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kry1300

Message 1 of 3, Sep 4, 2011

This essay on Engine tuning for the Yamaha xjr 12/1300 relates primarily to the carburetted bikes, and more specifically the 36mm original mikuni CV carb versions that come with the pre 04 (i think) bikes, although the same tuning philosophies can be applied to both the 37mm carbed and EFI bikes the specifics on jetting with the later carbs is quite different and i dont know enough about the specific arrangement of the EMS on the EFI bikes to comment.

It’s generally accepted among xjr enthusiasts across the globe that 125rwhp is readily achievable with a well tuned and fairly standardised and well known set of external engine mods, which equates to a rewarding 25% increase in peak power over the stock bike (about 100hp) and noticeable gains in mid range torque and throttle response for a minimal (comparatively) monetary outlay, coupled with a noticeable and predictable decrease in fuel economy.

These mods are basically as follows and will be termed for the purpose of this article as the standard tune up; 1, a freer flowing exhaust (similar results can be gained from both a modified standard exhaust system or a complete after market one), 2, the airbox mod (consisting of the airbox inlet bellmouth cover being removed and the tube [bog roll] that goes up the inside of the a/c being removed while leaving the bellmouth itself intact to locate the a/c, and an after market reusable a/c such as the k&n unit or other brands), 3, a stage 1 or 2 dynojet (or other brand) carb kit dyno tuned to the final spec of the bike, 4, the 36y carb to engine inlet trumpets from the yamaha fj 11/1200 bikes (being the predecessors of, and engine donors for, the xjr) and, 5, the 4 degree static ignition timing advance mod in either, buying an after market timing plate, or modifying the original one, forms.

More gains are claimed from some by going to individual air filters and the stage 3 dynojet kit (the main difference being that this mod requires a change to the pilot air jet to compensate for the loss of vacuum to the carbs associated with the use of an airbox), but i personally haven’t come across power figures that back these claims up without bigger cams or internal engine mods, all of which equate to considerably more expense and engine work than the standard tune up.
The highest output figures ive heard of for a xjr with individuals added to the standard tune up and backed up with dyno figures is about 130rwhp, but this bike had a fully ported head as well, suggesting that there’s either not a lot to be gained from the individuals and head porting or that perhaps this particular home done porting job was poorly executed (the worst thing that can be done when porting heads is to remove to much material, simple match porting will provide at least 75% of the total gains available from extensive porting).

In this case (to me at least) the 5hp gain is nowhere near as cost effective as the standard tune up (personally i don’t think id bother with individuals unless i was going to bigger bore flat slide carbs, but this is also a significant extra cost and i wouldn’t bother unless i was bombing the engine for full on drag use for instance), ultimately this is the beauty of the standard tune up, being that it is quite easily achieved by someone who is reasonably mechanically minded (or at least willing to learn) without any major engine overhaul procedures required.

Ultimately I’m of the impression that 140rwhp is readily achievable by adding a decent set of high lift cams, 39-42mm flat slide carbs with individuals and the otherwise internally unmodified engine of the basic standard tune up (which is basically what is done with one of the classes of the legends racing cars in the US), add 11 or 12/1 (or higher) compression (stock = 9.6/1), more powerful spark from dyna coils or full ignition system, good quality porting, balancing and blueprinting, maybe a big bore kit (but these can sometimes greatly reduce reliability with the xjr) and race fuel and theoretically figures up to 200hp are probable from what would be a largely standard looking naturally aspirated xjr engine that wouldn’t look out of place in a registered xjr, although engine life would probably drop considerably but people chasing hp figures like this aren’t generally too concerned with things like engine life, excess heat and fuel economy.

