Slow speed jerking / hesitation

[Beenthereuk]

Hi all,

The only problem I have on my GSX750F is uneven engine speed below 2000 RPM.
Its practically impossible to ride for more than a few meters at less than 10MPH without the bike hesitating and surging mildly though repeatedly. So I spend a lot of time pulling on the clutch around town.

Above 2k it is totally smooth, with acceleration and fuel consumption (~40mpg) roughly what they should be.

It has an Arkapovic street legal carbon fibre exhaust fitted with no baffle. And is probably stock apart from the small changes I made (see sig.).

I had a mechanic synchronise the carbs and all filters etc were replaced. It also has new plugs and a recent oil change.

Any ideas?

Tony.

[jbo197]

Clean the Carbs ~
Any mods done to exhaust will require re-jetting

[Kreylyn]

Stock sprockets?
Checked for vacuum leaks?
Idles smooth/steady without load?

Krey

[Beenthereuk]

I think the sprokets are stock - it seems to run out of gears at 40mph.. I find it easy to roll along in 5th at 30mph.

The idle is not as smooth as I'd like - misses a beat out of every two or three. And its almost impossible to throttle at a constant low rpm - at 1500 say, it just wants to speed up until it hits 2k +.

The exhaust is supposed to bolt on without the need for rejetting but I do have my doubts.

The only other thing I've noticed is that I get mild misfires when decelerating with the throttle closed. Runs really well above 2k - pulls to 10k (thats as far as I've taken it.. I used to ride a CB500..) without missing a beat.

Could something like this be caused by leaks in one of the manifolds?
Is it likely to need rejetting?

Cheers,
Tony.

[Kat.45]

post a youtube vid if u can

[Kreylyn]

I think the sprokets are stock - it seems to run out of gears at 40mph.. I find it easy to roll along in 5th at 30mph.

I don't understand... At 5k rpms in 6th gear, what speed are you traveling?

The idle is not as smooth as I'd like - misses a beat out of every two or three. And its almost impossible to throttle at a constant low rpm - at 1500 say, it just wants to speed up until it hits 2k +.

I don't know if this is a throttle cable routing issue, user issue (well running bike doesn't take much throttle to make a big change, and seems difficult to control if your not used to it), or the cable isn't adjusted/mounted correctly making it too tight.

You should be able to smoothly acelerate/increase the RPMs smoothly all the way to redline. The RPM range should not matter.

The exhaust is supposed to bolt on without the need for rejetting but I do have my doubts.

The only other thing I've noticed is that I get mild misfires when decelerating with the throttle closed. Runs really well above 2k - pulls to 10k (thats as far as I've taken it.. I used to ride a CB500..) without missing a beat.

Could something like this be caused by leaks in one of the manifolds?
Is it likely to need rejetting?

Cheers,
Tony.

The "misfires" is generally refered to as Afterfires. It's a common symptom of a lean running motor. Too lean of a mixture means all of the fuel is not burnt in the motor, and some gets into the exhaust... where is is burnt off causing the small popping after burning of the hydrocarbons.

I would say if you have put an aftermarket header/exhaust system on the bike... you probably need to tune the carbs for the new change in flow. Best suggested option would be a jetkit if your state inspections will allow for one.

Exhaust leaks can increase the afterfires because they allow more Oxegen into the exhaust. If the header is removed, you should replace the gaskets. They are inexpensive, and designed for a single mounting use.

Krey

[Scout]

Did ya try adjusting the idle knob? It's inside the fairing hole, on the right side, I think.

[scottynoface]

Am I the only one not surprised that a 750cc I-4 is lugging below 2k rpm? It's not a Harley...

[hardlydangerous]

back the mixture screws out 1/4 to 1/2 turn

[Kreylyn]

Am I the only one not surprised that a 750cc I-4 is lugging below 2k rpm? It's not a Harley...

My bike never did it... until I upped the tooth count on the rear sprocket.

Krey

[clark827]

My bike never did it... until I upped the tooth count on the rear sprocket.

Krey

triple

[scottynoface]

Interesting, mine did, but it was a 600. Same with my 636.

I guess jerking is pretty extreme though.

[Beenthereuk]

Kreylyn: Its no cable issue, of that I'm pretty sure. I meant to say that it has ooodles of torque so that I can easily drive slowly around town in a high gear and, funnily enough, when I do so, I get hardly any jerking. The bike jerks worst at low speeds and in low gears. My problem is I do a lot of urban riding, in traffic mostly, and this problem at low speed is a pain.
It also bothers me that I cannot hold a fixed engine RPM between 1100 (at idle, no load) and 2k.

Scout: yes I played with it and found that it settles best at around 1100RPM, any lower and there's a good chance the engine will stall if left to idle when cold.

BTW I forgot to mention that, when I rode it in subzero temperatures recently, it stalled on me several times and a mechanic here concluded it suffers from carb freezing - as far as I know, this model has no carb heaters. But thankfully, the temperature is likely to remain above zero C. now till December; we only get a few weeks of negative temperatures here in London so I can live with that.

From all your comments it sounds like it is running lean but I will make a video and put it up soon so you can see and hear what I mean. I'll also count the teeth and post here in case the sprokets are not stock - might be that the previous owner changed them causing some strange oscillation in the gearing at low speed.

Thanks for all your comments. Much appreciated,
Tony.

[Beenthereuk]

OK here is a vid of it running when warmed up - you can just about hear the engine pulsing/surging.

http://www.veoh.com/browse/videos/category/lifestyle/watch/v19872701cNCR2sDn

I also counted 49 teeth on the rear sprocket and from what I've read it should be 45 as stock setup is 15-45. So its been geared-up for acceleration. Which probably explains why I seem to run out of gears at 45 MPH..

I clocked it doing 40 mph @ 3000 rpm on a flat road in top gear - how does that compare with everyone else's?

Fuel consumption is 100m miles from full to the start of the red line which I've just discovered is only 16L (has another 5 in reserve). So it averages 100/16= 6.2mpL or just 28mpg (imperial) !

[Kreylyn]

OK here is a vid of it running when warmed up - you can just about hear the engine pulsing/surging.

http://www.veoh.com/browse/videos/category/lifestyle/watch/v19872701cNCR2sDn

I also counted 49 teeth on the rear sprocket and from what I've read it should be 45 as stock setup is 15-45. So its been geared-up for acceleration. Which probably explains why I seem to run out of gears at 45 MPH..

I clocked it doing 40 mph @ 3000 rpm on a flat road in top gear - how does that compare with everyone else's?

Fuel consumption is 100m miles from full to the start of the red line which I've just discovered is only 16L (has another 5 in reserve). So it averages 100/16= 6.2mpL or just 28mpg (imperial) !

You should be getting almost 2X the MPG from that bike. Closer to 45-50mpg.

Krey