Carburetor needle adjustment?

[bearsfolks]

My 83 R80RT was worked on at a shop a couple years ago. As an experiment the mechanic lowered the metering needles to lean out the midrange. I think it is now too lean. I have to run with the choke for at least 2 miles on a warm day before it will run right. It feels like it is running out of gas at partial throttle. How difficult is it to raise the needles on the slide? My Clymer manual is not very clear and talks about loosening the manifolds and removing the fuel lines and choke and throttle cables before removing the top of the carb. These are Bing Constant velocity carbs. If I remove the screws holding the diaphram covers will the thing explode like a Jack-in-the-Box?
Thanks.

[20774]

I've done as you suggest before, loosening the 2 or 4 screws holding the top of the carb and diaphragm in place. Nothing will sproing on you...just lift off carefully. When reassembling, make sure the needle finds the jet at the bottom of the carb throat before you ram things home. Also, the diaphragm has tabs on it that fit only one way.

The needles on my /7 carb move in and out with 90-degree twists. I'm not sure how yours work...I've heard about other carbs that have a spring clip or c-clip of some kind. You might want to measure the amount of exposed needle before and after you move them just to be sure they are set the same.

If you're lucky, all this fiddling may not require a resynch of the carbs, but it gets tricky. You'll have to loosen the jamb nuts that hold the throttle cable in place. These jamb nuts are what's used to set the balance between the carbs...moving them technically changes things. But you can perform the surgery, check that what you've done helps things, and then go back and perform the resynch later...certainly recheck the settings.

Kurt in S.A.

[tuber1]

nah, you can take the top off, but the cables are attached to the cover,look where they are screwed in. the needle should have 3 adj spots; on the old /6 we used to drop the needle to improve mileage, so ppl could brag about 50mpg, but sometime you get too lean under load. the needle has to be turned and lifted very littl...

[pmdave]

The needles on most Bings are held in position by a spring that fits into slots on the needle. The slots are positioned at 90 degrees to each other, so you insert the needle, turn until it clicks, then turn again until it clicks in the next slot, etc.

It's VERY easy to get the needle in the wrong groove. I second the motion of measuring the needle length installed in the slide, and making sure both needles measure the same.

Bear in mind that the pilot jet also affects mixture. You might consider leaving the needles where the are, and going up one size on the pilot jets (the ones at the front of the carb inside the bowl)

It should go without saying that if the carb isn't clean or adjusted correctly, fiddling with the needles/jets isn't going to solve everything. Clean out the main jet, clean the enrichener valve and replace the gasket, check the float level, and ensure that the tube in the head is tight and the rubber hoses are good. While you're at it, replace the air filter (if it's old and yellow)

pmdave

[20774]

pmdave -

You mention the pilot jet...I'm not sure what that is. IIRC, there are basically three jets - the idle jet, the needle jet, and the main jet. The idle jet is in effect up to about 1/4 throttle. The needle jet comes into play from about 1/4 to 3/4 throttle. Above that, it is only about the main jet. The original poster discussed running out of gas on partial throttle...sounds to me like the problem is the needle jet and the position of the needle relative to the needle jet. Moving the needle is a gross change in mixture. Changing the needle jet results in smaller increments which might improve the lean situation while maintaining reasonable gas mileage.

Kurt in S.A.

[RandyB]

The best advice I've received (so far) has been to return every thing to factory settings, then find somebody with a colortune and twinmax/carbstix or similar. Don't forget to level them.

[ISAMEMON]

love them colortunes

[RandyB]

This was the first time I'd seen them. Turns out the factory setting on my carbs was a bit lean. Neat gadget.

[pmdave]

The pilot jet is also known as the "idle jet".

Although popular wisdom is that the pilot (idle) jet only functions at partial throttle, it does not "shut off" at higher throttle settings.

Years ago, when I had some carb problems I couldn't solve, I talked to the "Bing Man" at a rally. In a few minutes he set me straight, provided a different jet, and the engine ran great.

pmdave

[MANICMECHANIC]

The main jet is connected to the needle jet, also called the mixing tube. These devices interact. The idle jet(low-speed jet) is always in the mix, also. It works with the idle mixture screw to set your low-speed mixture. As the throttle opens further, the needle withdraws from the air-mix tube causing more fuel to flow through the main jet, until, when the throttle is full open, the main jet does most of the work, with the idle jet just barely in the picture. On other types of carbs, different needle profiles are available that will permit the finer tuning of the carb over the throttle opening, and also take into account the altitude and atmospheric conditions. For most purposes, unless you've got lots of time and money, it's not worth the hassle. Sometimes good enough is good enough.

[Stuff2c]

www.factorypro.com
Then look in "product support" CV Carb tuning.

The best guide on the web!!

[20774]

I'm just going on what the Bing manual says. They call it the idle jet. There's another statement in their book regarding the effectivity of the idle circuit. "When the throttle valve starts to open, fuel begins to enter the air stream from the main fuel outlet while at the same time, it begins to cease flowing from the idle circuit. In this manner, we achieve a smooth, unhesitating transition from idle to part throttle range. How smoothly this transition occurs is dependent upon (1) size of jet, (2) idle air adjusting screw setting, (3) the underside profile of our slide, (4) the combination of needle jet and jet needle, and (5) mixture correction components."

It sounds like if everything is optimum, the idle cirucit/jet is ineffective with more and more throttle. If not optimum, then I guess it stands to reason that the idle circuit could still be contributing...

Kurt in S.A.

[pmdave]

I know what the books say--that the idle jet only functions at low power.

The symptoms on my '78 R100s were a slight hunting at WFO with the needles dropped to enable smooth running in midrange, or with the needles raised one notch, smooth running WFO, but too-rich at midrange. Increasing main jets one size caused too-rich at low end, and not much better at WFO

I explained my problem to the "Bing Man", and he suggested that I probably didn't run much at WFO, until I mentioned pulling a sidecar. He suggested larger idle ("pilot") jets.

Increasing the pilot jet one size allowed the needles to be dropped into the larger main jets, and have smooth running throughout the range. What really surprised me was that the engine ran cooler, had smoother power throughout the range, and also got better fuel mileage.

Again, if you are not happy with the mixtures, I suggest contacting the Bing Agency and asking for advice.

pmdave

[Stuff2c]

I've been trying to get my Ducati dialed in and have found that there is NO one fix/setup that works for all. It's definitely an art with a bit of science and LOTS of trial and ERROR. I can now remove and reinstall my carbs, fully chance the jetting in so little time that it would make a NASCAR tire team envious.


And I think I'm getting close to getting it right.

[plaka]

you can remove the top of the carb w/o touching the cable adjustments. On push/pull chokes loosen the retaining screw for the end of the wire. pull up on the wire. On spring return chokes, unhook the spring, lift up on the choke arm and rotate the barrel end of the cable out of the arm.

On throttles lift up on the throttle arm and rotate the barrel end of the cable out of the arm.

Replacement is the reverse.

I use independent chokes on my carbs (38 and 40 mm bings) which simplifies things. One knob and cable per choke. No springs on the 40's. As easy to reach and use as the fuel petcocks.

The needle and needle jet have a 'curve' of fuel vs slide position. The bottom line is always the cross section of the orifice available to flow fuel. That section is the area of the needle jet opening minus the section area of the needle sticking into it. I haven't measured them up and plotted the curves but it so simple to mess with either one (especially the jet) that you can nail it with trial and error. So long as WFO on the needle is a bigger effective jet than the main, you're good.