The 'build thread' of a Kawasaki Z250 C, a total beginner's point of view...


New Member
Well, just an update on here.

Bike started, with lots of vibration, think the throttle was stuck (or maybe not) but the result was the bike revving up to 6k with no intention of stopping...and with that came smoke from the right hand side (same as spark plug) and another burning smell...

I still haven't worked out how to post a video properly yet, so will do it when I work it out. Have tried over on the 1800 section.

Will start to read up on these symptoms and see if I can get to the bottom of it.

Pretty happy still, though it seems like its still quiet far from being road worthy.


New Member
It runs!

So, it runs! No diaphragm issue after all, but sounds like carb/jetting issue now.

Had a friend over today to help out and test ride the bike. We found the smoke I was complaining of earlier to be coming from various parts of the exhaust, starting at the manifold. This, coupled with some very clunkly engine sounds - literally like things falling apart inside, led to a load more oil - the prescribed 1.4L was definitely not enough. After a couple more tries, the bike sounded a lot better, and stopped smoking from the manifold - now it smokes white from the exhaust, which, according to the manual means there's too much oil, but for now, I think that's preferable.

The idle with choke on is still incredibly high - around 4000 revs, but not as bad as before. The down side is, without the choke on, the bike struggled to idle. Got it there after it warmed up a bit, but it wasn't consistent and seemed to struggle.

My friend was able to ride it around the block a couple times and complained of the bike just totally loosing power whenever he tried turning the choke off. To me, this sounds like the bike is struggling with the pod filter I've fitted and the extra air that is bringing in. Even with the re-jetting I've done, using the sizing I found on KZrider (130 primary and 70 secondary) it sounds like its not quiet there. Will have to read up on jetting and carb tuning and see what can be done. Any tips?

Meanwhile the vibrations are still epic.

But we're on the right track.

Let me know if you spot anything in the vids that might identify more things.

Oh and here's the plug chop results - rich.



New Member
I've been reading up on how carbs work - these have been a great source of information: - Too bad its missing the section on High Speed and power systems

and a 1980's Cycle World article called "In search of a Free Lunch" -

So I'm going to just think through what I understand, as there are some things which don't quiet make sense to me yet. Bear with me.

1. Carburetors operate on the difference in air pressure, with the low pressure spaces forming vacuums which suck at high pressure spaces.

2. Initially, the low air pressure formed in the carb throat/bore is caused by the intake of the engine, or the downward motion of the piston.

3. This low pressure affects the fuel in the float bowl and draws it through jets, starting the whole process.

4. This also relies on the Venturi Principle, which states that when an air passage narrows, moving air flows faster, making it an even lower pressure...

Now, in the context of jetting and pods:

1. Pods provides less restriction than stock air boxes, and more air gets to the carb bore. (More air does not equal lower pressure does it? So I am thinking that the issue why more air enters the carb is because there is more surface area for it to enter from, thus when the butterfly valve opens (controlled by the throttle) then more air is coming in...ok that seems to make sense.)

2. So then, shouldn't more air create a higher atmospheric pressure than before? Or is the low pressure even greater? I think what I'm asking is, does the engine suck in air at a greater rate because there is more air, or if the rate that air is sucked by the downward movement of the piston remains unchanged, which in my mind, means you will now have a higher low pressure than if you had an airbox, as there is now extra air?

3. Leaving that confusion alone:
lean - not enough fuel, too much air
rich - too much fuel, not enough air
It sounds like the mix of air and fuel occurs in the same volume - in the jet passages? so more air = a more air and hence less fuel. This is why carbs usually run lean. Hence jets have to be increased to allow more fuel to pass through. The aim of jetting therefore, would be to try to replicate the fuel/air ratio obtained when running the stock air box, at the different systems.

4. Therefore, if I'm running rich - I need to use smaller jets to decrease fuel flow. If it is the pilot system, I will also screw out the pilot screw.

