I took my low mileage ZX6r to the MoT man the other day and while he passed it with no advisories when we went to load it in the van he mentioned a slight misfire. It was running okish but he was right, at about 4000 RPM there was a noticeable misfire that had to be sorted. My immediate thought was spark plugs – not fun on one of these. Then I remembered, it had new plugs in it back last year and it hasn’t done any miles since. Upon investigation it became obvious that carbs 1 and 2 were over fuelling so they had to come off for further investigation.
What I use to investigate such things is a priming bulb from an outboard engine – it allows me to build pressure in the fuel lines without putting the carbs back on the bike only to find there is still a problem. I take the carb bowls off, put the carbs upside down on the bench and then connect my primer with a couple of bits of tube on it, I can then see if any fuel is getting past the needle valves.
On some carbs – the ones on the ZX included – the valve seats are removable, they have an o ring to seal them to the carb body. It was obvious that my needles were sealing but fuel was seeping out from the needle seat, so I removed and inspected them. In this photo a new o ring is shown top right, the old ones are from carbs 1 and 2. When it’s right it’s surprising how much pressure those needles will hold even with just the weight of the floats holding them down against the fuel pressure. Any seepage and you need to sort it before going any further otherwise you are just wasting time.
Incidentally if this is a really bad case what can happen is that the fuel gets in to the engine and finds it’s way in to the oil. If this has happened on your bike check the oil level, if it looks high drain it and refill, change the oil filter too. I had a Triumph 900 in once where this had happened and the big ends were ruined as a result. You can also get a hydrostatic lock where the fuel sits on top of the piston. When you try and crank it the engine locks up. If that ever happens stop what you are doing, take all the plugs out, stick a rag over the plug holes and crank it over. You will get a fountain of fuel out of each affected cylinder. Let it crank until it’s dry – obviously you need to stop the fuel first so whip the carb(s) off before you do this.
They were not split while in the carbs but they broke when being removed and that – ladies and gentlemen – is a fine example of what ethanol in fuel does to rubber components.
I will now spend a couple of frustrating hours putting the horrible sod back together. Before I get too far I will double check that all carbs hold up against the pressure of the bike’s pump before I fully reassemble it. These bikes are a real pain to work on and it can get frustrating when you spend hours putting one back together only to find you still have a problem.
Here’s a nice shiney new needle valve and seat as an example of one of the removable type. They often have a tiny filter on the fuel feed side which is great but when they cause trouble the problems can be huge or even terminal
The other bit that causes bother is the black tip on the needle valve itself. The smallest particle can cause serious bother here as it only takes a miniscule bit of detritus to cause all manner of mayhem. Have a look to see if there is any sign of wear on it, a jeweller’s loupe or strong magnifying glass is a big help especially for the older and more distinguished gentleman whose eyesight may or may not have been affected by years of intense studying of hard core pornographic material.
That aside, the bike is back together and running perfectly without any hint of misfire or other naughtiness so I went back to the SV650. I wish I hadn’t bothered. I got the indicators done and a couple of other minor bits n bobs then made the fateful error of investigating a fairly significant leak of exhaust gasses from the rear cylinder. What an absolute nightmare. Anybody that tackles this job needs a knighthood, in my opinion. Either that or sectioning under the mental health act. It’s a bastard of a job especially when the captive nut on the header pipe starts spinning so you can’t get the bolt out. It ended up being an angle grinder job and now I have to figure out how to repair the damage I did. I did, however, find the cause of the leak – some lazy fucktard had decided fitting a gasket was too much bother. If I ever find them I will kill them. Very slowly.
My despair was soon diminished by a visit from my mate Grub, who was looking for a CDI unit. He turned up on one of his more famous creations. This bloke builds some seriously cool schmutter.
Article provided by David Powell of Boston Bike Bits.
