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Tuning your XJ   |   XJ Electrical diagrams.   |   Ignition Problems
Tuning your XJ
XJ's can exhibit some strange behavior if the carburetors are out of tune.
Some of these are;

*When hot the engine has an uneven (hunting) idle.
*When power is first applied the engine gives one or two coughs before the power comes on
* Unable to get the bike to idle with intermittent problems with engine racing up in revs with no control.

These symptoms can also be caused by vacuum leaks caused by either cracks the rubber intake boots between the carbs and cylinders or at the gaskets between the boots and the head.
Cracked boots can be repaired by cleaning out the cracked areas with a solvent that leaves no residue and then rubbing in a petrol safe silicone rubber or similar sealant.

I will assume the bike is original in the air cleaner jetting and exhaust system. This method would work on any setup but the jetting has to be correct for the system parts used or you might have problems.

It is a long process and you will need some specialized tools but these tools will for the most part be able to be used on any petrol engine, save you a fortune in workshop costs and should last you a lifetime.
The tools are not that expensive in comparison to paying for a shop to do it, (about two or three shop tune-ups)
You will also get a great feeling of satisfaction because you did it yourself. My XJ's idle as smooth as glass at 1200 rpm without a hint of hunting (running rich chugging sound) and I get about 16-17KM/L from my carbureted bike and 20km/l from my XJ750-D Fuel Injection.

Firstly I found that carb mixture relates to vacuum directly, so if the mixture screws at the front of each carb near the choke - enrichment valve are not exactly matched on each carb you will not get the best result from synchronizing the carb linkages.
Also being able to work with gauge readings for all 4 carbs at once is a great advantage.

If you choose to do only the carb vacuum sycronisation follow instructions with the ** However I would recommend the whole job as the result is much better.

It is best, but not essential if you know for sure everything is OK, to fit new plugs, clean out the carbs, lube the butterfly shafts and set the float levels. Also adjust the valves before you do these adjustments.

You will need;

**A set of vacuum gauges or carb sticks allowing one for each carb, for an XJ you need four. One will do ...I will explain later.

**A large room fan to blow at your motor to keep it cool while you work. The more air and workshop ventilation the better. The exhaust fumes are bad for you and will upset readings you are taking, also you don't want to overheat your bike or you will do it lots of damage.

An exhaust gas analyzer or a Gunson Color Tune plug. Again four of these would be nice but I sure can't afford that. You can either test for CO (carbon monoxide) at the exhaust ports or use the color tune plugs The color-tune plug is a spark plug with a window in it so you can see the colour of the flame in the cylinder while the engine is running. The flame color changes relating to mixture.
If you use an analyzer make sure the metal sniffer pipe it uses will go through a 6mm bolt hole. These can often be purchased secondhand as obsolete models from automotive workshops. Now days they will be using 4 or 5 gas analyzers where for the purposes I outline here one that will do CO (carbon monoxide) only will do. An analyzer that can also handle HC (hydrocarbon) is handy and will show up carb faults that CO will not show.


**A YICS tool if your bike has the YICS port in the cylinder head.

This you can make, see........


Personally I found the result was better if I didn't use the YICS tool but
you may want to experiment for yourselves.

A digital temperature gauge with a probe you can clamp to the cylinder
This is not essential but can be of use.

A long slim small bladed screw driver. The mixture and sync screws for the center
two carbs are hard to get to.

A 10 mm, preferably ratchet wrench.

A heat source such as a gas plant or Primus torch with a big hot flame.

An electric drill with a 5mm drill and a 6mm thread tap. These hopefully
will not be needed.

**Either a small fuel supply tank of 4 litres or so, or a long fuel line that you can run to the bike tank while it is sitting on a work bench. Make sure the supply is kept above the carbs so you get a solid fuel flow.

**Ok lets get to business.... set up all the tools in your work area ready to go...speed is important because the work has to be done while the bike is hot and at a reasonably constant temperature.

