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Merc 15 hp is getting the best of me, I'm at a loss, won't run right (S/N 0T533347)

Does anyone know if item 41 in this parts diagram should be on my engine? I see it in the service manual, for my SN and year, and just found it in the parts diagrams online under "carb linkage" section. It is not installed in my engine. It shows it between two carb gaskets and the reed block...still just digging away seeing if I can find anything that tips me off. I've only had the engine for a year, so if previous owners missed something, I wouldn't know it unless I look for it.

Looking at the parts diagram through Merc, it's listed as No Longer Available, and also with a Comment of "Restricted 6", which I believe means it restricts this carb downward towards 6hp from the higher horsepower versions. But it's listed under the 15hp section, so I guess I may be learning how to read the parts diagrams.

Thoughts? Thanks.

restrictorplate.jpg
 
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Thanks. As far as I can tell too there isn’t a separate gasket between the reed block and cylinder head. So all seems right there too.
 
Here's another question...What is this brass screw circled in red, that looks like a jet, on the top right of the carb? The parts diagram doesn't show this, at least that I see. I don't recall, without pulling the carb, if it is a passageway or not. Assume it is.

carb screw.jpg
 
Made some progress. Another new set of plugs, but this time running it in a bucket of water, and not on a hose. I was able to turn it and the backfiring stopped as did the stalling. The first time I adjusted the mix screw, I landed at 2.25 turns out. I did it again a few hours later, and landed on 1.5 turns out (which also happens to be the service manual spec/starting point). I'm not sure what to think.

The plugs still look different but I think better than before. The bottom cylinder came out wet and likely fouled, and the top one much cleaner but looked normal to me (vs very dry earlier in this troubleshooting endeavor). Maybe the mix adjustment testing and long idling did this to the bottom plug and I should replace them again? I watched a Dangar Marine video, and he had plugs on a motor that were identical to my situation, and after all his research on the motor, there was nothing wrong with it.

I did have a few observations that I'm curious about, if they sounds normal or not.

1. When initially pumping up the bulb, maybe I pumped too much, but I noticed excess gas running out of the back of the carb into the cowl.

2. When pumping the bulb, I could hear gas/air swishing inside the fuel pump until it seemed to full up. Normal? I don't recall ever hearing that before, but never was really listening.

3. When using the primer, there is a small hole on the right side of it that faces the fuel hose fitting. I noticed gas expelling from that hole when the primer arm lifted. Excess gas? Is that what the hole is for?

4. As mentioned, first time I tuned it at 2.25 out, then 1.5 (leaner). Why would I get two different spots that seemed ok at different times.

Here's a pic of the plugs, and two videos.

new-plugs.jpg

1.5 turns out: https://www.dropbox.com/s/cbaj9xhja6yw5ra/1-5screw.MOV?dl=0
2.25 turns out: https://www.dropbox.com/s/kl7x6g1m2f8gn1e/2-25screw.MOV?dl=0


I still just don't feel confident in the motor, it still seems to idle weak. I wonder if the CDI box is faulty. It does hold idle of around 750 in forward, and jumps to about 800-850 in neutral, but still feels finicky.

Thanks for any new thoughts.
 
1. If you pump the bulb and gasoline is coming out somewhere, you need to trace where it is coming out...this is not right.
2. Are you sure you weren't hearing gasoline going into the fuel strainer bowl or the carb bowl?
3. If you have gasoline coming out of the hole on the side of the primer, you have a problem. That hole is there
to allow air into and out of the space ABOVE the primer diaphragm. You have a leak.
4. You are the running the engine at idle...you have to get the idle mixture set correctly. Follow the manual
on this....set the mixture half-way between loading up from too rich and slowing down from too lean.
The adjustment should be made very slowly.

Also, your cooling water stream seems very weak....there should be a solid stream of water coming
out.....even at idle.
 
