1986 Suzuki NZ250S surging at 4,500 rpm

[Chalmes]

This bike has done this the entire time i've had it (almost 6 months). The situation is as follows:

At home, start the bike up on choke, let it warm up for a couple of minutes and close choke.
Get it out on the road. It's sluggish while i slowly get up to 4,500 rpm. Once it gets over 4,500 rpm, it takes off.
Change gear, revs drop below 4,500 and its sluggish again. Get to 4,500, takes off.
Part or full-throttle doesnt seem to make much difference.

Once the bike has been out on the road for a good 5 minutes, the transition seems less sudden, but it is still barely noticable. And its worse on colder days. Also, on cold days, if coming to a stop while in gear, it can backfire through the airbox (only occassionally, have had this happen maybe 3 times)

Has factory everything.

I currently have the NSA pipe blocked on either end as with that in its normal position, when the bike takes off it threatens to throw me off, its like riding a bull it is that extreme.

Also, with the NSA valve connected up, it can get into a cycling on/off power (with me not changing the throttle) where it oscillates at that 4,500 transition which makes it quite dangerous in traffic.

I've had the carb off the bike and fully cleaned and replaced seals etc. Nothing seemed obviously wrong with anything.

It *feels* as if the slide is perhaps sticking, and then suddenly releases, giving me the power expected. I guess with a CV carb the slide should gradually move up with the building engine vacuum. so if it sticks a little while the vacuum is increasing, it will need to catch up a little bit which can give the violent burst of power.

WoT once the bike is warm, it runs really nice.

Any thoughts, comments, suggestions of things to try?

Cheers

Jon

[Chalmes]

A quick followup....

I started the bike today with the choke on the second notch and noticed that it was revving right up on 4,500 rpm. This may or may not be a co-incident.

Jon

[F5 Dave]

Lean. Sounds like an airleak. manifold perished? Airfilter holed or non std? Not sure what you mean by NSA.

when warm lean cond not as apparent. Try airleak search. Could try adjust float height so petrol level is higher &/or raise the needle with a washer if no clips, but you'd have to ask why before doing that.

[Ultrasonic2]

sounds lean to me .

Has it been modded ?

putting shims under the needles should help your issue but if you need shims under the needles than the main jets maybe to small also.

i had to increase the main jets and add washers. i had to get shims from the china cos washers from the hardware store were to thick for my little bike

[ducatilover]

I'd check for air leaks.

While the bike is idling, spray some CRC around the inlet rubber and if the revs rise or get smoother, you have an intake airleak.
I'd also strip and re-kit your carb, the NZ250 are old and it's probably in need of it.
Is your slide opening smoothly? Youtube "CV carb slide test".


Good luck.

Edit: Do the revs drop slowly at the lower end of the rev range when in neutral?

[Chalmes]

Cheers for the replies.

I've had the carb off and done seals/o-rings.

Will check that youtube clip out, it does feel like a sticky slide.

I'll richen it up a bit....ive been playing with the pilot screw a little to try and force the problem one way or another, but now its probably just over-lean causing the backfiring.

NSA is the New Super Acceleration pipe that is a 'feature' on this carb. It runs from the engine side of the throttle (from memory), though a one-way valve and onto the top of the slide diapghram. I believe it is there to assist the slide with lifting under particular conditions, but it just makes the 4,500 power issue worse, so better blocked until thats sorted.

Circlip on needle is currently in the middle position.

No mods at all.

Rev's are quick to react, no slow dropping in neutral.

Jon

[Chalmes]

A side thought from a friend.

Maybe the generator/igniter/coil is having issues, and it isnt until 4,500 rpm that it can start to put out a decent spark?

Jon

[Ultrasonic2]

it's to lean put the clip on the needle one lower (one closer to the pointy end) to pull the needle out more and make it richer. though ultimately you'll need bigger jets aswell.

[Chalmes]

Yep, i'll be messing with the pilot screw once i get the bike back together again to get the tuning right, but i'm beginning to think thats quite a separate issue from the 4,500 rpm power surge.

I've tested the carb for slide sticking by running a shop vac on the engine side and opening the throttle, both slowly, and rapidly to see how the slide moves. Seems to move just fine in all cases, no sticking whatsoever.

When its back together i'm going to throw a meter across the battery and see what voltages the generator is putting out at various rpm's, see if theres some tie-in there.

Jon

[F5 Dave]

Pilot screw won't help too much once away from just off idle. Needle.

[Chalmes]

Checked the float level (after the last mechnic 'set' it) and it was 1mm higher than factory spec.

My understanding of CV carbs was that the pilot screw/jet has a fairly major effect throughout the rev range



Even so, would an incorrect needle height produce the 4,500 kick in the pants surge that is my major issue? And if so, then why?

Jon

Edit: Here is where I read about the relative importance of pilot jets:
http://www.mychinamoto.com/forums/sh...e-different...

[ducatilover]

Pilot screw won't help too much once away from just off idle. Needle.

I keep telling people this, they won't listen....


Set the float height, get a new needle and seat for both the bowl and slide.
Set the needle at the factory position (probably will have a .5mm washer in it)
Set your valve clearances.
Replace your plug and gap correctly.
Replace ignition lead.

Set your carb up correctly after that.

[ducatilover]

Checked the float level (after the last mechnic 'set' it) and it was 1mm higher than factory spec.

Didn't notice this post. The float will be okay 1mm out.

My understanding of CV carbs was that the pilot screw/jet has a fairly major effect throughout the rev range

This is not true. It only adds a very, very small percentage of the total fuel going though, hence being called an idle screw/circuit.

Even so, would an incorrect needle height produce the 4,500 kick in the pants surge that is my major issue? And if so, then why?

Jon

It's possible, yes. If your needle is too low, or your slide spring too stiff, the mixture will not richen up as fast as when you you have spaced the needle or put a softer spring in. If your power just flattens out, you're too lean, if it bogs and stutters/surges, you're too rich.

Edit: Here is where I read about the relative importance of pilot jets:
http://www.mychinamoto.com/forums/sh...e-different...

Slides lift up and down at idle, this chap seems to think they only move when you whack the trottle open. They're constantly moving, every time the intake valve opens/closes it changes the vacuum between the carb and combustion chamber, thus altering the slide height. (It's usually a small lift, but, it's there)
The idle circuit does not control a lot at all.

[F5 Dave]

if it gets way too lean it will stutter & be bitchy. Can be hard to tell from rich stumbling.

I'm still wondering - why? Is pipe leaking/non std?