Stable idle?

[Jakobi]

Originally Posted by Neil E.

To verify: idle screw = slide stop screw (not air screw)

I would expect that if we are trying to obtain an idle with the slide as closed as possible, the pilot jet would have to be larger and the air screw turned out more so the idle air is supplied via the air screw instead of the slide.

Orrr.. The a smaller pilot with the air screw in a little!

Its a mind game trying to work it out with the slide (notch/non notch) needle diameter, pilot, air screw, slide position/idle screw.

It was made even worse because these bikes shipped with a F diameter needle which had to have the idle screw wound all the way in (slide lifted) to sustain even the lowest of idles.

Going to a smaller diameter needle and paring with a larger pilot seems to work best.

[Neil E.]

Originally Posted by Jakobi

Going to a smaller diameter needle and paring with a larger pilot seems to work best.

That sounds like a mind game right there. Shouldn't that be larger diameter needle (straight section, to reduce pullover) and a larger pilot (with air screw out more)?

It is interesting that JD recommends a 38 pilot for the 36 carb and a 40 pilot for the 38 carb. My bike has always felt rich at the "just off idle" point. I'm trying to find a combo that provides a reliable idle and goes a tad lean just off idle. A longer L1 dimension is the only way I see this possible, since the effect of the slide cutaway occurs mostly from 1/8 to 1/3 throttle.

[Jakobi]

You're right. My bad! I meant leaner (larger) diameter needle.

[farmerj]

Neil,

You sound like the perfect candidate for an RB carb job. I haven't done it on my GG yet - but I'm planning to. It really helped on my KDX re: low idle power & smoothness. Ron says: Modify low speed circuitry This part of the modification along with the other mods really brings the air mixture screw into play. With only very small adjustments, you can keep your bike running at its peak as you change altitude or with weather changes during your ride. FYI, http://www.rb-designs.com/gasgas250-300.htm

Jeff

[Neil E.]

Jeff,
I'm sure you're right about the value of the RB work. After looking at his site, I see that he bores the carb out to 39mm. I'm not after top end power. The divider plate is good. The modification to the low speed function might just be removing the needle shroud during boring. I'm not in a rush to send my carb over the border at this point.

Now for the rest of the story:

After finding out how much play there is in the slide stop screw, I decided to make up a new brass screw. I could feel the plastic screw change position when the slide put pressure on it. My lathe didn't show a .8 mm thread pitch on the gearing chart so I used 32 threads per inch (same pitch until you get to the fourth decimal place). I machined the thread until a test nut (M5 x .8) just barely started on.

I made a new holder for a 10-32 expanding die so I could open it up above the M5 pitch limit. By reducing the diameter a bit at a time I got the thread at an exact slight interference fit in the carb. It takes slight screw driver force to turn, but doesn't wiggle at all.

I set the air screw at 2.5 turns out and with the new screw installed I had a rock solid idle. It let it idle for a few minutes; no variation. It doesn't drop RPM and flame out (this behaviour only started after I removed the stock screw disturbing the sealer).

Our resident "el carb tuner supremo" Jakobi has mentioned he has had a speed drop and flame out also. Since he has done mucho adjusting, I wonder if the sealing compound on his slide stop screw has loosened up.

While I haven't measured the carb hole, the screw is very loose at only a few turns in. It gets less play as it travels into the carb. I believe that the hole is cast not drilled. It has a taper from the casting angles and is maybe 60% thread at the start, then improves to 100% at the end. The taper is why the screw can move around if the sealing compound is gone.

[GMP]

I've seen brass screws with locknuts, should do the same thing with some teflon pipe thread paste. You can probably find some at Mcmaster with a knurled knob and cut/dress the end.

[farmerj]

Good work there, Neil!

[lankydoug]

Originally Posted by GMP

I've seen brass screws with locknuts, should do the same thing with some teflon pipe thread paste. You can probably find some at Mcmaster with a knurled knob and cut/dress the end.

From experience I would not get any teflon tape or paste any where near any fuel system. If your careful when you put it together you may keep it out of the fuel passages but once it has been apart and together a few times I have seen strands of stringy teflon cause more problems then any possible benefit it could have gained. Just my 2 cents.

[Jakobi]

My speed drop an flame out doesn't happen on its own though Only if I back the idle out too far (closing the slide) and starving the cylinder. Lifting the slide too far results in a too high idle. Its like theres a 1/8th of a turn on the idle screw which is 'the sweet spot'. I can see if the screw wasn't secure and able to move on its own how it could give you an unstable/variable idle.

[GMP]

Just a coating on the threads should not hurt anything, and the paste I'm thinking of is just that, a paste, not stringy like the tape. Much like what is there stock. Even so, whatever trace could possibly come off would go right into the airstream. I see your point though if your talking about trying to seal something in the float bowl upstream of the jetting. If you could find a proper M5 screw/bolt with a shank, an o-ring could also be used.