115 4 stroke loosing RPM and misfire

[Sparkieboat]

I think you should build yourself a homemade 4 cylinder spark tester..use a block of wood and some bolts and a ground strip...use your imagination..that way you can test spark on all 4 cylinders at once..I am not sure how you preformed the last spark test. take some pics or short video and post it on you tube. that would be interesting to see...remove all plugs for test. thats what I would do anyway

[99yam40]

I would test the charge coils and other inputs to the cdi along with the out puts of the CDI like the service manual says to to rule all that out before replacing your coils

[rodbolt17]

like I say you CANNOT use a single lead tester to properly load and test BOTH ends of a waste spark coil.
electricity is rather lazy.
it really doesnt care which it uses for a ground path.
the TCI ign isnt NOT dependant on both pulser coils to fire.
its a 12 VOLT TCI system, no CDI assy.
if you lose a pulser it fixes timing and the other pulser takes over for 2 cylinders.timing is fixed and top speed cannot be reached.

[glfolts@gmail.com]

Ok, I know you are the expert and will build the double tester. I did not entirely understand your post, I did not think the tci used ground, but rather the flow was from 1 to 4 and the ground was the path between the plugs. I will make sure it has good spark from 1 to 4/ 2 to 3 out of the system

I did not quite follow the pulser comment, but am interested in more understanding. I have checked the timing and it seems to advance.

I will check the pulser output specs from the manual to see it they are in limit

Thanks for the advice!

[rodbolt17]

lets get very clear on ONE thing.
voltage does NOT FLOW.
voltage is a simple measure of electrical pressure.
the ignition coil is a simple inductive transformer.
the TCI circuit path on the primary is from battery positive from the main relay through the ign coil primary to a TRANSISTOR swtch in the ECU.
hence TCI ignition.
transistor controled ignition.
if one end of the coil secondary has 50 times the resistance of the other end its quite possible,actually probable, that one end isnt going to function ike the other.
unlike old two stroke smokeies the pulsers DO NOT fire the ign system.
they DO tell the ECU where the flywheel is and the ECU,based on other sensors as well, determines ign timing.
however the pulser coils are physically mounted 10*BTDC.
if a sensor or 1 pulser is lost the ign base timing locks at 10*BTDC.
idle speed goes to about 1100 RPM.
this alerts the operator something broke,or should.
depending on WHAT failed ign timing may or may not advane and top speed may or may not be reached.
its a very simple system that can almost troubleshoot itself.

[glfolts@gmail.com]

Succes

Hi all,

Success!!! I finally persevered and got my Yamaha 115 running like a champ. The problem, or better stated problems, were 1) a bad low pressure fuel pump (LPFP) diaphragm 2) partially clogged fuel injectors.

I first found that the motor was making oil (more oil on the dip stick than I had put in). It looked like there were three probable cause (all though a few other options exist) fuel pump diaphragm, thermostat, or bad rings/ cylinders. I re performed a leak down test, removed the thermostat and tested its basic operation (hot water and a feeler gage), and tested the LPFP. To test the LPFP, I removed it, put 7psi pressure on the input side of the pump and plug the outlet off with a finger. It leaked like pretty bad out the shaft (the side that connects straight to the oil sump). New diaphragm was less than 30 bucks and fairly easy to install.

To find the bad injectors, I performed a “load balance test”. I had done this before and note picked up the problem, it must have been getting worse during my test runs over the past month. At idle, I unplugged the injector power connector one at a time and listened for RPM difference, indicating one cylinder not pulling its load. In hind site, an RPM meter would have likely caught the problem much sooner. In this case, cylinder 1 (the one that was never fowling and the plug looked great) would cause the engine to stall, and 2,3,4 would only drop the RPMs. Part of the symptoms I have been having was cyl 2 and 4 fowling. To see if I could get the problem to move, I swapped injector 1 and 2. The motor now stall on cylinder two and not one. BIG SMILE HERE.

Thanks to some great forum posts, I had already researched cleaning the injectors. I took a piece of fuel line (the 5/16 I had removed from the boat), a can of carb cleaner, 12v batter, alligator clips, a catch pan, and some vise grips. I could plug the injector snuggly into the hose, use the vise grips to squeeze the hose around the small straw on the carb cleaner, pressurize the hose (press the spray nozzle), and flush through the injector by tapping the 12voltrs (alligator clips on the connectors). I did not hold the injector on, but tapped short bursts repeatedly. I first did this the right way (the way the fuel would normally flow) and then back flushed. Injectors 2 and 4 had very reduced flow until back flushed. You can find videos on this and posts, most use a syringe.

Took it for a ride and I have normally, since I bought the boat gotten 30 MPH, I got 35 MPH on a windy crappy day. Well it was crappy to the rest of the town, but the smile on my face was ear to ear.

Thanks for the help.

[Michaelboli]

I truly appreciate this forum topic.Thanks Again. Fantastic. Sparlin

I truly appreciate this forum topic.Thanks Again. Fantastic. Strode