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devilray snob
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Discussion Starter · #1 ·
Motor: Yamaha 50TLR 2-stroke
Prop: Powertech SCB3 15pitch
Extra info: Motor is in second mounting hole. Motor does this trimmed all the way down, and also keeps doing it with the trim bumped up slightly. I also have a 14pitch prop being sent to me by PT this week.

Edit to add: PT is sending me the new prop because Ken @ propgods said the 14p will put me around 5300-5400RPM WOT instead of the 5100 I am at now.

When running the skiff with two or three people and a normal load, I will get on plane and yadda yadda and keep the throttle at 3/4 for cruising. This puts the RPM around 4200-4600RPM (5100RPM is WOT with this prop).

While running, the boat will suddenly lose speed and the tach will read 4000RPM. I can lay the throttle down all the way to WOT and it wont budge, but sometimes it will. Other times, I have to back off the throttle and then gun it again to get over 4000RPM.

I'm thinking this is a cavitation/slipping issue of the prop?

Could it be the powerpack going bad? Bad spark plug?
 

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Stripper in my own Mind!
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2,537 Posts
I would test each plug and make sure they are all firing correctly. For reference, Yamaha cdi and electronics rarely are the culprit for an issue. Their electronics are solid but there is a bad egg every once in the bunch. Cut or Creek will get you tightened up...
 

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If it was cavitation I think you would recognize it fairly easy. Check plugs first. Maybe a clogged carburator or pinched fuel line. If you are using ethanol you could have a bad tank of fuel.
 

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If it was cavitation I would think your rpms would jump when the prop lost its grip with the water.
 

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Matty - i have no where the expertise of cut and creek but I have done all the maintenance for years on my Dad's not so micro boat (twin 150 HPDIs). I know HPDIs are completely different beasts then your carb 50. We have limited RPM issues a couple times a year and most the time its related to one of the 10+ various filters that HPDIs have. rather than throwing parts at it and replacing everything shotgun style I learned to use a mityvac vacuum gauge and start getting readings before and after the various filters of the fuel system. typically I look for readings between 1" and 4". when I get a reading below 1" I search for an air leak, if greater than 4" I know it is pulling too much vacuum and there is a clogged filter upstream from where I got that reading from.  I keep working upstream till I no longer get the high vacuum at which point I isolated the restriction and r&r the part. not sure what normal vacuum readings should be for your motor but you can probably forum search iboats or tht and find out. Not sayin your prob is fuel related but just providing this info as an idea for you to consider and it doesnt cost a thing to perform just some time, assuming you have a mityvac, got mine at harbour freight cheap. it could help you rule out the fuel system as a potential prob at a minimum. The hard part is you will have to take these readings on the water at the trouble rpms this may be tough but dooable if your vac gauge tubes are long enough.

If it were cavitation wouldnt you hear it and wouldn't your rpms increase, not stay limited to 4k?

Did you check to make sure when moving your throttle the linkage under the cowling moves as well?

I know you will get great advise from cut and creek but to me since it is intermittent it sounds more electrical then fuel. Just throwing out some ideas for you dude. Good luck with it and let us know what you find.

http://forums.iboats.com/engine-fre...ow-testing-your-boats-fuel-system-283363.html
 

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devilray snob
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2,398 Posts
Discussion Starter · #6 ·
If it was cavitation I think you would recognize it fairly easy. Check plugs first. Maybe a clogged carburator or pinched fuel line. If you are using ethanol you could have a bad tank of fuel.
The fuel lines from the tank to the carbs are all fine, no blockages.

I dont use ethanol, this is happening with 91 octane ethanol-free.

I would test each plug and make sure they are all firing correctly. For reference, Yamaha cdi and electronics rarely are the culprit for an issue. Their electronics are solid but there is a bad egg every once in the bunch. Cut or Creek will get you tightened up...
I tested all 3 cylinders with a basic lightbulb spark tester and they are all working.
 

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Considering the history of the engine, fuel, fuel, and more fuel (probably carbs). It's not cavitation or slippage as that would result in higher RPM's with decreased MPH's.

Me and Cut have said it a hundered times on here Yamaha Eletrical compents don't break if you start there 99.9% of the time your chasing your tail.
 

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devilray snob
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Discussion Starter · #8 ·
Considering the history of the engine, fuel, fuel, and more fuel (probably carbs). It's not cavitation or slippage as that would result in higher RPM's with decreased MPH's.
Damnit...I gotta clean the carbs AGAIN?