1. If you pump the bulb and gasoline is coming out somewhere, you need to trace where it is coming out...this is not right.
2. Are you sure you weren't hearing gasoline going into the fuel strainer bowl or the carb bowl?
3. If you have gasoline coming out of the hole on the side of the primer, you have a problem. That hole is there
to allow air into and out of the space ABOVE the primer diaphragm. You have a leak.
4. You are the running the engine at idle...you have to get the idle mixture set correctly. Follow the manual
on this....set the mixture half-way between loading up from too rich and slowing down from too lean.
The adjustment should be made very slowly.

Also, your cooling water stream seems very weak....there should be a solid stream of water coming
out.....even at idle.

1. When I pump the bulb, if I keep pumping, I get gas leaking out of the back side of the carb (where you can see the plate). So maybe my needle in the bowl isn't sealing properly when the float is up? I should mention that I did buy a new needle, and it looked different than the one that was in there. I'll take a pic. I figured they just updated it. Ordered the part that was specified in the parts diagram.
2. Definitely hearing this pumping sound in the fuel pump.
3. Thanks, and I thought so. Perhaps I need a new diaphragm for that. I will disassemble and inspect it closely. But there is definitely gas coming out of the side. I'll get a short video.
4. To adjust mixture, yes, running at idle in forward gear, I adjusted as you said, going slowly. Followed the manual and your previous suggestions about small adjustments and giving it time to adjust/settle.

I'm going to check my needle in the bowl and that primer gasket/diaphragm.
 
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Well no leaks from carb throat anymore. Maybe it was a stuck needle or something. But here’s the primer leak. I knew this couldn’t be ok/right.

https://www.dropbox.com/s/eokf3wfb8hs0l2j/primer leak.MOV?dl=0

I just ordered a new one. I pulled it and can see how worn it is all around. Not sure if this would have an impact on how it runs at idle, but certainly something else that isn't right. This was one neglected outboard before I got my hands on it.
 
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You definitely need to fix the primer leak. There is a gasket, too.
Maybe post a picture of the old and new needles...were they different lengths?
Perhaps that is why the float level was incorrect at the start.
Did you readjust the float level with the new needle?

You might raise the water level in your bucket....it should be well above the water pump.

Actually, from the video, the idle doesn't seem too bad. It is a 2-stroke engine.

Would be good to put it on the water and see how it does with open throttle.
 
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The needles were identical. I thought they were different. When new, the needle tip was gray not black. Probably coated in something. I compared them side by side but here's a pic.

needles.jpg

And yes, I double checked the float height after putting the new needle in. Thanks for noticing the water height. I didn't think of where the water pump is, vs the pickup...

So in looking at the primer diaphragm, the diaphragm itself seems ok. But the gasket has an obvious break in it. Unfortunately you can't buy just the gasket. I was going to quickly get it ordered, but then I saw that. I'd be wasting a bunch of parts I already bought separately. It may be the only option for me, unless you think I could just make my own gasket for this using actual gasket material from the auto parts store, not RTV gasket maker.
 
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I meant to ask this too. Could that leak in the primer housing be considered a significant air leak? Also what would this indicate: when first starting and running at a high speed idle, sometimes it will start to slow down and ultimately stall. It won’t hold the high idle. What I need to confirm is if it still does his after my mix screw adjustments and fixing the leaking primer.
 
This motor is really being a pain. Darn thing won't run now. Been sitting a few days, and now I can barely get it idle, and it does what it does, it slows and then dies. On fast idle to start, it sounds more like a regular idle speed. I'm grabbing the new primer and gasket tomorrow from West Marine (they have a Sierra kit, I just don't care to wait for parts to be delivered again). I figured while I waited, I'd check the timing at idle, just to confirm 8 degrees BTDC. But the thing won't run long enough or stay running. I know I could drop it off somewhere, but I'd like to learn and solve it. I'll wait to get the primer fixed but I doubt that's it, but you never know.
 
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I give up. Whatever is wrong, it's beyond my ability or desire to solve at this point. The new primer is installed, and while it stopped the leak and maybe improved cold start performance, it still won't run right and dies off like it's not getting fuel. Small squirts of carb cleaner will spring it back to life for a second or two then it dies off again. I have cleaned that carb probably 10 times and blew out every passage. I know it's clean. I'm wondering if it could be a crankshaft seal leak or something. I called a local shop to see if he can take a look. So frustrating.
 
