15HP Johnson runs great on muffs / bogs at the lake

Any input would be greatly appreciated...

I've been working on a 76 Johnson 15hp short shaft, pull start. This engine had been sitting for 3+ years before I got my hands on it.

Problem:
It will run fine on muffs in my driveway. Starts on first pull, revs, pumps water great from pee hole...etc.
However, for some reason when I place the boat in the water I start to experience problems... At first it will start rather quickly (1 or 2 pulls with choke out) and run fine for a couple minutes. After moving the choke back to the IN position and applying a little throttle, it bogs down immediately. After which it is nearly impossible to get it started, and takes me several (10+) pulls to get it running again.

Once I finally get it running again, while holding any CONSTANT throttle position (in neutral) it will bog down after a few seconds. Although I have noticed that I can keep it running a lot longer if I continuously play with the choke (pulling out and pushing in).

Observations:
*Could be irrelevant, but lots of water coming from the exhaust relief holes under the powerhead while running.
*Somehow the increased back pressure of the lake severely effects this engine. Could it be a weak fuel pump? Or possibly an ignition problem? ](*,)

Things I've done:
Rebuilt the carburetor with new float
New fuel lines
New plugs (checked for spark by simply grounding out the plugs to the engine block)
New water pump (pumps great out of the tell tale)
New thermostat
New exhaust housing (original was cracked)
Inspected fuel pump (seemed to work well enough?)

Sounds to me like you still have an engine running lean under load. I think you still have carb. issues. Did you completely disassemble the carb, soak it, then wash it out with clean water and blow all passages out with compressed air before reassembling?

If not carb. issues then you will have to make sure your timing is advancing when you accelerate.

When I redid the carb I made sure to clean it as thoroughly as I could. I didn't use water, but instead i used a parts cleaner. I used the evinrude rebuild kit, which came with a new plastic float. Both jets are very clean, and I reassembled with a new bowl gasket. One strange point, after i installed the new "slow speed idle" screw seal, it made the screw a lot harder to turn. (Hard enough that I can no longer use the plastic adjustment knob) I'm guessing its just from the added reaiatance of the new rubber seal.

I'm starting to wonder if it might be timing, like you suggested.

I have a 15 HP also and it did the same thing last spring. I was about ready to pull my hair out over it. What I found was the plug on the fuel line was not going on far enough and it would run out of gas. I replaced the connecter on the fuel line and it worked. I still don't know what happened to cause the old one to go bad.

sawmill said:

I have a 15 HP also and it did the same thing last spring. I was about ready to pull my hair out over it. What I found was the plug on the fuel line was not going on far enough and it would run out of gas. I replaced the connecter on the fuel line and it worked. I still don't know what happened to cause the old one to go bad.

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They have an O-ring in them and ethanol in fuel swells them to the point they will come out of the channel that retains them and if they go back in the housing that will happen.

Also they cost a little more but if you use OEM fittings instead of WalMart or any replacement end they tend to last longer.

Succotash said:

When I redid the carb I made sure to clean it as thoroughly as I could. I didn't use water, but instead i used a parts cleaner. I used the evinrude rebuild kit, which came with a new plastic float. Both jets are very clean, and I reassembled with a new bowl gasket. One strange point, after i installed the new "slow speed idle" screw seal, it made the screw a lot harder to turn. (Hard enough that I can no longer use the plastic adjustment knob) I'm guessing its just from the added reaiatance of the new rubber seal.

I'm starting to wonder if it might be timing, like you suggested.

Click to expand...

Sometimes if you have an orifice really plugged in a carb. spray parts cleaner especially if you didn't use a real good spray carb. cleaner just won't get it.

The absolute ideal thing is the dip carb. cleaner and put the carb. in and let it soak a few hours then wash it down with soapy water and then blow it out with compressed air as I stated in the first post. I know that everyone doesn't have access to this so you have to make due with what you have.

