tuning info

[Tom-madbiker]

right the chop is thro the mot but when you get higher up the rev range it starts to crackle and die i have lifted all the needles 1.5 mm and gone up a main jet size to a 130 the bike is a z750 and is running open pipes and filters there are no baffles in the pipes at all could it be a lack of back pressure

[Tom-madbiker]

just checked the plugs and they are a nice brown colour

[Mitch]

right the chop is thro the mot but when you get higher up the rev range it starts to crackle and die i have lifted all the needles 1.5 mm and gone up a main jet size to a 130 the bike is a z750 and is running open pipes and filters there are no baffles in the pipes at all could it be a lack of back pressure
Mrs R's old ZL600 did that, especially after it shot the baffles out of the Kerker Supertrapp it had on :eek: going down the road quite happily to the first forum party, then all of a sudden... Bang! - half a dozen shurikens whiz past the driver behind her's head!!!! :D

[contail]

what filters have you got on.
had a flick through my carb book. you cant get a very good indication from plugs any more due to unleaded fuel additives.
also. if your running filters that let a lot more air in, the top end will go realy weak to faid away (like you ran out of fuel) try tapping the filters up a bit. exhaust shouldnt be to much of a problem. does it play up when you rev it in nuetral as well
i had z1000j chop.straight through pipes. standard carbs/jet sizes. ran it with out filters for a day. she would fly up to about 6000rpm then that was it. when i tried to open it up more she died on me. thought ide forgot to turn fuel on. i put filters on and sorted it.

have you checked your tap and carbs for blockage. the flow could be restricted.

i know its a pain, but if you put every thing back to original does it run ok. if it still cuts out ide check your ignition system.
you probably know all this already, but if not i hope it helps some way

[pyro]

Bad top end is main jets, could be main jets too small or too much air (not quite the same thing), try wrapping a bit of tape around the airfilters to reduce the air flow, cover up 80% of them and keep adding/reducing it to try various amounts of air and see what happens, if it runs badly at the bottom end but improves at the top (with most of the air flow reduced) then you need bigger mains, if it runs better right though the range then you need to restrict the airflow somehow. This is a cheap trial and error method.

[PILRCGeff]

Unlikely to be the exhausts as too little back pressure is normally a problem at lower revs, not high revs.

[johnr]

z750 four cylinder or z750 twin cylinder?

[Tom-madbiker]

its a 4 cylinder but i think ive got it running pretty well now got 135 mains in also restricted the airflow a bit but now it seems to rev like crazy at high speed i think the rear wheel is too small

[Grav]

When you say Revs like crazy, what are the revs and what speed in top gear please.

My Z750 does 70 mph at 5,000 rpm in top gear (point of reference for you from a standard bike with a Harris exhaust)

[Taff]

Seem to remember its pretty old aint it, old enough to still be running points, or it got electronic on?
As said earlier, full wallop is when open drag pipes come into their own, about the only time like.
The fuel starvation suggestion is good, at full bore you need a hell of a lot of juice going thru, especially if rejetted and the flow opened up as it is.
You keep mentioning this small wheel and high revving, why dontcha bite the bullet and whack a smaller sprocket on the back, and do it large, like 6 teeth, least you'll know then.

[Grav]

I once owned a GPZ550 fitted with K&N's and a seriously modified motad ( the baffle got twatted out with a big chisel :D ) It had a main jet size of 142.5 and went like fuck on standard sized sprockets. :D

[Taff]

Y'know you said elsewhere that it shoots flames out the exhaust on changing up a gear.
Good fun and all that, but it aint fkin right.
Dunno what that signifies, but somebody with more jap 4 knowledge than me will.
Grav, pop theself over 'ere....

[Grav]

You say you have raised the needles up by 1.5mm, this could be causing the backfire and high revving probs. When you shut the throttle off to change up, the needles cant shut the main jet which allows too much fuel to flow through and doesn't all burn in the cylinders, flows through the exhaust ports and ignites in the exhaust giving you that luvverly afterburner effect.

Try dropping the needles back down to where they were.

[pyro]

I'm a bit confused over mentioning of revs v speed thing here, fuel / ignition wont affect speed v revs, unless the clutch slips, cos its fixed, the only way to change the ratio is to alter the sprockets sizes. (or take the gearbox apart and change cogs)

[Tom-madbiker]

sorry my fault mixing up two threads

[John Hopkins]

When you say Revs like crazy, what are the revs and what speed in top gear please.

