Low speed carb tune (to crackle or not)

[Vintageman]

I need some advice for carb tuning experience

I have serveral 2 strole bikes . Some when you are just cruising (throttle open a little no hill just rolling resistance) the speed is constant, ride is easy, the sound is smooth, speed is steady, no crackling, purring. In fact if resistance on road gets lower the engine will rev up to redline if that position is held, perfectly.

Others crackle. You know that crackling exhaust sound. It is running along, ho;img speed, but not smoothly. It will not rev up anymore if resistnace is less at that position.But, If you give it a tiny bit more throttle. It smotths out then, but you are accelerating too, not like my first paragraph where it is smooth and holding speed steady. .

How do I remove the crackle tune. Is it too rich or is it too lean? Is fuel droplets and not atomizing Pilot air not enough. Or lean Not enough fuel. Arg! Or something else too. I think this is fine tune I wish Inew if this is too lean vs rich

Have I explained this well... I hope.

please advice

[tz375]

Do the ones with chambers crackle and the stock pipes with airboxes purr by any chance?

[Vintageman]

Tz375,

Thanks for a reply. Again, I make a mess of my English, typos, and on top of the fact it is hard to describe a sound in words.

All my bikes have chambers and all my bikes have stock air filters and intake passages. These are all VM carbs

Crackle is not the best word, but how it sounds to me. It does not run a consistent frequency of sound. If you could, at that time, lift the back tire off the ground, the ones that are crackling along probably would hold the same speed/rpm and not climb to red line. The others that purr would run up to redline while holding the same thottle position.

Note, that the ones that make a crackle/inconsistent sound, all you need to do is open the throttle just a tiny tiny bit more and it purrs. But, now you are NOT holding the same steady speed for you are accelerating slightly. So for these to purr, your riding style is to accelerate a bit let off accelerate a bit let off…. Repeat this. Or just hold steady and crackle along. The ones that purr just hold steady and keeps that one speed, purrs and is so ready to rev up given a slight change in road resistance. Perfect Tune!!!!.

Note that this state of tune I am discussing, is only when one is cruising a street or road real easy like. A lot of riding is like that me. A little more aggressive riding all is well. Since I have some that purr and other that don’t I have become aware of this and wonder why. I would like all to purr. Maybe it’s like trying to ask a four stroke with a fat cam to idle smooth.

But unlike a cam it does not matter if I am trying to hold 3k RPM or 5 K RPM. It is not RPM related it is small throttle position with light load r. It is when trying to cruise real easy with steady throttle position . A position where throttle is not opened much.

To possibly fix, with out spending a lifetime at it, I was trying to understand if this is a lean or rich case. Or, maybe the fuel is not atomizing well with air. ( I should check needle jets again to ensure it is smooth and shinny inside and not rough…look under magnifying glass). The pilot jet air jet combo is not right?. Maybe carbs a bit out of sync?

I run fresh batt, points, new coils in some cases, and Iridium plugs (rarely if ever do I foul plugs once going. Sharp blue spark from Iridium’s).

I think it all has to do with fuel/air mixture?

GT185 = Purr
GT380 = Purr
GT550 = Crackle
Yamaha R5 = Purr
RD400 = Crackle
Rest I can’t remember now

Thanks

[Suzukidave]

It just about sounds like your describeing " 4 stroking " and i think thats pilot jets ?

[tz375]

It's really hard for most f us to describe sounds and the way something responds. It sounds, pardon the play on words, as though you are describing riding along with the throttle either closed or just cracked open.

In that position, Dave is correct, it's running on the pilot jets and it could be a lean OR a rich stumble and it shouldn't be too hard to work out which. Adjust the air screws in 1/4 turn and try that and then another 1/4 turn to confirm. And then try 1/4 turn out from where you started and then a further 1/4 turn.

With any luck, that should identify which way improves and which makes it worse. If you got really lucky, that would be enough to fix teh issue, but you may have to change pilot jets.

If the stumble is at slightly wider open throttle it's probably the slide cutaway and that's starting to get a bit more expensive to fix, so let's the hope the pilot jet setting fixes it.

Of course, I am assuming that the carbs are opening at exactly the same time and that there's no synch issues to take care of. It could be that simple.

[Vintageman]

Too rich or too lean?

Pilot Air screw out more than it should be or Pilot in more and no difference I can note. So if Pilot Jet do I need to up or lower jet size. Wish I knew which way first. Maybe I’ll check to make sure air passages are perfectly opened
I had two identical low miles 1982 Ski Doo Everest. 503 Rotax twins, Single 36mm VM carb very Low miles, sat a while. both ran well, but one just ran a bit rougher in a similar light load cruising situation. If I did not have two I would have accepted it.
But since I did , I swapped parts between the two and it was the carb. It was the Needle Jet. It was eroded. I could not tell until I had a microscope to view it and The surface was sean as lightly pitted not smooth ( I saved it somewhere for it isn’t much!). It was not egg shaped just corroded a bit.

Changed the Needle Jet and smooth. So fuel rich/ not atomizing ( I am just remembering this now and may be a potential issue).

I should have changed all brass carb parts, Not always are the Needle Jets/ Jet Needles available.

