Yamaha 150 2 stroke idles on 4 cylinders

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AndyB1574

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Jan 13, 2012
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Hi all, I'm new to this site, I hope someone can help.

I have a 1990 6 cylinder which idles on four cylinders.
I want to know how this happens, a car mechanic friend told me there
are different reeds in the top carby, which only open up above
a certain rev range, to lower pollution around launching areas. Is this correct?
If so can you change the top reeds so it idles on all six?
Thanks, Andy.
 
9

99yam40

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Re: Yamaha 150 2 stroke idles on 4 cylinders

That is something I have never heard before,
I do not think it is true, but see if someone that works on these motors all of the time will jump in and say for sure
 
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AndyB1574

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Re: Yamaha 150 2 stroke idles on 4 cylinders

Ok I hope so. At idle with air intake removed he sprayed aerostart into top carby, it seemed to not be sucked into the engine, but when he sprayed into bottom two carbys it definitely went in
 
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99yam40

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Re: Yamaha 150 2 stroke idles on 4 cylinders

have you checked to make sure all of the cylinders are sealing properly?
A compression or leak down test should show that.
 
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AndyB1574

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Re: Yamaha 150 2 stroke idles on 4 cylinders

Thanks again for your reply. We have tested compression, all cylinders are about the same and I believe the engine itself is fine. It has an occasional miss, and will stall when it's cold, but runs and idles fine when warm. It is a bit of a nuisance if the boat ramp is busy and you can't really sit there and let it idle for 3-4 minutes to warm up - so I figure if it can idle on six it would be better.
 
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99yam40

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Re: Yamaha 150 2 stroke idles on 4 cylinders

Some motors a designed to not run on all cylinders at idle, and we have no idea what 150 you have.
If it is having problems find out what the problem is and fix what is wrong instead of trying to modify it.
 
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Inducted

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Re: Yamaha 150 2 stroke idles on 4 cylinders

Carbureted motors do not drop cylinders at idle. Some fuel injected models do. If you are spraying starting fluid into the top carb at idle and it is not being sucked in then check your throttle valve setting. It is probably not syncronized properly. Also check your carb as the idle circuit may be clogged. good luck
 
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99yam40

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Re: Yamaha 150 2 stroke idles on 4 cylinders

Like I said it is best to always post the model#, which you still have not.

A Yamaha service manual for your motor would give the needed info for the link and sync that needs to be done every time the carbs are reinstalled
 
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99yam40

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Re: Yamaha 150 2 stroke idles on 4 cylinders

6G4 seems to be a 1983 model, that is an old motor.
Hard to believe you found a service Manual for it, was it a Yamaha one or aftermarket?
 
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AndyB1574

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Apparently the 'D' means its a 90 model. The manual is an original one I believe, it covers from 2 hp right through to 300 hp. It's very detailed and has 630 pages with about 500 that are irrelevant to me. Only cost $8 though!
 
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yamatech43

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Re: Yamaha 150 2 stroke idles on 4 cylinders

ok, no it is not designed to do this, no motor ever had reeds designed that way and this motor is older(way older) than to have the computer drom cyls for idle....sounds like the reeds are stuck on top cyl....
 
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99yam40

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Re: Yamaha 150 2 stroke idles on 4 cylinders

6G41983150A/150CETO

Strange to have two cylinders with stuck reeds,
what would cause them to stick closed?
 
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jeffersondodson561

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Re: Yamaha 150 2 stroke idles on 4 cylinders

AndyB1574 said:
Hi all, I'm new to this site, I hope someone can help.

I have a 1990 6 cylinder which idles on four cylinders.
I want to know how this happens, a car mechanic friend told me there
are different reeds in the top carby, which only open up above
a certain rev range, to lower pollution around launching areas. Is this correct?
If so can you change the top reeds so it idles on all six?
Thanks, Andy.
Click to expand...
I think you have a fuel delivery problem in the idle to off idle circuit of the carb. I would get a can carb spray and squirt a short 10th of a sencond burst into the throat of the carbs that are not hitting at idle. If the rpms pick up, increase, you need to go through the carbs. If you have a fuel delivery at lower rpms you could have a problem at the higher rpms too. This could lead to engine damage eventually
 
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AndyB1574

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Jan 13, 2012
Messages
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Re: Yamaha 150 2 stroke idles on 4 cylinders

Turns out the problem was in the idle circuit. The jets were blocked up and the sync hadn't been done properly. We ended up getting a professional to sort it out and it runs absolutely perfectly now. He checked compression and it was 100 on all 6 cylinders and he thought the motor was in very good condition for its age. You clever people who diagnosed this problem over the Internet led us to take it in to this guy, and we are VERY HAPPY FISHERMEN now. Thanks all
 
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