2002 Suzuki GSX750f acceleration problem

[Landyman]

Hi.
I need some help and advice with a continuing problem with my 2002 Suzuki GSX750f . I have reset the tappets, renewed the carb diaphragms, replaced the float bowl seals, clean the jets, checked the float settings, reset the pilot to factory, checked the ignition coils resistance and the TPS settings. All of the hoses are leak free, the carbs have been balanced using a Multi-Tune Pro Manometer and the battery is full and happy.
The bike starts on choke and ticks over without a problem, once warm the engine fires up on the button and the rev range is great..... UP TO 5000 RPM after which it starts to die, cough and if I do manage to get it further up the scale, it won't/can't rev beyond 7000 RPM. The motor is sweet as a nut on the low rev range and the individual cylinders test out at 160/170 with a proper compression tester (Snap-On). I have tried all the test fixes such as spraying easy start into the carb mouths when accelerating, pulling on the choke, tightening all hose fixings and clamps, cleaning the timing points, checking and cleaning all electrical connections, replacing the spark plugs with new and checking for leaks on headers and exhaust. ALL TO NO AVAIL!!
I'm a retired engineer (mechanical and on the spanner type) but this has me pulling my hair out, I have had bikes and cars all of my adult life but with this I am struggling, can anyone out there help PLEASE?
Kind Regards.

[that_impulse_guy]

weirdly enough, a good friend of mine has a 750 teapot that does exactly this. We've been troubleshooting it for ages.

Done what you did+fuel tap. Once its back on the bench, best we could figure out is that it has a k+n so we may go play with some jetting. It seems to go above 7k on smaller throttle opening if you go slowly with the throttle (our one), but wont go there on full throttle
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Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[Nobby the Bastard]

Is this a new issue?
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[MarJay]

Sounds like the main jets are blocked. It's possible to have a blockage that is not visible with the naked eye, so ultrasonically cleaning the jets is pretty much the only way to cure it. Looking at your post, you've tried a tonne of electrical and generic fixes, but haven't had the carbs off.
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Landyman]

Hello Again
Just to confirm, the carbs have been off and all the jets have been cleaned with carb cleaner and compressed air, after which the jets could be held up to the light and checked with a magnifying glass to confirm that they were clear. It is also worth mentioning that the previous owner had the carbs ultrasonically cleaned. I am however open to any suggestion at this point and will remove the carb bank again just to double (treble) check. Another point to mention is that when warm, pulling on the choke to try and improve fuel feed when accelerating does nothing more than stall the motor, as would be expected when flooding the carbs. There is no smoke at the end can, nor is there any evidence of over-fuelling. The motor runs sweet, the exhaust is clear and there are no unrecognisable noises. I'm led to believe that this engine is bullet proof and that is is virtually unknown for either the Ignitor or the coils to fail, can anyone confirm this or tell me the results if they do fail?
Kind Regards

[MarJay]

If it were a coil it would run on 2 cylinders and fail when hot rather than at a certain RPM. I have heard stories of people cleaning individual jets with carb cleaner and compressed air and not having any luck, and then only when the jets are ultrasonically cleaned do they actually fix the issue. I did also think that it could be worn needle jets but that would be overfuelling rather than underfuelling. It definitely sounds like underfuelling when it comes onto main jet.

I've had plenty of 'The previous owner did' issues lately, with my KR1S, with my GSXR750, with my GSX-S1000F, with my house, with my car... you name it. The only way to verify is to watch it being done. Often people will butter your bread when trying to sell a bike (unless you know the previous owner personally of course)
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Landyman]

Thanks for your replies, I have the carbs off again and I'll do another full strip, it might also be worthwhile putting new main jets in so that I can kill or cure but will definitely remove another possible cause. As for the needle jets, what I didn't check was that they are hooked up to the third notch as specified in the Bible of Haynes, so that's another avenue to investigate. The good thing is that I have another bike to ride so there is no rush with this and I can spend the time getting it right (and learning even at past retirement).
Thanks Again All
Kind Regards

[Robby]

Need a bit more information on the origin of the problem. Has the bike ever behaved in your ownership, was it laid up for a long time. Is the problem kicking in right from starting it up, or does it lose power and fail to rev high after 10 minutes of normal running?

