Topic: 750 Needle Clip Position-- Please Clarify

[GammaFlat]

I was looking at some old notes on the mods to my K6 carbs... I had drilled out the top (away from threads) larger holes to .035.  HondaMan recommendation from a few years ago regarding K6-ish bikes:

"I drill the other, smaller holes (those are .025" on the K6 I have here at the moment) if the bike is still running too rich in the lower midrange regions. The RPM range where the aeration is occurring corresponds to the (vertical) height where those holes are: at lower RPM, the air enters in those little holes nearest the nut, and increases with RPM toward full throttle. The tradeoff for enlarging the holes is a slight dead spot in throttle response between 7/8 and full throttle, but I've never heard anyone complain..."

Someone here (maybe HondaMan here too) mentioned in previous posts that chamfering the holes helps prevent buildup  of crusty crap in these holes in the future.  

Enlarged holes (not perfect but ran lots better):


Also.. have you looked at your needles?  Is it possible they are Keysters (aftermarket replacements from rebuild kits)?  Keysters have a different taper (I'm told).  Original Honda needles are marked "27201" while Keysters are one less digit I think. The Keysters are supposed to be impossible to get tuned correctly.  If you have Keysters, do what you can to get Honda needles.  Honda needle:

[Popwood]

I was looking at some old notes on the mods to my K6 carbs... I had drilled out the top (away from threads) larger holes to .035.  HondaMan recommendation from a few years ago regarding K6-ish bikes:

"I drill the other, smaller holes (those are .025" on the K6 I have here at the moment) if the bike is still running too rich in the lower midrange regions. The RPM range where the aeration is occurring corresponds to the (vertical) height where those holes are: at lower RPM, the air enters in those little holes nearest the nut, and increases with RPM toward full throttle. The tradeoff for enlarging the holes is a slight dead spot in throttle response between 7/8 and full throttle, but I've never heard anyone complain..."

Someone here (maybe HondaMan here too) mentioned in previous posts that chamfering the holes helps prevent buildup  of crusty crap in these holes in the future.  

Enlarged holes (not perfect but ran lots better):


Also.. have you looked at your needles?  Is it possible they are Keysters (aftermarket replacements from rebuild kits)?  Keysters have a different taper (I'm told).  Original Honda needles are marked "27201" while Keysters are one less digit I think. The Keysters are supposed to be impossible to get tuned correctly.  If you have Keysters, do what you can to get Honda needles.  Honda needle:

Thanks, GammaFlat. I verified the needles are indeed stamped with the correct number and not Keysters. Wish it were that easy. Besides, I'm pretty sure most all the problem is in he pilot circuit. My plug chops in mid and upper range circuits showed good read on plugs.

As a kid (or adult for that matter) I never did the spaghetti in the nose trick so the technique you describe for getting around the angle in the passageway hasn't quite sunk in yet. And were you using nylon leader or nylon coated metal leader fishing line? I have the wire wound guitar D string and might give that a go. Thoughts?

[GammaFlat]

As a kid (or adult for that matter) I never did the spaghetti in the nose trick so the technique you describe for getting around the angle in the passageway hasn't quite sunk in yet. And were you using nylon leader or nylon coated metal leader fishing line? I have the wire wound guitar D string and might give that a go. Thoughts?

I never did the spaghetti thing either but I've seen it plenty - yuk. 

What I used was a nylon coated, steel leader (which seemed like it didn't like carb cleaner much - when I was done, the coating looked like it was getting soft).  I cut off the end and put a small kink near the end.  The small kink helps it make the 90 degree turn at the top of the emulsifier tube opening.  It also helps to rotate the wire as you get to the 90 degree turn.  But again, the 90 degree turn is the tough part.  Maybe I'll try to get some pictures or video later today for clarity. 

In terms of a guitar string, you might try an "E" or "A" but a "D" might be overly effective and it might be too easy to remove carb bits.  Whatever you use, be gentle. 

