Q: My Yamaha YZ125 two-stroke MX bike is really annoying me, after I’ve just spent loads of money on new parts!

Being a two-stroke and due to many reason it recently destroyed a piston and the barrel coating. So I bought a new piston and had the barrel plated again, all using genuine parts. I did notice there was some damage to the cylinder head (little indentations around the combustion chamber), which I cleaned up nicely.

So after I rebuilt the bike I took if for a ride and it ran hot and sounded ‘funny’ at high rpm – the jets are the same as I used before. So I stopped the bike and haven’t run it since, because I just don’t want to spend any more on an old MX bike.

So, do you guys have any idea what I could have done wrong, or anything else, before I throw it away forever?

Martin.

A: I’d say the problem lies within the cylinder head that you described you, ‘cleaned up’. The small indentations will glow red-hot around the edges when the motor is up to operating temperature. This will cause detonation problems, which is probably the noise you’re hearing at high rpm levels. If you keep riding it will eventually destroy the motor, melting the piston and throwing hot metal all over the crankshaft- then you might as well throw it away if it’s indeed that old.

The only way to cure this is to take off the head and get an engineer to machine the damage out and, machine in the correct angle for the squish band (normally 7-degrees). If you have to take off too much material to achieve this you might have to machine the top of the barrel as well to compensate, to return to the correct and specified compression ratio. Not an easy job at all, so be careful and choose the right people to attempt it please.

Q: I keep reading about closed-cartridge and open-cartridge forks found on modern day motorbikes. So, could you please take a few moments to explain what the difference is and why the closed-cartridge version is supposedly much better than the other?

Thanks for a great site as well, the best we have in SA by far.

Chris

A: Open cartridge forks have basically the normal fork internals we’re all used to seeing. The damping rod sits in the oil that is put into the fork tube when you have a service for example, which most people can do in their own garage.

But now, as you say, we have closed-cartridge suspension used by all Moto GP and World Superbike machines and indeed top of the range off –road motorbikes as well. This is in effect a sealed unit, set at the factory that drops inside the fork tube and cannot be serviced or messed with easily. Then separate oil is placed around the outside like a conventional fork, so there’s basically now two parts to the fork.

This prevents foaming and air contamination and keeps the damping process constant at all times, which is what racing demands of course. It basically makes the front fork now act in a similar fashion to the rear shock and is much more effective. The down side is that these types of forks are more expensive to produce. Hope that clarifies things, Chris?

Thanks for the support and comments as well.

Q: I’m the happy owner of a 1998 Yamaha R1, one of the first ones with good old carbs instead of fuel-injection. It has now covered 63,000km and gets serviced at the correct intervals. But now I have a problem…

Every time I ride the bike it starts to smell of petrol and begins to splutter and misfire. When I take out the plugs two of them (1 and 3) are black and sooty. I changed the plugs and it’s OK for a while but then the same thing happens again. I’ve had the carbs taken off and cleaned but the problem always returns.

So I thought I’d ask you guys to hopefully send me in the right direction and thanks for a great site, by far the best we have in SA.

Alan Comb

A: I would say the problem lies with the needles and/or seats that allow the fuel to pass into the float bowl, especially with the amount of kilometers the bike has covered. You should also make sure the float heights are set correctly, which is very important. These settings can easily be found.

You can find carb kits that will replace and rebuild these parts on the Internet, which might be the best option here? They’re not too expensive and just make sure a competent person installs them. Then I’m sure everything in the fueling department will be fine once more.

Q: Hi there, I’m in need of some professional assistance, obviously?

My K3 Gixer (GSX-R) will not rev past 9000 rpm under load on some cold days. This is an intermittent problem, which confuses me. The bike has done 73,000 km and it’s been well taken care of.

The timing chain and tensioner were replaced around 60,000 km. The spark plugs are iridium and there’s no back firing. The oil is a synthetic 10W40 and clean, as are the oil and air filters. It has an Akrapovic full system and the exhaust valve is set to be always open (disconnected).

It starts easily and revs to redline through the gears (most of the time), when it doesn't there is no sputtering or anything that would indicate ignition issues. It just revs cleanly until 9000 rpm and then seems to starve for fuel but there are no kinked pipes and the fuel filters are clean. The petrol tank was refurbished around 25,000km because the bike stood for a long time and there is no visible contamination. Have you guys experienced this before and, if so what can you advise me to do?

Thanks.

Steve.

A: We would suggest that this problem, because we’ve had it before, is the fuel pump accumulator, which is probably partially blocked or full of various contaminants. Or maybe a new one is needed due to the mileage, hopefully not?

But first, the entire fuel pump can actually be taken apart by a very competent mechanic, but please be extra careful not to damage the intricate parts whilst taken apart or rebuilding!

You can buy a complete new pump but you’ll probably be horrified at the price, hence the advise to strip and clean it first. Ask the price from a Suzuki dealer and I’d imagine that part would possibly be around 7-8k!