Gz125 (again) power issue related to revs

[MarkGoy]

Hello,

1st, thank you to this forum, i came here a while back with the same issue and was told to fix my carb leaking first before anything else.

Well thanks to your help its fixed, no more leaks, so thank you, sincerely.

//

Right, to business, GZ125 K3 made 2003.

I plonk it in first and pull the accelerator, it gets to X revs and just cant pull anymore, making a weird judder noise from the engine.

Its exactly the same in all gears, but not with the clutch in or neutral, I can rev to my hearts content like that.

I sellotaped my mobile to my satnav to try and get a good video of the noise / speedo.

link follows.

https://www.youtube.com/watch?v=4FJ4TsGl7d0&feature=youtu.be

I hope this gives a good amount of detail?

thanks in advance.

[jaffa90]

Hello again, did you set the float height as in pic?
Another thought, is the exhaust restricted?

[MarkGoy]

Hi again! haha

erm, i never got to the bottom of that issue.

the book is very unclear and it seems impossible to set it as shown.

i'll take another look though shortly.

video gives no clues?

//

ok i double checked, it is still impossible

[jaffa90]

Ears not so good but it sounds like it has limited power rather than losing power, if possible ride the bike again and hold when it starts acting daft for a while then stop, remove the spark plug when cooler to see the colour.
If its clean then its short of fuel, if its black / sooty then you have fuel there but it could be exhaust restricted or ignition timing not advancing.

[MarkGoy]

Should I insert a new spark plug for that test, or clean one up first?

Did you ask about a limiter? If so not factory standard as far as I know.

Yeah the video isn't the best, it's the best quality I can manage.

Edit : I should explain that while I had that carb issue the bike was running lean and subsequently the spark plug blackened.

I'll put a new plug in and have a go tomorrow.

Ta

[jaffa90]

""""""""Edit : I should explain that while I had that carb issue the bike was running lean and subsequently the spark plug blackened.""""""


A spark plug blackens with soot when it`s running rich or a weak spark.

What colour is the plug now and yes clean it before the next plug inspection (chop).

[MarkGoy]

Sorry I have been drinking, I meant rich not lean.

Thanks for correcting me.

No idea what it looks like currently will check before I do anything.

Tha.ks

[Robby]

Does it have a rev counter?

I'm wondering if you're just hitting the limiter. How fast is a 125 meant to go in 2nd gear?

[MarkGoy]

No rev counter unfortunately.

I understand your point, and I was thinking the same, but I can't exceed 45 mph in 5th, im pretty sure it should go faster than that.

And regardless of that the engine doesn't sound right, to my understanding anyway?

[MarkGoy]

Thinking about it, top speed should be (in gears, according to current limitations), ~12 mph, 22, 32,42 52.

Which would be close to top speed, I would assume.....

I'm still sure the engine doesn't sou.d right or has correct top speed per gear..

[MarkGoy]

Ok, i managed to get it done today.

I changed to a brand new spark plug, took bike on road for about 5 minuets, I kept the bike in its silly state for about 30 seconds in total.

I find holding it like that scary, the bike judders about and the noise is horrible.

so, pic 1) old spark plug (from before carb fix, hence looking rich)

pic 2) brand new spark plug before fitting

pic 3) spark plug after 5 mins ride + 30 seconds daft engine mode.

[jaffa90]

Pic 1 is not running rich, pic 3 is short of fuel.
Well done for doing a plug chop as they say.

[MarkGoy]

Ive had a look at the chart,.... is pic 1 normal then?

that would make sense as i avoid the stuttering part, presumably the engine for the most part stays is normal operation...

//

Short of fuel.... is that explicitly short of fuel or can it be too much air too? as in a leak somewhere?

if its the case that it is too little fuel... it could be float height.

I assume the diaphragm , needle jet/ jet needle operate normally, other wise not enough air would get in and the mix would be rich?

could also be incorrect size main jet? maybe too small?, ive had them cleaned twice so they're not blocked...

could be a blockage between the carb and engine?

