help me diagnose an issue

[pompousporcup...]

Gpz5 starts and runs alright however once the engine has been run for a little while it doesn't rev as well and the exhaust note sounds crap..not crisp and responsive. When engine is cold sounds a lot better

Spark is good, valve clearances checked, carbs cleaned and balanced. Idle set correctly

I'd like to get it running cleaner.

[CaNsA]

Leaky carb rubbers?
air:fuel mix out of whack?
Fuel tank breather pipe blocked?
Fuel filter replaced?

[pompousporcup...]

Don't think its the tank breather, at least not this time as it runs ok...seems a bit hesitant when hot at lower revs in say 3rd at 30mph so I tend to stay in second. Tiny throttle and its fine until around 3/4k rpm then picks up

I've adjusted the mixture screw to 2.5 or there and got a blue spark at idle with the colour tune device

There is a slight fluctuation at idle on the rev counter.. so possibly a leak somewhere but has only started doing it recently after 100s of miles. The rubbers looked in good condition

Fuel filter is perhaps 500 miles old

[bladeblaster]

sounds like a fueling issue.

Is the fuel filter an in-line filter, and is it supposed to have one? They can play havoc with gravity fed systems.

[Pete.]

Get it warmed up then get your face round by the exhaust outlet. If you're choking on fumes it's running rich.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Rogerborg]

Did you inherit this issue with the bike, or was it running fine but has started misbehaving recently?

Did you make any changes before it started happening? I'm looking at "valve clearances checked" with a little suspicion.

Any chance the choke cable has got pinched and the choke isn't coming off fully? Probably not since you're been at the carbs.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Diggs]

Degraded coil. Swap for a decent one and see how it goes...
____________________
Now - Speed Triple, old ratty GS550, GSXR750M
Gone (in order of ownership) - Raleigh Runabout, AP50, KH125, GP125, KH250, CBX550, Z400, CB750FII, 250LC, GS550, ZXR750H1, Guzzi Targa, GSX750F, KH250 x2, Bimota SB6R and counting...

[orac]

the fact that it happens with hot is sort of helpful.
how does it start when hot, easily needs a little cranking or is complete bastard?
compression test, take readings when cold and hot. if you get less when hot either the valves are incorrectly shimmed (which is my current guess) or you are getting blow by on the rings. I would think that you would be getting some oil from the exhaust if you had blow by - you can also do a leak down test to check for blow by.

I don't know what the plugs are like to get to on that but hot plugging to check mixture could be helpful.

incorrectly shimmed valves wont always show up on compression test, I have not done enough to be able to advice on that.

when you say carbs are clean, did you check float height, are the mixture set correctly, are the slider needles set correctly are the diaphragms in good condition. I had issues with a single diaphragm which stopped the bike running correctly when hot, the hole barely made it all the way through the membrane and took nearly 15 minutes of inspecting (by stretching in front of a light source) to find.
____________________
Current rides - 2016 Triumph Street Triple Rx, 1994 Suzuki Bandit 400 VM, TGB 204 Classic 125cc
"with nothing left to lose, there is everything to gain. It's not the size of the dog in the fight, it's the size of the fight in the dog"

[stinkwheel]

I'm pretty sure you've already checked the air filter?

The issue you describe (pulling well at high revs when cold, not so good when hot) is classic of an over-large main jet if you were setting a bike up.

So too much fuel, not enough air at wide-open throttle.
____________________
“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.

[pompousporcup...]

Not checked the filter in a few hundred miles so will have a look see.

Carbs were completely stripped and cleaned so it's unlikely an issue with them although I'm not ruling it out. Jetting it stock.

Coil..could be, the leads and plugs are pretty new with less than 1k on them.

Valve clearances are, as far they can be, correct. They were checked, checked again and then rechecked again with a size up and size lower.

I'l go and inspect the air filter and reset the idle mixture and do a general bolt/plug/rubber check

[pompousporcup...]

Filter was a bit dirty, I've taken a newer one out of the spares bike and have given it a clean

Set the idle mixture again using the colour tune and also changed the plugs in case it was a duff plug causing issues

Fuel filter is a zzr600 one that's in the fuel rail. A known mod as the gpz never came with an inline filter. I've checked it this evening and it wasn't clogged up or anything.

Nothing like reassembling a bike before leaving for work -_-

[pompousporcup...]

Ignoring the spammers. I put it back together this morning and rode to work. Still happens a bit but really If I use the rev range its not such an issue...it's like it's bogging down a bit under certain throttle/load situations but maybe I'm expecting too much from the bike and its heluvalot 47hp

Il keep adjusting the mixture until it's right and update when I've found the issue

[orac]

All carbed engines have a flat spot somewhere in the rev range. The gs5 was resolved with a10% up jet on the mains,up jet on the primaries extra half turn on the mixture screws and a washer under the slider needles to raise them a half notch
____________________
Current rides - 2016 Triumph Street Triple Rx, 1994 Suzuki Bandit 400 VM, TGB 204 Classic 125cc
"with nothing left to lose, there is everything to gain. It's not the size of the dog in the fight, it's the size of the fight in the dog"

[Vincent]

Does the bike have the standard pipe/s? Could they be blocked?
____________________
Space Is Deep

[pompousporcup...]

no standard pipes.. stainless aftermarket set with a viper end can. Swapping this one over from a motad system the top end felt better(as much as it could from the engine)

Pretty sure they're not blocked but worth a check next time they're off. It rides fine and screams up to the red line.. its just a bit hesitant between 3/4k rpm at some throttle position

[orac]

seems the pipes had an affect but full standard google indicates a flat spot @ the 4-k5mark.

retune and service should sort it providing its not been fubard by a previous owner

heres something google threw up
https://www.bikechatforums.com/viewtopic.php?t=80961
____________________
Current rides - 2016 Triumph Street Triple Rx, 1994 Suzuki Bandit 400 VM, TGB 204 Classic 125cc
"with nothing left to lose, there is everything to gain. It's not the size of the dog in the fight, it's the size of the fight in the dog"

[pompousporcup...]

[bikenut]

sitting in traffic it gets hot, fuel vapour lock or similar ?? but check ign advance when hot, to rule that out !!

exhaust headers by head glow red hot when bike misbehaves ?? show signs of too much heat ??
____________________
nuts about bikes

[pompousporcup...]

little update, thanks all for the ideas and advice.

i fixed this by trying to do an oil change.. because of the manifold design i couldn't get the sump plug out properly so ended up taking the exhaust off and when i'd finished the oil change i put the exhaust back on and used copper ring gaskets instead of reusing the organic gaskets i'd fitted a few hundred miles earlier

hey presto. the jumpy revs went away and its running better where it used to stutter (the correct gear means i never have this issue)

only now the flat spot is a bit further up the rev range -_-