Low speed carb tune (to crackle or not)

[Suzukidave]

I have read that just from the up and down action and vibration of the needles can wear the needle jet out of shape so it flows wrong and this would show up at small throttle openings .. just a thought .. a easy test would be to retard the timing a degree or so to see if it would settle down the poor slow steady runner ? easy enough to put back if it didnt help .

[Blokhead]

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.

Early GSXR 750, and 1100 CV carbs have this same problem. The more miles, the worse it it is. People couldn't figure out why they never could get the carbs jetted perfectly at light loads and steady throttle. Turns out, the needles wore the needle jets oval shaped. I used to have pics of this, but can't find them right now. New needle jets fixes the problem. On some really high mileage gixxers the needles themselves, are worn through the hard anodized coating on them, and have to be replaced.

[Vintageman]

I put in all new jets, Needle Jet, Jet Needle Main, and Pilot in the T500.

It purrs now! No crackling. What a difference at small throttle positions.
Please note that I did up the pilot from a 30 - 32.5.

To try and be fair to see if needle jet only fixes things I started with 2.5 turn out on Pilot air versus 1.5 and it was better at the start.

I went to 1.5 o n Air screwand maybe even a bit better but only when very small throttle position accels. I did several small throttle opening accels up to redline and very smooth. The JEMCO chambers definitely make a power surge when they come into play at maybe 5000 RPM.

Right about there is an occasional stumble, flutter, pinging (when real light throttle) and maybe a bit too lean pilot still. I think l I'll try a 35 pilot. I find the air screw really does not much change until you get say less than half turn out and idle drops off. The pilot jet is what is important I suppose and maybe 3/4-2/5 turns is just fine or minor compared to jet itself

If I can get my microscope to take a pic of the corroded Needle Jet I will. It is not the Mileage wear as the case others have mention it is actual corrosion/pitting from sitting many years.

This T500 I just recently finished and really only ran well at say 1/4 throttle and beyond before. Always had good top and still does

Onto the RD400 next. This one may just be pilot but will inspect all jets here too.

Thanks

[Blokhead]

Found a couple of pics. Needle jet doesn't really look that bad, but it really screws up the carburation.

[Vintageman]

Blokhead
That one is real bad due to wear. There is another type of wear caused by corrosion inside just below opening. It should be shinny and smooth. You have to see inside. Tolerance are so tight on this parts that seems to be important issue that can affect proper metering or atomization.

Update, I tried a 35 pilot on mine and actually now too rich. I went back to new 32.5 and two turns out. If you turn the idlee up say to 2K RPM on the carb you are setting pilot screw go slow, you can tell the optimal (highest RPM) position so much easier. It is still very subtle.

If remember correctly the original 30 pilot was highest idle rev at 1 turn out on pilot air screw. Yes, I did not go back to 30 pilots again so maybe 32.5 versus 30 helped too

I due suspect some of issue in this cases was the pitted Needle Jet internal orifice, not egg worn like your important example.

So be aware of this. If your easy cruising is bit crackly (crackle crackle) not perfectly smooth... vroom, vroom , it may be the needle jet orifice erosion and or pilot jet too small or both.

The 159 series works well too (can't get 188) in the T500. The round main jet screws in just as well as the hex. In fact, I had to go back and verify the 159 is designed for hex not round mains for the round fit so well and did not harm any threads.

I did use a 115 hex which is about a 100 round. I wanted to go a few percent on mains to be safe with chambers and I had new 115 hex. It works well. I hope I never have to take the carbs a part again. But much easier to take off battery box too if you do on this bike to get boot to fit best.

Thanks for everyones help.

[tz375]

You are correct. 188 take a large round main jet with 0.8mm thread pitch and 159 series take a large hex with 5mm x 1.00 pitch. A Round jet should not fit into a 159 unless the threads were damaged from a prior owner forcing a hex jet in there. or the jet threads may be loose/damaged.

[Madbuffalo]

I'm rebuilding a Mikuni VM32ss carb for a friend and this thread got me looking at the needle jet. He was having some issues related to the main circuit and after seeing it's corroded I'd better replace it. So where online should I look to find a 0-8 needle jet for that carb, the bike's a '75 Yamaha DT400. I've tried the Sudco website but keep getting lost. Some help from a more experienced member may help. The parts guy at my local shop told me it was discontinued, but this is the same guy that kept trying to tell me there was no such thing as a needle jet and that the needle slides into the main jet "You're looking for the main jet". I gave up on that battle.
Thanks guys.

[tz375]

If you remove the needle jet, it will probably have the series number 159,188 etc and the size P-4, Q-2 etc.

Then go to JetsRus or SUDCO or other jet store and order that jet. Here's couple of links.


http://www.motorcyclecarbs.com/Mikuni_N ... ts_C36.cfm" onclick="window.open(this.href);return false;

http://www.psep.biz/store/mikuni_needle_jet.htm" onclick="window.open(this.href);return false;

http://www.sudco.com/vol33/60.pdf" onclick="window.open(this.href);return false;

http://www.jetsrus.com/a_jets_by_carbur ... mikuni.htm" onclick="window.open(this.href);return false;

Prices and shipping costs vary a little.