[jaffa90]

Pic 1 is weak enough but passible, would be better with a biscuit colour on the white bit (insulator). I`m surprised there`s a c.v. carb fitted and not a direct cable lifting the carb piston / slide.
If the fuel level is correct then it`s possible that the carb slide is not lifting high enough under vaccuum. The diaphragm rubber has to be sealed around the edges and puncture free to operate.
A simple test is to lift the carb slide manually and watch it slowly going back to rest.

[MarkGoy]

pic 1 spark plug hasn't done many miles if that makes a difference?

Aaaah, so if the carb slide doesn't lift high enough, enough air cant pass over the jets, pulling sufficient fuel into the engine, I hadn't thought of that. I assume that's effectively hitting the top of my rev range.

I've done a quick test, by you saying "watch it slowly going back to rest." I think its not right.

maybe i haven't fit the diaphragm properly, I got a new diaphragm last year, no holes or anything like that in it.

https://www.youtube.com/watch?v=kGimQLpl7fQ

[talkToTheHat]

[Robby]

The spark plug looks like you've had a lean mix, and you've been cocking around with the carb, so let's consider that area.

3 areas to consider:
1. Fuel supply/flow to the carb. This is easy. Assuming a normal gravity feed with a fuel tap, does fuel freely piss out of the end of the fuel hose when you pull it off the carb. Direct it into a bottle, pour it back in the tank afterwards. Let it flow half a litre or so - if you only do a few cc of fuel then its just the bit in the pipe and filter if fitted.

2. Air leak on the carb rubber. Does the carb sit into the carb rubber in a secure way, assuming there is a big jubilee clip holding it in place, is this clip tight? It is possible that the lip of the rubber got folded under the carb during fitting, causing a big air leak. Also look for splits in the rubber. Easiest to see with the engine running, because the rubber will pulse and flex a bit.

3. The carb itself. Take it off the bike, drain out the fuel into a clean container to look for contaminants, then get the carb on a clear, well lit work bench to go through it piece by piece and rebuild it to factory spec. In particular you should be checking:
a. The condition of anything rubber.
b. jet sizes, condition, cleanliness
c. Needle height and straightness.
d. Mixture screw setting.
e. Float height.

Clean any passageways you can find with a can of carb cleaner, using the straw nozzle to get in there. Wear eye protection, carb cleaner will get in your eyes and it stings.

[jaffa90]

"""""""""I've done a quick test, by you saying "watch it slowly going back to rest." I think its not right. """"""""""
I agree it`s not right so you need to find out why.
Twisting the throttle cable opens the butterfly valve which causes a vacuum above the diaphragm and should lift it providing it`s not stretched or punctured and is sealed around the edges.
Google c.v. carbs and how they work.

[MarkGoy]

talkToTheHat :

Thanks, I understand your point, but I disagree, I believe the GZ to have a bit more power to give, and i can prove that with the spark plug test.

I'll add CDI unit to "things to check", thank you.

Robby :

1) One of the first things I checked, it is gravity fed and it does work fine.

2) Carb sits secure, yes clips on both ends, both tight, not possible for rubber seal to be folded I always check before attaching clips, I haven't checked for splits while the engine runs I will add that to the list.

3) No contaminants, Ive had the carb off and stripped many times each time I've checked, already been rebuilt to factory specs with suzuki parts.

a) all good, or replaced if otherwise.
b) factory settings jets for this carb, placed during rebuild
c) by needle do you mean jet needle? I assume so again factory set, definitely straight.

d+e ) set to book settings but the book doesn't match the carb, can't work out why. I have also tuned the mix screw just in case. Its my understanding that mix screw only affects low rev range.

I have already cleaned the carb with carb cleaner and pressurized air, and Ive had it cleaned professionally.

jaffa90 :

I understand the basics of how carburetors work, I'm not sure on the exact differences with constant velocity other than on this model I control a butterfly valve rather than the slide bit where the jet needle is...

I'll do more research on the subject.

Thank you, ill be back when I either get stuck or solve the issue (but the bike still runs bad).

//

thanks to all.

