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Yamaha 140hp 1993 Loosing power under load

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Post Options Post Options   Likes (0) Likes(0)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Topic: Yamaha 140hp 1993 Loosing power under load
    Posted: 27 Oct 2015 at 5:33pm
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My First Post. Guess I have to start somewhere. I apologize in advance - I've had a few let downs and disappointments by service men. I've tried to document everything 

I've recently got myself my first boat. I've grown up around boats, and finally had a chance to get one for myself. It's a 1993 140hp Yamaha. Searching the internet and that motor almost dosn't exist. Seems it is an au/nz only release.

I bought it with a known engine problem. of which
 the description at the time was "No power under load - maxes about 3000rpm and won't plane"

I've done a lot of work myself and diagnosis the abbreviated version below: 
* new fuel hose
* new connector to tank
* new fuel pump
* had carbs off - taken to  a yamaha service center to have them apart and cleaned. 
* New fuel tank (long story there) 
* New Plugs
* Checked the oil system.

After all the above, it runs much better, up to about 5200rpm and sits there for a few seconds then looses power down to about 3000rpm 

I've drawn a conclusion, and had a few people agree that I'm experiencing a fuel issue. 

I don't know what else to do myself so 2 weeks ago i had it at a shop - they took one look and sold me a fuel tank - without even testing it under load (which was one of the reasons i took it to that shop) After getting the boat back it's worse than ever - barely get any revs before things start coughing and spluttering (Although it will still plane) if i leave it coughing and spluttering, it will eventually die altogether, never did that before it went to the shop.

While i was out testing - i tried flicking auto choke (thinking that if it was starving, the choke would richen the mix up a bit) but that instantly made things worse still, actually killing the engine completely in a split second on one attempt - it was really struggling that time.)

I mentioned earlier i had the carbs 'done' I asked  a yamaha service center to clean them up and inspect them. they told me they weren't that bad - wouldn't be the cause of my issue. I never got a receipt showing what was done, but i've since been advised that they should have done a rebuild at the same time just because they were off and everything was apart anyway.

So i guess overall I've lost faith in mechanics. 

Has anyone got any advice on things to check or try - Or a reliable mechanic that will actually test and diagnose under load and not make wild guesses.
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Post Options Post Options   Likes (0) Likes(0)   Quote pjc Quote  Post ReplyReply Direct Link To This Post Posted: 27 Oct 2015 at 5:49pm
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had a simular problem with a small motor ran till warmed up then would loose power,turned out to a capacitator/ristor thing breacking down when warm.holiday marine on the shore fixed it and never had a issue again,back in 1995
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Post Options Post Options   Likes (0) Likes(0)   Quote 2 Meke Quote  Post ReplyReply Direct Link To This Post Posted: 27 Oct 2015 at 6:48pm
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I had a similar issue with an old Yamaha 140 on a previous boat.
It had a "limp mode" that when something went wrong it would go into limp mode to get you home (3000rpm)
It could be something as simple as a thermostat or impellor. How is the temp and are there any warning alarms set up?
The limp mode will; also kick in if there are issues with the oil injection assuming your engine is oil injected.
And finally  I had an issue some time ago with a grey fuel line I brought from a local shop. The internal lining slowly broke away and got into the carbs. Cleaned them out and before long was experiencing problems again so cleaned them again and discovered the issue was the pipes. Needless to say I changed these and had no further issues
I'm sure there are others out there more knowledgeable then me but that's my 2 cents worth. Good luck.
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Post Options Post Options   Likes (0) Likes(0)   Quote Dagwood Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2015 at 3:44pm
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Talk to Rhys at Fishing Boats NZ on the North Shore.
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Post Options Post Options   Likes (0) Likes(0)   Quote Gareth15765 Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2015 at 7:50pm
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also pay to check your power reeds behind the carb if suspect change them very crucial part of a 2 stroke.
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Post Options Post Options   Likes (0) Likes(0)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2015 at 10:14pm
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Thanks for the reply guys. I've gone to reply and typed up a message twice now and walked away before finishing it. 