If it runs ok for a while then feels a bit like it's running out of petrol, it could be running out of electricity due to a failed reg/rec or stator.

If you have a 7000rpm "wall" that is always there then I would be looking at the ignition pickup coil(s) for a single thing that can affect 2 or 4 cylinders.

CDI/ECU/black box failure is unlikely, particularly seeing as it runs otherwise. They don't tend to break, and when they do they tend to really break.

[Landyman]

Good Afternoon.
I only purchased the bike at the end of November 2023 after selling the exact machine 2 months earlier and regretting it as it was ridden away. I believe the it was laid up after the last MOT in March 2023 and it has 26000 miles on the clock with full service history and receipts, the GOV/MOT site checks out as does the HPI check. The fault is from start-up and doesn't improve or get worse as the bike warms up. I was told at purchase that the carbs had been ultrasonically cleaned but would require balancing. I have done everything I can think of but, apart from a much smoother and regular tick-over after balancing, the problem is still there and the same. Rev up and it responds without delay, shut down and it does so to the tick-over without issue and, as I have already mentioned it is quiet and sweet. It came with an aftermarket can, which I have replaced with the original and the carb jetting is as quoted in Haynes. I hope that this additional info sparks a light with someone out there and, thanks again to everyone trying to give me a leg up.
Kind Regards

[Easy-X]

Some sort of timing issue? I know on the old bikes I've worked on the points have a spiny-weight system that mechanically advances/retards the timing based on RPMs. Dunno what the equivalent would be on a (relatively) newer bike

I only say this as it vaguely sounds like the wall the 50cc scooter bois try to break through on a regular basis.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Landyman]

Regarding the timing thread.
The timing is from fixed points that I have cleaned and checked along with the connections they make to the Ignitor. All of the ignition side has been checked out with a multi-meter and found to be in keeping with the spec provided by Suzuki and Haynes. The two coils provide the spark to each of the four cylinders after receiving the signal from the Ignitor, all of which perform brilliant until I try to open the throttle at and beyond 5000 RPM. Thanks however for your ideas, it is appreciated.
Kind Regards

[stinkwheel]

Air filter in good order? Just because you didn't mention having checked it.

What you describe sounds exaclty like what happened to a GPZ500 of mine when it ate its own air filter which had been disgustingly manky, I suspect it was the original factory one and the engine finally had enough and sucked all the foam through the mesh and through the engine. (this was a bike sold to me as freshly serviced).

Anyway, that is bye-the bye. New filter fitted and it hit a brick wall at about 6k rpm. You could pull through it eventually and it went suitably batshit if you could get the revs up to 8k. Not ideal for riding because you were either bimbling or thrashing it about flat-out.

The eventual diagnosis was blocked power jets in the carb, they had filter foam lodged in them. These are the little brass jets mounted round the edge of the carb facing back towards the air filter. They bleed more fuel into the system at specific midrage revs before you are fullly onto the main jet and needle. They aren't removable and are jetted internally in the carb. I cleared them by a combination of compressed air applied directly to them and careful poking through with nylon fishing line.

Looks like your carbs have them, may be worth a look. They may even be removable on yours.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Landyman]

Good Evening
Again, thank you for all the responses, all of which will be considered as I try to make some sense of what is and isn't happening. I admit that I haven't mentioned the air filter which is brand new and at present out of the story but makes no difference with air box on or air box off. All of my work at present is without the air box and filter due to the need to pull back the carbs from the rubber boots to enable access to the vacuum take of nipples for the manometer. I have tried a number of mixture screw settings to compensate for this but without a successful outcome. The 'factory setting' is screw them in till they just bottom and then back off cylinders 1 and 4 two full turns and cylinders 2 and 3 1.5 turns. When I started on this little project the screws were all set to 3 full turns out, with the air box and filter in place, it still died at 5000 RPM. To date I have tried every combination of settings from factory to 3 full turns, no luck on any, however, the dead spot remains at the same rev range.
Kind Regards and Thanks.

