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Old 07 January 2009, 16:49   #1
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90 model 5hp yamaha will only rev past idle with partial choke on

Has anyone a sollution to this problem this motor idles fine but when you go to rev it up it just dies unless you turn the choke slightly on i have pulled the carby down 2 times now to clean it and looked at the fuel pump it looks ok though the rubber diaphram seems a bit stiff it has 85 psi compression i dont know what the minimum compression on pull start single cylinder 2 stoke should be but my manual tells me over 100 psi minimum for a electric start can anyone help thanks
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Old 07 January 2009, 16:51   #2
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Has anyone a sollution to this problem this motor idles fine but when you go to rev it up it just dies unless you turn the choke slightly on i have pulled the carby down 2 times now to clean it and looked at the fuel pump it looks ok though the rubber diaphram seems a bit stiff it has 85 psi compression i dont know what the minimum compression on pull start single cylinder 2 stoke should be but my manual tells me over 100 psi minimum for a electric start can anyone help thanks
Should run at 85psi-though it's a bit low. Try holding the fuel tank above the engine-if it carries on running with the fuel supply higher than the carb then it's the pump.
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Old 07 January 2009, 17:15   #3
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Thanks Nos4r2

it has an over head tank fitted which i emptied cleaned and refilled it could you tell me what would be the minimum compression for this engine and can it be accuratley tested by the pull starting as i cant find much information on compression testing a pull start engine best regards
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Old 07 January 2009, 17:22   #4
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it has an over head tank fitted which i emptied cleaned and refilled it could you tell me what would be the minimum compression for this engine and can it be accuratley tested by the pull starting as i cant find much information on compression testing a pull start engine best regards
Compression readings depend on how fast you turn it over. I'd want around 100psi really-but if it ticks over in gear in the water it's not compression that's the problem
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Old 07 January 2009, 17:39   #5
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I had problems similar to this when the engine would starve of fuel when you tried to rev unless the choke was on and it was a blocked jet, have you stripped out all the jets etc and blown them all out and used some wire to push through them?
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Old 07 January 2009, 17:45   #6
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second that, sounds like the main jet is blocked
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Old 07 January 2009, 17:49   #7
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second that, sounds like the main jet is blocked
That's what I thought-but he said he'd cleaned the carb out.
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Old 07 January 2009, 17:57   #8
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Thanks Chris

i have the carby off again and was going to try and soak it can you suggest any solutions for cleaning carbies that i can leave the carby soaking in that wont damage it thanks
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Old 07 January 2009, 18:11   #9
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Thanks but cleaned the main jet

I pulled the float out checked the needle undid the main jet and pulled out the venturi under it they where all clean if it i have it in pieces again now for the 3 time i was going to leave it soaking but would like to know what the best sollution would be to leave it soaking in for a couple of days cheers
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Old 07 January 2009, 18:22   #10
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Have you removed the jets and made sure they are clear? It's worth putting compressed air through all the passages too.
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