5. If I'm running lean - I will need to use bigger jets to increase fuel flow. With the pilot system, I will be screwing in the pilot screw to also decrease air.

Symptoms of a rich running carb:
- boggy/sluggish acceleration
- black spark plugs

Symptoms of lean running carb:
- backfires when throttle is closing
- white/powdery plugs
- requires excessive choke to start

So at the end of all this, I am slightly confused because:

1. My spark plug shows I'm running rich.
2. My bike does not run without the choke on - it will stall

I'm thinking perhaps I'm not getting full power to my coils perhaps? That would explain the plugs.

The combination I'm running of the 130 main and 70 secondary seems to have worked for a couple guys at kzrider, so I'm not sure - I understand that jetting is particular to bikes and its not a one size fits all thing.

So, any advice would be great - and definitely let me know of errors in my thought process.


New Member
Ok still haven't had the chance to check voltage to see if its the coils, but did try adjusting the pilot screw to better its running.

In the process, I dropped it and bent the tip, so put in a pilot screw which came with the kz400 carb rebuild kit. Its a slightly longer screw from the thread and above, but the actual pointy bit and all is roughly the same size.

Started very easily, and idled well at first. Could get power to the wheels without choke at less than 1/8 throttle - getting it to roll along the verandah, but when I revved it a bit harder, it stalled with a very loud metal clang.

Got more difficult to start after that, i tried adjusting the pilot screw a bit while it was idling, but as I was doing so, it slowly stalled and white smoke came out from the pod filter.

And then starting getting more difficult from there on...

Until it pretty much refuses to start now.

The metal clang does keep happening when it stalls - I tried to record the sound, though its not as abrupt or loud in the moment I captured. Is it just the clutch?

The spark plug is fouled pretty bad. I'm thinking this might be the reason for the starting issues, though the metal sounds I'm hearing gets me a bit worried that there may be other issues internally as well.


New Member
The sound is nothing unusual. The metalic 'clank' is from the starter clutch engaging. As the engine was stopping, the last spark sent the crankshaft rotating backwards. The starter clutch engaged, and the engine stopped. You have had it running at high revs before, so the engine is at least in fair shape. You just need to get the mixture right. Keep cleaning and checking the spark plug.

- Dwarf


New Member
Hi Dwarf,

So it looks like the spark plug fouling was from excessive engine oil levels. This is what pointed to that:

I decided to drain the oil and start again, and while I was at it, I redid the float level as well, as I was never sure of it, despite many attempts. So, after having the correct oil level (dead on between the two lines) and having the fuel sit 1mm below the float bowl gasket (manual says 2.5mm +/- 1mm, but I think its close enough) it starts incredibly easily, even with the fouled spark plug. Hear the starter motor go and its running.

Idles well around the 1.2k/800 mark - starts off around the 1.2 and slowly drops lower. First time round I let it idle for a couple minutes and it slowly revved slower and slower until it stalled. Second time around I let it idle and revved it when it started dipping to about 800 and it would pop back up to 1.2k. I'm thinking with some tweaking of the pilot screw that might be fixed?

There was still white smoke coming from the exhausts, which surprised me as I know the oil levels are good - dead on centre between the two lines. So perhaps it just needs to burn off. Only let it idle for about 5-10 minutes all up.

What a big differences these little things make. And how ironic its taken this long, essentially fidgeting with the same several elements.


Well-Known Member
Hi Sham, That's what bikes are all about. Things need to be spot on to work. Well done. You have learbned a lot so far.


New Member
Thanks Teazer,

Looks like they're also about patience...

After last night's goodness, all has reverted back to normal in the non working sense.

I started the bike up in the morning and let it idle, just to see how it would perform. Started off great - it always seems to idle a bit higher at the beginning before quickly coming down to just below 1k. I messed around with the idle screw to get it to the 1.3k mark the manual recommends, and it was happy with that. All the meanwhile, white smoke continued to come out from the exhaust.

