[bobbler]

Really hoping one of you knowledgeable folk on here can help as I'm having issues with the bike popping on tickover (tickover problems seem common for the nx650 by the looks of things). Sometimes the revs will hang high after small throttle input and other times the revs will drop and die but on the occasions that it does tickover there is an erratic popping coming from the exhaust.

I've tried to take a video to show what I mean if anyone can help.

https://youtu.be/neIKu6YyR3g

The current setup is an iridium spark plug, direct mount Ram Air pod filter, stubby 2 into 1 scorpion exhaust (no baffle), 60 pilot jet with pilot screw around 1.5 turns out and a 170 main jet fitted after the carb was cleaned. There are no air leaks around the inlet manifold that I can find (squirted while running with WD40) and the bike runs fine other than the tickover issue. I'm guessing I need to replace the pilot jet again but without a gas sensor I don't know which way to go.

Other work already done in case it makes a difference. New CDI unit and coil pack fitted by previous owner.

I haven't yet shimmed the needle but I'm not sure that would that really affect tickover as I always thought that was controlled by the pilot jet and adjustment screw?

Cheers.

 

[philippo]

Hi bobbler,
The standard jetting on the 99 dommie is 145 main and 50 pilot with the screw out 2 turns.

Given the pod filter and open exhaust though, despite your bigger jets,
My guess would be that it's running lean in the pilot fuel circuit.
Try winding out the pilot screw to richen up the tickover, half a turn at a time.
If you get to the maximum pilot screw adjustment and it's still a problem, you'll need a bigger pilot jet.

Seeing as you've eliminated potential air leaks, ignition, sparkplug issues the general rule of thumb is, a hanging idle after a blipped throttle is due to lean mixture. A sluggish pick up on a blipped throttle is due to over rich mixture.

Oh, just thought of something . . .
Make sure the throttle barrel on the carb is returning smoothly to the closed position after your twist grip input. Are the throttle cables unkinked, lubed and operating smoothly. Sounds like a no-brainer but sometimes it's easy to overlook the bleedin obvious. ;-)

 

[bobbler]

Thanks, the cable has been checked and returns fine. The return cable was removed by one of its previous owners.

I'll try to richen the mixture before getting a larger pilot jet, I ended up at 3 turns out the other day but the revs still hung and there was a strong smell of fuel so I tried leaning the mixture out a bit. I'm trying to get hold of an emmisions tester just so I know if I'm on the right track

 

[philippo]

Ah, so you already tried richer on the pilot screw. It's probably not that simple then. Damn.
Have you stripped the carb and made sure there are no blockages?
Pilot jets are notorious for getting blocked with crud/varnish from old fuel.
A partially blocked pilot jet would make the carb seem like it was set too lean.
And you'll be chasing your tail trying to set it up / compensate for the blockage.
Is the choke working properly?
When you smelled the fuel was the bike fully up to running temp?

The exhaust gas analyser is deffo the way to go.
Can you get the bike on a dyno?
In a single dyno run you'd see immediately where the problem is manifesting.
Takes out all the guess work. You mixture ratio throughout the whole rev range is right there in black and white.
Sometimes its the exact opposite of what you think. Could too much fuel from the main circuit be compromising the pilot circuit? Maybe.
The 170 main jet does sound a bit on the huge side given that the standard main is only 145!

Hopefully one of the site veterans with more experience than me will be along later today to give you some better ideas.
Good luck ;-)

 

[bobbler]

The bike had just finished a 30 minute ride 70/30 dual carriageway/town riding so temp was around 90/100C.

Jets were brand new along with mixture screw, o-ring, carb needle and float valve. The previous owner said he had it ultrasonic cleaned and although I have no proof it was gleaming inside when we stripped it. While there I also changed the fuel line just for good measure. The only thing I haven't done is shim the needle or drill the slide as per Dave's Mod.

Starting this morning on around 2 turns out and the bike needed choke to start and warm up but it still had its moments where it would only drop to tickover if I brought the revs down first by staying in gear and slowing the engine otherwise the revs had a tendency to hang quite high.

It seems to run ok on a 170 main with plenty of power higher up the rev range but in saying that I have had the same thoughts as you. If I refit the baffle it feels rich and really sluggish with very little power up top (WOT at 60mph in 5th on a slight incline would not accelerate yet around 3/4 throttle did).

I've spoken to my local garage and they have advised the same re dyno time so it looks like a dyno run will be on the cards and go from there.

 

[philippo]

I'd be fascinated to see dyno run/gas analysis printout on a '99 dommie.
Keep us all posted.

 

[Jeff Kirby]

Did you ever this fixed...and can we see the dyno test?

 

[bobbler]

I ended up doing Daves mod which helped a lot and it went over the dyno at the weekend.

For some reason I posted the results on another post but here it is. I'm told without a rev counter the torque figures may be out but the hp at the wheel looks ok for a 20yr old bike.

 

[Jeff Kirby]

Thanks, as you say, looks respectable. What did it cost for the dyno run and where did you have it done? Wouldn't mind running mine for a laugh

 

[bobbler]

It was put over the dyno at dragon tuning in Durham Jeff. Sorry I don't know the cost as it was part of a bike deal I did.