[talkToTheHat]

Have you misread my point? I'm fairly sure you do have a problem. 2nd gear should take you to 32 mph, which will read at about 36 on the speedo.

Also check you have the standard count of teeth on the front and rear sprocket, it's fairly common to downgear the GZ in an attempt to get less glacial performance.

Give me a minute and Ill FFT the audio and give you an revs chart
____________________
Bandit. does. everything.

[talkToTheHat]

Ok, something's definitely up. The fundamental frequency of the strongest signal peaks at 125Hz.

For a 4 stroke single,
rpm=f * 60 * 2
because 60 seconds in a minute and 2 revolutions per exhaust pulse

That's 15000rpm which is unlikely

It's likely therefore that we're hearing a pulse once per rotation and you're peaking at 7500rpm.

The engine should be giving you 9000 RPM, which is approximately 32mph in 2nd and 65 in 5th.

Is the phone physically attached to the bike or the rider?
____________________
Bandit. does. everything.

[MarkGoy]

Yes I did miss read, Im sorry I jumped to a conclusion.

I haven't come across these concepts before, I think I understand what your saying but not the "how or why".

I'll take a look into it.

I'm impressed you got that from the video audio.

I literally all cellotaped it to the satnav, so it's juddering and shaking with the bike.

Thank you.

[MarkGoy]

"The fundamental frequency of the strongest signal peaks at 125Hz. "

Do I have this right? at the strongest signal, or clearest? part of the audio the lowest (presumably consistent) frequency you could find was 125hz.

So the rpm its doing at that present time, would be worked out by ..... I think that 125hz comes from 125 explosions per second which can be heard, which would make 7500 explosions per minute.

But as you say, the fuel mix explodes in the engine once every two cycles (explodes, exhaust, intake fuel air mix, compression).

Meaning that 7500 explosions, multiplied by two revolutions per explosion = 15'000 rpm.

Ok, I think i've got what your saying now..

//

One explosion per cycle sounds bad, very bad...

The only thing I can think of is fuel air mix is entering engine when it shouldn't be..

is that a timing issue? but if it was a timing issue, wouldn't the bike not run at all?

I'm a little scared right now..

//

Edit : had a quick think... I don't think it could be timing because the engine runs fine out of gear.

So what's the difference between being in gear and not, I'm thinking resistance to the action of the piston...

The difference in gear ratios would create different resistances to the action of the piston, which would by my top speed in 2nd is roughly 25mph and my top speed in 5th is 48ish...

Unless gear ratios don't work like that...

The normal resistances that come with a gear box would be overcome by the fly wheel...

What if the up stroke of the compression never fully gets there, the spark ignites, there's a very weak explosion... if the piston never gets to t.d.c what happens to timing....

If it just tried that step again, the exhaust gasses would never leave.

But you said one explosion per cycle, which would mean the exhaust is vented and it switches straight back to the fuel air mix step, if possible.

But it must be possible, assuming you are correct in saying the explosion happens once per cycle, the piston becomes stuck between those two steps.

So the fuel air mix it pulls in is being mixed with part of the exhaust gasses, showing it as not enough fuel air mix?

Would that not leave a different residue on the spark plug? Or does exhaust gas cause nothing on the plug..

Currently I'm now thinking, what if... on the down stroke of this 2 stroke action the piston pulls in air fuel, the spark happens part way through, the piston then on the up stroke it vents the exhaust gas? Rinse repeat?

Sorry about the very very long message, my brain is exploding with thoughts and ideas, I've just written down all my thoughts over time.

But if that motion is right, the air/fuel intake valve would be open during the explosion and some of the gasses would travel backwards into the carb, slowing down the flow of air into the engine or at least causing eddies/ mixing with fresh air/fuel. Subsequently causing a pressure drop in the venturi of the carb, dropping the slide... if this is true.... the engine receives less flow of air/fuel and revs slower? Putting itself under that problem rev range? Then as the revs increase the issue would start again...

[talkToTheHat]

[jaffa90]

EEEEEEEERRRRRRRRRRR as my last post as well.