I can't reiterate enough that I am betting it is a fuel delivery problem. Go back to basics. I reread your list of things that you have already done and I am asking this not to belittle you but you did put a fresh fuel mix in it this year? Fuel is sorry and doesn't carry over well even with additives. Then check your connections as stated in by another poster. Usually if it is a fuel pump, but not always your primer bubble will go soft if it isn't working correctly. That is another problem area I didn't ask about, is it staying pumped up when this problem occurs?

Putting a new seal on the low speed air adjustment will restrict it some as to turning, if the old one was hard and worn out then it wasn't doing its job. Those plastic tabs usually are removed the first time someone rebuilds a carb. anyway. That screw shouldn't be excessively hard to turn though.

I would look very hard at fuel. Checking the timing advance on muffs is darn near impossible. You can do it running if you have a barrel to put the motor in while attached to the boat but you most likely will wind up wet and frustrated.

Similar prob . . . my carb needle was sticking to the seat. Now I run it dry before I put it on the trailer.

nccatfisher said:

Sometimes if you have an orifice really plugged in a carb. spray parts cleaner especially if you didn't use a real good spray carb. cleaner just won't get it.

The absolute ideal thing is the dip carb. cleaner and put the carb. in and let it soak a few hours then wash it down with soapy water and then blow it out with compressed air as I stated in the first post. I know that everyone doesn't have access to this so you have to make due with what you have.

I can't reiterate enough that I am betting it is a fuel delivery problem. Go back to basics. I reread your list of things that you have already done and I am asking this not to belittle you but you did put a fresh fuel mix in it this year? Fuel is sorry and doesn't carry over well even with additives. Then check your connections as stated in by another poster. Usually if it is a fuel pump, but not always your primer bubble will go soft if it isn't working correctly. That is another problem area I didn't ask about, is it staying pumped up when this problem occurs?

Putting a new seal on the low speed air adjustment will restrict it some as to turning, if the old one was hard and worn out then it wasn't doing its job. Those plastic tabs usually are removed the first time someone rebuilds a carb. anyway. That screw shouldn't be excessively hard to turn though.

I would look very hard at fuel. Checking the timing advance on muffs is darn near impossible. You can do it running if you have a barrel to put the motor in while attached to the boat but you most likely will wind up wet and frustrated.

Click to expand...

Thanks for your support nccatfisher,
I am using fresh gas mixed at 50:1 ratio. I think I'm going to go check all my fuel connections, as my primer bulb does not remain hard while running. It's a little firm, but not as hard as it was when I prime the engine. I suppose I could have air getting into one of my lines, or a weak pump. All my fuel lines, included primer bulb are new, but I reused the original "fuel connectors" that lock onto the tank/powerhead. I will check all of those again carefully.

As for checking the timing, I will need to do a little research prior. I'm not exactly sure how to do this.

Again, thanks for the advice everyone!

Is the carb float adjusted correctly? IIRC it should be level with the carb when turned upside down and the free end could be slightly higher. If you don't have compressed air, a can of air like ones used to clean computers usually works well for blowing out carb passages

sawmill hit on something that happened to me, the fuel connector that plugs into the motor could have a bad seal allowing air to be sucked in while running. But usually that will leak when pumping bulb.

might be as simple as carb adjustment, air:fuel ratio is different on the muffs without a load, versus in the water. this is why carb adjustments should be made on the water. most carbs baseline for the air screw is all the way tight & then back it out 2.5 turns

i'd start with that, as it seems like you've exhausted all other options. my immediate thought was a bad primer bulb, but you said that's new. not sure if you're running a fuel water seperator, but replace it if you are. also check the fuel filter if you haven't already.

did the motor come from somewhere that has a different altitude than where you're running it?

if you're up in the mountains, you're going to need more air versus someone @ sea level

in the water your thru-prop exhaust is combatting water pressure thus changing back pressure. the motor will run more efficiently with proper back pressure, just fiddle with the air screw & i bet you'll get it going if it's not the fuel pump. could also be the wrong spark plugs in there, i'd check that out too

edit:

re-reading the original post about the choke issue, leads me to think you're sucking air or lacking fuel & that the problem is NOT related to being deprived of air. you need to change the air:fuel ratio to have more fuel & less air, either by fixing a leak, removing a stick or clog, or adjusting the actual jetting and/or air screw if there's not a fuel system issue. if it runs forever on full choke, you DEFINITELY have an air leak, & i usually find that the carb fuel bowl gasket is the culprit. on full choke, the motor should bog out due to lack of air once it warms up. hopefully it's not the timing [-o<

First thing you should do is make sure your fuel line from tank to motor is NOT backwards.

