BIKES: 600 SRX 850 TRX 600/650 XR 250 TTR 1000 FZR STREET TIRES CARBURETOR  
  Tuning the Yamaha 600 SRX

 

Here is an awesome list of answers to SRX's owners most frequently asked questions. The original document can be found on the "thumper page"

This is an article on various aspects of tuning the Yamaha SRX-6.
I'm currently in the process of turning my second SRX, a much-abused junkyard dog, into a good street sportbike. This article will, in part, follow that story (I make the mistakes, so you don't have to :-).

My first SRX was a racebike, and went through a few rounds of modifications before the current owner got it sorted to its current, wonderful, state. I'll describe what was done there, as well.


Suspension Upgrades

SRX's benefit quite a bit from suspension mods. The stock setup is fine for commuting or touring, but for serious sporting work, it falls flat. You won't believe how bad a stock SRX is until you ride one that's properly set up.

Tires

I'm slow enough that the stock Excedras were adequate, so the read this section with that disclaimer in mind.

For street use, I've found the stock sizes to be plenty adequate (100/80 front, 120/80 rear). For racing use on stock rims, going up one size (to 130) in back is a good idea if you can turn competitive lap times. Oversized tires, esp. large fronts, will slow steering, and SRX steering is none too quick to begin with.

Metzeler Comp-Ks come in the right sizes, and work well, street or track. They provide what I consider to be an acceptable amount of mileage (about 5-6K miles from a rear), and have a good profile, providing neutral, quick steering.

Bridgestone makes a new bias-ply tire, the BT-35 Battleax (as opposed to the radial Battleax, which is the BT-50). It's available in the stock sizes, and though I've not yet tried them on the SRX, they've done nothing but improve the two bikes I have tried them on. They are certainly more than adequate for the street.

One combination I can recommend against is over-sized Dunlop 591s, (110/80 front, 140/80 rear). This is what my junkyard dog was fitted with when I bought it, and they slowed the steering way down. As soon as I get things together, these are coming off and a set of Bridgestone's going on.

Forks

The mild solution to buy the springs that Progressive Suspension offers and fill the forks with 10w oil to the stock level. According to the Yamaha shop manual, this is 310cc.

The fork spring kit from Works Performance was what went into my first SRX, and it worked great. However, they're a bit more expensive than the Progressive's, and don't seem to offer much improvement over them. They are adjustable, but chances are, you'll never need to (or want to, you need to remove them to adjust them).

Either are much better than the limp stock springs, which are soft enough to cause a nasty head shake when braking into turns.

The wild solution is to fit a new front end. The most common swap is the entire front end off the FZR400/600. This is basically a bolt on proposition, once you've located the parts. The FZR steering stem is longer than the SRX stem, so you'll need to either have the SRX steering stem pressed out of the SRX bottom triple clamp and into the FZR clamp, or do what Mark Sievers did and just use three top bearing adjustment nuts. This swap will give you stouter 38mm forks and a 3x17-inch front wheel. Damping and springing will be both be too stiff for the SRX, so these will have to be altered. You're on your own here, until I can get more info. Stay tuned.

Note that this swap will also require a rear-wheel swap, since the FZR front wheel is 3 inches wide, and the stock rear wheel is only 2.75 inches wide. See below.

Shocks

Mild: Ohlins or Works Performance twin shocks. Note that I've had bad luck with the service at Works Performance. Their shocks are fine once they're set up, but they rarely come from the shop properly set up. Lindemann can set them up right the first time.

I've got a set of Ohlins shocks coming, and I'll relate my experiences with them when they show up. They appear to be on a slow boat from Sweden. Noleen is the US importer. Reach them at (619) 246-5000. They're in SoCal. Order well in advance.

By the way, make sure you let the sales types at Noleen know exactly what bike you're talking about. They have a huge catalog that appears to cover Japan-only bikes as well as US-bound models. The sales girl insisted that my bike was an '89 and a monoshock (as they were, in Japan). However, after talking to the "tech guy", I convinced her that my bike is indeed an '86 and has twin shocks. Seems her catalog only goes back to '89. :-) Our bikes are the same as the SRX-400 (another Japan-only bike), and so are the shocks, which they do list correctly. Of course, a single shock may just show up at my door...

Note: since I first wrote this, I had to cancel the order from Noleen. After waiting for 12 weeks, the importer still didn't have them. My SRX is still bouncing around on the worn-out stock shocks.