[Madbuffalo]

Thanks Richard

[Vintageman]

Madbuffalo;

What series Needle Jet does the DT400 have? Did Yamaha OEM show this parts avaible? That is usually a clue if you can get or not. I am finishing a 76 GT250. Can't get that needle jet. I think 174 series. The say it is same as 169 series but 0.040 Shorter at top shoulder so it will set in too far (could just raise needle clip one or two and be OK maybe but below carb body and maybe turbulance issuee) I found a 0.030" spacer I'll place under main jet washer and should work OK)

If you ever work on the little bike with say 24 mm and less carbs it a %$&$ to find and Needle Jets

Tz375

I think H2Rick? has a paper on net saying rounds kind a screw into 159s. Even though your right about thread spec. If you did not know that, tried it, you would note no issue, at least with the T500. Try it if you have not. My 159 were new and so were both the round and hex jet. After looking with a magnifying glass no thread damage on any parts I could see. I would have no worries about it. But you can cross between the two types... if you trust that.
Got my 77 RD400 out of winter storage and yes it crackles at light load. One of the 175-P2 were eroded heavily just like the T500. The other just some.
Only 25 Pilots. Again this bike has chambers. Even with idle up on carb bit I was trying to adjust for maximum rpm, I could not make the Pilot air screw affect Idle speed. In fact even closed fully it dropped very little if at all. I think they are way too small for chambers.

My 73 GT550 purrs (had new jets) So just the RD400 left to solve low speed light load smoothness

New Needle Jest ordered, and I 'll try new 30 Pilots. to start.

I'll let people know and will still try to get a digital pic of those eroded Needle Jets.

[Suzukidave]

Madbuffalo;
[I found a 0.030" spacer I'll place under main jet washer and should work OK)

I was wondering what the spacer under the main jet does ?

[Madbuffalo]

The jet is a 159 O-8.

Here's what I found from the links:
motorcyclecarbs.com - $31.45 shipped
psep.biz - not offered
sudco.com - Just too difficult to navigate
jetsrus.com - $23.44 shipped

It's my friend's money so based on those prices I'd better investigate the issue a bit for myself before ordering one. Thanks again Richard for the links, They're saved to my computer and I'll be using them in the future.

Josh

[Vintageman]

SuzukiDave

The shim under Needle jet after the main lifts the Needle Jet. The 169 top shoulder is thinner by 0.040" only real diff. and thus sets into carb too far by 0.040". The 169 is stated to be the same as the 172 series (Not Available) but it is not.

This 0.030"shim should lift it OK while still allowing enough thread to screw main without modifying cab body. The air passage into the 169 is longer so its lines up well with carb body air passage even when lifted. It will be like raising needle 0.010" but not bad for I will put chambers on the 1976 GT250

So I think the 169 can work with this little trick... I hope.

[tz375]

The key to that last reply is that it isn't a washer under the main jet, but a tiny ring fitted into the top of the carb throat so that it raises the needle jet slightly. The mounting spigot/flange/ring towards the top of a mikuni needle jet determines how far in the jet sinks.

For example,

169 is 1.5mm tall/deep.
175 is 1.5mm
176 is 2.5mm
182 is 2.5mm.

175 and 16 are essentially identical but they are not interchangeable because one sits down a little far. It will work in a pinch but it's the equivalent of one needle clip higher.

If I had a 172 I'd measure it and add it to the database, but I don't. I would try a 182 in place of a 172 but you would probably need to skim 1mm off the bottom face where the jet washer fits. 182 is 1mm taller than a 172 overall

[Vintageman]

Tz375,

Yes that would do the same thing, but there is no tiny thin ring available unless you machine something or hire someone to make

If you think about it, simply putting a washer of the same diameter as Needle Jet body with a hole to allow main jet threads to pass lifts the Needle Jet just as your suggestion

I found a readily available washer that does just that. I only had to drill hole a little bigger and sand outside a tiny bit and it fits. This is only 0.030” shim versus 0.040”, but overall looks good

Now the issue when raising the Needle Jet is the Main Jet threads only has a turn or two of threads into Needle Jet.

So next I found a thinner washer to replace the existing brass one just after main jet. I suppose you could machine the carb casting some too or shave the existing washer some, but no need with thinner washer.

The only thing I question is the material type of this new washer and shim washer, Nickel Plated and Stainless. Hope that doesn’t bother anything with fuel or dissimilar metal affects???

If you did not shim the needle jet and just relied on raising C clip on Jet needle for same mixing, I think the needle Jet opening is too low below the throat of the carb and that may affect flow. So I do think it is wise to raise it a bit. The Air emulsion inlet is oblong on the 169 and still lines up well when lifted some