I've been wondering how the shop made it considerable worse, when they spent bugger all time on it - and I wonder if they may have tinkered with the pilot screws. I checked them and they were set all different, between 1-1 and 1 3/4 turns. 

I've just found a workshop manual that covers heaps of detailed steps including the pilot screw turns. all 4 should be set between 5/8 and 1 1/8 (There was 1 of 4 screws within this range) 

So I am really hoping that this simple thing solves my issues. Eagerly awaiting the weekend to try out these changes. If that doesn't do it, I'll check the reeds and possibly rebuild the carb myself so I know it's done right (now i have a manual to follow!) 
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Post Options Post Options   Likes (0) Likes(0)   Quote Gareth15765 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Oct 2015 at 12:06am
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the pilot screws generally run idle rpm and up to where the main jet takes over so shouldn't affect your top rpm range

check the main air screw there should be a setting range for that I have had a top end issue years back with motocross bike where the main jet dropped out and was absolutely useless at hi rpm but that should have been checked with service

does the butterfly on each carb open fully ?

can you get to top rpm in neutral ??

apart from a failing electrical component breaking down under load cdi unit or the like I would defiantly remove all carbs and remove the power reeds and thoroughly check for chipped or cracked or broken reeds these act as the intake valves on a 2 stroke and can be very moody if poor condition 

just been surfing some other sites this looks interesting it is a 50hp but worth a look

I just got done fixing the very same issue with my 50hp 2-stroke Yamaha. I was having two issues actually. Poor idling and no high rpm. I pulled the carbs and cleaned them and this solved my poor idle issue but still no high rpm. Thinking about it I realized that low oil makes the engine throttle back so I decided to pull the oil level sensor out of the oil tank to simulate low oil and much to my surprise there was no change. This got me tracing out the wiring to the sensor and I discovered the power feed wire to the sensor actually runs around and under the front of the motor to the starter side. As I was pulling on this pink colored wire to trace it out I was surprised to find the end of the wire was corroded and disconnected from the quick-connect that provides the power to the sensor. I spliced the wire back together with a heat shrink connector and now my motor runs like a champ again. I guess the fact that this wire is under the motor where it is likely to get wet is why it corroded and finally just came apart while I was running high rpm on my last trip out. I spent 2 days working on this before finally narrowing it down. I guess I didn't notice the corrosion because this wire was tucked under the main harness connector near the starter.

worth a look there too good luck
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Post Options Post Options   Likes (0) Likes(0)   Quote Grasshoppa Quote  Post ReplyReply Direct Link To This Post Posted: 29 Oct 2015 at 6:21am
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you should state where you are before asking re reliable mechanics
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Post Options Post Options   Likes (0) Likes(0)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Posted: 29 Oct 2015 at 9:30pm
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Ahhh good point RE Location - I'm epsom, Auckland area. No issues to travel to wider auckland area. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Durban Quote  Post ReplyReply Direct Link To This Post Posted: 29 Oct 2015 at 10:25pm
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When it starts loosing RPM at that point get someone to prime the bulb , this will give you an indication where you are at if the RPM improves .
Sounds like you have Bogging down symptoms normally caused by insufficient spark on one cylinder or fuel and or both . The fact that it reaches 5200 rpm tells us one of the 2 mentioned is going missing . A power pack that fires intermittently will cause the spark failure .   
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Post Options Post Options   Likes (0) Likes(0)   Quote waynorth Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 10:48am
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You are probably well past this point AS but just a thought - does your fuel run through a filter/water separator ? You didn't mention checking/changing that in your first post, and I once had very similar symptoms & the problem was the filter almost full of water. Petrol floats on water, & at low revs, it drew enough fuel through the separator to run OK, but as soon as the revs went up it died, then happily restarted & ran at low revs again.
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Post Options Post Options   Likes (0) Likes(0)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 11:09am
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Theres no seperator on the fuel line. I've checked through all the fuel lines inside the engine from the entry point to the carb, and replaced everything outside i.e. tank + hose + connectors. The filter and hoses have been checked/cleaned.