The sparkplugs indicate that it might be running on one cylinder.----Does spark jump at least a gap of 3/8" or so , yes or no.------What are the compression numbers on this motor ?
 
The sparkplugs indicate that it might be running on one cylinder.----Does spark jump at least a gap of 3/8" or so , yes or no.------What are the compression numbers on this motor ?

Yes, it will jump a 1/2" gap, no problem. I just double checked that the other night too. And compression, while not perfect, is decent at 105psi, and the same in both cylinders. And when I pulled a wire when it was running, it would change and slowly die off on only one cylinder, and either cylinder. The plugs are more balanced now too.

This has something to do with fuel delivery. It has to. When I prime it, she fires right up. When it starts to die, if I press the primer, she brings back to life for a bit, then slows down and dies. I just disassembled the fuel pump to take a look. No visible tears or holes in the daphragm, which is new about 2 weeks ago. The fuel pump itself, the plastic piece, seems fine, the reeds look good, and no cracks. New gaskets too. New fuel lines. The bowl float has been adjusted to be in spec per the service manual.

Is there a way to test a fuel pump, or remove it from the equation? I read about gravity feed or something, put the tank and ball higher than the motor?
 
If motor responds to operation of the primer bulb the fuel pump on the caeburetor is NOT working.-----Gasket between carburetor and block the correct one ?----Also look into crankcase compression issues.----Fuel pump is driven by crankcase pulses.
 
buy a complete carb kit for your motor stop buying pieces. A good kit will have the.fuel pump and primer parts. Soak the carb in a good cleaner and reassemble with the correct parts. Make sure you use the correct base gasket and install it correctly. When you put the base gasket on make sure no holes get covered. Check the spring and ball under the primer as those like to jam up in the spring. 2.25 turns out is fine for the low speed screw you also need to make sure all the linkages are correct and do a link and sync. Where did you find a 1300 rpm idle speed that will shorten the life of the lower unit nicely. ALL final adjustments MUST be done with the motor warm and running in gear NOT cold.
 
Racerone, thanks. Yes, the gasket between the carb and block is the correct one and installed with the proper orientation. I understand how this fuel pump works off pressure, so is there a way to test that pressure? This isn't something I saw in the service manual.

Flyingscott, pretty much everything in the carb is new now with the exception of 2 parts, so I'm not going to buy a whole new kit just because. I'm sure I spent a few extra bucks the way I did it, but it is what it is at this point. The carb is clean. I cleaned it, ran it through an ultrasonic cleaner, and made sure all the circuits were clear. I bought a new carb bowl because I couldn't get one cleaned out. The linkages are right, with one exception. I want to confirm the idle timing, but I can't get the motor to run long enough to warm up to do it. The fuel pump gaskets and diaphragm is new (could be bad, who knows). The gaskets are on right, holes are not blocked. The ball and spring are good (and new). I bought a gasket kit, and then a few parts. The only thing not replaced is the mixture screw/needle and the plastic fuel pump part itself (which perhaps is the problem). All the rest if new. And I would make the adjustments with a warm motor, if I could keep it running.

So far I have not replaced anything that didn't need it. I had wires that didn't test properly, they had a break, so they got replaced. The primer diaphragm was leaking so it was replaced, etc. I'm not throwing parts at without trying to test, diagnose, and then solve. That's what I'm asking here...is there any way to test the fuel pump...off to Google that to see what I can find.
 
To check the fuel pump do this. Pump the primer ball while it is running if it stays running the pump is bad. Do a compression test before anything else to rule out an internal engine problem.
 
To check the fuel pump do this. Pump the primer ball while it is running if it stays running the pump is bad. Do a compression test before anything else to rule out an internal engine problem.

