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GSXR 750 1991 low speed is "herky jerky"

239 Views 10 Replies 4 Participants Last post by  Awaytogetsun

I have a nice 750 that i bought this winter. It had not been ridden since 2018.

It ran but a little rough, i cleaned the carbs, put new orings on the airscrews and synched the carbs with my carbtune synctool.

Ended up with airscrew 1,45 out, but when hot it took som time to come down to low idle.

It ended up being the throttle cable that were to tight, after i slacked it up by the handlebar it works great and returns to idle directly.

It runs supergood and is a really nice bike to ride.

Its not much of a problem but when i ride it in slow speed (20- 40 km/h- 12-25 mph) the bike is very herky jerky/ uneven/ rickety rackety when i have a steady throttle. If i gear it down it doesnt change much,

As soon as i give it gas its fine again and runs very smooth.

Is it something i can do to fix it or just run it like it is?

It its original except a micro 4-1 exhaust.

Thanks Marten
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throttle bodies may need synced up? TPS? idle speed? valve tappet clearances? valve timing? plugs? ignition?
I think the “main air jet “ which is under the diaphragm is the same as a main jet. I replaced the ones in an 86 LTD that I couldn’t get to run smoothly. Something to consider if nothing else is working
Thanks for the tips. I will consider changing the main air jets. They are pretty steep OEM. Any advice to were to buy them?

Not in Sweden, LOL! Nah, seriously, if you choose to fiddle with them, I'd take one out and go to a shop and see if they happen to have any. In my case it didn't take more than 1 size. I can't remember if I had to go larger or smaller. It was around 35 years ago that I was messing with that bike and it still had stock carbs on it. I also had the benefit of having a bunch of mains sitting around which made it seem like a reasonable thing to try since I had exhausted every other possibility.
Are you sure all of your diaphragms are in good shape? When we're talking about carbs this old, there are a lot of ways for them to start going lean with air leaks.
Also, I realize in your original post, you said you cleaned them. I don't know you, and you don't know me either, but there are "cleaned" carbs and then there are carbs that are truly CLEAN. Not trying to be a dick, but it is possible that there may be some stubborn dirt in the pilot circuit. I'm not positive that the MAJ will cause the degree of poor running you mention.
A dirty pilot circuit will (surprisingly) still allow a bike to idle, and once you get past it and up onto the mids, it will run fine again...leaving you with dirty running between idle and you're describing.
I've had good luck with acetone getting passages clear and I'll usually squirt alcohol through where the pilot jet screws in to make sure I can see plenty of alcohol coming out of the pilot holes at the spigot, the area where the needle jet goes, and the air jet(s) back at the flange.
Again, not trying to assume what you may or may not know, but I've chased my tail many times until I decided to get real picky and careful about making sure there was clear flow every way I could figure out...and I say this because this is stuff I didn't do the last time I opened the RS 36s on my 87 and only bothered to clear the pilot jet figuring "it'll be fine"😄 I should have taken the extra pains.
If you do choose to use something like acetone, be sure to remove all the rubber items from the carb bodies. I'm sure that is obvious, but again, we don't know each other(y)
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haha!! Hmm no the diagrams i didnt look to much at. I get what you mean, i would say the carbs are clean and not CLEAN=)

I made sure the brass where free from dirt and sprayed carb cleaner in the passages.

But as you say i could have made a better job.

I am quite happy with the bike for now so i think i give it a real go when the bike season is over.

Thanks man.
Your pilot screws don't seem like they are that far out. If you don't want to take everything apart again, and you are able to get your hands in there, you could try opening up the pilot screws to see if things smooth out. It's a major PITA to get in there, but may be worth a try.
Did the problems show up after the installation of the slip-on or did it come with the slip-on? Has the bike ever run properly for you?
If opening up the pilots doesn't help, unfortunately, you'll have to start taking things apart to investigate.
Sometimes prior owners will attempt modifications and simply give up...then the bike sits for a while and and eventually gets sold.
The slipon was there from the start, and i just owned the bike for about 3 months and started riding it about 3 weeks ago.

I would say it hasnt run perfect in my hands, its good but not primo.

I dont know much about the history of the bike other than it sat inside since 2018.

As you say i have adjusted the pilot screws and its tight as ....

I will try to recollect my pilot screw adjustments: First after sync i hade them at 2 turns out, it was smooth and idle returned as i should. But when the bike was warm it got a very high idle.

I turned the pilot out another half turn to 2,5. Then i tought the bike was crisper and the sound was more "growly". But now the idle when warm went even higher.

After that my friend adjusted the throttle cable and i turned the pilots back to 2 turns out. Idle went back as it should.
Bike went a little flat compared to 2,5 turns out.

Conclusion: The high idle could have been the throttle cable all the time. Or??
I will try to go to 2 and a quarter turns out and see were that takes me.

Other conclusions you draw from this?

I will report back.
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If you’re not interested in taking things back apart, and now that you’ve gotten your throttle cable squared away, try opening your pilot screws back up. Trying 2 turns is fine, but 2.5 is generally considered the “right “ setting when one has the proper pilot jets in place.
It is normal for things to seem fine idling in the garage but have a hanging idle once you go for a ride if, IF, you’re close. I’m going through that right now with a Bandit 1200 that was way too rich on the pilot screws. BTW, the Bandit is fine now, it was at 4 turns but only needs 3. :)
Now that you know you had a cable problem, try your pilot settings again. You shouldn’t be too far off presuming the only mod is a slip on.
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These old GSXRs seem to get really out of sorts once you go to a full exhaust and individual filters. Even the fabled tuners of old will lament how difficult they can be, even from bike to bike.
That’s why I mentioned the MAJ first off…but it seems you had a another issue that you’ve figured out.
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For what it's worth cleaning the carbs I recently had surprising results using Startron as a fuel stabilizer for a carbed 250. Went to use it after sitting outside for weeks in hot, humid weather and the fueling was better than when it was parked.
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