Suzuki GS550E — Quarter Turn Throttle Conversion.

 Converting your throttle tube to a 1/4 turn.

I'm pleased to say the GS550/673 is going really well — the engine has been faultless so far. One thing that is annoying me though is how bumpy the front end is, and it's really obvious since sorting the Bandit. The 1200 front end is now feeling incredibly refined and plush after a fork oil change, so I wanted to change the springs in the front of the GS to try and help her out.

As luck would have it, a pair of new Progressive springs came up on eBay for $110 delivered. Result! Although the general consensus now is to fit straight-rate (linear) springs, this was a bit of a bargain. However, they got stolen on the day they were delivered by some scrote who told the postie that they lived at my house. And that was the smaller of three parcels! Luckily I got my money back via Paypal, but that's besides the point. I was not impressed.

Back to square one. With a bit more reading up on the GSResources forum, I decided to try linear 0.9kg/mm springs from Sonic Springs. Apparently they can transform the handling compared to, what are now, forty year-old forks and springs. Can't wait!

Now I was in the mood, I took the opportunity and drained the fork oil through the drain screws, and refilled with a mix of 10/40 and ATF (165ml) as per the manual. I'll then do them again when the springs arrive to flush them out once more. This should remove some of the sludge buildup. 

I also fitted my new handlebars, these have a lift of 4" instead of the 6" of the standard bars, and don't pull back so far. The riding position is much better for me and allows you to lean further forward. While I was in there swapping levers and switchgear etc., I decided to try the throttle tube upgrade. 

Most riders out there have heard about the R1/R6 throttle tube upgrade allowing you to get to full throttle with just a quarter of a turn. Much easier then having to reposition your hand while the throttle is 3/4 open to get it round to WOT. 

There is a cheaper method though! Acquire a piece of PVC tube, fit it over the original throttle tube, and you are grabbing the holeshot all day long!

After a bit of looking around in the garage I came across an offcut of PVC pipe in a box (this is why we never throw things away!). I'm guessing you can get various thicknesses, but this piece was 30mm I.D. and 34mm O.D. Some of the throttle tube conversions I've seen online look much thicker — but a measly 2mm will certainly improve things. 

Mine needed to be around 8mm wide to clear the inside of the throttle housing, you will need to check the clearances and work with what you've got.

The rough edges from the hacksaw smooth off nicely with 80 grit emery sheet on a flat surface. No one will ever see it admittedly, but it's nice to get it right.

Measure and cut the right amount out to fit nicely. I used PVC glue on mine to make sure it all stayed exactly where it was supposed to, most just slide it on and rely on the fact that there is nowhere for it to go. Personal preference.

I also smoothed the whole thing down on the end on a flat plate with 180 wet & dry. I prefer to know that there is nothing in there to rub or bind when you're needing to open or close the throttle.

And with that it was back into the throttle housing, I didn't even need to adjust the cable on this bike. It's not quite a 1/4 turn throttle, but the difference is huge. So much nicer to ride and, as much as thicker tube would reduce the angle even more, this feels really good as it is.

Valve clearances.

With a few miles under its belt now, it was time to recheck the valve clearances and rebalance the carbs. I bought a few more thinner shims from Wemoto should I need them. Following the Suzuki procedure properly allows you to measure two valves at the same time, making it quicker, and actually gives a more accurate reading. 

I went through them all a few times to give me as wide a gap as I could get away with, with a few being slightly more than Suzuki recommend. After a bit of jiggery pokery, and finally finding my valve compressing tool, everything was adjusted and recorded for next time. I shouldn't need to open the top up for some time now.

Valve     Shim          Clearance

Ex 1      2.65mm     0.003"

Ex 2      2.65mm     0.003"

Ex 3      2.60mm     0.004"

Ex 4      2.65mm     0.005"

In 1      2.45mm     0.004"

In 2      2.50mm     0.003"

In 3      2.45mm     0.005"

In 4      2.50mm     0.004"

I normally don't record all this information and next time they're due, you end up needing shims you don't have. If you know which shims were last fitted, and the clearance, it gives you a fair idea on what you're going to need. I'm on the ball this time. 

Suzuki recommend 0.001" to 0.003" (0.03mm - 0.08mm), but shim and bucket always wear tight so these shouldn't need adjusting for a long time. I was concerned that they might be slightly noisy, it actually doesn't sound any different. Result!

Carb balancing was next and so much easier with the correct adjusting tool! If you like to balance your own carbs, get this screwdriver, the accuracy you can achieve quickly is unreal. It cost me around $50 off eBay and, although a bit pricey for a no-brand tool, will probably last forever because it rarely gets used.

With a few slight adjustments, and mixture screws all set around 2.5 turns out, it was all put back together. It sounds healthier at idle speed now, before there was an annoying chatter from the engine if the idle dropped to around 1,100 RPM. That was a little slow admittedly, and made worse by the carbs being out — it'll be set back at 1,200 RPM once I've road tested it.

Hopefully my springs will arrive soon and I can get a decent road test in!