I am still not sure if too rich or lean or droplets versus fine atomized mist.

Keep letting me know what you think. Carb jetting is a pain in %^& for me. Nor do I love taking carbs a part over and over.


Thanks

[Vintageman]

I benched sync is the term I suppose.. and carefuly as I could.

Or like on the GT550 used the dots on slides very carefully.

I suppose I could raise or lower one slide (twin) and see if I can make it better worse.

Yes I agree rich or lean I think they have the same affect down low.

Yes I am just riding slow and easy, very small throttle changes, and lower range of opening. If I did not have some bikes that purred I would not have noticed this type of tune I suppose

Another way I can expalin it. Say you are in nuetral and Idling. Some bikes just open throttle a tiny tiny bit, hold and the engine will rev up smoothly, tach no jitter, all the way to redline almost a run away,

I think I like lean low throttle response??? Maybe the crank is out of balance some too as I right, I'll have to see if I feel vibration. I am pully straws and going nuts too

It got cold again in NH. I'll try carbs again.

My T500 is having same issue and may start here since I just got it running.

I'll try to get a video maybe.

It is subltle is not major, but when some bikes run perfect here (IMO) All should be able to

[tz375]

I would start with one bike and make it an easy one - so it's the RD400 to start with because it is so easy to change jets on that one - once you get the airbox back out of the way.

Those engines are touchy on the needle jet and there are not a lot of options available. You could try two easy changes and see the effect of each. Try going up one size on the pilot jets OR raise the needles one clip position. RD400 pilot jet is #25 in most models and we went to #40 on an F model with DG pipes and a stock air box. It was slightly rich at that, but safe.

[Vintageman]

Tz

That makes sense

Yes, Air box is a pain. I did raise and lower needle jet. Not it. This bike's midrange (fiber reeds) and top is real nice.

After some reading last night, I found article that says if you raise the idle speed (say 2k RPM) on the carb you are trying to set Pilot air screw for max speed/lean you can better notice the changes with screw setting. I did this on the T500 and yes I could better tell. On that bike starting 1/2 turn out it reved up to where 1.5 and 2 turns was best and beyond 2 turns no diference. So set at 1.5 as it is to be per spec as well.

I can't remember the pilots I used on the RD400, but I'll try tuning air screw again as above firts, If not a fix I'll change pilot richer. I also think if I take it apart I'll get new Jet needle and needle jet if I can. I have an nice electronic microscope at work and will take pics. Believe me on how just a little pit of pitting can really mess up smoothness

I also remembered on the R5, that purrs, was originally a too lean pilot with D&Gs and bucked regularly. I think I went up to next size on pilot and fixed 90%, still lean side but real responsive and Vroomy sound, but it was before too. I hear people on air cooled RD site for example copy the R5 carb somewhat and mod their RD350/400 accordingly (Dave Frisk or something like that change they call).

Well The RD400 is not out yet for the season and the T500 is so I'll try this first. The GT550 is out to

The 73 GT550, I wish I could find a lesser cut round slide. That one I have the pilots a bit rich and the needle clip stock 3rd for best MPG. I had a ping when I backed off from 3/4 throttle to 1/8 quickly. 80% gone. I think I'll raise the needle and go back to stock pilot and put up with a couple more mpg loss for low speed cruising smoothness. The issue on that one I feel is right between the two circuits pilot and needle: round slide cut.

I have extra stock slide. I had thought about milling some off bottom for I can't change the angle to be less of course, but could lessen its height. Not sure if the slide will come all the way down. Ever done this? Any 2.0 cuts available for it?

Thanks. I'll let you know

[tz375]

Ah the old "Dave Fix" That one makes no sense. They start with a bike that runs fine and then port it or change exhaust and can't jet it right so they change to a piston port set up. How does that make any sense? The PP set up is designed to be leaner at the bottom than a reed valve set up to allow for the reversion at low speed on a PP motor.

So they then try to lean that out slightly and still end up with a rich bottom end. Why not just jet it correctly to begin with?

The answer is that a tuned RD really needs different needle jets . E and F models in the rest of the world got a 284 series needle jet that looks like a cross between a Primary and Bleed type and has a taller shroud that changes the fuel slope. That was based on work done by Leon Moss at LEDAR in the UK. LEDAR used to sell replacement needle jets and they were the fix for all RD/LC/RZ (YPVS).

I have never seen 284 offered by the aftermarket, but it's possible that Yamaha may have some I guess. Last I heard, LEDAR jets were available for the LC and those might work. IVAN was thinking about making 284 series but I have no idea if there was any demand.

[Vintageman]

Yes,

My 77 RD400 has the stupid four stroke looking factory Needle Jet. It has to pull air into via a small tube from air box. Must be like breeathing through a straw

I do see that Needle Jet and Jet needle are at HVCycle for example for my 77 if it proves to be warn.

So what year would the E and F be. These were sold in Europe when USA said no more after the Daytona in 79 (Isn't that even different yet).


Do you recall if orfice was the same on the 284...same jet needle used. Did it still pull air to emulse the fuel via the tube from air box?