You will need to remove the four 6mm bolts in the bottom of each header pipe. These bolts are where you "sniff" the exhaust fumes with the gas analyzer. They are normally rusted in so do them first while the engine is cold.
Heat the bolt heads until they glow red and carefully crack them free with the 10 mm wrench.
I found when they cracked free they came out easy but the odd one will possibly break off. if they do you will need to drill out the remaining bolt, re-tap the hole and fit a new high tensile bolt.

**Also I found that the four YICS galleries that link with the main YICS port in the head have a point that blocks with carbon. Remove the caps from the ends of the YICS port. Using a length of steel 1/16" welding wire with a bend at a right angle of about 5mm you can feel along the port with the point of the hook pointing upwards and will find a small hole in the top of the port, next to each carb. Use the hook to poke the four holes clear. If its badly blocked, slide the thin screwdriver in under the wire to apply more force to the wire at the blockage.
Once all four holes are clear use the wire bent double with some rag hooked in the end as a pull-through to clean out the YICS port of rubbish. Some solvent on the rag will help to get it really clean. Refit the YICS plugs.

**Go for a ride on the bike and get it well warmed up and return it to the work area and put the bike on the main stand. You will need to setup the vacuum gauges, connect each one in turn to the number four spigot on the rubber connector between the carbs and cylinder head. Start the bike and adjust the needle valve fitted in each gauge until you get a near steady needle on the gauge then zero the gauge to a known value (anything is OK as long as you remember the reading) make sure all the gauges read the same. It is OK if the needles wobble about 1/4" or less but be careful not to restrict the gauges too much or they won't respond well.

Turn the bike off.

**Working fast, remove the tank.Set up the temporary fuel supply and remove the four header bolts.

Turn on and warm up the gas analyzer (if used) or fit the color tune plugs. You can use one plug and one vacuum gauge but you will have to swap them between cylinders a lot and it will not be as accurate.

**Fit a vacuum gauge to each of the four vacuum ports.
Remove one of the YICS port caps and insert your YICS tool (IMHO optional).

**Set up the fan close to the center front of the engine and turn it on as fast as it will go.

**Start the engine and let it re-warm for a minute or so.

**Set the idle for 1400 RPM exactly.

Insert the analyzer into number one exhaust port 6mm bolt hole and check the reading. Normal is around 2.75% CO (carbon monoxide). Adjust the number 1 carb mixture screw until you get 2.75% CO. (note the adjustment is really
fine so be careful). Repeat for all four cylinders.

**Now check the vacuum gauges and see how well balanced they are. If they are not the same start with number 1 and 2 carb. Adjust the screw between the two carbs on the butterfly shaft and set it so the same reading is on each gauge for one and two carb.

**Then adjust the number 3 and 4 carb the same way

**Now adjust the two pairs of carbs with the screw between number 2 and 3 carb to give the same readings over all 4 gauges.

**You may need to redo the settings sequence to fine tune the readings.

**At this point the engine may be getting hot so check it and maybe take a break to let it cool. If it starts to smoke around the back of the engine its far too hot SHUT IT DOWN!! and leave the fan on.

When the temperature of the engine is back to normal recheck the CO readings again.
I found that the CO shifted the vacuum readings and visa-versa so you will probably need to go over each setting sequence a few times and you will find the readings will all balance out. As this happens you will also notice the engine start to run dramatically smoother.

**You may need to readjust the idle speed to keep it at exactly 1400 rpm while you work. It is important that the gauges are always checked with the engine at the same revs.

If you are like me your smile will get larger as the bike runs smoother :-))

Regarding the temperature gauge, if you fit it before you warm up the bike and check it while you are riding you can get an idea of the normal running temperature and then keep an eye on it while you work. This is not important
but could prevent overheating and the tests should be done at as near normal running temperature as possible.

Well that's about it. Put everything back together and try out your handy work. Look for flat spots and check the color of your spark plugs after some running. They should be gray around the electrodes. If they are black try for a CO reading around 2.25% and if they are white try CO around 3.25%.

If you have flat spots try a richer mixture IE 3.25%.

Note a small adjustment on all these settings makes a lot of difference so be careful and good luck!