Slide sticking in carb R25/2

[Kurt in S.A.]

At the start of the year, I was out on my R25/2 for a tank of gas.  Near the end of the ride, it was evident that the throttle stuck because when I rolled off throttle, the bike didn't slow down.  Eventually all I could do was pull the key, pull the clutch, and glide to the side of the road.  I was able to get a friend with a truck to get us home.

I investigated and found that the slide was stuck at the top of the carb bore.  I had been running lots of throttle to get up a hill.  I took things apart and cleaned the slide and bore.  Didn't help that much so I used some fine grit sandpaper on the slide to get things to fit.  After trying things out, the slide seemed to work well, pulling it up and the internal spring pushed it back down.

Yesterday, I was able to get out again.  Throttle seemed fine in the garage.  Also seemed fine during the first part of the ride.  Near the end, I noticed that the slide was stuck again, but I was able to free things...somehow.  From that point on as I turned for home, I keep the RPMs low and didn't get to a situation where I would be using lots of throttle.  Got home, parked the bike, shut if off, and rolled the throttle full on...slide sticks again.

So what could the issue be?  I will recheck the cable routing but don't think that's my problem.  Could it be heat?  About 18 months ago, I had to hand-sand the carb flange that bolts to the phenolic block and to the head...it appears that it had warped.  Usually if that warps, the issue might be leaking air.  Could the warping of the flange distort the body of the carb?

Thanks...Kurt

[Kurt in S.A.]

[rolf.soler]

If it was heat, both parts, body and slide would (and actually do) expand. Only if the slide gets hotter than the body it could stick.., however, as the heat comes from the cylinder head via the body the slide can't heat up more than the body...
However, the body could be distorted slightly. You can measure if slide / body are round or slightly oval..
But I guess iI would first try a new spring. Maybe its lame

[strichzwojan]

Hi Kurt,

I had the exact same problem with my carb two years ago. Normal starting, but after 40 km the throttle cable felt like being close to damage, as if there was sand in the cable. It was a tiny pimple in the slider tunnel. It took me two months to discover the problem. Now its OK. I give the slider and the tunnel a massage with a "graphite-indexfinger" every now and then...

Jan

[Eppo]

Hello Kurt,
in normal use the carburator should not get warm/hot.
Because of carburation the fuel needs energie and the only one available is thermal energie so the carburator gets colder. Because of this it could happen that carburators can freeze in deep temp. conditions (winter).
If your carburator gets hot and after some waiting time you can drive normal again, then there is something wrong with the thermal isolation.
Your work on the carb. flange could cause this failure.
Good luck.
Eppo

[Kurt in S.A.]

Eppo -

I don't know if the carb is getting hot, I just wondered.  You wrote "Your work on the carb. flange could cause this failure."  Can you expand on that a little?  Are you saying that something I've done might have caused more problems?

Thanks...Kurt

[Eppo]

Well,
you wrote that the last work you did was on the carb. flange.
So maybe you damaged or forget the isolation plate.
Only a guess by myself.
Or it could be something with the hook up after your work on the flange.
If the carburator works fine before your work,
than there is a huge possibility that that sticking is caused by something that happen after or while your work on the flange.
Greetings Eppo

[Kurt in S.A.]

Thanks -

I worked on the carb flange in September 2020 and have ridden nearly 700km until I had the failure on the road.  The "isolation plate" is in place.  As I said, the flange was not completely flat and I just sanded the mating surface to the plate to make it more flat.  I try not tighten the nuts that hold the carb on too tight.

It is clear that the slide sticks at the top of the carb bore, so something is wrong with the interference there.  I will probably order a new spring for inside the carb along with a throttle cable.  But I guess I need to figure out why the slide sticks.  I will attempt to measure the diameters of the bore and see if it is not perfectly symmetric anymore.

Kurt

[strichzwojan]

Thanks -

I try not tighten the nuts that hold the carb on too tight.  AS SMOOTH AS POSSIBLE.

I will attempt to measure the diameters of the bore and see if it is not perfectly symmetric anymore.   EVEN THE PISTON IN ITS BORE IS NOT SYMMETRIC... TRY GRAPHITE!

Kurt

[Kurt in S.A.]

Any recommendations for the graphite?

Kurt

[strichzwojan]

Normal graphite powder you can get in the hardwarestore or "Baumarkt". I had no acceptable results with PTFE-Spray, but graphite was fine.

[Kurt in S.A.]

Thank you.  I would have thought that the powder would wear away somewhat quickly.

Kurt

[strichzwojan]

Thank you.  I would have thought that the powder would wear away somewhat quickly.


Kurt

Good combination. It clings to the rougher parts where it is mostly needed.