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Help 1989 GSXR 250R - carb tune?

Discussion in 'Suzuki 250cc In-line 4's' started by John McLeod, Apr 17, 2019.

  1. John McLeod

    John McLeod Member

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    Hi guys/gals, I have a bit of a problem that I’m hoping I can get some help with


    I have recently bought a 1989 GSXR 250R GJ73a and I think it is running too rich. It will start from cold with no choke and idle great with a very responsive throttle, but won’t idle when warm – it will bog down and die unless I give it a bit of throttle. For the life of me I can’t see any air/fuel mixture screws on the carbs. There is only the one idle adjust screw. I assume they are aftermarket carbs – they have “Mikuni Corp” and “made in Japan” on them but no other markings or adjustment screws that I can see or easily access. Are they in a hidden spot?

    I did take the carbs apart to clean when I bought it and they were pretty clean and the jets weren’t blocked. Someone has drilled about 10-15 holes in the top of the airbox, and the jets are 47.5 and 105 on the outer carbs and 110 and 47.5 on the inner two, with all pilot jets as 12.5.

    I checked the plugs and all have a spark but are very black and carboned up. I also found that one cylinder wasn’t running so I am going to check that carb float bowl to see if it has fuel in it, and if so then I’ll take the valve cover off and check for stuck/sticky valves - I assume running rich is clogging up the valves


    I think someone has restored this bike as its in great nick cosmetically and all the rubber on the engine is new and it has very low mileage compared to others I've seen. The previous owner said he bought it with an ignition problem and replaced the coils, and this idle issue is new (but the bike sat in his garage the majority of his ownership).


    Any ideas on if my carbs can be adjusted or if I should cover the airbox holes and re jet back to factory spec? or other ideas?


    Thanks
     
  2. Cbuzz

    Cbuzz Well-Known Member Dirty Wheel Club

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    Start putting the stock jets back into it & sealing the airbox. Otherwise it will be tough to get the mix right.

    Get some good pics of the carbs up as well.
     
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  3. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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  4. John McLeod

    John McLeod Member

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    thanks guys, I'll try to get some pics up soon. Can someone tell me the stock jet sizes? I'm sure I saw it on here before but cant find it again...
     
  5. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    pilot screws

    pilot.png
     
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  6. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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  7. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    Yea correct .... I did it in a hurry without my glasses on .... :lolsign: ..... old age I think its called :commando:

    Ill go back to servicing my walking frame :D
     
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  8. John McLeod

    John McLeod Member

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    Awesome thank you!! Looks like it might be a bit tricky to get the middle carbs while they are on the bike but I’ll give it a go and post an update
     
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  9. John McLeod

    John McLeod Member

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    Sorry for the slow response its been a busy couple of weeks. I've taken the carbs apart again and carb 1 had some gunk in there (must not have cleaned it well enough, or may have been sitting in the pipes) so I'm hoping that was causing the one cylinder to not run.

    After seeing all of these issues, I don't trust whoever has touched these carbs in its previous life so I have a few more questions.

    Those parts diagrams are super helpful. Turns out my 110 and 105 mains were the wrong way around so I'm glad to have fixed that. I've found a site to get some more jets so will replace my starter jets and pilot jets to the correct size.

    A couple questions on the starter jet, are they a different shape/size or is it only the hole size that is different? The site I found (MikuniOz) has only one "starter jet" which is smaller and a different shape to the one that is in there already and the one. The site has a "main jet" which is the same dimensions as what is in there, am I able to use this "main jet" in a size 50 as my starter jet? or does it need to be the different shape?



    And what is the orientation of the jets? I've added a photo of the way they are now, with the carbs sitting upside down (float bowls on top)

    I'm also not sure on what height the slide pin thing (sorry don't know its name!) should be, its currently sitting on the second notch (see pic)

    Finally, the last owner has removed all of the hoses from the carbs (except the fuel line) - are these important? they have also added little blue clips onto the vents to block them on the diaphragm covers (final pic)

    thanks
    John


    Sketch.png IMG_1517 (Small).JPG Sketch1.png
     
  10. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    The 'slide pin' is the jet needle and the 'thing' is the clip .... the clip in the centre notch will be the stock setting

    Those vents are breathers for the vacuum/diaphragms and the float bowls so shouldnt be blocked off

    This shows the jet orientation ... upside down to your photo

    Be careful removing the pilot jet as they are very soft brass and the head crumbles easily

    Jets.jpg
     
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    Last edited: May 2, 2019
  11. John McLeod

    John McLeod Member

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    Awesome thank you! I wonder why they have changed so much on these carbs..?

    Ill take off those plugs on the breathers. Do the other missing tubes matter (in the red circles in the pic)? I saw on another thread that someone had taken theirs off.

    Stock jets have been ordered, I cant wait to get them and put it all back together. I've missed out on summer and the autumn frosts are now here, there are going to be some chilly hands in the mornings once I can finally ride this wee beast
     
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  12. John McLeod

    John McLeod Member

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    An update on how I have gotten on - a bit of a long one


    This bike is like the gift that keeps on giving, so many days of intense excitement – “this is the day I finally ride it”…. Oh no one more thing to fix. My stock jets finally arrived and I put them in, put the tank on and tried to start the bike up. It started up after a bit but I noticed liquid streaming out of a hole at the bottom of the muffler…. And by liquid I mean petrol….


    During my maintenance of this bike I cleaned out the tanks for rust and removed the petcock filters that were inside the tank and as they were ripped open and no longer working. I called the local Suzuki dealer for some replacements but he did not have any. His suggestion was an inline filter before the carbs to catch any residual rust before it goes in and clogs the carbs. All done and I was good to go… or so I thought.


    When I put the fuel tank on the bike, I noticed that the fuel line was leaking fuel even though It was in the on (not prime) position. All my dirt bikes have on off and reserve, so prime and vacuum fuel tanks are new to me and I thought nothing of it. The residual rust had gone through the petcock and held the little spring inside the petcock open so it was permanently in the prime position…This fuel on prime managed to make its way into the cylinders (stuck float maybe?) and filled the engine with petrol… I have seen this filter removal suggestion on a few other forums – removing the in tank filters and adding an in line one to replace it. I would not advise it!


    New oil, oil filter, spark plugs and a drying session later and now the bike doesn’t like to start… a shot of engine starter in the airbox fixes that and its running (almost) perfect. Still not running on cylinder one – the Suzuki dealers mechanic suggested a good long run to clean out any old carbon build up in that chamber and to get things moving since it has sat for so long. I did that the other night and what a blast it was!!! Even at 75% power this thing was so much fun. Sounds like an F1 Car and you are still happily within the speed limit. I can definitely understand the saying its better to ride a slow bike fast than a fast bike slow – go the 250’s!!


    Its still not running perfect yet, it doesn’t start without a squirt of engine starter, and all my tinkering with the carbs on and off has meant my boots are now leaking air (slow to rev down, revs up by itself, spraying engine starter on carbs increases RPM)

    So this weekend ill get them apart again and look for cracks and try to get it properly sealed – I think a new air filter is in order too, this one is at least ten years old…
     
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  13. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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