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M90A part throttle issue

whaler_warner

Contributing Member
My 90 has been running great all season, however last trip out without family on board i wanted to get some WOT runs and noticed minor possible missing/surging at WOT but was fine at part throttle.

I stopped for a while and when i started the motor would barely get onto the plane and was like it was running on 2 cylinders but seems to idle fine, i thought it was maybe a blocked main jet so after getting home i stripped the carbs which were all clean as a whistle and all jets clear.

(no alarm buzzing and low oil sensor is disconnected) the issue doesn't appear to be am RPM limiter just major loss of power.

I checked compression and all fine at around 125psi each, fueling is fine (new fuel pump, fuel lines, filter) and plugs were all wet indicating it had been running rich and fuel trapped in the airbox too.

Any ideas what it could be? there seems to be spark to each cylinder although one plug cap was very loose and corroded so i have cleaned and up a ensured it now sits snug, running on muffs it doesnt seem any different and its only under load i seemed to have the issue.

I have checked the resistance beween the plug cap and a ground and all seem to have the same resistance, not sure if this is checking primary or secondary coils.

Any ideas?
 
Did you change the kind of oil you are running? Some oils will leave more residue on the plugs. Veryify timing (link and sync), and make sure choke is releasing OK.
You tested secondary half of the ignition coils. You could also check the rest of the ignition, using your Factory service manual as a guide.
 
Thanks Paul When i purchased the motor it had been stored on a pallet and oil drained, i used a TCW3 oil but an Ebay non-OEM brand as i wanted to check everything worked first as it had not been run in years.

I have noticed this thing drinks oil compared to previous 2 strokes, maybe using 3l of oil in maybe 100l of gas although id rather it use too much oil than none at all!

Timing and carb all appear in sync, I did notice air/fuel mixture screw was adjusted slightly different across all three and wasn't sure whether to reset to factory specs after the carb clean I did initially, however been running great all season until now.

I wasn't sure if anyone had a similar problem or any known issues for these motors

bottom plug has a fair bit of carbon build up
 
That sounds like a lot of oil. 33 to 1. I would expect more than 50:1. I would start with a fresh set of BP8HS-10 spark plugs and see if there's any change. I would also blow out all carb passages with acetone-type generic carb spray. Be thorough there -- use up to a can per carb. Then see what's what with the oiling. The variable ratio oiler should be about 50:1 at speed, and less at idle.
 
Ok, so i thought i had solved this problem found that the spark plug cap on top cylinder was loose and not clicking on, i removed the cap closed the gap on the connector and it clicked on securely, next time out ran perfect but didnt run WOT as kids were onboard but plenty of punch onto the plane so i was happy.

Went out on the longest trip of the year in company, ran great the hour trip out but on the way back same issue came to a standstill as waiting for other smaller boats to catch up and when i accelerated it spluttered a bit and then ran half power for the whole journey back, no misfire and idles fine but seems like its running a cylinder down but only under load?

As its end of the season i will strip carbs down and give another good going through, change plugs and the oil tank is getting low so i think i will bleed through and replace with quality branded oil rather than the cheap ebay TCW3.

I am still leaning towards ignition coils being the problem and breaking down under load, but don't want to waste money?
 
You can check coils with a good analog ohm meter, but if they only fail hot, you may not get good info cold.
 
Fuel pump?


Its had a new fuel pump this season and the fault doesn't change when manually pumping the primer bulb.

Now we are in off season I am going to run it until it faults and then remove each plug cap to narrow down which cylinder is causing the issue and narrow down from there by switching the coils to see if the fault follows the coil
 
Ok so chatting to an old school marine engineer today about my issue and hes convinced its my fuel pump. Although i replaced the pump last season it was a non-gen part through a local marine factors, he says that the stuff they supply is Chinese rubbish and would bet money the diaphram is weak and deformed.

Anyone had similar issues? I recently purchased a cheap ebay carb rebuild kit for a 6hp Johnson I salvaged and after 5 mins run time the carb was leaking fuel and every gasket had melted/deformed! My friend has also found his impeller purchased from the same factor disintegrated after just one seasons use whereas the previous yam gen part was perfect after 5 years.
 
Hmm. Could be. I use OEM parts when possible, be it a Tohatsu, Yamaha, or a Chevrolet. I know for sure that Tohatsu OEM water pump impellers are far superior to aftermarket items.
 
Hopefully pic uploads but stripped the fuel pump and diaphragm looks warped compared to the OEM one which luckily I kept fit parts after busting the barn, also the non gen part first have a spring/diaphragm but instead a sheet of clear plastic?
 
356040001MFUEL PUMP ASSY.$85.13


Item 1-4:
334040050MDIAPHRAGM$28.88


Item 1-12:
356040300MDIAPHRAGM B SET$24.77

002-21031-2_FIG05.jpg
 
Thanks Paul, what do you think of the state of the pump diaphragm in the picture is it normal to be bowed out like that when the original is flat?

In any event I have just ordered an OEM Yamaha part which lists same part number for the tohatsu and at the very least I feel better for having a better quality part.
 
Ok just to resurrect this thread, the motor has been running flawlessly all season and i thought the new fuel pump had solved the issue however last trip out i suddenly lost power again and would struggle to get on the plane, no rpm limit just a loss of power and when feels like a loss of one cylinder.

Went out last night to try and diagnose.
1. ignition fine to all three plugs (tested strength across a gap using a tester I purchased) and plugs all look ok having been changed at the start of the season.
2. when at idle the motor will run on one cylinder and tested each one individually
3. noticed when running more fuel in the water around the motor than I would usually expect
4. with motor idling I disconnected the linkage between all three carbs top and middle carb I could open each butterfly and the motor would rev higher, bottom carb it would bog and revs dropped, It therefore must be an issue with this cylinder.

Next step was going to be to strip the bottom carb as maybe a main jet was blocked (carbs have been cleaned to within an inch of their life in the last 6 months, along with completely new fuel system, lines, two filters inc water separator) however I changed the plugs as I had some spare and immediately the problem was fixed itself and I could rev each cylinder independently.

This seems bizarre as the plugs looked ok and the only thing I can think is my generic TCW3 2t oil is fouling the plug? I have noticed I seem to be using a lot of oil so maybe this is the problem? whats your thoughts?
 
Thanks Paul, what i will do is put the old plugs back in and check the fault follows the plug. I will also check and measure the oil pump linkeage to make sure its all within factory spec.
 
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