Revved it a bit and noticed 'dirty' smoke coming from the exhaust around the 3k mark. Not sure if its brown or black but lots of it, after which, the bike started to want to die on itself, and while I could keep it running by revving it more, it got harder and harder until it died.

Pulled the spark plug out again and noticed it was carbon fouled. Reasons for this seems to be the following:
1. rich mixture
2. weak ignition
3. poor compression

I tested my compression and got 110 PSI, which seems to be what I'm getting these days with a warm engine.

Cleaned the spark plug (just brushing some of the carbon off) and tried starting again. Bike did not want to start and I believe started backfiring. I'm calling it backfiring because I saw fuel mist shoot out from the jets holes in the carb, on the intake end, after removing the pod filter. This action produces a far amount of force and even managed to shift the carb loose from the manifold...

So reasons for backfiring according to a quick search on the internet are:
1. lean mixture
2. incorrect ignition timing

So continual issues which I am now facing are:
1. Bike idles fine, but has a tendency to want to die after giving it substantial throttle and revving past the 3k mark. As I am running the pod filter this most likely has to do with my jetting (running 130 main/68 secondary - originally 98 main/68 secondary)
2. White smoke, initially thought to be a result of overly high levels of oil, but after running it for a good 10 minutes or so on idle, the smoke is still there. May be pointing to issues with piston rings or valve seals?
3. Starting is unreliable and gets more and more difficult after each attempt.

Having seen the big difference the correct oil levels made to starting the bike, and seeing the continual white smoke, and linking that with the difficulty in starting (but not the backfiring, which is the main issue now) I'm thinking there may be an issue with the oil seals? I have read that I should be able to read the oil levels on the bike after letting it cool down for a couple minutes, but my oil levels never seem to return to the gauge very quickly - could this be pointing to an issue to how the oil may be impeded from returning back down?

Anyways, I think I should look into the backfiring first. Will try changing the secondary jet back to the 70 (one size up) and try to check the ignition timing. However, the manual says I need a strobe light, so I'll have to look into it. It also confuses me why this might happen now, when it was starting fine before. It definitely isn't the spark plug, as even though its fouled, I can see a clear spark from it.

Well, Swivel sent me some good motivation. A kz250 featured on pipeburn, from an Aussie builder!



New Member
Thinking its the fouled plug...makes more sense why it would work and then suddenly not work...


New Member
Woohoo - super happy. No more plug fouling! It wasn't the main or secondary jet, the valve clearance or anything like that - it was just a loose pilot jet!

Checked the power going to ignition coil as well and it was surprisingly good - battery was reading 12.5 or so, coil was reading at 12.4. Took a while to get a reading, grounding it to the frame was giving me readings of 0, but the front forks did the job.

Road around the driveway a bit and it was great - bike was very consistent, didn't want to stall or anything like before. No white smoke or brown/black smoke, even at the higher ends of the revs.

However, there are new issues - noticed I am getting a bit of an oil leak, and the cam chain tensioner casing (?) is cracked. Not sure when that happened, but it was definitely after I checked the crankshaft timing a couple months ago. Will have to see if I can get a new part. Does the crack indicate that there are other issues I need to be aware of?

But then I got greedy and thought I'd take off the centre stand seeing as I can't use it. Bad move. It turned out to be surprisingly difficult and I needed to remove the exhaust as well to slide the metal tube out to free the it. Ended up stripping the hex nut at the exhaust mount near the rear tire, dissembling the exhaust mount only to find the spring was too difficult to remove, tried to put it back together and realise the spring was then too strong and couldn't quiet get the mount back in place. On top of that, I cut the centre stand spring, so now it drags along the ground. So - no more riding until I can get it off. I may have to just grind off the thing. Bah.

But that aside, pretty happy. I can now focus on other aspects of this baby.

Thanks for all the help and encouragement.
Video of the bike idling.


