Throttle needle vs airscrew adjusting - 2stroke

[mcferreira100]

Can anyone explain the difference between adjusting a throttle needle up/down versus the airscrew on the carb? The way I understand is they are both affecting air fuel ratio.

[DirtyDT]

The airscrew adjusts the idle speed only. It raises the slide and allows more or less air into the carb. It should only be used to adjust the idle speed when stationary.

The throttle needle alters the fuel/air mix and is most effective at mid range throttle. It acts as to restrict fuel. As you open the throttle the needle rises allowing more fuel into the carb (richer). The needle is tapered so as it rises it allows the fuel to enter the carb quicker. On a needle there will be a number of slots to allow the initial setting. If you visualise that moving the circlip in the slot down (towards the tapered end), that will actually raise the needle at idle and therefore, because of the taper, allow more fuel in. If you move the needle circlip up (away from the tapered end). That restricts the fuel more (because of the taper) and allow less fuel in.

Once the throttle is getting mainly open then the needle will not affect the fuel flow and the restrictor becomes the main jet ( that sits below the needle).

On idle the pilot jet is used as the restrictor.

So:

Airscrew - just for Idle

Pilot jet - Just for idle and low revs

Needle jet - mid range revs

Main jet - higher revs

[YPVS TONE]

The airscrew doesn't raise the slide, that is done by the throttle stop screw. The pilot airscrew alters the mixture for the idle circuit. The screw is usually a tapered screw which restricts or derestricts the air for the idle circuit which usually means when you screw it in it makes the mixture Richer or when you screw it out makes it leaner. Most manuals will give you a base setting to start with i.e 1 and 1/4 or 1 and 1/2 turns out from being screwed in until lightly seated. As with most carbs there is a certain amount of overlap between the different circuits.

[Airhead]

needle position is effective from approx 1/4 to 3/4 throttle

airscrew (mixture) is effective from approx idle to 1/4 throttle

[dt502001]

And to totaly mess you up the pilot jet also effect's the wide open throttle mix of fuel and air.

It play's a very small part but does effect the over all mix at wide open throttle.

So yet again I will say for a street bike best to run it a we bit fat/rich and find a failrey even mix,your never going to get a 2t twin to run smooth at 3500k rpm to 4000k and not hunting for revs.

Instead of having a boost bottle they opted to use the joined ballence tube from both intakes to try and even out the back pressure between the 2 cylinders,,, and the rd 350 suffers for lacking a power valve for mid range performance below 5k rpm,,,, as 5 k is the ideal mid range for your bike.

KEV if you want to jump in and tell him how a 81- 82 rd 350 runs now would be a good time,,, IE ride it like you stole it and rev the duck out of it all the time afer the break in !

I am going to leave it at this if you keep fucking with you jetting without doing some deep research your going to blow your bike up long before you you get the jetting right if you keep riding it,and

Thats why I won't build a 2t for anyone unless I break in the motor and am involve in the cleaning and setting on the carbs,and why 2t got such and bad rap for blowing up. To many people who have no idea as to how to set them up and drive them thinking " it runs so all must be ok" and fiddel with shit they know noting about.

You haven't even done a plug chop to know where you stand on the mix and have no base point to start from to know where to go from.

Instead of creating many threads of how to help you get your bike set up keep them all in one,having the spring off the oil pump durring break in would have had it rich some times and who knows all the important info,,,,,, your jumping at straws and I have tried to help you,and others won't have read that so how can they help you when they don't know all the details?

[DirtyDT]

The airscrew doesn't raise the slide, that is done by the throttle stop screw. ..................

That's what I get for doing something else whilst typing an answer.

[mcferreira100]

Jeez.. relax DT50..u having bad day or what? If u don`t feel like commenting anymore just ignore the post.

For your information I havent actually fiddled with anything except that airscrew..done 500km now and it hasn't blown up yet(if these bikes will always have a 3500rpm flat spot then its running fine..not going to cut perfectly good plugs to pieces).

Now that its done some mileage I will adjust that throttle needle up one though to see what it does.

[KevtheRev]

Ehh, you don't have to cut up plugs to do a plug chop . It just means examining the electrode ends to see if they are in the right temperature range . It's the only surefire way to tell if a two stroke bike is running properly .

[mcferreira100]

I googled this procedure to do a plug chop.

1. Get new spark plug(s), but don't put them in yet.

2. Warm up motor to operating temp on old plugs.

3. Install new plugs.

4. Accelerate through all gears to top gear with throttle pinned.

5. When you hit the top of the top gear, hit the kill switch and

pull in the clutch.

6. Remove the spark plug(s) and ride home on the old plug(s).

7. Hold the new plug in a vise and use a hacksaw to cut the threads

away from the center electrode.

8. You should see a brown 2mm "smoke ring" at the base of the

white center electrode if the main jets are dialed in perfect.