If it is backwards, the vacuum in the carb will suck gas up after a number of pulls and begin running. The moment you go to WOT or close to it, the primer bulb will squeeze shut and bog the engine. You can idle and run slow with a backwards fuel hose but can't run WOT.

If that's not the case, I would flush the hoses and fuel filter again.

If that doesn't fix it and your confident the carb is clean. Check for a spark on both cylinders. Chance are you could be running on one cylinder.

Use the acronym FAST.
Fuel, Air, Spark, Technology - in that order.

chevyrulz said:

might be as simple as carb adjustment, air:fuel ratio is different on the muffs without a load, versus in the water. this is why carb adjustments should be made on the water. most carbs baseline for the air screw is all the way tight & then back it out 2.5 turns

i'd start with that, as it seems like you've exhausted all other options. my immediate thought was a bad primer bulb, but you said that's new. not sure if you're running a fuel water seperator, but replace it if you are. also check the fuel filter if you haven't already.

did the motor come from somewhere that has a different altitude than where you're running it?

if you're up in the mountains, you're going to need more air versus someone @ sea level

in the water your thru-prop exhaust is combatting water pressure thus changing back pressure. the motor will run more efficiently with proper back pressure, just fiddle with the air screw & i bet you'll get it going if it's not the fuel pump. could also be the wrong spark plugs in there, i'd check that out too

edit:

re-reading the original post about the choke issue, leads me to think you're sucking air or lacking fuel & that the problem is NOT related to being deprived of air. you need to change the air:fuel ratio to have more fuel & less air, either by fixing a leak, removing a stick or clog, or adjusting the actual jetting and/or air screw if there's not a fuel system issue. if it runs forever on full choke, you DEFINITELY have an air leak, & i usually find that the carb fuel bowl gasket is the culprit. on full choke, the motor should bog out due to lack of air once it warms up. hopefully it's not the timing [-o<

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Thanks for the input everyone, today I'm going to buy a large drum so I can actually run it with the added back pressure of water. I bought the engine just a few miles away, so I'm guessing that it had been set up to run at my current altitude.

When I apply the choke to keep it running, its more of an erratic behavior. I can't simply pull it out to keep it running, I have to quickly open and close the choke while also applying various throttle inputs.

Today I will check:
Fuel line connections
Carb bowl gasket
Test various slow speed needle settings
Ensure primer bulb is facing correct direction
*Also want to do compression check

Just thought I'd share with you guys how my exhaust housing looked upon inspection... I searched around and have yet to find another one cracked like this. I'm not sure what had caused it, but I replaced it with another I found on Ebay for about $25.

check the coils, have they been replaced? points?

as for the carb, remove the needle and look carefully at seat. you should see a plastic piece inside, the needle bearing. sometimes the needle bearing gets old and brittle and can cause air leaks.

dino

She's lean. Plain and simple....but why? Could be:

partially obstructed carb
partially obstructed fuel lines
failing fuel pump
filter screen plugged
air leak

I am about 95% confident she's starving for fuel based on the fact you can pull the choke lever out and it'll pick up a little. But since these things actually do have a choke plate at the front of the carb, you can't just run it with the choke pulled all the out or it'll flood it...so you have to push it back in for some air to get in there. Then it runs a little longer until it goes so lean that it won't self-sustain until the choke lever is pulled back out.

Just did mine the other day....the fuel pump died. Easy fix on these, as is most anything on the Johnny Rudes. I found a ton of rubber in the screen too, so I replaced the fuel lines under the cowling (they crumbled..literally).