[stinkwheel]

Mixture screws are for idle. They do nothing at all much above 1/4 throttle.

You are talking midrange which is mostly throttle valve cutaway, needle jet and throttle needle position and taper... If it's a carb problem at all.

If it starts and idles and trundles around a carpark, stop messing with the slow running circuit, it's not that.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Landyman]

Hey Up!
That's a great bit of information and something I didn't fully understand, but I do now! I'm beginning to think that with the information I get from this Forum, I might have a chance of getting it moved on from a trundle around a car park to riding the Fylingdales by the end of the month.
Thanks Again, please keep it coming and I'll listen and learn.
Kind Regards

[MarJay]

Power jet carbs! Yes so the way carbs work is that you have an idle jet that does the idling and very low RPM, then a needle jet and then eventually onto mains at high RPM. Power jet carbs work a little differently and the power jets add an extra layer to that. I’d definitely be looking in the power jets to see if there is a blockage.

Stan Stephens once told me that on an F2 350 power valve if you were to say put a ball bearing inside the power nets and make the mains a lot bigger you’d get a massive improvement in top end power because the power jets were designed to over fuel the bike to allow it to meet its warranty mileage witkout blowing up.
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Robby]

Power jets would make sense if the problem is only occurring under load. If the bike is sitting on the stand in neutral, it should be able to hit the redline without needing much throttle at all, certainly not enough load to bring most of the fun stuff in the carbs into play.

Simple test. If you charge the battery overnight, does the problem go away for a few minutes?

[WD Forte]

Dont know the bike but had a cursory glance at some wiring diags and
pics and from what I've seen it appears to have a common reluctor type pulser/signal generator.
Basically a single coil around a core supplying the ignition control with pulses

I'd check the resistance to see if its in spec as it's an old bike and it may have deteriorated over time and may not be giving as strong a pulse as it did when new.
Dont know the exact specs but from experience with other pulsers
I'd expect to see a resistance of say 250-400 ohms or so?
Tens of or kilo ohms would look dodgy to me

This may not be the problem but it only takes a couple of minutes to
test
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bikers smell of wee

[MCN]

I worked on a fast rescue boat many sun's ago, the report was that it bogged down when given the beans in the water.
It used to sit next to the helideck in baking sun week after baking week. It would sit so pong up there that when the idiots plopped it in the water and cawed the engine Ower, it would start then promptly run out of petrol.
Petrol would be acquired (by semaphore, as we were on a jack-up drill rig in the Persian Guff, and choppers cannot carry petrol.)
So a week later 25l of petrol would be delivered and the boat petrol tank filled. It was a RIB with a plastic portable fuel tank.
By the time it was my turn to work on it, the cards were caked with dried petrol. They looked like they'd been mixing cement through them.
The cards cleaned, the engine was still gutless.
It was a little 3 cylinder Honda outboard. Cylinders mounted vertically and the ignition control on one side with a block holding three coils in one block.
By phaphery, I managed to feed no 1 with no 2 coil and no 3 with no 2 and combination of swaps.
That confirmed that two coils were phuqued.
The engines are reliable enough to be great at starting and running but if any ignition component is weak then the engine will not perform 100%

Long story short.

Fit new coils.

They are not dead expensive.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Landyman]

Good Morning
The carbs are going into carb clean soak today and for the next 24hours but I will post a couple of photos of the jets etc with my next note. Meanwhile, here is the full official spec and set up for the carbs, please let me know if something here hits you in the face as being an obvious path to follow.

*I have asterisked the settings on my carbs*

Kind Regards

Stock Settings for GSX750f (Katana)
Specifications and stock settings for the Mikuni Carburettors found on the Suzuki GSX750f (Katana) Motorcycle.
Note: When tuning the fuel screws start at the stock setting, or a known good position. Make changes in increments of quarter or half turns on the screws.