Wild: Monoshock rear end. This is usually accomplished with a swingarm from an FZ600 or an FZR400/600. This requires a fair amount of expertise to get working, which I certainly don't have. Lugs for the top shock mount and the linkage will need to be fabricated and welded to the frame in the right spots. Fortunately, there are cross members on the frame in the right places. The swingarms will fit with a bit of grinding and shimming (small amounts, like .010"). A shock set up for an RZ350 (length and damping) is apparently the perfect fit. This is, of course, usually accompanied by a wheel swap. More info when I get it.

Wheels

To fit the latest rubber (or at least more recent rubber), a wheel swap is a nice modification. Swapping the whole front end is generally easier than trying to fit a wider wheel into the stock fork. The rear can be done with the stock swingarm. Up to a 5" wide wheel will fit, according to my sources (one WSMC racer fit a 5" TZ250 wheel). The 4x18 rear wheel from the FZR4/6 is a common swap, and requires only that the stock spacers be milled a bit to center the wheel. The rotor and brake hanger from the SRX can be used. The rear caliper and the torque arm will need to be trimmed a bit with a grinding tool to clear.


Engine Tweaks

According to dyno tests by dearly departed Cycle magazine in 1986, a stock European SRX (608cc vs. the our 595cc) made 28hp at 6500rpm. Ahem.

With remarkably little work, an SRX can be made to be fast enough to make a brisk sporting pace possible. With some effort, they can go quickly enough to suck the headlights out of stock FZR400s. One can't expect much more than that. Absolute tuned-to-the-gills grenades-in-the-making SRX engines can approach 70hp, according to English tuners NWS. My wild speculation is that a reasonable street motor will top out at 40hp.

Easy Stuff

The modifications described in this section are doable on a fairly modest budget and won't affect reliability too much. They're a good place to go for a street engine, and a good place to start for a race engine.

Exhaust Systems

Buy one. Now. It's incredible the difference just uncorking the exhaust (and a bit of carb fiddling) will make all by itself.

There are at least three off-the-shelf systems available. One is from White Bros., and is a single downpipe that terminates just aft of the clutch cover. The two exhaust ports are joined just outside of the head (about 2-3 inches) into a 2.5-inch downpipe. A short stub SuperTrapp can made of brushed aluminum handles silencing duties.

The second is a SuperTrapp slip-on, made of stainless steel. I don't care for this pipe as much as the White Bros. It's heavier, and actually harder to mount. Pretty, though.

Finally, Kerker makes a pipe. I've only seen photos of this one, and it seems to use the same downpipe as the White Bros. kit, with a different, non-SuperTrapp silencer.

I used the White Bros. pipe with the stock 8 discs in the can on my first SRX with reasonable success. Regardless of what pipe you go with, you must do some carb work, as well. The stock setup is marginally lean with the stock pipe, and does not run at all well with a pipe in place.

Tuning the stock carbs

This is possible, but a real pain in the ass. DynoJet sells a kit. However, the oddball carbs on this bike are neither Keihins nor Mikunis. The left carb is a copy (more or less) of a Mikuni slide-throttle carb, and will accept standard Mikuni jets, it has been rumored. The right carb is something made up by a bean counter at Yamaha. You'll notice that this is a CV carb and has no float bowl, but takes its fuel feed from the left-hand carb's bowl. No one (it appears) makes jets for this carb. DynoJet's kit includes a pair of small drill bits so you can drill out the main jet in the carb to the new size. I've found that even with individual K&N filters and a pipe, this kit makes the bike too rich above 1/2 throttle even on the leaner setting.

On the subject of individual air filters, I'd suggest using UNIfilter-type foam filters instead of the K&N type. The K&N's don't fit all that well between the frame rails and with the carbs so close together. The foam filters can be squished to fit a bit better.

When using any individual filters, the airbox must, of course, be removed. Since the airbox also contains the battery box, you must either cut up the box, fabricate a new battery box, or go to a battery-less system using a battery eliminator.

Carb swaps

Sudco will sell you Keihin CR Specials that bolt on to the SRX. These are smoothbore slide-throttle carbs. They are available in a number of sizes, but 31mm and 33mm seem to be the most popular. These are very nicely made carbs, with lots of parts available to get the entire throttle range working perfectly. However, be aware that there are some concessions to be made. This is not a simple bolt-on kit. They just barely fit in the space available. The choke is all but inaccessable, the idle screw even less so. Getting the throttle cables to work properly can be quite a chore.

Before I purchased this kit, I talked to the current owner of my first SRX, who had already fit this kit to that bike. He assured me it was a simple bolt-on affair. After I began running into a number of minor road blocks, his story changed. "Oh yeah, I had to do that, too," I kept hearing. Apparently, his idea of "bolt-on" and mine were a bit different.

