Dommie cuts out at low revs - Need help!

[Lourdite]

Guys I need a bit of help. I have owned an 1988 NX650 for 1 year now and it hasn’t ever run properly and I am out of ideas. I welcome any suggestions you can offer. The bike starts properly when the choke is open full and the revs are high, as soon as I start to close off the choke the bike runs rough then starts to pop as if there is a build up of fuel, the pops become closer together then it stalls.

So far I have swapped the coil, carb, CDI, it has had a new camshaft and rocker arms, tested wiring loom for continuity, bypassed ignition, swapped stator. I have not been using the tank, I have been supplying fuel from a funnel and hose so it’s not a fuel supply problem, new plugs, air filter, HT lead, new battery.

Any ideas, so far I have an expensive paperweight!!

Cheers

[Rubberchicken]

Sounds like the mixture is somehow off. Does it rev ok, or is it messy all over the rev range?

Is it stock or has it got a different exhaust and/or air box mods and such? (A ratty old leaking stock exhaust may cause trouble too.)

Cleaned the carb? Which size jets were in there? Blocked slow jet perhaps, that'll mess up the low revs.

Check the underside of the rubber thing between cylinder head and carb, that has a tendency to split on the older dommies.

(The one on the left is new, the one on the right has a bunch of hair cracks.)

[kitped]

There is a thread somewhere about the exhaust valves being an issue, the can cause stalling at low revs, I think but may be wrong that the exhaust valves gap should be greater than the books says. I can't do a search at the monet but wil try and look later.

[Cozmo1589]

I had a similar problem and even when the carb was overhauled the bike still did the stall on occasion. I was told to try the premium high octane fuel and a iridium spark plug. Have never looked back, the bike runs great now

[Lourdite]

Sorry forgot to mention, I have replaced this pipe, mine did have a small split in it.

[Lourdite]

Kitped,

Thanks for the advice, I'll have a look for a thread on exhaust valve clearances. In the meantime if you know what the gap you have used is I would be grateful if you could let me know.

[catteeclan]

Inlet .10mm /.004in
Exhaust .12mm /.005in

[Rubberchicken]

The trick with the right hand exhaust port is that it has the automatic valve lifter. Setting it at exactly TDC would cause weirdness, IIRC that one needs to be set just a bit past or before TDC.

But I get the feeling that there's 2 different "cutting out" things going on:
1. There's the legendary Dominator Stall, especially at or approaching traffic lights with closed throttle. Bike will otherwise idle well, just cuts out every now and then. This is normal.
2. Running rough at low revs is an indication that something more is amiss.

1. can be fixed by turning up the idle speed a bit or by carefully adjusting the idle mixture (that impossible vertical air screw on the underside) and/or fitting a slightly larger slow jet if necessary. Decent spark plug will help, too.

[Lourdite]

The trick with the right hand exhaust port is that it has the automatic valve lifter. Setting it at exactly TDC would cause weirdness, IIRC that one needs to be set just a bit past or before TDC.

But I get the feeling that there's 2 different "cutting out" things going on:
1. There's the legendary Dominator Stall, especially at or approaching traffic lights with closed throttle. Bike will otherwise idle well, just cuts out every now and then. This is normal.
2. Running rough at low revs is an indication that something more is amiss.

1. can be fixed by turning up the idle speed a bit or by carefully adjusting the idle mixture (that impossible vertical air screw on the underside) and/or fitting a slightly larger slow jet if necessary. Decent spark plug will help, too.

Thanks mate, I will have a look at these, if I could only get the bike to run at low revs I would be happy, when I turn the choke down it is dead in less that 120 secs!

[Rubberchicken]

Yeah that would be a case of number 2...

What's the minimum revs (roughly) at which it does keep running?