Personally the 125rwhp of the standard tune up is plenty for me and it’s quite easily achieved without any necessity for pulling the engine itself out of the bike, apart or with any internal engine mods.
But i think this hp figure can also be achieved cheaper and without advancing the ignition or using a specific dynojet (or any other brand) jet kit as per the standard tune up, while also improving cruising fuel economy and power delivery over the stock bike.
Firstly because advancing the static ignition timing doesn’t affect peak hp figures (mainly midrange torque and throttle response), which are achieved basically from the main jet and your engines ability to move more gases at a higher rpm, which is improved with the a/f airbox mod, 36y’s and a freer flowing exhaust.
And secondly because increasing engine performance increases fuel consumption, but only when your using the engine in the rev range of the increased performance, so if you increase performance across the board (with the static ignition advance and jet increase in both the mid range and top end) then you have increased fuel consumption across the rev range of the engine.

So what im saying is that in terms of performance and economy on most modern larger capacity (particularly inline 4 cyl) jap bikes its possible to have your cake and eat it too, based mainly on how you ride the bike and how you tune it while otherwise using what are well known cheap performance mods.

The following basically explains how and why ive come to this conclusion.

My 01 xjr runs 11.6 quarter miles (12.0 stock bike) with 16/38 gearing, pulls hard to 9000rpm (about 7500 stock), keeps up with most things on the road (to a point obviously) and will get 400k’s between refills (all using 91ron fuel) at open road speed limits with 18/38, 20/38 and 20/37 gearings (interestingly there doesn’t appear to be a dramatic increase in fuel economy with the ratio’s over 18/38, there’s some but its not massive).
Now my bike doesn’t have the ignition advancer mod or a dyno tuned dynojet jet kit and runs on 91ron fuel, yet its slightly quicker than an 05 xjr that ive compared with at the drags that has the full standard tune up arrangement while using 98ron fuel and a similar gear ratio to my 16/38 that i used at the drags (i think he was using 17/41, very close).
We both ran 12.0’s on our first runs, we were doing 11.7’s after a couple more runs and both ended up with best times in the 11.6’s for the night (indicating that rider ability in this context is pretty similar), the difference being that my best was an 11.62 and his was and 11.69, clearly indicating that my bike has just as much power and performance potential as his whilst being less modified.

My bike came with a remus full 4into1 exhaust system (can be exy but i reckon similar flow can be achieved with known mods to the standard cans or any other after market exhaust), bored out standard inlets (similar dimensions to 36y), a partial airbox mod including k&n a/f, 100 main jets, the idle air bleed set at 2 ½ turns and otherwise was and remains engine internals wise in stock bike condition.
The airbox now has the inlet bell mouth cover removed and the bog roll cut out from inside the a/f as per the standard tune up, the ignition is unmodified apart from the colder 9 heat range plugs i use instead of the standard 8’s.
the carbs (original 36mm) currently run 112 main jets (stock 95), the stock jet needle set in the leanest position (clip highest instead of the stock middle of the three positions) and the idle air bleed adjustment screws set at 3 turns instead of the stock bikes 1 ½, this is how i ran the engine at the drags when getting the 11.62 et.

So from the carb settings you can see that i run it richer on the main jet (which everyone does), which richens it up in the higher rpm for wide open throttle work giving more peak power, and leaner on the partial throttle and idle air metering jets, which most of the cruising and around town riding is done with, which gives me my touring fuel economy.
In addition this tuning provides a more progressive (less abrupt) power delivery and throttle response both from a closed throttle and a partial throttle positioning, making it an easier bike to dawdle through car parks with (compared to both the stock bike and the standard tune up) and in transition from deceleration to acceleration, and masks (in the opposite way to the standard tune up) a known and commonly detected flat spot in the mid range of the power delivery thought be designed around emissions testing requirement of the time.