It definitely stays running if I pump the primer bulb. I have to pump it hard though, as the bulb becomes pretty firm, not soft, as I pump. So I don't feel like I am really pumping, as much as I am keeping a constant pressure on the fuel...if that makes sense. They are small pumps of force. But as it would start to stumble, I'd press/pump and then it would catch and speed up. I could repeat it at will. Given that the fuel pump diaphragm and gasket is new, but not the fuel pump body, I am wondering if it could be that part, or, as mentioned above, if compression is an issue, that I'm not getting a strong enough pulse for the fuel pump to operate properly.

I did do a compression check a few times, and I get 105 psi in both cylinders. Slightly below spec (115-125 per service manual), but they are both the same and not less than 100 psi. I do have a cylinder leak down tester, so I plan on doing that tonight to see what results I get. How hard is to it to fix the compression? Is it just new rings if the cylinders aren't scored? That's one thing I've never done...

And Wave Hunter, I have not checked that. I am pretty handy, but learning as I go a bit, so I will need to figure out where to check for that and look at the parts diagram to understand what I'm after. I will check it out.

Thank you both. This has been an adventure, but I am learning a ton that should serve me well in the future.
 
Here's an update with some progress I believe, at least when it comes to diagnosing and ruling things out.

First, I did a cold leakdown test. I've read that it should be done warm, but, well, I can't keep the darn thing running. Worst case my readings would look worse than they really may be. The good news was, assuming I did the test right, is that both cylinders appear in very good shape, including head gaskets, etc. Cyl 1 and 2, when each was at TDC, only read about 10% loss. That is excellent.

leakdown_cyl1.jpgleakdown_cyl2.jpg

Then I moved on to the fuel pump pulse. I'm not sure how I can measure this, but with the carb removed, there is a very obvious and strong pulse of air when I pull the starter rope, even not pulling that hard. The top red circle in this next picture is the hole I'm talking about. Of course the fuel pump itself could be having an issue still even with the new diaphragm...

reedblock.jpg

I do have a question about the lower red circle in the above picture. That port lines up with the carb port where the bleed line connects. Should the bleed line be full of fuel? When I was pulling the motor over gently to test the fuel pump pulse, gas was coming out of the bleed line. Is that right? Does that tell me anything? This hose:

carbhoses.jpg


And beyond that hose, regarding the reed block, there are two little holes in that little insert (bottom red circled item in the reed block pic above), and it seems like it has some type of plastic piece inside it that moves around. You can literally push it around to block the tiny top hole, or move it to block the tiny bottom hole (with a small piece of wire inserted). Is that supposed to be there? Could this be a blockage of some sort? This is just a bleed drain from the crankcase right, yet that hose seems like it had gas in it.

And lastly, where's the anti siphon valve on the motor? Looking at the service manual, many of my symptoms have an items listed as a faulty anti siphon valve, that to test, you could remove it and try to run engine. I just can't find it on the parts diagram.

Hopefully this information tells someone something...

Thank you all!
 
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In the service manual, see under Powerhead, "Bleed System".
There are a couple of check valves.
I'm wondering if your bleed hoses are routed correctly....in the last picture, there is a bleed hose going off to the right of the photo.
Is that normally connected to something?
That hose also looks to be larger than the other bleed hoses.
Was it replaced?
 
In the service manual, see under Powerhead, "Bleed System".
There are a couple of check valves.
I'm wondering if your bleed hoses are routed correctly....in the last picture, there is a bleed hose going off to the right of the photo.
Is that normally connected to something?
That hose also looks to be larger than the other bleed hoses.
Was it replaced?

Thank you for replying.

That hose going off to the right is just where it's sitting while disconnected from the bottom of the carb. It normally connects there. I will check the check valves. As for the hose being a different size, I just looked closely and it sure does seem larger. If it was changed it was done by a previous owner. I'm not sure that could create an issue though, could it? I do see that two of the bleed hoses have dashed lines on them, blue and yellow.

I will look closely at the service manual again on the bleed hoses. I was also talking to a local marine mechanic/friend, and he is convinced it's running lean, as some have said, and if the air pulse for the pump seems strong, that it's either the lower crank case seal, or the idle carb jet. I just cleaned that little tiny tube again, to be 100% sure it was clean. I will see what I can find for lower crank case seals, besides pulling the power head.

