Suzuki NZ250 that has me beat...

**Conquiztador** · 20th May 2007, 12:30

I have a Suzuki NZ250 that is driving me crazy. (it's my sons) It shoots out the exhaust. Starts and idles perfectly. But when reving she starts to shoot. After about 5 minutes she starts running OK and no problem before stopped and left for a short while. Then it starts all over again.

I diagnosed carbi problems so I replaced carbi with same type. Did not help. I recond carbi. No change. I replaced with other type that I know is workng well. No change.

So I came to conclusion that I am obviously having a problem with valves not closing properly and therefore petrol going out in exhaust. I replaced head with one I know was good. No change.

I then came to conlusion, that if it was not head or carbi, then it must be electrics. So I placed motor in another complete frame I had that had all electrics intact (and that had been running sweet until motor died) Yep U guessed it: same problem.

So what left? exchanged spark plug. Twice. But no change. Ahaa I thought, the only electrical bit I have not replaced is the pickup and magneto coils. So I replaced them on the motor. But still same problem.

So now, the only thing that is left is the bottom end incl. piston and barrel. All electrics, carbi, head, spark plug, muffler has been replaced.

I have the bits to put together another motor, and it is what I will do next. But I hate to be beaten. And if it all works when I replace the motor, then I have the bike back up and running OK.

But I just want to know what is causing this??? Any ideas??

**xwhatsit** · 20th May 2007, 15:25

What is to `shoot'?

**Jantar** · 20th May 2007, 15:41

Originally Posted by xerxesdaphat

What is to `shoot'?

To take aim, then squeeze the trigger.

**JimO** · 20th May 2007, 16:08

i bought one of those beasts last weekend for my boys to learn on and havnt had any probs but yours sounds like its stuffed so i suggest you GIVE it to me for spares

**Conquiztador** · 20th May 2007, 18:38

The manual is being sent on Tuesday. I have 3 of these NZ250's. Two are complete and working brilliantly. But this one is a mystery. She is the one that I am setting up as a retro bike. But the motor is driving me crazy. Spending too many hours working on her. But hate to be beaten... Was just hoping that someone out there had some ideas that I have not considered.

**JimO** · 20th May 2007, 19:01

if you ask me flames coming out of the exuast would be the coolist thing to impress the chicks at the school car park

**imdying** · 22nd May 2007, 13:39

I know you said you changed the head, but have you checked the valve clearances?

**Conquiztador** · 22nd May 2007, 17:21

Originally Posted by imdying

I know you said you changed the head, but have you checked the valve clearances?

Yep. They have been checked. Cheers.

**riffer** · 22nd May 2007, 18:45

Could there be a buildup of carbon in the zorst which builds up pressure?

Once you get enough pressure built up it goes bang...

Does it do this with the zorst off?

Damn. Just reread your post - you've replaced the zorst.

Maybe you've got a cracked piston...

**Conquiztador** · 22nd May 2007, 21:10

Originally Posted by riffer

Maybe you've got a cracked piston...

Did not even consider that. But it would make sense. As the only thing that has not been swapped is the bottom end of the motor, piston and barrell. I have had the piston out and I did not notice any damage to it. But maybe there is a hairline crack somewhere that I have not noticed? I will inspect and get back. Thanks for that!

**Big Dog** · 23rd May 2007, 00:00

Essentially to get this symptom you must have unburnt fuel making it to the exhaust or there is no resistance to the combustion exhiting the exhaust.

If you know your head is good and your clearances correct, you need to look at why there is too much unburnt fuel hitting the exhaust.

If it is only during the choke or warm up stage then you probably have insufficient spark or insufficient air when warming up.
Insufficient spark can be caused during warm up by any number of expensive parts and I don't know where you want to start but cleaning and gapping the plugs would probably be it.

More likely is insufficient air to burn the available fuel.
Try starting the bike with no air cleaner and an open air box.
You may need to lean out the mix on choke.
Or your air cleaner may be filthy, or otherwise obstructed.

Also may 70-80's bikes had air flow meters on the air cleaner. These are only any use in sub zero conditions if you are going to choke it anyway and often seize in the closed position causing engines to "gasp" often resulting in flames during start up.

Living in the the north island? Not planning on visiting the south island in winter? Bypass it.

**Conquiztador** · 23rd May 2007, 00:32

Your comments are appreciated and I will investigate. The spark plug is fine. have replaced with new and gapped as per specs. The valve clearances are also fine. Triple checked them. And both heads doing the same. The filter on the aircleaner has been cleaned and I have also swapped it. But it is a big box under the seat. And I shall look in to what else hides in there. Said that, I put the motor in a nother frame that had earlier held a motor that did not have this problem (but blew up...), and the problem was still there even if I used all the bits apart from the blown up motor. (Carbi, air cleaner, electrics, tank, muffler etc.). Thats why this thread, I had run out of ideas. But thanks to U guys I am getting some new things that I can look at. Will keep U posted re progress. Cheers.

**Kickaha** · 23rd May 2007, 08:20

Originally Posted by Conquiztador

I then came to conlusion, that if it was not head or carbi, then it must be electrics. So I placed motor in another complete frame I had that had all electrics intact (and that had been running sweet until motor died) Yep U guessed it: same problem.

So what left? exchanged spark plug. Twice. But no change. Ahaa I thought, the only electrical bit I have not replaced is the pickup and magneto coils. So I replaced them on the motor. But still same problem.

I take it you have checked the timing ? (both valve and ignition) it might pay to do it with a degree wheel and dial guage to make sure it is accurate

**Conquiztador** · 23rd May 2007, 08:53

Originally Posted by Kickaha

I take it you have checked the timing ? (both valve and ignition) it might pay to do it with a degree wheel and dial guage to make sure it is accurate

Yep, been there done that. Even adjusted valve timing differently to what is recommended just to see if any diff. But same problem. Tining on these are set and there is no adjustment. But with the problem going away after a few minutes and she then runs sweet I would not think it could be the timing anyhow.