electrode.
These are good tips especially if a lot of trolling is involved.
Hope that helps,
US Member - 1 Year
Topics: 52June 17, 2017 at 6:00 pm #59831
Thanks guys for the spark plug tips. I never thought about gap alignment before, although I have played around with changing the gap on problem motors with mixed results, and I have put a higher heat range plug in one cylinder if the plug consistently comes out looking wetter/blacker than the other and this often works! Orientation of the electrode may explain why a 9.9 Johnson I worked on 3 years ago went from terrible to perfect idle when I put in new plugs, although I did change from Champion to NGK. Maybe the alignment just happened to get right with the new ones?…although NGKs just seem to work better in some motors.
DaveJune 18, 2017 at 12:03 am #59845
If you look at rudderless notes. He is running a NGK b6 on top and b5 on bottom. I am guessing he is running B6HS top B5HS bottom You might consider trying what he has learned about the plugs.
I think I was running a heat range of B7HS in both plugs and I used them in all 3 motors both cylinders which is a bit cold for that motor in my humble opinion but they are being ran in Key West its a bit hot down there and I pulled the thermostats on them one of them had sand up to past the bottom cylinder in the water jackets when I pulled the head. If you hit a sandbar down there it will fill one up with sand quick. I decided to pull the thermostats to help more water flow and less likely hood of that happening. Again just my humble opinion my wife hit a sand bar in VA Beach once and filled the motor water jackets with sand so fast that we had to walk like 8 miles down the beach to base to get the truck. I bet I pulled the heads on that 140hp 3 times to finally get all the sand out of water jackets. It would start and circulate water great then 2 minutes later stop. The sand got hot and formed like concrete in the water jackets and would stop water flow.
You might try his heat range on your plugs might help dial it in.
I did find out the BR7HS didn’t work as well the R is for resistor. and wasted power less spark.
You don’t want a resistor in the 9.9s just my humble opionion. As NGKs numbers get lower the plug gets hotter. A 7 might be a tad cold for that motor? again just my humble opinion. A ngk B6HS may give you better results it would be closer to an 8 in Champion.
Like a champion L77JC4 the heat index on it is almost an 8 which is way hotter than a NGK 7 again my humble opinion. I would try what rudderless did he has 3 years of messing with these. Don’t re-invent the wheel.
Grinding the side electrode and orientation when plug is tight in cylinder are great old timer tricks to keep plugs from fouling especially when a lot of trolling is involved.
Hope that helps,
JoeJune 18, 2017 at 2:01 am #59853
The 74,75,76 had a head that would spray intake charge right at the plug tip. Later heads had the deflector to keep this from happening. The 93 and up the plugs are aimed away from the intake charge. The older head was higher in compression so I use it. I only use the hot plugs on that head. I aim the ground electrode towards the intake charge in an attempt to shield the center electrode from the charge. The tip of the plugs are always black but further in they are running clean.
Shorten the ground electrode so it will pass by the center electrode at .025-.028in…kinda like a surface gap plug but with a ground electrode.
I am using red Atom point eliminators and really like them. If they cook I will make a couple like in the make your own Atom thread. They are mounted outside on the motor next to the spark coils.
I feel the ngk’s have a little less thermal mass at the working end and heat up quicker.
The bottom cylinder runs cooler, puts out less power. The upper cylinder drains condensed fuel into the lower cylinder and the upper has a much shorter intake runner. The cc reeds burnt on the upper much faster than the lower cylinder also.
I’ve only got 3 years onto this motor..members like Pappy have had a lifetime practically. He knows more than he lets on I betcha…June 18, 2017 at 12:46 pm #59865
I hope we haven’t bombarded you with to much info. I was challenged with these motors not the normal plug and play omc motor.
You do have access to extra carbs so you might try that route first. I think that would be the best 1st step I would take.
Make sure the carb butterly is closing all the way when inspecting the carb your going to try or the one you took off that isn’t idling correctly.
Don’t be afraid to turn the low speed screw out more than the manual says to get a good idle I had to like 4.5 turns out on 1 engine. It was my thought that they sure put a bunch of threads on the needle for some reason. You might also make sure reed valves are closing completely by inspecting them with a bright light or see if one might be physically damaged or not closing as-well as the others.
I looked up the sprark plug listed for a 1977 9.9hp its showing a L77JC4 which I cross referenced to a NGK B7HS . The B6HS would be a tad hotter if your already using the B7HS and can’t get it to idle well.
Hope that helps,
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