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Discussion Starter #1
Hi guys. I've recently bought an older bike - a 1990 GSXR750. I'm use to modern machines and I'm trying to learn about the motor in this one. I bought the manual, but I've come up with some questions. I had a mechanic look at it a few weeks ago. He was going to replace the plugs because they are carbon fouled ( I think the bike is running a little rich). He didn't have the right plugs in stock and said the plugs in the bike were the wrong size for the year and said I either had a 1992 motor or a 1992 head on a 1990 motor. Is that possible and how can I tell if I have a 1990 motor or a 1992 motor. Also the manual indicates the idle knob between carbs 2 and 3, however I have a knob (screw) on a long stem sticking out the side of the motor. I looks like an idle type adjustment to me. And yes I know where the choke is. How do you adjust the mixture. The manual says it is factory set and you shouldn't adjust it, however in the trouble shooting section it clearly suggests that the bike is running too rich. Thanks guys.
 

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the 92 and 90 750 use the same plug stock - ngk cr9ek or cr10ek.

the best (and easiest) way to tell the difference between the 90 and 91/92 cylinder head is to pop off the valve cover and look at the valvetrain. the 90 uses only one cam lobe per cylinder per cam, with forked rocker arms and threaded valve clearance adjusters. the 92 has two cam lobes per cylinder per cam, with individual rocker arms and shim adjustment for valve clearances.

about the idle adjuster - it sounds like your carbs have a remote adjuster.....to my knowledge, the slingshot carbs for gsxr's never came with remote idle adjusters, only a thumbscrew. when you turn the adjuster, does it change the idle?

however, slingshots meant for katanas (gsx-f) do have remote choke and idle adjusters. it's possible somebody may have retrofitted your carbs with a remote idle adjuster, or you have katana 750 carbs. katana 750 carbs are 36mm slingshots.....the 90-92 gsxr's are meant to use 38mm slings (with the exception of california model bikes but that obviously won't be the case with your bike), so if you have the katana carbs, your motor is being restricted a bit up top.....

you adjust the pilot (idle) mixture with a screw at the bottom of each carb, forwards of each float bowl. i don't know if australian bikes will have a cap (to prevent tampering of mixture) over the mixture screws....u.s. bikes do.

to adjust the mixture for the midrange and higher rpms, you will need to take the carbs apart.

keep in mind, dark plugs could also be indicative of a clogged air filter, for example....
 

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Discussion Starter #3
Thanks for you help and advice. I'm going to put a slightly hotter plug in and a new air filter. Change the oil and filter, then run the bike for a week and see how the plugs come out. I spoke to a Suzuki mechanic today. I explained what the bike was doing and said it sounds like the bike is loading up. What does he mean by that. You see, sometimes when I go to take off from the lights, the bike kind of wants to die. It doesn't and then picks up and is fine. Also if I blip the throttle when standing at the lights the bike wants to die when I release the throttle. I can always catch it, but it's not right. Again, thanks for your help.
 

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yeah, it does sound like it's running rich down low.....try a set of ngk cr9ek's (1 heat range hotter).

if that doesn't help, it could be that a previous owner has fiddled around with the mixture.
 

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Discussion Starter #5
Thanks Jeff. I chucked the hotter plug in, cleaned the air filter, changed the oil and oil filter and adjusted the idle. The bike is running better. It still boggs down sometimes at take-off. I'll let it settle for a couple of days. It hasn't been riden hard or long for quite some time. Maybe I need to put some hi-octane fuel in and go for a thump in the country to blow the cobwebs out. Again thanks for your help.
 
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