XR600 Throttle Action

[SteveJC]

Anyone got any good tips to lighten the 600's throttle (and clutch for that matter!).

Despite fresh well lubed cables the controls are noticeably heavier than the BRP, which isn't the lightest of throttles anyway.

Interestingly I just found out that it appears to be "working as designed" (ie heavier) according to the "Comparison" in the BRP article here:

http://www.off-road.com/dirtbike/jan2000/xr650r/specs.html

But I have to ask the original question anyway? ?

Steve

[miguelitro]

keep an eye on ebay cuz i've seen some clutch extenders that give you more of a mechanical advantage over stock. i've never tried one however.

never heard of lightening the throttle pull. dont know if i'd mess with the return spring.

XR600's were not designed to be easy or light in any way. that is just there disadvantage. the trick to ridin em is making their advantages work for ya.

my two cents cuz i love my 600

[lucky_logger6]

For the clutch, MSR makes a perch and lever with a mechanical advantage option.

[Jeff@TheQuadShop]

SRC (Summers Racing Components) has products for both of those issues.

[HairyScary]

There was some funky stuff recommending a small hole drilled in the slide to help reduce the vacuum that jams the slide into the carb body when the throttle is closed and the engine is providing braking force (highest vacuum conditions). This is assuming you are talking about the throttle being "heavy" when transitioning to on throttle from being fully off/shut.

I'd saved the links - they came from a TT thread in this forum. Can't vouch for how it works.

http://www.enduro-stammtisch.de/xr/xrtune.htm

http://www.danzl.at/wolfi/xr_vergaser.htm

[SteveJC]

That is genius - dam those Germans! ?

I obviously see the need for a decent spring but can't help thinking the Honda designers over-engineered it a bit?

Will certainly look at this and if it works OK will try it out on the 650 as well.

And the hole in the slide is rather interesting as the over-run is definitely the worst time for heavy throttle, will give that a leeetle experiment too....

Thanks for the post Hairy - the wonders of sharing info on the net and all that. ?

Steve

There was some funky stuff recommending a small hole drilled in the slide to help reduce the vacuum that jams the slide into the carb body when the throttle is closed and the engine is providing braking force (highest vacuum conditions). This is assuming you are talking about the throttle being "heavy" when transitioning to on throttle from being fully off/shut.

I'd saved the links - they came from a TT thread in this forum. Can't vouch for how it works.

http://www.enduro-stammtisch.de/xr/xrtune.htm

http://www.danzl.at/wolfi/xr_vergaser.htm

[HairyScary]

It seems like the problem isn't anywhere as bad on the 650R when compared to the 600R. I've wondered if the 650R has the ACV spring set so that it does more which cuts down on engine braking (and the resulting high vacuum conditions). THere is probably room to experiment in that area as well.

[yzfxr]

i adjusted my spring similar to the german website, but the idle would hang after throttle release. thumper racing sells a easy pull throttle kit that works well. it basically increases the diameter of the pulley. i've heard of people grinding the spring coils down slightly as well.

[SteveJC]

Hi - if you mean the air-cut-off valve then that is another subject, but related.

On several of my bikes I have disabled it, as with a properly jetted bike the ACV tends to pump in too much fuel, and the result is a dead feel to the throttle when blipping during down-changes and reapplying after throttle-off. Exactly the same conditions as we are talking about of course, but different cause / effect.

There seems to be a theory around that the ACV somehow reduces engine braking, but in my experience it has no effect, if anything it *increases* braking.

To disable the ACV you just cut approx 5mm bit of plastic pipe and put it inside the spring (pic available) - this stops the valve opening, and stops it blocking the air-supply to the pilot jet during over-run (which would otherwise give a very rich mixture to prevent popping). And thinking about it, the ACV disabled would at least partially reduce the vacuum problem as it provides a small passage for air to pass through (but I haven't noticed).

An article here explains how the ACV works:

http://www.4strokes.com/tech/carbover.asp

Disabling the valve gave a noticeable improvement in smoothness on-off throttle to my XR650, and a small adjustment on the fuel screw got rid of the popping. But it still has the mechanical problem from the high vacuum, which is less than the 600, BUT it snatches more through the transmission in comparison.