The most drastic thing you need to do is relocate the breather fitting on the top of the oil tank, since the new right-hand carb's float bowl needs to go there. This isn't all that hard to do, but you'll need a friendly machinist or a set of taps. You must remove the oil tank from the bike. It will come out, but requires some careful arranging. The carbs have to come off, and it helps to remove the intake manifolds, as well. If you can split the tank, do so. Mine was glued together with something I couldn't undo. Cut off the brass fitting and tap the hole for a small bolt to plug it. Epoxy would do as well. Next, drill a hole in the top of the tank, and tap that to take a small angle fitting. It appears to be much easier to find SAE fittings in the US, so I used a 1/8-inch NPT brass fitting, with a 1/8-inch tap. You'll need to either put the hole on the left side of the tank, or use a longer breather hose. Clean out all of the metal shavings carefully. If you couldn't split the tank, do an extra good job of this.

The welded lip on the bottom of the gas tank will rub against the throttle linkage. Some not-so-gentle massaging with a soft-faced hammer or a pair of pliers will be necessary.

The Keihin carbs don't come with much in the way of instructions. There are three hose fittings on the carbs. The one between the float bowls, near the carb mouths, is the main fuel fitting. Run a hose from the bottom fitting on the petcock to here. The other two fittings are air intakes for the enrichener circuit. Most people I've seen have just left them as is, but you may want to plumb them into a filter.

The jetting that seems to work for mildly tuned motors is:

62-65 slow jets. 128-132 main jets. 230-240 air jets (the jets poking out of the carb mouths). YY8 needle.
Jets come in steps of 0, 2, 5, 8, 10 (i.e., 60, 62, 65, 68, 70, etc.).

Sudco sent me carbs "all set up for your bike" with 70 slow jets and 120 mains. The bike idled at 4,000 rpm with the idle backed all the way off and bogged horribly past 1/4 throttle. Not good.

You should also check the float level. With the float bowls off, hold the carbs upside down. Pull up the float with one finger so that the tab just kisses (but doesn't depress) the little plunger in the needle valve. Measure from the highest point on the float to the gasket surface. This should be 14mm. Adjust it by bending the tang until this is right (mine weren't).

All of this took several calls to Sudco to sort out. I also have to thank Mark Sievers for letting me know what his jetting was, since his worked fine out of the box. Sudco can do a good job, they just didn't in my case.

Finally, you'll run into some troubles with the throttle cables. They're a tight squeeze under the fuel tank, and the optimal routing is not obvious (I'm still undecided). Make sure the cables are well lubed. At least two people I know have decided to dispense with the closing cable, since the return spring on the Keihins are pretty strong. This does make running the cables a bit easier and, paradoxically, tends to prevent the opening cable from hanging up. However, be aware that slide-throttle carbs tend to stick open. The vacuum cocks the slide slightly in the bore, and the resulting stiction will sometimes overcome the return spring force.

As I write this, I'm still fiddling with mine, so I can't tell you what the improvment is. According to three people I've talked to, they make a very big difference. If you find all of this a bit daunting, stick with the stock carbs. They also require fiddling to get close to right (I've never seen them exactly right), but it is less expensive.

Several weeks later: I still can't get mine to start with any regularity, and have nearly given up. I suspect there's some other problem somewhere. This bike wasn't in great shape to start with, and I may only be compounding an existing problem.

Interesting note

One SRX builder I talked to had a problem with the power falling off at high revs. Despite a great deal of internal engine work, it wouldn't make more than 45hp, and was prone to a light misfire at high revs. I speculated that it might be ignition related (wild guess), and a coil and CDI swap fixed the problem.

Hard Stuff

I'm going to wuss out and recommend you call White Bros., (714) 692-3404, if you decide to start changing pistons, rods or do head work. I've never touched the insides of either of my SRX's, and I don't plan on doing so.

The stock engine is tuned quite mildly (a compression ratio of only 8.5:1, for instance, which is lower than the car I own at the moment). Big power gains are possible (50-100%!). However, note that one weak link in the SRX engine is the connecting rod. If you start to rev the engine past the stock limit (which is very easy to do with large carbs, and absolutely required if you want the most out of this engine), then you need to replace the rod with a stronger part.

One other weak area in high-revving engines is the magneto rotor on the left end of the crank. This is also the ignition timing rotor. When run at high revs for extended periods, this can crack and break free of the splined end of the crankshaft. In the two instances I know of this happening, the rotor simply stopped turning (it didn't hole the cases or anything). This, of course, stopped the ignition, and therefore the bike.