15 HP with no power and low rpms

[scout-j-m]

I was able to take the boat out this morning for some fishing and to try out the 9.9hp with the 15hp carb. It ran almost identical to when the 9.9 carb was on and ran well. I only saw a couple hundred more rpms up to 5600 and maybe 0.5-1 mph gain but it still ran good so I guess that at least proves that the 15hp carb is good and isn't the source of my 15hp lacking power. I forgot to take the 15hp fuel pump with me to try that out but will do that next. I'm making some progress here at least.

And if anyone has any ideas why I didn't see the expected rpm gains from the 15 hp carb I'd like to hear them. I know im missing the reed spacers and tuned exhaust tube to go the full way to the 15 hp but I thought the carb alone would do more. Is it still possible the carb has a minor issue like the jet possibly being the wrong one? In a previous post I mentioned I thought it was a jet from a more modern 15hp carb and was a couple thousandths larger than what it should be. I'm certain the throttle plate was fully opening at WOT too.

 

[scout-j-m]

Tried to pull the flywheel yesterday to check the ignition components under there but didn't have the right size bolts in my puller kit. I'm going to have to figure out what size they are and buy some at the hardware store soon.

I also did a cold compression test to make sure it didn't break a ring or anything in the test runs I have been doing and got between 105-110 psi on both cylinders.

Moving on to test the fuel pump like I mentioned earlier sometime this week when I go fishing too.

 

[Sandor27]

Check the reed valves on the lower cylinder. There may be a cracked reed allowing too much fuel in to the lower cylinder causing the black plug, low power, and the unburnt fuel in the exhaust...

 

[scout-j-m]

Check the reed valves on the lower cylinder. There may be a cracked reed allowing too much fuel in to the lower cylinder causing the black plug, low power, and the unburnt fuel in the exhaust...

Thanks. I was wondering if the reeds may be an issue. By the looks of the intake manifold bolts though I may have to lift the powerhead some to access them. Can anyone confirm?

 

[scout-j-m]

It's been a while since I have fooled with this motor but with the days getting shorter I have found myself with more free time in the evenings after work since it's hard to get on the water unless it's the weekend. So the other day I finally pulled the flywheel on the motor so I could diagnose why I lost spark.

Nothing blatantly obvious stood out. However, there was some residual goo on the flywheel where it rests at the charge coil with no throttle advance. Also, the part number on the coil was 511441. A Google search as well as a part search on a few reputable parts sites leads me to believe it is possibly a for a foreign OMC model. It definitely didn't match the OEM, Sierra, or CDI Electronics part numbers from the parts diagram. I tested the resistance of the trigger which seemed good. When I tested the coil I didn't get a reading on one winding, however, my multimeter has been unreliable lately. I'm going to buy a basic analog one which is better for resistance anyways and recheck but likely will be ordering a new charge coil. I don't know if it could possibly limit my rpms in any way but I need to get spark back before moving on.

If I can get it back, I will swap power packs and fuel pumps and test and then pull the intake to check out the reeds if that doesn't work.

 

[flyingscott]

Do an open air spark test make sure the spark can jump 7/16". The ignition components under the flywheel are very reliable. I would look at the cdi box if the spark test doesn't check out. What is the compression on that motor.

 

[scout-j-m]

Do an open air spark test make sure the spark can jump 7/16". The ignition components under the flywheel are very reliable. I would look at the cdi box if the spark test doesn't check out. What is the compression on that motor.

I tried an open air gap test again the other night. I started at 1/2" and worked my way in, however, it wasn't generating spark even when I got the gap almost all the way closed.

I have a known good powerpack I can swap out which will be helpful. But I'm thinking that since the coil resistance is potentially showing open it may have gotten fried when I when I was probing the wires as it was running (I mentioned it in another post buried earlier somewhere).

Compression was even and around 110 psi I believe.

 

[scout-j-m]

I got my new meter in and checked the resistance of the sensor/trigger, charge coil, and ignition coils - all were in spec. The powerpack is not possible to check as far as I understood, so I swapped it out with a known good pack from another motor. Got spark back and it is nice and thick with a good snap easily jumping a 1/2" gap.

Now the question becomes, could the old failing powerpack be the cause of my initial problems of not climbing past 3000 rpms under load? Or was it just a coincidence it failed on me while testing and troubleshooting? At that time I had good even compression, good clean carb which worked well when put on another motor, and good spark (at least at cranking speed).

 

[scout-j-m]

Been a while since I bumped this thread but wanted to try to see it all the way through to full repair. Per my last update a powerpack swap at least got me nice thick spark back at cranking speed. I ran the motor in a tub recently and it ran well and seemed to have good power. I am going to lake test it on my 1648 flatbottom here in the next few days. It has my 9.9hp's carb on it so if it is running well it should match the performance the 9.9 had on this hull which was about 16mph at 5400 rpms.

 

[eseibel67]

Your trouble-shooting road has been very long and winding, you are getting very close to the end. Co-incidences of having multiple issues at the same time are always possible, however they are unlikely. Your highest probability of narrowing down the problem is going with the facts. The spare 9.9 helps a lot. I read through this thread quickly so I may have missed something, but based on all you have been through so far, as someone suggested earlier I suspect a problem with the #2 leaf valves. And from what I remember, removing the intake on these low-profile engines is difficult or impossible. Last time I did it I lifted the powerhead. Good luck and please post final results. You're almost there.

 

[scout-j-m]

Your trouble-shooting road has been very long and winding, you are getting very close to the end. Co-incidences of having multiple issues at the same time are always possible, however they are unlikely. Your highest probability of narrowing down the problem is going with the facts. The spare 9.9 helps a lot. I read through this thread quickly so I may have missed something, but based on all you have been through so far, as someone suggested earlier I suspect a problem with the #2 leaf valves. And from what I remember, removing the intake on these low-profile engines is difficult or impossible. Last time I did it I lifted the powerhead. Good luck and please post final results. You're almost there.

Thanks for the encouragement. I hope it doesn't come to having to removing the powerhead, but if it does I will use that opportunity to replace the water tube grommets and maybe rebuild the upper shock on the leg with some automotive bushings.

 

[scout-j-m]

Victory at last!

I lake tested the motor yesterday after work and it opened up wide open and climbed to WOT rpms. I saw 17 mph but didn't have my tach on to get rpm readings. This was with the 9.9 hp carb on it so my speed is right on par with what the 9.9 hp motor was producing on this boat.

To recap what I did... I changed the plugs, installed new OEM coils and plug wires, and put on a known good powerpack. Based on no performance changes from the new plugs, wires, and coils I would say that I had a bad powerpack all along. Based on the unburnt fuel on the lower plug, it seemed to be failing at higher speed and producing very weak to no spark on the bottom cylinder. Whatever was causing this problem eventually totally failed when the powerpack died altogether.

Now I just need to order a new/used powerpack and put the 15hp carb back on and this motor should be good to go for another 35 years lol.

I appreciate the help from everyone!

 

[oldboat1]

Well done! Curious to see what performance changes you see when switch back to the 15 carb, so hope you post. Lot's of discussion on those carb switches -- normally involving 9.9 motors (strikes me your's might be a better test for isolating carb differences).