Back to pilot jet. If you find the the factory pilot air jet gives best lean fast Idle RPM does that imply OK. If still too lean and you do go bigger would you expect to end up say a turn out more? Does the pilot air jet (be one in there) ever need to be adjusted? I found it interesting that after 2 turns out it had no affect. I suppose the design is such after that screw make no restricyion diference or the air jet is the limit?

Also if the pilot jet is eroded a bit is the pilot circuit sensitve to that? Is it worth putting in a nice new shinny one? I am sure about that when it comes to needle jet and jet needle.

[tz375]

All RD's use a 4 stroke type (primary) emulsion tube/needle jet, but there are several in that range. The only two I know that incorporate a spray bar are the 284 series fitted to European and Australian E (78) and F (79) models. I didn't own a US spec RD400 so I have no first hand knowledge of what was fitted there. Earlier RD400 and 250/350 used a 175 series with no Spray Bar and they flow differently.

Air is drawn in via the main inlet opening. There are a couple of small air jets/drillings in the inlet/mouth of the carb. Those also have to be cleaned when a carb is cleaned.

The pilot jet has most effect at idle and just as throttle is opened and at that point the slide cutaway starts to have an effect as well. There is quite an overlap between circuits. I tend to get the right size jets and air screw settings for idle first and then I see how it works as I crack the throttle open. If it bogs at all at tiny throttle openings, I usually change the air screw setting or pilot jet. We're talking modified bikes here but pick up is more important to me than a steady perfect idle.

If I need it very rich at idle and then it's too rich as I roll it on, I may have to fit different slides or take a file to one. It may be possible to machine 1-2mm off the bottom of a slide and still have it close properly. And it's easy to get a larger cutaway size with a file. if you need to go that way.

It's also possible that on a particular bike, the pipes will not allow it to ever run clean at certain revs or certain throttle openings, but you can usually smooth things out and make it more rideable. Off hand I can't remember ever owning a bike that was happy running on almost closed throttle. Well OK my VFR did and a couple of Triumphs weren't too bad, but all my 2 smokes get cranky at zero throttle on the and a blip is a god way to clear its throat. RD had a drilling above the exhaust port to make slow running smoother and that hole gets plugged with carbon and oil.

[Vintageman]

Tz375 (et al),

I have several that run amazing at low throttle opening. Yesterday I took my T350 out first time this season (A GT250 with all T350 motor stuff and RD400 D&G chambers).
This one adds to real smooth at low throttle openings even up to high RPM.
The ones that are smooth have nothing to do with RPM really ( I am sure I need to be above say 3K RPM for example … I never tied to lug, just normal riding experience)
They are just jetted right at small throttle position it seems that works at a wide RPM range perfectly
I find for cruising streets and roads this is must and makes the ride very enjoyable.

I am now thinking crackling is lean situation. Again, if I just open the throttle only wee bit more it smooths out perfect, but also pulls slightly versus holding a steady speed with light road resistance. I think I am explaining this reasonably? I want it to purr at light load and hold speed

I have all new Jet parts for the T500 and will work on this next It has JEMCOS and will try this first
I will let you how I make out. Maybe some just don't like to run smooth at low opening as you said. I wish I knew if I was too rich, lean or not atomizing fuel if it is a jetting issue.

Does any ones GT/T500 run smooth (steady frequency exhaust note, not crackling/popping sound) at small throttle opening as I am trying to explain????

[stcyr]

Does any ones GT/T500 run smooth (steady frequency exhaust note, not crackling/popping sound) at small throttle opening as I am trying to explain????

When coasting or just holding small throttle settings while idling, 2strokes will normally pop and crackle but under a little load, they should purr. Tune for performance, not sound. Feeding any engine extra fuel under no load conditions just to remove the popping is a waste.

[Vintageman]

stcyr,

I am not talking about no load, or coasting. Although too lean crackle may not be wise hear either, but I don't care so much as long as I don't hole a piston

I am talking/care about cruising with very light load, What I mostly do. You know: a flat smooth road. Holding a steady speed and with small throttle position. Not accelerating or decelerating… steady and easy and smooth no crackling when doing this

Most of my bikes purr hear, but a couple crackle in the case above. It is a fine line, but when you have some bikes that do run smooth here it's a great state of tune IMO.

I am not sure if too lean, too, rich or fuel not atomizing as well as it should. But still believe it can be corrected... I hope. Maybe it is bike dependent

I tore down my 75 T500 carbs.

When appling a light into where the main jet screws in and then looking into the throat of the Needle Jet from the top, with a magnifying glass one of the inside surfaces looks a bit moon surface like. The other was not as bad, but still not perfect.

A new one is perfectly smooth comparably

I’ll try to take a picture with a microscope at my work.

The Jet needles were very good, but one had bit of a scratch early on.

I am awaiting a new rubber boot between Air Filter and carbs, since the one I had was hard.

I’ll let you know if this is the issue, I am optimistic since I did have this happen to me before with bad needle jet on two identical snowmobiles.

If not I'll work on Pilot Jetting ( I did replace this with new ones a well).

All Jets used are true Mikuni parts

Does any body T500 run smooth as explain in this light loading small throttle condition above?

I'll let you know soon as I know.