Dellorto Phbl 26 Carb - What Jets?

[toman21]

Firstly i would invite you to read this link : http://www.advrider....ad.php?t=861765

This is a segment from the link above

"Basically, one way to test the fuel screw circuit is during a sudden 100% full throttle snap from idle check. The vacuum of the engine will draw fuel INITIALLY from the fuel screw circuit (the progression circuit in delortto speak) and if too lean, the engine will bog and or die. If too rich, the revs will just build slow. Its very easy to detect a lean bog , so thats what you use as a indicator - so you start from a known lean condition and work towards getting rid of it. Ideally, what you want, is enough fuel to prevent a lean condition during this wide open snap of the throttle and "just" eliminate the lean bog and alittle bit more for safety (said in Italian this sounds better)The safety adder portion should actually be tested as well - you do that with a throttle chop. Find a big steep hill (or a cranberry bog) that you can climb under WFO throttle and listen for any lean pinging after you chop the throttle at the top. If you hear any pinging, you need to richen the fuel screw more. Use this hill to verify your main jet too while your at it!

My Procedure on PHBL:

Make sure bike is good and warm. Never play with the setting with a cold bike.

Turn fuel screw in to about 1 turn out or so. (or whatever setting you know will produce a bog - you'll know it when you hear it)

Turn down idle speed to very low - enough so it doesnt stall, but very low.

Wack throttle wide open very quickly. Really make sure you get a good fast full turn snap. Listen for obvious lean bog.

richen fuel screw in 1/2 turn increments until the lean bog goes away. It will become a more subtle transition but still there.

repeat with 1/4 turn settings near point where lean bog goes away to improve setpoint accuracy. Once you find that gray zone where the bog is "just" gone - add another 1/4 to 1/2 turn richer to the fuel screw.

I find by lowering the idle way down, this really helps fine tune the "just" gone or subtle transition and I find I can get away with only adding a 1/4 turn*. The PHBL has a very sharp and defined transition so is very easy to set, so even 1/8 turn resolution of the transition is attainable once your used to it. (Unlike a VHST or even a Keihin that are alittle less defined IMO) If the weather gets really hot and humid, your safety adder from setpoint may grow from the 1/4 turn to the 1/2 turn range. (*test on hill - once found use that or be safe and use 1/2 turn)

The above can take some seat of the pants practice, but once you calibrate yourself to your bike, and you become repeatable, its' really a quick and easy thing to set just right. I will test it anytime the weather changes dramatically. Hot and humid vs cold and dry would be at least 1/2 turn different - so worth doing.

Theres a video example of this is one of the jim snell motor video's. probably the last one near the end.

After all is said and done, you should be in the 3 to 3 1/2 turn out range on the fuel screw on a average day near sea level. If not, your either jetted wrong or the carb/jets are dirty. (on a GG 250 -300 anyway)

If done correctly, the bike should have instant throttle response and the revs should drop quick too.

Std jetting recommendation from GGUSA is:

2004-05 TXT280-300 PRO WITH PHBL26 (26mm) SEA LEVEL TO 3,000 FEET

MAIN: 118 (115 OK, NOTHING SMALLER!)

PILOT: 36 (38 OK)

NEEDLE: D36 (3rd, or 2nd CLIP POSITION DOWN FROM TOP)

FUEL SCREW: 3.5 TURNS OUT

FLOAT HEIGHT: ARMS IN LEVEL PLANE WITH FLOAT BOWL BASE (PARALLEL)"

I found that i had a lean bog when on a 33 pilot when i quickly opened the throttle from idle. There was no bog actually driving about but the lean condition could be affecting bottom end grunt. I had been using the 33 for a while so was used to the smooth bottom end and power (or lack of).

So i then used the technique above to get rid of the lean bog and found that it dissipated when i had a 36 pilot at 3.5 turns out on the fuel screw. But then i drove the bike to find that the bottom was no where as smooth, it was as if the bike could put along as slow. The idle and driving about was a lot more "PUTTY" My problem is that i don't know if that it normal and i am used to the 33 being so smooth.

I live in the UK and the bike is a sherco 290 2008(no air leaks, clean filter etc etc) What i have heard is the the fuel quality has decrease over the years thus the lean bog on 33. There are many that say that a 36 is what is needed now, but i am not sure if people actually use it. However in my case it does seem to be true as it cured the bog.

I would like to know what set up you are using

Thanks

[copemech]

Long story made short, the bikes will run ok on the 33 pilot, and Leaner is Smoother, yet you may not achieve the other desired traits.

Beyond 3.5 turns out seems little difference, as the jet is maxed out anyway, you may try to lean the 36 down to 2.5-3.5 turn range, or whatever level you can accept as idle may get a bit floaty, got to find happy spot!

Just how often DO you wick it wide open while riding, prolly not at all normally!

I run a 38 jet(on our fuel) at around 2.0-2.5 turns out(as lean as possible), stops the pinking as well, yet it is more sensitive on the day. Not too poppy, yet that is what clutches are for!

[toman21]

Right oh, i try the 36 again. Is "Pinking" engine rattle when you come off the throttle, which is cured by more fuel???

[copemech]

Yes!