Is there a way to test for the mentioned electrical fault?
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Post Options Post Options   Likes (0) Likes(0)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 11:13am
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Quote I was pulling on this pink colored wire to trace it out I was surprised to find the end of the wire was corroded and disconnected from the quick-connect that provides the power to the sensor.

I'll double check that wire - I remember disconnecting it when I had the carbs out last time. 

Quote can you get to top rpm in neutral ??
The engine revs fine in Neutral in water, and also in gear on land. In both cases it goes up to 5500rpm without issue - With still more left but I'm not keen to overrev the engine due to not having a load on it.

N.B. Have only run the engine up to those revs without load twice, and only for a couple of seconds at a time. I've read that it' bad practise. 

Quote does the butterfly on each carb open fully ?
err - i assume so. When i had them out Everything seemed to move freely. 

I haven't come across anything in my manual (yet) on how to check or confirm this. 

Quote When it starts loosing RPM at that point get someone to prime the bulb , this will give you an indication where you are at if the RPM improves .
Tried this last time i was out. Made no difference at all. Flicking the electronic choke however, made things much much worse instantly. Seems to point towards running rich?
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Post Options Post Options   Likes (0) Likes(0)   Quote CEEBEE Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 5:05pm
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Another question...when the engine starts to "falter" is it misfiring or simply losing power on all cylinders?
 
Also when losing power does the rev counter drop suddenly or progressively with engine rpm?
 
Can you accelerate just to the "power loss" point / back off, then accelerate again?
I STARTED THE DAY WITH NO FISH AND I STILL HAVE PLENTY LEFT
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Post Options Post Options   Likes (0) Likes(0)   Quote Durban Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 5:34pm
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Good ? Ceebee . at this point feel the pressure in the prime bulb if its not firm you have a fuel starvation or vacuum problem.
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Post Options Post Options   Likes (0) Likes(0)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 5:37pm
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Quote Another question...when the engine starts to "falter" is it misfiring or simply losing power on all cylinders?
 
I'm pretty sure its loosing power on all cylinders - but could be mistaken.

Quote  Also when losing power does the rev counter drop suddenly or progressively with engine rpm?
I would call it quite sudden, but this clip may help explain better. (Video taken prior to the engine being in a shop - currently it's worse. )
https://www.youtube.com/watch?v=KScJpgb8mLw
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Post Options Post Options   Likes (0) Likes(0)   Quote Betty Boop Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 5:58pm
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This is an interesting thread as I have an almost identical issue with my 2000 Optical ignition carbed 150hp Johnson only at WOT.......5000rpm.
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Post Options Post Options   Likes (1) Likes(1)   Quote arcticShadow Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 8:05pm
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Walu, Hopefully you will get some ideas on what to look for :-)
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Post Options Post Options   Likes (0) Likes(0)   Quote Ohsoslow Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 8:27pm
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Going off that video, it screams a fueling issue to me.. Fuel starvation or air in the lines. I would be checking low pressure fuel pump/s, and all hoses, connections, primer bulb, and filters.

If it was spark/electrical id be expecting more stuttering, a change in sound (dropping a cylinder or two ) or a sudden drop in rpm to a limp mode point.

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Post Options Post Options   Likes (0) Likes(0)   Quote samax Quote  Post ReplyReply Direct Link To This Post Posted: 30 Oct 2015 at 9:41pm
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I had a similar issue with the boat at home and that old grey fuel line breaking down on the inside and clogging the line however recently grandad had an issue with his 70 yammy doing a similar thing to yours under load.
He had one cylinder honed etc and a new Cdi or something like this put in as his old one was just about buggered anyway after 3 or so months of the same thing back and foward to the shop they worked out (or said) that the adjustment for the spark timing had been reversed and instead of advancing the spark they had been retarding it retarding which caused a similar thing to what you are experiencing in the video. Sorry I know it is a bit vague but hope this may help
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