1998 to 2006
750cc
Type BSR36SS
Pilot jet
Austria ans Switzerland 12.5
*All other Euro models 15
US and Canada 12.5
Jet needle
Austria ans Switzerland (clip position) 5DF19-53 (3)
*All other Euro models 5DF-52 (3)
US and Canada 5DF21-53
Needle jet
*Euro models P-0
US and Canada models P-0M
Main jet
Austria and Switzerland
carbs 1 and 4 120
carbs 2 and 3 117.5
*All other Euro Models 117.5
US and Canada 117.5
Pilot screw setting (turns out)
Austria and Switzerland 3
*All other Euro models
*carbs 1 and 4 - 2
*carbs 2 and 3 - 1 1/2
Canada models 3
US models 2.5
*Float heights 13.0 +/- .5mm

[sickpup]

If it has Mikuni carbs they are likely BST34's which are pretty easy to work on and reliable.

Its easier to set up the carbs with a CO2 meter. iirc you need to be aiming for around 3.5% (could be wrong, been a long time) at idle. It does effect the whole of the rev range.

Make sure the emulsion tubes are still round, the needles wear them out.
Check the fuel tap flow. The vacuum diaphragms fail.
What exhaust does it have fitted?
The coils are crap, check and maybe replace with any with a 3ohms primary winding iirc.
Plug caps fail on these.
Make sure the plug hole drains are clear, any water here causes problems.
Check HT leads.
Try blowing through the main jets
Check the carb floats are set right
Is it a UK bike or an import?

[Landyman]

Hello Again and Thank You for your response.
As Previously posted, the carbs are now totally stripped and have hade the most rigorous cleaning you could imagine. The emulsion tubes, main jets, choke jets, pilot screws and pilot jets have all been removed,soaked in carb cleaner, polished and checked for size. The float chambers have been cleaned and the needles have all been stripped and checked for wear. Following this, everything has been blown through with compressed air and anything brass has been polished. I have ordered new main117.5 jets and a complete set of o-ring seals together with new inlet boot 0-rings. To be further sure, I have constructed a small 'goal-post' gauge with a crossbar hight of 13mm so that I can be assured all of the float levels are set bang on. The exhaust is original, the bike is a UK model and all of the resistances are to spec as issued in Haynes Manual.
I will hopefully have it all back together early in the week,I have changed oil and filter just to ensure that everything that moves is getting the best I can give. As a temp measure I have sprayed all of the HT leads, coils and plug tops with silicon spray to provide a temporary seal against leakage, the next spend, should what I have done fail, will be two new coils and four new plug caps.
Watch this space!!!!!
Kind Regards

[Landyman]

How it looks just now.
Kind Regards

[Landyman]

Good Morning
I think that we must have almost exhausted this topic without coming out with a definitive solution that will get me running as should best, here is my latest post to try and explain what is happening and is aimed at the more senior end of the audience. If you remember the days before electronic ignition, computer controls and fuel injection, you will understand what I am trying to say what is happening. In those days the ignition system was a feed through the coil which, via low and high tension leads then fed the distributor which in turn through the action opening and closing points provided the spark to each cylinder, usually in the firing order 1-3-4-2. The whole sequence was kept in balance by a little silver cylinder wired into the points known as the condensor which, when it failed, caused the spark to become erratic, resulting in loss of power under acceleration accompanied by backfiring as unburnt fuel was intermittently fire. The condenser was a quick fix at around £2.00p but, when it went wrong, it stopped the show.
These are the symptoms I am suffering with my bike! By weekend I will have it all back together and will, I would like to think, be able to provide a positive out come however, I'm not holding my breath.
Kind Regards

[Nobby the Bastard]

from the days of cdi onwards (basically electronic ignition) they became quite robust and due to the fact there are no moving parts (or even repositionable parts) they tend to either work or not work, and very little likelihood of your problem being ignition related.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050