Low speed carb tune (to crackle or not)

[Vintageman]

Tz375.

Just realize I am not sure what the OEM Needle Jet number is for the 76 GT250. Please for give me if I said it wrong. But I know that the 169 works as my last reply.

Overall the 169 was shorter 1mm
And mounting spigot/flange/ring was 0.040 (1mm) thinner

I'll double check the number and repost

[tz375]

Hi, I can't tell you what goes into a GT250, so that would be interesting to know for sure. It wouldn't surprise me if it was the same series as the GT550 (172). If someone has one they would like to measure accurately, I'd appreciate it.

We tried various washers under the main jet and the problem is that the needle jet is pulled down by the main jet and it will usually slide down in use. We ended up in one case with tiny rings around the flange on the needle jet, until we found a different jet that could be modified and was a slightly better fit.

We also tried an O ring there and of course it expanded and squeezed out.

Agreed that if a needle jet sits too low it has an impact on the flow rate/fuel curve. Taller spray tubes discharge relatively more fuel at high revs than a lower spray bar. I wouldn't expect much of a change in air flow difference though from one to another.

[Vintageman]

My 1976 GT250 is the later version (see GT250 Bulletin) and had 172 O-0 Needle Jets.
The new 169 series I inserted a 4mm washer. I had to drill the hole a little bigger to allow jet to pass through and file the outside diameter so it would slide into carb body.
I picked two washers that measure 0.036" so only off by 0.004" since the 169 shoulder is 0.040” thinner. Only difference.

The main jet can't pull down the needle jet since the 4 mm washer is directly under it and same diameter as Needle Jet. It makes the needle jet stand longer against the main jets larger washer(I choose a thinner one here too for allow main threads to reach better).
The air jet inlet still lines up well since the 169 series emulation air passage since it is oblong versus the 172's I took out where a bit smaller and round.

Have not fired this bike up yet… almost done

As far as the RD400 low speed crackling issue, the new Needle Jet (Non Mikuni though Oh well still shinny new) made it smooth. I also upped the pilots from 25 to 27.5. Pilots play a big role here too as you guys have stated
The RD ran much smoother. But now I heard an occasional skipping at low rpms. This was masked by the more dominant jetting issue before I suppose, but could now be felt clearly at low rpms: above 4.5K RPM up was fine and smooth

I changed the points with true Yamaha parts and bought two new aftermarket coils.
That did it!!!! It purrs even with the slightest throttle without any skipping, low speed and as it revs up.
So Needle Jet if eroded can be an issue (I could not get a good picture, I will need to section it to show erosion)
Large Pilot Jets help... Particularly with chambers.
I think if it too small pilots you can't really find a spot on pilot air screw that maximizes RPM at low speeds. After about 1/2 turn from closed they all (i.e. tried three size when doing T500) dropped RPMs as you close air screw since no air at all I suppose.

So I think steady crackling during light load or inability to climb to high RPMs with a constant small throttle position in my cases was a case of too lean…. Skipping was ignition.

The T500 all though smooth now, has a bit of a skip down low similar to the RD. I just bought a new OEM coils from Paul Miller and true Suzuki points elsewhere. If it helps I’ll state.

The GT550 all though smooth slow (previously reamed Needle Jet to next size to smooth bores and fix pinging) can’t climb to high rpms with small throttle. I upped the Pilots one step and will try that. They were stock… bike has chambers as well.

Running perfectly at small throttle position to me is awesome when cruising back roads! Love the two strokes!

Thanks

[Phil C]

very interesting thread. I have owned a few 2 strokes over a 40year period. I can tell you that I tried on a few occasions to try to eliminate the crackle or missing at low to medium cruising speeds and I was successfull by leaning out the fuel-air ratio by lowering the fuel needle (works on a Villiers 2T motor and also on my T500). The problem is that the motor makes less power and bogs when you snap open the throttle. Similar levels of success were achieved by leaning-out the idle screw but with problems:-The Villiers 2T(250 twin) motor would never start when hot and the T500 tickover speed was allover the place until it was returned to the stock setting.

Phil C

[Vintageman]

Phil C.

So far I have been the opposite. I had to richen the pilots for low end and make sure the Needle Jet are perfectly smooth inside.

For example, my 73 GT550 stock is 27.5 Pilot and ended at 35. This allows me to open throttle just a bit and the bike can rev to redline without crackling or pinging. Float height make big difference

I just finished the the 1976 GT250 (with 169 Needle Jets). It has D&G chambers for an Rd400. I tried it without chambers and then with. The chambers makes a crazy difference more power and much more top end. In fact this later design, has no low end torque, but has a definite powerband and pegs the tach easy. It is getting only 31-32 mpg (check when all stock jets). Not ridden easy, but notallows hard either

But at light throttle it crackles heavy. Open a little more and its smooths, but accelling too/ can't just cruise without crackling (unless one gear higher than I like and make it lug). My whole reason for this post. I went from 30 pilot stock to 35. I think it helped, but still crackles, but at 35 pilots I have to run >2 turns air screw for max rpm at idle.
Maybe I still need to go larger yet? I should try need clip as you said, maybe leaner, maybe richer. But I must say this little 250 accelerates quite well. The extra boost ports sure makes a differnce on the 76/77 gt250, but very little low end torque. Hard to drive this thing slow... wants to go fast. maybe this one is just going to crackle no matter what I try.

[Vintageman]

I definitely find it hard to tell if too rich or too lean. Not good a plug chops (unless I live in a flatter area and less traffic there too).
The 1976 GT250 I had started with 30 Pilot, then 32.5, then 35 No change really it crackles (or maybe better yet Gurgles). Yeah plug chops seam to show dark plugs If you give a whisker bit more throttle, no smoothing out. If you give even a bit more it accelerates real hard (fast little guy).

I went back to 30 and Needle Clip 4 truly better. I then went to pilot 27.5 and 5th clip (yeah, should change only one thing at a time). The mpg went from 31 to 42 mpg! Smooth, but lost some of the strong acceleration pull , a bit flat. I miss that hard pull. Let off throttle and nice Vroom----- Vroom ------ Vroom ---- sound out of exhaust (before it was just quite)

Stayed at 27.5 and back to 3rd clip. Still same low as above, maybe too rich after that. Your snap thottle back to from 1/2 to 1/4 it will gurgle for a second before it cleans up.

I put needle to fourth clip, but was dark (deer jumped in front of me, near miss) and really did not try it but optimistic. Since I raun the 169 Needle Jet and shimmed it about 0.012’ too low versus OEM, 4 th clip may better. If I get mid thirties MPG and good accel, I settle here or go back to 3 clip with 0.012 shim and done either way

The whole point of this reply.

-It should run smooth(er) slow… even if it runs well running briskly.
Trying to tell if too rich or too lean (for me) is hard. Couple more dozen bikes maybe. Crackle (lean) versus Gurgle (rich)??? Yeah what the heck does that mean/sound like

-Corroded parts (e.g. Needle jet) can make it very hard to tune. But very low end is Pilot Jet

-Float height makes a big difference. Just 0.020” for example. If too lean or too rich across the board try adjusting float height.

-If you have for example 35 pilot or 25.0 and tune for max Idle RPM, the air screw is always somewhere about 2-1 +/- 1 turn. So for me the air screw just tunes what pilot is in there and if you have wrong size pilot for your tune goal, running air screw out or in really doesn’t help you unless you are near spot on with the Pilot size


Curious about others experience