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Thread: Main jet on small single 4stroke

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    Default Main jet on small single 4stroke


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    Hi, I just bought a Suzuki gz250 single 4stroke, used to have a gn250 & tu250x years ago and they share the same engine . The gn and tux had no problem getting up to 75mph. The gz has trouble getting to 60. (Supposed to hit 75 aswell).
    I checked the valve clearances. The plug is a bit black sooty. The carb is clean but the main jet is new and a #135 when stock is #120!
    Everything is stock apart from the main jet. Like I said it's gutless above 55mph and also has a wee stumble around the 3/4 range.
    Are most of my problems the #135 when it should be #120?

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    Default Re: Main jet on small single 4stroke

    Also my plug is non resistor and tis supposed to be a resistor type.

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    Default Re: Main jet on small single 4stroke

    Two cheap things to change. Change them and see what happens! And the plug... Cleaning a fouled plug is often a false economy.

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    Default Re: Main jet on small single 4stroke

    Pull the air filter out and give it a good ragging., might pull through and confirm your diagnosis of too large mj

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    Default Re: Main jet on small single 4stroke

    Quote Originally Posted by sadlonelygit View Post
    Pull the air filter out and give it a good ragging., might pull through and confirm your diagnosis of too large mj
    I did that and it bogged terribly from 3/4 throttle up with a good dose of backfire.

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    Default Re: Main jet on small single 4stroke

    Is the option there to drop the needle one ring or is it already on the bottom one. If you can drop it one see what happens.

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    Default Re: Main jet on small single 4stroke

    One is assuming it is on a standard exhaust?

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    Default Re: Main jet on small single 4stroke

    Everything is Standard including exhaust. The needle circlip has only one position, ie only one notch.
    I'm getting a #122.5 main jet. I'm hoping the 3/4 stumble is the too big a main jet kicking in, and the poor wot aswell. But as I've said in an earlier post should it behave as it did with air filter off with too big a main jet?
    I'm just worried that there was a reason previous owner changed to a #135.
    Apart from the 3/4 's stumble it really behaves like a 125cc and gives a very good 90+ mpg.eventhough plug is a bit sooty.

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    Should Get Out More maccecht's Avatar
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    Default Re: Main jet on small single 4stroke

    Good thread here. Does mention a European model having a 122.5 Main though so do you have a manual to check the model of carb? Someone may have fitted a carb of another bike??
    http://www.gz250bike.com/forum/showthread.php?t=447

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    Default Re: Main jet on small single 4stroke

    Another thing, I might just be getting paranoid but when I was checking the valve clearance, there was 150 written on the intake valves acess cover in large yellow marker. Could someone have put a 150cc in? The mind boggles.

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    Default Re: Main jet on small single 4stroke

    No the carb is stock a F03, and the stock jet for that is #120, so I said too meself sure a#122.5 should be ok as well.

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    Default Re: Main jet on small single 4stroke

    Stock carb is a bsr32ss
    Wont run without air filter fitted so explains why ran like shit without one fitted.
    Have you unblanked the idle adjuster as well?

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    Default Re: Main jet on small single 4stroke

    If you can find the jet sizes and carb settings online or in a workshop manual I'd say go back to standard. Make sure your air filter isn't dirty and clogged. Fit a new plug by all means but that probably isn't the issue - modern engines with unleaded fuel do tend to look a bit dark, especially after town running. Do you have any suspicion the engine could have been apart? If so check the cam timing in case somebody has put it back a tooth out.

    Regarding the 150 question, Wikipedia suggests the GZ150 engine was fuel injected. In any case there is usually some moulding on the barrel that says the capacity.

    If all else fails don't throw time and money at it, flog it on and buy a better bike.

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    Default Re: Main jet on small single 4stroke

    SLG is right so what year is your bike and is the carb ID stamped on the side the correct one
    https://www.manualslib.com/manual/10....html?page=104
    Top end for this bike flat out and a good tail wind is 71mph thats 115kph so dont expect warp speed

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    Default Re: Main jet on small single 4stroke

    It has bsr32ss F03 which is stock. I have access to the fuel screw. Air filter is new. I'd be happy enough with 71mph, just to join in on the slip road and changing lanes on the dual carrageways. Should have got a gn or tux.
    Will update when I get the #122.5.

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