Now i “probably” wouldn’t be able to lean out the jet needle and idle circuit safely (without the tune up being to lean) if i advanced the ignition timing and ran the hotter (stock 8 heat range) plugs, because these cause the engine to flow more requiring the mixture richening associated with the dynojet kits of the standard tune up and the associated increase in mid range power and throttle response (particularly when combined with the other standard tune up mods), even at cruising rpm’s my fuel mixture settings may be too lean with all these mods combined.
Where as the stock bike settings on ignition and jets are already quite generous without the colder plug so using the 9 plug allows me to safely lean the engine out in the lower rpm range for the purpose of fuel economy while compensating with the richer main jet whenever the throttle is opened wide.
Incidentally the exhaust pipe runs a dry black with my settings (and has never gone a lighter colour) and the plugs are dark brown with this tune up so there is no indication of this being a too lean mixture, infact im sure i could use the stock hotter (8) plugs safely with my settings but this would also equate to a slight increase in fuel usage, and of course the colder plug also has the effect of richening up the mixture a little in wide open throttle high rpm use compensating for not using as large a main jet as i possibly could.

The main reason that i tune (and can tune) the carbs this way is because the stock electronic ignition is already well suited to it and has an inbuilt extra advance system anyway run via the throttle position switch (tps). I found this out after talking to an auto electrician about what the tps does on the xjr.
Basically the ignition monitors the difference between the tps and engine speed and when a pre set disparity is detected it momentarily engages (triggers) an alternative (more aggressive) ignition advance curve (aiding acceleration) until that disparity no longer exists and then immediately returns to the standard curve, this is actually very common technology with electronic ignitions which has been taken to a higher level with modern total engine management systems (ems), to me at least this made the ignition advance mod unnecessary and allowed me to pursue a more personalised engine tune up that to date has been successful.

There is also the potential for the trigger point with the tps to be tuned (adjusted) advantageously but this is not something that i have looked into any further as of yet, maybe ill look into it next year after ive finished my current drag tuning plans, the tps will probably be the last thing that i try to tune with the engine.
Im planning to fit genuine 36y’s, as big of a main jet as it’ll run on (at least 120) and run the standard 8 plugs (or possibly even 7’s) over some drag strip tuning ill be doing this summer (ultimately im trying to crack a 10 with some other bike mods to go with these), then when ive maximised this tuning and finished at the drags for this year ill put the road gearing back on it to see how much difference it makes on the road (compared to now) and to see if ill need to wind the tune up back at all to maintain my current cruising economy. If i do have to wind it back i may fit a stock (unbored) set of inlet trumpets to see if this improves my road economy any and how much it affects top end performance as well, compared to both my current tuning and the tuning that ill end up with after the drags.

Also, i think there’s merit in the idea of having a fresh air duct running to the airbox bell mouth, not pressurising it but just ensuring that there is a supply of air coming from in front of the engine directly to the bell mouth that hasn’t been heated by the engine, a suitable duct to mount on top of the oil cooler hasn’t eventuated as of yet.

So put simply i think that the 125rwhp figure can probably be achieved with a simpler and cheaper set of mods than some are using with the standard tune up (i.e. free up the exhaust, airbox mod + a/f, 36y’s? and bigger main jets [jets can be bought directly from a mikuni supplier in Queensland much cheaper than Yamaha]) and fuel economy can be improved over stock simply by using the 18t (or 20t custom) front sprocket and leaning out the stock jet needle and idle air circuit.
The whole thing could probably be done for less than $500 over a period of time, with the main outlays being a/f, jets and 36y’s, providing you can fit these items and mod the exhaust and airbox yourself.
The main differences being that the static ignition advancer and dynojet kits are not necessary, top end power with this tune up appears just as good as the standard tune up but power delivery is more progressive (especially from a closed throttle with the bike leaned over in a turn), fuel economy can be better than on the standard bike, and its a cheaper option than the traditional route of the standard tune up which generally includes a fair bit of dyno tuning in order to maximise the available jetting.
Ultimately you do your mods, adjust the jets, set the air mixture screws and balance the carbs and bob should by rights be your uncle.

Anyway that’s my take on tuning my bike, ill update this thread with news as i learn it (wont be doing much till December probably though) and feel free to enquire about or criticize my ideas, nothing i like more than a healthy discussion on xjr’s.

Spot ya.

Chris.