9. if the smoke ring is darker than chocolate brown or taller

than 2mm, you're rich on the mains, but if it revs clean to full

throttle, you can leave it there and be on the rich side.

10. If the smoke ring is smaller than 2mm or lighter than chocolate

brown, you're lean on the mains, go up a size and try again.

[DirtyDT]

Put new plugs in

Ride around for a few miles

Find a good stretch of road where you can ride at high revs in the same gear (doesn't have to be top)

Ride for a couple of miles in that gear at high revs - a straight, fast road with a lay by or pull in is ideal.

kill the switch and pull in to the lay by - Make sure it is safe to kill the bike engine safely as you will be in coast mode and use the kill switch, not the ignition.

Take out the plugs - they will be hot

Check the plug colour.

[KevtheRev]

I googled this procedure to do a plug chop.

1. Get new spark plug(s), but don't put them in yet.

2. Warm up motor to operating temp on old plugs.

3. Install new plugs.

4. Accelerate through all gears to top gear with throttle pinned.

5. When you hit the top of the top gear, hit the kill switch and

pull in the clutch.

6. Remove the spark plug(s) and ride home on the old plug(s).

7. Hold the new plug in a vise and use a hacksaw to cut the threads

away from the center electrode.

8. You should see a brown 2mm "smoke ring" at the base of the

white center electrode if the main jets are dialed in perfect.

9. if the smoke ring is darker than chocolate brown or taller

than 2mm, you're rich on the mains, but if it revs clean to full

throttle, you can leave it there and be on the rich side.

10. If the smoke ring is smaller than 2mm or lighter than chocolate

brown, you're lean on the mains, go up a size and try again.

Well I never , I never heard of that method before . I use the same method as DT above .

[mcferreira100]

Well an interesting day with highs and lows. Set the throttle needles one notch up for leaner mix. Started bike and it was making weird sucking noise and backfiring out one exhaust (almost sounded like something wrong with reed on one side). Took it slowly down the road but noise persisted. Wondered if I put slide in wrong (I`ve heard this can happen). Checked slide was ok but then noticed pipe between carbs was off..my bad. Problem sorted.

The flat spot is better on the new needle setting. I set airscrews back to 1.5 turns out straight after reassembly and went another 1/4 turn out after a run. Flat spot is virtually gone.

I do notice though that after 7000rpm it seems to bog down a little when I would think it should pull even stronger.

[Preload]

Well an interesting day with highs and lows. Set the throttle needles one notch up for leaner mix. .

The quote in red do you mean you lowered the needle! ie set the circlip one slot higher? this would weaken the mix.

The needle slots on Mikuni carbs are spaced, every space is an 1/8th more fuel or less fuel dependent on which way you go.

A tip a lot of 2t racers are richening up there bikes due to the ethonol in fuel these days as they were nipping up. So due to this I run my 350B needle on the 4th slot from the top which is slightly richer than standard.

I have the Mikuni manual somewhere!

[mcferreira100]

Clip is on 2nd notch from the top..ie: leaner mix. Yea 4th slot is running pretty rich.

Must say I don`t intend to go racing or anything like that and I tend to ride like an old man (this bike not actually suited to my riding style). Just ringing its neck now and then during run in and thereafter just to clear its throat.

I would like to know as I`ve been told the rev counter is slow due to standing so long..could anyone with and RD let me know what revs their bikes are doing at say 62mph? Will give me an idea what this rev counter is doing. Could be thinking I`m at 7000rpm when I`m actually at 8000 (I could have been in the red already without knowing it).

[KevtheRev]

Depends on the gearing . You'll find that the power runs out a good bit before the redline , even if it does get louder .

[mcferreira100]

At 62mph its running 4000rpm in 6th gear.

[Preload]

I know its a different model but my 350 A/C is around 5000 @ 60 I would think the gearing would be close, My rev counter was running slow as well but its getting better. The grease get hard over time if not used so I left the bike in sun which helped. You may have an RPM gadget in your garage they are built into strobes, multimeters etc these days.

The needle setting on my A/C is only 1 groove different to standard so not overly rich about 12%, on your LC I wouldn't go leaner its not worth it, also its not just the mains that is effected by ethanol all the jet are running leaner, the racers recon ethanol reduces the octane about 10% which makes my bike 2% rich from standard.

[mcferreira100]

Makes sense as it seems to run out of puff after 7000..which if rev counter is slow..would mean its actually at 8000.

Would hate to change needle setting back to middle setting as the bike is running a lot better. As I said the 3500rpm flat spot is all but gone..and I ride around that rev range most of the time in traffic.

[dt502001]

Properly set up thoes bikes will scream to 11k rpm that's 2k into red line,so even if your tack is slow wich I find hard to belive that it could be out more than a few hunderd not a full 1000 rpm,your bike is starving for fuel and will blow up eventualy!

[mcferreira100]

Would a standard bike rev like that though..you not maybe referring to one with modified pistons, jetting, exhausts, porting and so on?