Main jets on American import

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Keithfgb
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Main jets on American import

Post by Keithfgb »

I know there has been quite a bit of discussion over main jet sizes but I would like your learned opinions on which jets I should use on my gt550 b model American import. I have searched through the forum and going on some of your experiences I think I should use 105’s and 102.5 in the centre as a starting point. After trying everything I can think of(raising/lowing needle,cleaning and more cleaning etc) I am still experiencing surging/ bucking on over ride and deceleration. There is no leak on the inlet rubbers,the Cush drive is ok. Any help will be appreciated.
It'll go eventually.
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Keithfgb
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Re: Main jets on American import

Post by Keithfgb »

I forgot to say I have a standard air box and higgspeed pipes.
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BAZ
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Re: Main jets on American import

Post by BAZ »

Have you balanced the carbs..
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Keithfgb
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Re: Main jets on American import

Post by Keithfgb »

Hi Baz, I have set the throttle slides up using a feeler gauge. Everything else is as you set them up. I’m at the end of my tether now,the carbs have been taken off that many times they just jump off every time I open the garage door. Acceleration is great,tickover etc is ok. Before I lifted the needle you could not keep a steady throttle on the road,that’s better now but I still have the surging etc on letting the throttle off. You don’t feel confident going into bends because of it. Helllppp!!
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Harrymail
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Re: Main jets on American import

Post by Harrymail »

The main jet sizes you mention are for UK spec 550 A/B. US spec are different due to the US bikes not moving to chrome bores. If you look on Old Japanese Bikes field guide the standard sizes are listed. I'm on holiday otherwise I would of looked them up. It's also good practise to stick to genuine Mikuni jets. Check the balancing using an air flow meter.
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Harrymail
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Re: Main jets on American import

Post by Harrymail »

Had a look Keith Main jets US spec are 97.5 L&R , 95 Centre. Pilots #25.
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Keithfgb
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Re: Main jets on American import

Post by Keithfgb »

Thanks Harry,that’s the information I came up with also and it’s got 97.5’s in now but it’s not running correctly. After going through the previous threads I thought upping the mains might help. On the oldjapanesebikes site it does say 4th notch on the needle which I have now and it’s made a slight improvement but not enough.
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BATTS
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Re: Main jets on American import

Post by BATTS »

Surging on the overrun ie closed throttle can be a symptom of the pilot circuit running lean, increasing the pilot jet size or varying the airscrew from standard may help, changing the mains wont. Interestingly on another forum a 550 owner was experiencing similar problems, eventually tracked to an air leak, not on the carb rubbers but on the fuel tap vac line. Might be worth a look.
Chris
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Harrymail
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Re: Main jets on American import

Post by Harrymail »

I have #30 pilots on my 550A though it's UK spec. #25 is correct for US. There are sizes available inbetween though. Check you have genuine Mikuni first. As mentioned by Batts the air screws can be adjusted by 1/4 either way.
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BAZ
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Re: Main jets on American import

Post by BAZ »

Try balancing using a flow meter we do all like this and get great results.. jets won't make to much difference.
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