My Z750 does 70 mph at 5,000 rpm in top gear (point of reference for you from a standard bike with a Harris exhaust)

Thanks for that Grav...Thats very useful to me! my standard Gt750 should do the same. :D John

[pyro]

Thanks for that Grav...Thats very useful to me! my standard Gt750 should do the same. :D John

Er no actually, cos one chain and ones shaft.

And mine does 100 @ 5000 rpm. oh its not a Kwacker.

[John Hopkins]

First let's get this clear, I don't know anything...having got that out of the way. with suck, squeeze, bang, puff, bikes if flames come out of your exhaust it sounds like you have sticky valves. maybe carboned up..but for about £5 you can buy a compression meter on ebay, and it only takes five minutes to test. otherwise one day your bike may just go bang. John

[pyro]

First let's get this clear, I don't know anything...having got that out of the way. with suck, squeeze, bang, puff, bikes if flames come out of your exhaust it sounds like you have sticky valves. maybe carboned up..but for about £5 you can buy a compression meter on ebay, and it only takes five minutes to test. otherwise one day your bike may just go bang. John

Or an air leak on the exhaust
or over advanced ignition
or a plug not firing
or.......


Sorry John your first line got me going. :D

[John Hopkins]

Or an air leak on the exhaust
or over advanced ignition
or a plug not firing
or.......


Sorry John your first line got me going. :D

Hah even I know you cant advance the ignition on a gt750 ..and an air leak on the exhaust isn't going to make a difference if he has already knocked out the baffles...and he would hear a plug not firing all the way home. :D John

Now argue that pyro..you just like flames and don't want it to change..

[John Hopkins]

Er no actually, cos one chain and ones shaft.

And mine does 100 @ 5000 rpm. oh its not a Kwacker.

Ok smartarse you got me again my GT750 only does 60mph at 5000 revs in top gear. And since it hasn't got a fairing and I'm a very slight 3 stone overweight that may be normal. still I cant see it lasting much longer if I try to do the quoted top speed of 127mph. Probably tested with a fairing, going down a one in four hill, with a strong tail wind. :D John

[Tom-madbiker]

got a fifteen tooth on the front now goes great still a little lumpy at around 95 tho going up to 140 mains and maybe drop the needles back down

[pyro]

Ok smartarse you got me again my GT750 only does 60mph at 5000 revs in top gear. And since it hasn't got a fairing and I'm a very slight 3 stone overweight that may be normal. still I cant see it lasting much longer if I try to do the quoted top speed of 127mph. Probably tested with a fairing, going down a one in four hill, with a strong tail wind. :D John

I was wrong! (I'm never wrong so a first...alright second on here) it does 85@5000, dunno why I thought 100!
On the way back from the Ogri on the M4 I whizzed past everyone as they suddenly slowed down, I kept going and didnt see the "SPEED UNIT" van on one of the bridges...was doing around 135, oops

Are you sure about the quoted top speed though, I'm sure the GT I had was a bit faster than that, although it was a long time ago (1986/7??) I do remember that it handled like a pig at speed though.

[slob]

Can someone - namely Tom-madbiker summerise the symptoms (from both threads if relevant) the current siuation and the the steps taken to cure it.

There have been a lot of suggestions but I haven't the foggyest what does what so a quick brief as to what the solution was would be helpfull.

cheers

[pyro]

He upped the mains which increased flow of fuel at the top end and restricted the amount of air getting into the intakes (didnt say how) this seems to have improced the running.

[Tom-madbiker]

indeed i have upped the mains from 125 to 135 and have lifted the needles by 1.5 mm and restricted the airflow slightly with different filters but it is getting a bit hot and does run out of steam at about 90 still overtook tons of sportsbike riders on the way back from devon yesterday sitting in traffic :D

[pyro]

Does it run cleanly right through the range until it runs out of steam?
How much juice are you using?
Does it start without choke?

[Tom-madbiker]

its pretty clean up to the point where it runs out of steam no it wont start without choke it revs pretty clean when no load is applied and it uses about 6 litres for every 50 miles