What size fuel line should I b using for this 50 from the tank to the engine? Right now I have 5/16 line...seems small?
 

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Considering the history of the engine, fuel, fuel, and more fuel (probably carbs). It's not cavitation or slippage as that would result in higher RPM's with decreased MPH's.
Damnit...I gotta clean the carbs AGAIN?

What size fuel line should I b using for this 50 from the tank to the engine?
Probably man probably.

3/8 from the tank to the FWS, but the 50 uses 5/16 primer and hoses under the cowl, so I doubt that is your problem
 

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devilray snob
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Discussion Starter · #10 ·
I have 5/16 fuel line that came with the skiff and my fuel filter has 3/8 fittings. I shoved the 5/16 line on there but I think I might have crushed the inner liner of the fuel line when doing that causing blockages.

I'm going to check my fuel filter right now. I ordered 25' or 3/8 hose to replace the entire fuel line from the tank and a new fuel water seperator filter.
 

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I have 5/16 fuel line that came with the skiff and my fuel filter has 3/8 fittings. I shoved the 5/16 line on there but I think I might have crushed the inner liner of the fuel line when doing that causing blockages.

I'm going to check my fuel filter right now. I ordered 25' or 3/8 hose to replace the entire fuel line from the tank and a new fuel water seperator filter.
Things that make you go hmmm
 

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devilray snob
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2,398 Posts
Discussion Starter · #12 ·
I have 5/16 fuel line that came with the skiff and my fuel filter has 3/8 fittings. I shoved the 5/16 line on there but I think I might have crushed the inner liner of the fuel line when doing that causing blockages.

I'm going to check my fuel filter right now. I ordered 25' or 3/8 hose to replace the entire fuel line from the tank and a new fuel water seperator filter.
Things that make you go hmmm
:-*

Also, I switched out my fuel primer bulb for a super duper mercury one that is the best.

My fuel filter also had a small hole in it, it was leaking gas...probably taking in air.

To add to all that, after priming the fuel bulb and removing the QD fitting from the outboard, the fitting was making a hissing noise. The metal ball inside the QD wasn't closing all the way letting air in/out.
 

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Cert. Yamaha technician
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4,330 Posts
Considering the history of the engine, fuel, fuel, and more fuel (probably carbs). It's not cavitation or slippage as that would result in higher RPM's with decreased MPH's.

Me and Cut have said it a hundered times on here Yamaha Eletrical compents don't break if you start there 99.9% of the time your chasing your tail.
This

To me it sounds like you have one partially clogged main jet that is letting enough fuel thru under normal conditions but when you load it down with 3 people and try to run a higher rpm its a lot more load and it needs more fuel, more fuel than that one halfway blocked jet can allow.

/thread/ ;D
 

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Brandon, FL
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10,798 Posts
While running, the boat will suddenly lose speed and the tach will read 4000RPM. I can lay the throttle down all the way to WOT and it wont budge, but sometimes it will. Other times, I have to back off the throttle and then gun it again to get over 4000RPM.
Matty - a couple of simplistic questions:
Are you saying the tach does not move or are you saying that you can not hear the engine change speeds?

Do you have one of these fancy new tanks the govt requires? My portable tank went bad and got this new thing with these vapor preventers on it and my motor would not reach full rpm's. It took me some time to figure it out but I now simply loosen the cap because these things now operate with a small vacuum.
 

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devilray snob
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2,398 Posts
Discussion Starter · #18 ·
While running, the boat will suddenly lose speed and the tach will read 4000RPM. I can lay the throttle down all the way to WOT and it wont budge, but sometimes it will. Other times, I have to back off the throttle and then gun it again to get over 4000RPM.
Matty - a couple of simplistic questions:
Are you saying the tach does not move or are you saying that you can not hear the engine change speeds?

Do you have one of these fancy new tanks the govt requires?  My portable tank went bad and got this new  thing with these vapor preventers on it and my motor would not reach full rpm's.  It took me some time to figure it out but I now simply loosen the cap because these things now operate with a small vacuum.
The tach is a reflection of the motors rpm, so when the tach doesn't move I don't hear the engine revving any higher.

I have an aluminum forward fuel tank with side vent. If the vent tube was clogged with water it would be sucked into the tank and the line would be free to breathe again.
 
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