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Rob2810
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Joined: 05 Mar 2019
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PostPosted: 20:18 - 05 Mar 2019    Post subject: Mechanical question Reply with quote

Hi all got a bit of a mechanical question have got a suzuki gsx750f that has developed a bit of a problem when it gets to 120 mph it starts struggling like it's short of fuel its had
Cleaner in
New fuel pipes
Vacuum pipes
New diaphragms
Fuel filter
Any suggestions appreciated
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redeem ouzzer
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PostPosted: 20:22 - 05 Mar 2019    Post subject: Reply with quote

Have you cleaned the carbs? And I don’t mean just dumped some snake oil in the tank.
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Rob2810
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PostPosted: 20:43 - 05 Mar 2019    Post subject: Reply with quote

No not had the carbs off to do that was trying to eliminate other things first
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Grubscrew
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PostPosted: 22:12 - 05 Mar 2019    Post subject: Reply with quote

Sounds like a partial jet blockage, esp if the carbs haven’t been stripped down.
Purchase yourself a sonic cleaner, brilliant bit of kit, items like carbs come out like new.
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Hong Kong Phooey
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PostPosted: 22:54 - 05 Mar 2019    Post subject: Reply with quote

Fuel hose not kinked or pinched when you lower tank down?
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bugeye_bob
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PostPosted: 23:25 - 05 Mar 2019    Post subject: Reply with quote

Standard exhaust and air filter ?
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Teflon-Mike
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PostPosted: 06:03 - 06 Mar 2019    Post subject: Reply with quote

GSX750F... the old tea-pot.. oh-kay.... tough as old boot oil-boiler motor, I keep using the word 'old'.... hmmm, tea-pots and kettle's perhaps!

Right, first thoughts, were 120 mph? Where's he doing that then? Shortly followed by, so how fast does he want to go? Which is to suggest old adage before looking for a solution, make sure you actually have a problem.

Back to the owld.... I has to go check specs, 'cos I seem to recall that the thing was no tarmac-rippler.. well, like eva! Specs suggest that it has about 95bhp, which sounds about right for last of the line pre-water-cooled motor. Specs also suggest it has a top-end of somewhere between 130 and 140.... from a well fettled and prepped "press" bike, put through the traps in close to ideal conditions by a practiced pro..... run-wot-you-brung scores tend to be rather disapointing compared to the mag reviews, with typical examples of the breed in the hands of typical riders... even in the more favourable conditions of a sticky track..... So? a genuine 130'ish' would be more reasonable, 120... yeah that sounds a bit asthmatic, especially if thats the usually rather optimistic speedo reading.

BUT.... like I said, before looking for a solution, make sure you have identified the problem!

Before pulling carbs apart and 'stuff'.... check the tyre sizes! an over-sized front could give a 10% difference, and at that sort of speed that's more than 10mph.

Next up; the fugly fairings are 'supposed' to aid aerodynamics.... and they probably do, but.... without any at all, 95bhp isn't going to get you far past 120mph...... which begs next query, before pigging with jets an-stuff... has any-one 'foy-t'ad' the thing? Or put a tall or flip screen on it?

And follow on, if it's not been stripped naked... how large a fella are you, and are you getting 'tucked' out the wind-stream... and back to top... where are you trying to do 120? It can take a bit of winding up to get the last few mph out of almost any bike... are you sure it got the room?

And that begs the next conundrum... is it on stock cogs? If it's been down cogged for acceleration, it'll top out early. If its been cogged up, for more relaxed cruising, it will likely struggle, even more, to find the last few mph on the scale.

IF all of that is checked out and eliminated first.... my NEXT suggestion is the clutch... as you approach topping out, and the wind resistance is reaching the max force the engine can make... the clutch can slip... A-N-D the jerkiness of it letting go, then biting again, CAN feel a lot like a fuel block/starvation....

