Suspected Carb Problem

[dungbug]

Firstly apologies for my first post being a long winded technical problem, but I'm hoping someone will be able to ggive me some advice.
I have a 1999 Suzuki GN125 with a problem, she revs fine and pulls through the gears accept when I put it into 5th then it seems to 'bottom out'. The power seems to tail off and it de-celerates slightly, if I ease of the throttle and wind it back gradually it'll pick up again.

I checked the diaphragm & found this;

https://i16.photobucket.com/albums/b29/dungbug/DSCN5864.jpg
In total 7 holes in the diaphragm.

The diaphragm has been replaced with a genuine Suzuki part;

https://i16.photobucket.com/albums/b29/dungbug/DSCN5880.jpg

I also changed the spark plug and cap with NGK items as the old ones were looking, well, old. Also changed the HT lead as well with a new item.
There was some improvement after changing the diaphragm but she still losing power at the top end, I checked the plug and found that it's running lean;

https://i16.photobucket.com/albums/b29/dungbug/DSCN5906.jpg
It's actually quite white, checked it again yesterday after a run.

Something I have noticed is the power is still being lost every now & then, the engine note changes and the speed drops away.
Yesterday the carb was pulled off thinking that the lack of fuel getting trough could be either a blockage somewhere or need the mixture adjusting. Adjusting the mixture wont be easy as the mixture is fugged;

https://i16.photobucket.com/albums/b29/dungbug/DSCN5865.jpg
I don't know what/how some ham fisted monkey managed to do that to it. Someone has been inside this carb before, screw heads are buggered & the internals have a 'worn' look about them.

The carb was stripped down, compressed air blown through everywhere, checked the float operation (all good). Put the carb back on and the behaviour is still the same, still losing power at the top end. Also stripped the fuel tap off and cleaned it, she's using a standard airfilter BTW.
I'm out of ideas now, part of me thinks it's a mixture adjustment but she's hit 70mph a few times on the dual carriageway this week before geting bogged down and dropping to 55mph The engine feels strong, there's no rattles/knocks or smoke during start up or riding/revving.
I did check the spark yesterday and she's got an orangey spark (plenty of it) but I thought it would/should be blue.

I'm wondering if it'll be worth changing the carb for a new pattern part one. Finding a genuine Mikuni one seems to be nigh on impossible, Wemoto sell a pattern carb for my bike as well as several companies on eBay.
I've used pattern part carbs before (with aircolled VW's) in the past and found they weren't very good.
Has anyone used a pattern carb and can give me some feedback/advice? Or can think of something else I can try? Many thanks.

[Fizzer Thou]

With the spark plug looking as it is I would suspect that there is a leak between the cylinder head mounting for the inlet manifold.Several people have had similar problems to what you appear to be experiencing and have gone on to find that the seal has shrunk and/or perished,allowing too much air in to weaken the mixture and reduce power.On some bikes I have even found that the inlet manifold itself has split.

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/Workshop/Picture352.jpg
____________________
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Always have a 'Plan B'

[Ichy]

Firstly, thanks for the great photos! It makes a change for someone to actually come on here with an image that is in focus.

Is there any chance that what you think is a mixture screw is actually an anti-tamper plug?

The plug doesn't show a massive amount. For a proper diagnosis you need to ride at one throttle opening for a good distance, kill the engine and then check the plug. Something like a colourtune would be easier to read.

I would put it back together and then, once it has warmed up. Spray some WD40 around the manifold. If the revs pick up then you have a leak. It is worth doing the same around various parts of the carb, such as the choke, to check they are ok.

Have you checked the air filter?

Is the fuel flowing correctly and is there an inline filter?

It may be worth replacing the jets, they are usually around £1 each. Personally I would be looking for a 2nd hand carb if this one has been butchered.
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[dungbug]

Cheers gents, I knew they'd be stuff I forgot to mention. The air filter element was replaced about 3 months ago with a new item genuine Suzuki). The carb to head inlet rubber was replaced about 5 months ago, it did have an airleak. I thought it would have solved the problem only to find the diaphragm was holed as well, both have been fixed now & the problems continue.

This was the old inlet rubber~ the white cack was some sealant I used as a stop gap as there was a big crack on it;
https://i16.photobucket.com/albums/b29/dungbug/DSCN5488.jpg

The inner part was also perished (not overly clear in the photo);
https://i16.photobucket.com/albums/b29/dungbug/DSCN5486.jpg

Fitted back on the bike;
https://i16.photobucket.com/albums/b29/dungbug/DSCN5490.jpg

The photo of the plug was taken after being cleaned, gap checked and given a run up the dual carriageway for a few miles at a speed between 50~60mph.

I did do the 'WD-40 test' around the inlet, carb & air intake parts and there was no change in the engine note or speed so I don't think an airleak is the issue anymore.
I'm sure someone has been inside this carb before and been a bit heavy handed with it.
I'm unsure whether to buy a 2nd hand carb incase it's worn out, that's why I was thinking about the pattern carbs that are available. I can't seem to find any reviews/feedback on them though..............Which might say alot!

[dungbug]