I have just signed up for my first flat track event. Last weekend I watched such an event for the first time and the entire day I regretted not having signed up myself. It is by no means a “professional” event. It is meant for old street bikes older than 1985.

Luckily my 1977 KZ650 falls well within that range. Even though the event is mostly for fun and I could probably ride my bike as it is, I still see it as an opportunity to do a bit of work on the old girl and modify her a bit to suit the event a little better.

That is the entire point of having old bikes – getting to work on them and tinker with them to suit your needs. Now I have a very tangible use case and hence – a goal.


The first issue is tires. And more specifically tires for a 16 inch rear. Most tires for flat track, enduro or cross are 18 or 19 inch. It is so hard to find a good viable option for the rear that I considered changing the rim to 18 inch.

But luckily I found a good alternative. Pirelly makes scrambler/super-motard tires called MT-60RS. It is a road-oriented dual sport tire with a soft compound for extra grip. These are made in 150/80-16. Some way from the 130/90-16 that is original size for the rear, but I recon that by adjusting the rear wheel to be as far back as possible in the swingarm, then it will be possible to fit the wider tire.

Having settled on what seems to be the only option for the rear, the easy option will be to run the same up front. Unfortunately Pirelli does not make the MT-60RS in 19 inch – so no luck there. But the “normal” MT-60 is made in 19 inch so I settled on that as my front tire.


My biggest issue with the KZ650 is the lack of low-end torque. The bike is really powerful once it gets going and produces quite a lot of torque from higher in the rev range. But cruising around at “normal” speeds in low rpm, there is not that much “go”. Or at least not as much as I want.

The easy answer is to say that it is the wrong bike for that need. If low-end torque is such a priority, then buy a twin. And believe me – I have considered that and actually done so – but that is for a later story.

But why would I buy a “better” motorcycle when I can modify the one that I have.

Hence my decision to modify the KZ650 to suit my needs instead of buying a bike that suited a specific need better.

Cam chain

The first change was actually not a modification but more a servicing issue. I have had some noises coming from the top-end of the motor and suspected the cam chain as the culprit. This is no big break through as a lot of old Kawasakis have noisy cam chains. The issue is that I have adjusted the chain tensioner on two occasions and I still did not hear any difference.

But by total coincidence I read an answer on a forum that highlighted the importance of adjusting the chain with either pair of pistons at top dead center. I never adjusted the tensioner like that.

After setting piston 1 at TDC and loosening the bolt for the semi-automatic adjuster to take up the slack I could actually hear it taking up some slack on the chain.

Starting the bike afterwards I was happy to hear that the horrendous rattle at low rpm was gone.

Even more importantly riding the bike revealed that this adjustment had also given more torque down low. The reason for this is that when the chain gets loose the cams are actually “retarded” in their timing. This leads to a loss of low-end power as the cylinder filling and pressure is lowered.

I had looked at advancing the cams from standard to tune the engine more towards low-end torque than high-end power as I never really use the power up top any way. This servicing got me some of the way by at least restoring the engine to its original spec.

I may still end up advancing the cams even more. But for now, this will do.


This is probably the section where I need to let you in on a little secret. My KZ650 is not really a 650. It is actually a 750. Someone mounted barrels and pistons from a 750 giving it a bit more power. And along with the change they also fitted the 34mm CV carbs from the 750.

While I have these carbs dialed in completely and they do offer a smooth ride, I am not a fan of the CV-type carburetor and its vacuum controlled operation. Especially not when I consider racing the bike. There is always a little delay between turning the throttle and the bike accelerating as the vacuum needs to build to open the carburetor. Under normal driving conditions this makes for a smooth relaxed ride, but for racing where you want total control it is not the best solution.

As mentioned above I was also in the hunt for more low-end torque so downsizing the carburetors to deliver more low-end grunt is an obvious choice. I will lose a few top-end ponies, but gain torque and responsiveness in the rep-ranges where I use it the most with my riding habits.

After a bit of research both in terms of compatibility, price and size I decided to look for some 24mm Mikunis, which are standard on the KZ650. Properly tuned I am sure they will deliver exactly the grunt I am after. It will also be easier to buy intake boots, airbox etc.

Now all that is left to do is buy some parts and put in the effort to get the bike ready.

The result

One thing is what you plan to do. Another is what you manage to do.

I got some carburetors of eBay, but once I got them it was very apparent that they had not been operational for a very long time. Both slides and chokes were completely jammed. I used close to a full day with an ultrasonic cleaner to have then back to somewhat normal operation.

Carburetor boots/intake manifolds was easy to order, but finding an airbox for a reasonable price proved more difficult.

All in all I could see the plan slip and decided to wait with the carburetors. I had to make to with the CV-carbs for now. At least they run as good as CV-carbs can.

Fitting the tires ended up throwing a few curve balls or at least the rear tire did. First of all the tube fitted lost the valve once I tried to remove it. As the rim actually have “tubeless” stamped on it I decided to try and get some tubeless valves for it. I found some cheap 90 degree ones that are threaded and clamp together on the rim. This would make it possible for them to be used on the Enkei rims as they are quite thick in their material.

Trying to fit them ended up being a nightmare though. I had to drill out the rims to accompany the valve but as the rim is very thick the thread was almost too short and after two trial fits the thread has disintegrated on the aluminum valve. Oh well you get what you pay for.

Back to plan B – order a tube for it. My friends from the garage also recommended this as we would be running low air pressure on track to enhance grip. Running without a tube could see the tire spinning in the rim. With a tube fitted there is a bit more friction to help combat this.

Fitting the 150/80-16 to the KZ650 rim proved to be too much for my hands. As the tire is wider than recommended I could not manage to keep it compressed and inside the recess in the middle of the rim to allow it to stretch over the final part of the rim. So I ended up taking it to a local tire shop that helped fit it and did not even want to have any money for it. I just love that people still do that. He even had to struggle with his machines, so there is no chance I could have it fitted myself. That was a bit of consolation.

The front tire went on without any issues at all. The old tube did not disintegrate and the tire went smoothly over the rim.

With tires fitted and the plans for changing carbs before the race scrapped I finally had the bike ready for the race. So of course there are a few pictures of it fitted with the new tires. Sorry for the quality as they are taken with my phone and not the Fujifilm camera that I normally use.

Next post will be about raceday/weekend.