And its an oil-boiler.... what oil is in it? When was it last changed?
If car oil has been used... there's often copper based friction modifiers in there... bit like coppa-slip brake grease, fine on cars and even in cars with in-sump gearboxes where the oil would pick up phosphur bronze from the syncro-cones anyway.... b-u-t, not so great on a bike, with a wet-clutch sat in the oil, where you don't really want the fibre friction plates picking up low-friction additives so they dont have any friction!

And on the tea-pot, that actually begs another ponder-point; I seem to recall a common bug-bear in selector forks, and particularly, I think 3rs gear often not staying engaged, which oft down to knackered/bent/worn selector forks.... which are usually a bronze based alloy.. coppa-slipping the oil as they wear.... does it shift easy and stay in all cogs, under gross load? I would likely be dipping fingers into old engine oil... then cursing cos I forget to let it cool first! But smearing it on nice clean white kitchen roll, looking for the glint of gold and worn selector fork metal in the fluid.

So... starting with the obvious, simple and easy:-

Has the bike been layed up for winter? Have you dumped the fuel in the tank? Over-winter layup condensation on the inside of the fuel tank, will collect, and settle, the petrol floating to the top.

Following down, has any-one fitted an after-market fuel filter? These will often air-lock. My advice, if some-one has added one, is to dump it. They are oft more hassle than they save. But check the pipes for periushing, check hose clamps, check that the hose isn't kinked or pinched when any-ones taken tank off.

Working down... hmmm... carbs is scary and delicate... likely culprit by as said, I;d be in no hurry to have'em off.

Back to fuel tank and pipes. You dont say what year the thing is, and they made them an awful long time... earliest not so fugly faiuring models go back into the late 80's ISTR, so you have a bike that at it's youngest is over a decade, at its oldest over three, most likely somewhere in the middle, around 150-20.... so what state is the tank in?

On resto, I tend to POR15 the tank, as a matter of course. The cleaning in the instructions gets rid of silt and shite from the tank, the treatment puts lining on its walls stops it rusting or coming back. Costs about £50 but, done properly and diligently, using the whole kit and some cocum, saves SO many silly questions, faff and re-doing jobs, LIKE cleaning carbs, as old tank dumps old shite into nice cleaned carbs.... Treating tank also begs removing, cleaning, and insprctuing, overhauling or replacing the fuel-tap, as well as hoses including the breather.

Which is all 'hint' and stuff worth looking at, checking and at least bottoming before getting intimate with delicate diaphragms. If nothing else, thorughly cleaning the tank when you dump the old fuel is a good start.

Moving on.... normal service procedures follow... change the oil, change the filters, change the plugs; check the cable adjustments, check the tappets....

Eliminate 'so' many silly questions at source, and again, before looking for complicated solutions, be SURE you actually have properly identified the problem.

IF its still stuttering up top all that done..... then back to diagnostics, and you may have to go over some of the stuff you have looked at, like seating of the tank and routing of hoses.

Before puliing carbs, that clutch ~IS worth another visit... after you have flushed the oil. Is it a cable or hydraulic? Is it adjusted? Is it adjusted 'right'.. revisit and make sure it cant be slipping.. and in test, stu8ff the top-speed runs, try high gear and up=hill acceleration under high-load, see if you get similar symptoms pointing at the clutch, not the carbs.

Clutch is actually not all that hard to swap and overhaul, or all that expensive compared to torn carb diaphragms.... and since you are as likely to tear a diaphragm trying to tear down and clean a carbl hence make extra problems toy chuck into the mix.... discount all else first!

It probably IS at the carbs... and its likely that a float needle or three is sticking, and or the carbs are out of balence and or the chokes a bit icky.... BUT... lots of checks outside the carb before you need pull them off... check the air filter again... check the carb rubbers, look at cables check motion of butterfly's and slides.

And as Haynes warning, carb balence is the last jfinal setting up after all other (engine) service procedures.... with a set of guages, do they sinc up?

