Jerky low-speed running

[Blimp]

Hi. I've had my TRX for a few years now. It's on its second engine, following a disasterous attempt to re-shim the inlet valves. The bike accelerates well and pulls strongly, but hates holding a low speed at a constant throttle, for example maintaining the new 20 mph speed limits that are increasingly popular. Around this speed the bike shudders and jerks. Replacing the TPS improved the stumbling and hesitaton it had when pulling away from stationary. It's had a standard Keyster carb kit recently, but Classic Motorcycle Mechanics (I think) reviewed the TRX last year and suggested going lower on one of the jets, but I can't remember which (and I no longer have the mag).
Any advice would be appreciated. Cheers.
Blimp.

[dicky]

Standard jetting on the TRX is too rich, you can easily drop to #140 mains.
If you've replaced the jet needles then it might pay to put the old ones back in. The needle jet (ie the emulsion tube) is the bit that wears out, the standard needles are fine, and all aftermarket ones that I've seen to date have a different taper to standard, making the carb response very different.
Other things you should be looking at with the age of these bikes is all the rubbers. The inlet manifold boots and the vacuum lines to the fuel pump and tap may be cracked and leaking.

[Blimp]

Thanks, dicky
I'll compare the needles and swap if there's a noticable difference.A few days ago I dropped the needle to it's lowest setting and went 2 turns out on the pilot air screw. This seemed to improve the jerkyness at 20 mph at a constant throttle, but it still felt rough.
By the way, have you noticed the step on the base of the throttle slide, where the needle sits? It causes the needle to slant. I seem to recall reading somewhere that this was intended to deal with power pulses (or something). No one else has referenced it in any forum thread that I've seen, but I did see a YouTube video where some foreigner was making half-moon washers to level the slide seat out. Not being in English, I've no idea what was being said, or if it improved whatever issue he was dealng with.

[HolerTogni]

Hi blimp!

No one else has referenced it in any forum thread that I've seen, but I did see a YouTube video where some foreigner was making half-moon washers to level the slide seat out. Not being in English, I've no idea what was being said, or if it improved whatever issue he was dealng with.

That's a common mod among the German TRX riders and in the German forum there's also a measured drawing thereof in the download section.

This special shaped washer is intended to and helps reducing the wear of the needle.

Nevertheless, the needles keep wearing but a little less than without this washer.

Also installed greets from Munich!

[Blimp]

Hello holler,
Thanks for you response.
I fabricated some half-moon washers myself to see if it improved general running, but I suspected the step must be there for a reason as Yamaha were unlikely to produce a throttle slide with a flaw built in...so out they came, and the needles were allowed to wobble about enigmatically. I can't remember where it was I read the explanation offered for why the needles were deliberately offset, but I seem to recall it was meant to compensate for the pulses through the carbs. I don't know why you would want the needles oscilating inside the emulsion tubes, but I'm not an engineer...

[HolerTogni]

Hi blimp!

I am an engineer but not specialised in fluid mechanics - so I also don't know ...

"Keeping the needles straight " - greets from Munich!

[Blimp]

I composed a lenthy and witty account of my tribulations when changing the jets on the TRX to eliminate the richness caused by the standard 142.2 mains.It had everything : drama, pathos, farce (obviously). Upon completion I wiped the sweat from my brow and hit "submit", only to be informed that I neeeded to be logged in to post on the forum. I thought I was...Oh well, it's a small matter to log in. Now, where did my post go? I can't find it. Didn't think to save it, of course.
Anyway, to cut what was a long story short, I the Keyster 142.2 main jets I installed about two years ago with some 140s obtained from NRP Carbs in Manchester. I noticed that the "O" rings on the Keysters where a little ragged, and were much thicker than the ones supplied with the 140s. I remember the Keysters being difficult to fit at the time, and the were tight coming out. The Keyster "O" rings filled the groove in the jet, while the NRPs are half the thickness. Will they seal properly?
With everything back together I took the bike out for a run. It seemed a little more responsive, and the jerkiness encountered previously at low throttle openings didn't seem as bad. I've got a 16 tooth engine sprocket to try next and we'll see how that goes.
Blimp

[Blimp]

I mentioned in a previous post that the "o" rings supplied with NRP's 140 main jets were much thinner than those provided with Keysters carb kit. I emailed NRP, and they replied that Viton doesn't compress as much as the material used in Keyster's "o" rings, but they will "send some thicker ones out." I'm a bit puzzled by this. If Viton is less compressible, why are they sending replacements? The difference in thickness was so marked that I naturally wondered if the wrong "o" rings had been supplied by mistake, especially as the rings were not fitted to the jets but in seperate bags.
Any thoughts?