If you don't have or want a barrel that's in the way, it's possible to use a bigger rubbermaid container. That's what I use. My 9.9/15 works well, and my little 25 yammie works in it too, but the container is a little small for the old 90 HP Merc. It works but the water pump pees so hard that it will empty it fairly quickly. I got it at Dollar Tree of all places. So it wasn't expensive. Dollar General has them, and of course Walmart, Home Depot/Lowes, etc. It takes about 5 minutes to fill it enough to run the motor, as opposed to 30-40 min to fill a barrel. The only issue is that the motor needs to be mounted on something sturdy (transom or whatever)...because the cheesy rubbermaid containers aren't stout enough to hold the motor. But they do take up a lot less space than a barrel.

Exhaust housing looks like it may have been melted? Hard to tell. A lean engine can do that...as well as melt pistons.

I would start with the simple and replace the fuel filter and then go from there.

I would like to thank everyone for their responses in assisting me with this whole thing.

Well I've got some good news. Today I went out and got a large trash can, so I could simulate the back pressure at the lake. After doing making some adjustments to the slow speed needle, I managed to get it running pretty good. Now It will start on the first pull, and idle all day long in the water. However, If I maintain a high RPM for more than 30 seconds or so, it will eventually die out.

New observation:
After checking all fuel connections, I do not have a leak anywhere.
When it dies out, the primer bulb has no pressure.
Tank vent is open.
After priming again, it will start right back up.

Everything points to the fuel pump/filter. I'm guessing that it's somewhat restricted or becoming too weak to sustain any RPM higher than idle for an extended period of time.

Here is a short video of it running today. Seems like I'm getting quite a bit of water coming through the upper most exhaust ports, not sure if this is normal or not.

good job. nice running motor.

change the fuel lines, and rebuild the fuel pump. i think the diaphram might have a small hole or old and not as flexible to keep up with the pulse. or buy a new fuel pump. dont buy a cheapy (dont ask) stick with a omc or sierra at the least. the off brands are not so good.
i have rebuilt probably about 20 or so, its pretty easy. a lot cheaper than buying a new one.

dino

Acts like a fuel pump. Probably has a hole in the diaphram or at the least they are very hard from ethanol. Rebuilds are easy. Give you a hint, you may want to run your water hose slowly when your are running your motor in that trash can so as to not starve your motor for water. It won't hurt to let the can run over with water.

Sounds like a restriction in the fuel flow, maybe check all the lines ,bulb ( one of the valves could closing ) and the pick up in the tank. If it has a screen it could be partially clogged. Like whats been posted it wouldn't hurt to the diaphragm and spring in the fuel pump. It also has a screen as well.
If it does not have a thermostat that would explain the excessive water coming out of the exhaust relief. IMO nothing to worry about unless you run it in the winter. Some say ya gotta have a t-stat some don't. I only had them in the larger outboards I've had cause I did run them in the cooler months. Made them easier to start after warming up.

On a side note, nice Wrangler ! Got an older Renegade myself.

Charger25 said:

Sounds like a restriction in the fuel flow, maybe check all the lines ,bulb ( one of the valves could closing ) and the pick up in the tank. If it has a screen it could be partially clogged. Like whats been posted it wouldn't hurt to the diaphragm and spring in the fuel pump. It also has a screen as well.
If it does not have a thermostat that would explain the excessive water coming out of the exhaust relief. IMO nothing to worry about unless you run it in the winter. Some say ya gotta have a t-stat some don't. I only had them in the larger outboards I've had cause I did run them in the cooler months. Made them easier to start after warming up.

On a side note, nice Wrangler ! Got an older Renegade myself.

Click to expand...

I went ahead and ordered a fuel pump rebuild kit today from iboats, hopefully it will take care of the problem. I'm also going to inspect the pick up within the tank.

As for the thermostat, I just put a new one in it. I'm guessing the water coming from the upper exhaust ports is normal, I noticed the same thing happening on several youtube videos.

Here's how the jeep looks now, lol. It's been through quite a bit...