New Member
Oh and for future reference, for anyone interested, I'm running a Keihin CV32 (stock carb for the z250 D) with a pod filter. Main jet = 125, replacing the 98. Secondary jet = 68, unchanged.


New Member
Thanks guys, definitely been a steep learning curve, but the rewards are pretty satisfying.

Looks like riding with a broken cam chain tensioner is a big no no, so after the brief rendezvous, I'll have to let it sit for a while until I get some parts.

Guess it will be a good time to have a look at the other things, and see where I want to take this.


New Member
Finally managed to get the exhaust off and hence the centre stand. Now I need a new flanged nut before reinstalling the exhaust. Would be interested in maybe putting a new one on some time in the future, as this one is rusted pretty bad and again, remnants of this red gasket material which shows up everywhere on the bike can be found here as well.

Parts have been ordered, including a complete gasket and o ring set, which I hope will solve the compression issues. The range given in the manual was something along the lines of 129 - 180 PSI, so I am definitely a bit low.

Getting parts for a petcock rebuild to hopefully fix the leaks, as well as the new cam chain tensioner. Here are pictures of it dissaseembled. I'm wondering if the missing washer from the top screw was what led to the crack? Maybe uneven distribution of pressure or something?

Its a long wait now, so I'm starting to think about how I want the seat and possible relocation of electronics in relation to it.

This series by Kott Motorcycles was pretty good just to get an idea of how one goes about making a seat from scratch. I believe there are many other ways, but it seems doable if you had the right tools.

And a pic of the bike's profile, for me to start thinking about possible lines for the seat.



New Member
No bike update, but I picked this up yesterday amongst some trash.

I really like it, but I know its not really compliant anymore in terms of surpassing the recommended 5 year life span.

Its a AGV CX5000, from May 1988.

Cleaned it up a bit - the soft foam was removed and the lining ripped, but otherwise its pretty neat i think.

Good side project to restore it while I wait for parts? Be nice to have just to look at and hang up. Shame about retro helmets not being safe...


New Member
I finally got the chance to do the leak test on the bike, to try to get to the bottom of my low compression readings. Getting the bike to TDC again was a pain just in terms of the losing all that oil from the alternator, and how without the cam chain tensioner it was much harder to turn. At some points it even felt like it was stuck and I had to use quiet a bit of force. Now that I think about it, I was suppose to make sure the chain was tight, but without the tensioner I suppose it wasn't was it...bad?

Using a very small displacement air compressor with a air brush nozzle attachment, I shoved that into the spark plug hole and tried to listen for a leak.

A leak coming from the intake valve = an issue with the intake valve
A leak coming from the exhaust valve = an issue with the exhaust valve
A leak from the crankcase vent = issue with piston, rings and/or bore
A leak from the head gasket = an issue with the head gasket

...and to be honest, I'm not sure I heard a leak at all? The loudest "sound" was coming from the spark plug hole itself where the air brush attachment was. And as I tried to listen for a leak, that was really the only sound I could hear, though perhaps around the head gasket it got slightly louder? But don't think I heard anything from the intake, exhaust valve or crankcase vent. Which hopefully means any compression "issues" I might be having are either from a faulty compression tester, or can be fixed with a gasket kit anyways, which I'm waiting for.

So overall, if I haven't done any damage by turning the crankshaft without the chain being tense, then its all good for now and continue on waiting for these parts...


New Member
Parts arrived! Very surprised at how quickly they came, but I ordered the wrong Tensioner Spring!!! Swapped a 1 for a 0 and now I have an even longer spring than my original...

So I'm thinking I'll probably wait to see if I can get a new one before reinstalling the cam chain tensioner? What effect does a longer spring have on the actual cam chain tensioner? Seeing as my original cam chain tensioner is in pieces, and I had the original spring in there at the time which is only 2mm longer than the service limit, is it likely that this was the issue?



Well-Known Member
From your description, it sounds like there might be a leak at the head gasket. Those "tight spots" may be the cam chain slipped and valve timing has moved. Check that before you button it all up.

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