Steve

[thumpasaurus]

I fought this problem for years. It all boils down to the inner bore of the carb wearing out, allowing the slide to get out of alignment when at idle. What's worse is the throttle return spring is overly stout and makes the situation worse. One thing I did that helped was taking one turn out of the spring, by which I mean letting it unwind one revolution.

In the end I bought another carb (TM40) and throttle.

There are some solutions for the clutch involving a longer arm where it comes from the case. XRsOnly will have it.

[SteveJC]

Interesting findings over the weekend. I drilled the little hole in the slide but didn't really notice any difference in the vacuum-stickiness on the throttle.

Did half the spring mod - cut a 1cm slot in the throttle pivot. Noticeably lighter throttle. In fact very much lighter and super smooth.... BUT - I had also oiled the main pivot - ie the bit that sits in two bushes in the carb.

After an hour or two riding the super-smooth throttle began turning back to its original slightly sticky self, and consequently heavier action.

Hmmmm...

A light-bulb pops up after getting home and washing the bike, and strip the carb down again. I had noticed that the main shaft of the throttle pivot (ie the bit the big spring fits over) had some of its chrome rubbed off. By mimicking the cable action ie pushing the pivot at the cable point rather than twisting it I could feel the stickiness that the missing chrome was causing. By oiling it earlier it had overcome this, but eventually petrol vapor washes the oil away.

So, this is the main cause then - the shaft worn out - £60 for one of these so no thanks ?

Two options - (a) do the second half of the spring mod to relieve more of the pressure on the bearing - this is relocating the inner end of the spring, and is an easily reversible mod.

Combine this with a careful polish of the exposed metal on the pivot - its quite rough under the chrome.

Alternative option (? is to drive out the little roller pin and turn the shaft 180 deg so fresh chrome is on the main pressure points. The screw for the throttle slide arm won't fit properly then but could be drilled out and replaced with a larger bolt.

If the bike was much newer I would just buy the new part or get a better carb. So plan (a) for now - I have been exchanging emails with the German guy that put up the website and he says the throttle action is perfect despite the reduced spring effort.

We will see...

Steve

[SteveJC]

Just as a follow up

(On the XR600) I ended up turning the chromed throttle pivot 180 deg which certainly helped, and then lightly polished the inner carb body as the slide seemed to be a bit sticky in places.

Much improved, and ultimately could be lived with, but still heavy with the engine decelerating. Interestingly the throttle mechanics is now so tuned to perfection(!) that I can feel the slide getting stickier during longer down hills with throttle shut - ie not much petrol swirling around to lubricate the slide. Once back to normal accel/decel the slide loosens up a little.

Yeah, maybe a little obsessed but its my only bug-bear with both my 600 and the 650 - more so than the kicker even!

As for the spring mod - I ended up putting it back to stock as it hung quite badly (at lower throttle) and in some respects didn't feel quite so positive/crisp applying gas with the much-lighter spring. Maybe my imagination, who knows.

As for the hole thing, well it may have helped a little but for it to really work it would have to be very BIG and then it would seriously screw up the mixture as well.

Mods here in case you missed it first time

http://www.enduro-stammtisch.de/xr/xrtune.htm

Steve

[CrackiN]

I have done some tweaking to relieve the stiffness in the throttle action...I first let the carb spring slip back 1/2 round and anchor itself on the cable attachment bracket....This eased the tension and still allowed the throttle to snap closed...After some brain storming I decieded that the push cable was causing a lot of unneccessary drag on the throttle and since the spring on the carb closes the throttleany way, I couldn't see any reason for having the push cable, so I took it off....So now with the spring unwound 1/2 turn and no push cable the throttle is smooth and snaps closed. Does anyone see any problem with removing the push cable?

[marcelo1965moto]

I solved the sticky throttle problem in my XR 600 by filing the carb blade to eliminate the sharp lower corners... Works perfectly. For a detailed explanation and illustration check this link to my blog.

http://marcelo-bbt.blogspot.com/2008/07/honda-xr-600-sticky-throttle-problem.html

The solution works great, I have tried it for hours of trail riding with a hot engine and not one single sticky incident.

Rgds,

Marcelo.