Stable idle?

[Neil E.]

Is it the holy grail of 2T jetting? I just tried a #8 slide thinking it would help clean up the off idle jetting compared to the stock 6.5 slide. I couldn't notice any real difference except that it was much harder to get the bike to idle. I think this was a reslut of the new #8 not having a notch.

The 6.5 has a notch that measures .095 wide x .040 deep. This would allow the slide to be closed further (less pullover). Current jetting is 40P, AS 1.5, JD Red #4, 175 main. I rarely ever exceed 1/3 - 1/2 throttle when riding.

The AS is not particularily effective. The bike will idle OK for about 20 seconds, then the RPM drops and it flames out. As long as I blip the throttle once in a while it's OK. I don't usually idle for more than 15 seconds when riding.

Does this behaviour suggest a low float level?

 

[Neil E.]

stainlesscycle put in a stronger spring
http://www.gasgasrider.org/forum/showthread.php?t=10909

desertgasser300 found a loose adjuster nut
http://www.gasgasrider.org/forum/showthread.php?t=10912

I added a knurled brass cup to my slide adjuster screw so I could easily give the screw a bit of a turn out on the trail. It works very well and saves getting out a screwdriver.

However since doing the mod, I notice it's harder to get a steady idle. The stock plastic screw had a white thread sealer/locking compound on it. Since I removed and replaced the screw, most of this compound came off. I think the screw is not held as snug in the hole now. It is the stock screw with a brass piece added.

Any idea what this stuff is? I might try some watered down blue loctite as a replacement. I suspect the reduction of original sealant is having an effect on the slide screw position.

 

[Jakobi]

I have a number 7 non notched and have the same kind of scenario. Its taken alot of work getting the right needle diameter and pilot to work well. The idle screw itself is a very fine line between idling too high and flaming out quickly.

 

[GMP]

Your pulling over to the needle jacking the slide up with the idle screw. Maybe try a #7 slide. This new carb seems like a PIA, even on a 250, which have been traditionally easy to jet. Guess I'll find out for myself.

BTW you don't need the plastic "T"s on the vent hoses. They are "T"ed inside the carb body casting, run the horizontal lines down, vertical lines up.

 

[gasgasman]

BTW you don't need the plastic "T"s on the vent hoses. They are "T"ed inside the carb body casting, run the horizontal lines down, vertical lines up.

Those vent tubes may be some of your issues. The float bowl has to have atsmospheric pressure to work. Those "Ts" may have too much restriction in them.

 

[Neil E.]

The tees have a large bore and shouldn't be an issue for my riding. I connect all the vents and run a single line up to the airbox. I haven't had any issues after falling over on a hill (with the bike on it's side, wheels uphill).

When you look at how very fine the slide stop screw thread is, the slide is being moved extremely small amounts to make idle adjustments. The JD red needle should minimize pullover (assuming the needle jet bore is correct).

I am thinking that the slide screw spring pulls the screw back centering it on the thread pitch. The slide closes pushing on the screw and after a bit, the screw moves to the side of the threaded hole. This allows the slide to drop that extra thou or two, enough to change the idle speed.

A heavier slide spring might push the slide to a more reliable "home" position. I suspect the lack of sealing compound is letting the screw move.

 

[Neil E.]

Your pulling over to the needle jacking the slide up with the idle screw.

Let's review this. Lifting the slide means more air to the motor (needle is still on the straight section). If we need more air, is that the result of too large a pilot jet?

 

[webmaster]

To minimize pullover - you also need to go with richer pilot along with GMP suggestion to back out your idle screw.

jeff

 

[Neil E.]

To minimize pullover - you also need to go with richer pilot along with GMP suggestion to back out your idle screw.

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.

 

[GMP]

Yes, exactly. Raising the slide too much to achieve an idle is a crutch for a poorly tuned idle circuit.

 

[Jakobi]

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.]

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]

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.