GSX-R750 Bogging Down

[Alex A]

1992 GSX-R750 WN

On the last few rides, the bike seems to be bogging down a little on slight throttle openings, in lower revs, whilst under load. It's fine when in neutral, and once you open the throttle more than 20% or so, it pulls as strongly and consistently as ever. But at constant slight throttle the power cuts in and out a little. It's not drastic by any means (doesn't judder about) but it's noticably weaker and a little 'wheezy'. In short, it's not as happy to build revs under load from low down, and I need to slip the clutch a little longer with bigger throttle opening to achieve the same 'go'.

Great excuse to always thrash it, but a little annoying for getting about.

It may or may not be coincidence, but it seems to have come about since I ran out of petrol last week (low petrol light doesn't work). So I'm thinking that perhaps some cack from the tank has made it's way into the carbs and maybe jammed up one of the slides?

I haven't done anything yet, though battery/plugs/oil/filter/coils were all renewed about 1500 miles ago, so all fairly fresh. Carbs were also stripped, cleaned and balanced at the same time.

Before taking the thing to pieces, I'd like some suggestions as to the likely things to check. As it only happens under load, I may end up unbolting everything, fettling whatever might be the cause, putting it all back together, then riding to check, and I could end up repeating that process a number of times until it's sorted. I'd rather just do it the once, so feel free to suggest a few things to do whilst the tank's lifted before I put it all back together to test it out again.
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Current: Ducati Panigale R / Ducati Streetfighter S / Suzuki GSX-R1000 L5

[SW Motech Shop UK]

Try draining the carbs with the screws under the float bowls. You wont need to take anything apart to do that.
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https://www.motorcycle-road-and-race.co.uk and https://www.sw-motech-shop.co.uk/ and https://www.motorcycle-luggage.co.uk/

[0l0dom0l0]

sounds a little bit like a small peice of crap is blocking on or some of the jets. this would be sufficient with the small throttle openings as it probally wont be on the main jet!

might also be the fuel filters abit cloged up, you could try replacing that first? then draining the carbs and seeing if that makes a difference. if not i think its a carb strip down maybe?

[Alex A]

[temeluchus]

Check moisture hasnt gotten into a HT cap between the lead and cap. Corrodes the copper a bit and can make the bottom end a bit fluffy.
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Some shite cruiser. Now with guns and FREEDOM!

[temeluchus]

[benjami]

Had the same problem with the exact same bike, took the carbs off, (bit of a bitch due to tight space) and cleaned them thoroughly.

Sorted it after this.

[Alex A]

[Alex A]

OK, carbs out (very fiddly). They look pretty immaculate inside - though they should considering they were only stripped 1500 miles ago. I did notice one of two breather pipes wasn't connected to the carb assembly before I took it out.

Got some carb cleaner but no air line. Any tips?
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Current: Ducati Panigale R / Ducati Streetfighter S / Suzuki GSX-R1000 L5

[temeluchus]

Its also worth noting the GSXR750WN is also well known for burning valves on stock jetting, especially if it developes any kind of leak in the intake path.
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Some shite cruiser. Now with guns and FREEDOM!

[Alex A]

[Alex A]

[Alex A]

Maybe time for a new air filter too?

https://i512.photobucket.com/albums/t321/Furyous84/airfilter.jpg
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Current: Ducati Panigale R / Ducati Streetfighter S / Suzuki GSX-R1000 L5

[Alex A]

[Alex A]

Right! Just got back from work to finish the job.

Carbs thoroughly checked and cleaned and reinstalled. Airbox refitted with new filter. Tank and seat put back on.

Went for a quick run and I've got a triple

Right hand cylinder isn't firing. Fuel is getting through as it pops in the exhaust. Think I might have damaged/yanked the HT lead when removing and cramming everything back in under the tank. Will check the plug for a spark in the morning.

The good news is, despite running on three, it's actually running better and building revs in lower RPM much more happily than it did when I began (and it was running on all four then). For instance, if I held the throttle (in neutral) at about 5-10% before it would reach about 4.5k and climb no further. Now it runs right up to about 7-8k with the same throttle and doesn't need a whole lot more to have it running freely to the limiter. Get this cylinder running and I reckon it should be good to go.
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Current: Ducati Panigale R / Ducati Streetfighter S / Suzuki GSX-R1000 L5

[Alex A]

[Alexio]

[temeluchus]

Perhaps some nice dyna coils? The stock suzuki items arent great.
____________________
Some shite cruiser. Now with guns and FREEDOM!

[Alex A]

Been on holiday for a week so only just got around to lifting the tank again yesterday. Checked all the plugs and reseated all the HT caps, replaced the tank and she's back in business. Running very nicely indeed with the new air filter and freshly cleaned carbs

Thanks all for the help - much appreciated.
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Current: Ducati Panigale R / Ducati Streetfighter S / Suzuki GSX-R1000 L5