THEN it may be time to pull the carbs off.... carefully!

Check the state of the mounting rubbers either end, then drop the flat-bowls, check the float needles, check movement of needles, check float heights, then pop the tops, look at the diagphrams.. they tend to be expensive and a lot of lore about repairing torn ones with durex or superglue, which often isn't that wonderful even if it 'temporarily' seems to work.

Slides in alloy can get sticky, especially on CV carbs where full throttle doesn't mean full lift very often, and they can be a lot looser lower down than higher up, and get sticky when you open the taps.... like pushing the last few umph out of it at almost double legal limit speeds......

Again.. pre-emptive checks of exghaust are called for before stripping carbs... exhausts rot, after-market exhgausts often aren't optimal, and an AWFUL lot of presumed carb faults are often to be found in the 'zorst.. usually after a lot of head scratching and ping-fuckets from the flat-bowls, for the umptiueth time.

IE in short... DONT dive into the carbs assuming that's where problem lies... check everything else and the less obvious, First.
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bikenut
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PostPosted: 11:55 - 06 Mar 2019    Post subject: 120 Reply with quote

120mph, that's a bit fast for the roads here dude.....
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Evil Hans
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PostPosted: 12:48 - 06 Mar 2019    Post subject: Re: 120 Reply with quote

bikenut wrote:
120mph, that's a bit fast for the roads here dude.....


Whistle
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Teflon-Mike
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PostPosted: 00:07 - 07 Mar 2019    Post subject: Re: 120 Reply with quote

bikenut wrote:
120mph, that's a bit fast for the roads here dude.....

Looking at his profile, map suggests he's based some-where near Liverpudle.... which begs the thunk, "yer, actually I'd want to crack the taps on the M62 and get the chuck out of there as quick as I could too!" Laughing
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stinkwheel
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PostPosted: 19:23 - 07 Mar 2019    Post subject: Reply with quote

Check the fuel tank breather.

What exact symptoms. Stuttering/misfiring?

DOes it do it consistantly as soon as it hits that speed or is it a bit intermittant. 'tis the season for carb icing, that would be worse with low air temperature/high humidity and after a protracted high speed run. They can even conk out altogether as you close the throttle with this issue.
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bugeye_bob
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PostPosted: 12:26 - 08 Mar 2019    Post subject: Reply with quote

stinkwheel wrote:
Check the fuel tank breather.

What exact symptoms. Stuttering/misfiring?

DOes it do it consistantly as soon as it hits that speed or is it a bit intermittant. 'tis the season for carb icing, that would be worse with low air temperature/high humidity and after a protracted high speed run. They can even conk out altogether as you close the throttle with this issue.


When you ride the bike to the garage for fuel, does that cap hiss when you take it off.
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Rob2810
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PostPosted: 11:18 - 09 Mar 2019    Post subject: Reply with quote

No the breather is ok
My mechanic suggested it could be the floats being the wrong hight and therefore it being starved of fuel at that speed (not as I continually ride at that speed response to other posts ) thanks for decent response
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jaffa90
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PostPosted: 11:36 - 09 Mar 2019    Post subject: Reply with quote

Rob2810 wrote:
No the breather is ok
My mechanic suggested it could be the floats being the wrong hight and therefore it being starved of fuel at that speed (not as I continually ride at that speed response to other posts ) thanks for decent response

You say (got a suzuki gsx750f that has developed a bit of a problem when it gets to 120 mph ).
How can it be the float heights?
It does sound short of fuel or to much air.
Q, when it looses power is it ok at 100mph?
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Rob2810
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PostPosted: 11:41 - 09 Mar 2019    Post subject: Reply with quote

Everything else has been changed
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P.
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PostPosted: 13:19 - 09 Mar 2019    Post subject: Re: 120 Reply with quote

bikenut wrote:
120mph, that's a bit fast for the roads here dude.....


No it's not. You are just too old and frail. Calm your tiddies.
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