Do you know what this anti-siphon valve refers to in section 4, troubleshooting, for hard starts, stalling? Is this the valve in the fuel line bulb or something else?

service_antisiphon.jpg
 
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Checked out the bleed lines and check valves. The one in the lower crankcase is definitely good. Only allows flow out of case, not in. The check valve in the reed block also seems ok, but maybe not perfect. It seems to let a little air through back into the reed block, but not always. Not enough that I think it would be an issue. Just not a great design on that really, vs not working. I don't think it's mean to be 100% tight. Maybe I'm wrong though.

I also cleaned the carb AGAIN, and paid special attention to the idle tube, making sure it was clear and I could see light when I was done. One thing I did notice, not sure if it matters, was the way the carb gasket sat on the studs. It kind of blocked some of the air hole for the fuel pump pulse. I wonder if that was reducing the effectiveness of the fuel pump. I had 2 other new gaskets and they all seemed to sit the same though, but I made sure when I put it back together that I had the gasket shifted so the hole was unobstructed.

gasket.jpg

I'll give it another shot tomorrow, although I really don't think I'm done anything or found anything major to lead me to believe my results will be different.

I need to confirm my lower crankcase seal is ok I guess, next.
 
Thanks. I saw a different reply via email from you, so to answer those questions - these are all mercury OEM gaskets that seem to block the port a bit if you let the gasket just sit on the studs. And I am using the Mercury service manual, and sure as the sun come sup, compression numbers are published in it under "Compression Check" in Section 1, General Information

compression.jpg
 
I noticed that the idle wire does not appear in your photos.
Is that hooked up and adjusted correctly?

Also, the fuel pump check valve diaphragm and gasket should NOT be re-used once the fuel pump is disassembled.
 
The idle wire was there. Maybe the angle of the picture hid it, or I had moved it out of the way when the carb was off. And yup, do not reuse the fuel pump diaphragm or gasket. I did not. I have not taken it apart since I put the new ones went in. I was convinced that was not the problem.

Now, I am happy to report that I believe the problem has been solved. The motor is running great now, with easy restarts and no stalling. I did some digging last night and I believe I had a 3 fold problem, maybe just two, but I'm not arguing with success. First, there was the carb gasket slightly obscuring the fuel pump pulse hole. Maybe that had no effect, but I made sure when I put the carb on it wasn't shifted and covering the hole. Second, I cleaned the carb again, and I would bet my life on the idle jet being clean before, but it appeared to have some crud in it, so I cleaned the whole carb again. But that led me to wondering how any crud got back in the carb with about 10 cleanings and a new fuel filter, so I dug into the fuel filter system. I am guessing at some point a previous owner ran it without a filter in place, as I found about 4 pine needles and some gunk in the outlet port of the fuel filter housing, blocking fuel flow! I'm guessing that was the issue last year with it being tough to get going at times, even with a clean carb. I cleaned that up and made sure all fuel lines were 100% clear.

Now, once I got it running, my suspicion on engine timing was correct, the idle timing was a mess, basically running at TDC. Once it was nice and warm, I adjusted the timing at idle in forward gear to 8 degrees before TDC and it absolutely smoothed out. Forward idle RPMs around 775-800, and fast idle (not in gear) at 1700. I'll tune the rest with it on the Zodiac in the water at the marina when I launch it.

Thanks everyone for sticking with me and with all the ideas and guidance on things to look for. Here's the videos of the success...

Here's it running after starting it up after cleaning everything up. It ran for about 10 minutes with no issues.
https://www.dropbox.com/s/tp10z5l4fv2txw8/Fixed First Start.mov?dl=0

Here's a restart from off (it has never started this easy!):
https://www.dropbox.com/s/u0xddveeeab8n71/Fixed Restart.mov?dl=0

And here it is after I adjusted the idle timing:
https://www.dropbox.com/s/y4kpot0bpjt63kn/Fixed After Timing Adjust.mov?dl=0
 
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