It may be enough to get a wire or string in far enough so you can see it when you look straight up the hole where the emulsifier tube tops out.  I believe what you're really trying to accomplish is to get the passageway clear right at the base of the emulsifier tube.  My point is that the "floss method" may not be required to get you cleaned up. 

[GammaFlat]

Here are some pics of some of the particulars... 
disclaimer: Folks (who I greatly respect) here on the forum have warned against using a guitar string for cleaning your carbs because it's pretty hard steel and the carb materials are generally softer.  Exercise caution and use at your own risk.   

Here's the tool of the day.  I think it's an "E" string.  Notice the jiggety jig bend on the end (more on that later): 


Here's the entry point on a very dirty carb (parts carb for demonstration purposes only ):


Here's where you can see it through the emulsifier tube hole.  Note that the needle is not in and I'd be cautious about scratching the needle with the end of the wire.  Your wire is hard.  The needle... not as hard - be careful if you do this with the needles in place.  This is hard to see even under good light conditions - it was a pain to photograph.  The "jiggety jig" at the end gives you the ability to rotate the wire to clean/scrape crap out of the hole - especially right at the opening at the base of the emulsifier tube (top of tube). 

[Popwood]

Wow, GammaFlat, great photos. I'm sure it did take some work to get the lighting right on those close-ups. This helps clear things up on the cleaning technique.  In the first photo, the other hole/opening is part of the pilot air passageway circuit, correct? It connects the air screw adjustment opening to the pilot jet. Since I think most of my rich condition is in the pilot circuit, I really want to make sure that passageway is unrestricted. Assume your same technique applies or any thoughts on that?

One reason I am willing to keep coming back to these passageways getting whistle clean is because the bike, in the year + I've had it has tended toward rich especially on plugs 1 & 4 with some evidence on plug 3. With all the carbs set up the same, it makes me think one unpredictable variable is air blockage. When I first had the carbs off for my first cleaning of them a month + ago I discovered someone in the past had installed 95 main jets in three carbs and a 90 in another. Stock is 105. So I wonder if this could be a clue that someone was trying to make an adjustment for rich condition in the past. Granted, not in pilot circuit but at least in the overlap between pilot and 1/4 throttle+ range.

Another question. When I had the carb rack off Sunday I did not remove from stay plate and I did not touch the needle/slide; all that assembly was untouched. So putting rack back on I assumed a re-sync of the carbs was unnecessary. Would you agree?

Thanks again for all the help.

[TwoTired]

Another question. When I had the carb rack off Sunday I did not remove from stay plate and I did not touch the needle/slide; all that assembly was untouched. So putting rack back on I assumed a re-sync of the carbs was unnecessary. Would you agree?

Not this time.  But, I say this because the plugs aren't showing the same deposits.  Troubleshooting is about eliminating variables.

Also, you've had the carb off/on  enough times that you should check for intake runner leaks each time.  IMO

Cheers,

[GammaFlat]

In the first photo, the other hole/opening is part of the pilot air passageway circuit, correct?

No, it's the main.  [EDIT] on second perusal, I believe you're right.  The opening that does not have the brass fitting (larger opening) is the pilot.  The brass fitting is the main. 

Since I think most of my rich condition is in the pilot circuit, I really want to make sure that passageway is unrestricted. Assume your same technique applies or any thoughts on that?
One reason I am willing to keep coming back to these passageways getting whistle clean is because the bike, in the year + I've had it has tended toward rich especially on plugs 1 & 4 with some evidence on plug 3. With all the carbs set up the same, it makes me think one unpredictable variable is air blockage. When I first had the carbs off for my first cleaning of them a month + ago I discovered someone in the past had installed 95 main jets in three carbs and a 90 in another. Stock is 105. So I wonder if this could be a clue that someone was trying to make an adjustment for rich condition in the past. Granted, not in pilot circuit but at least in the overlap between pilot and 1/4 throttle+ range.

The path for the pilot is different and the air sort of makes a 90 degree turn.. after it enters through the hole on the intake side of the carbs, the air goes down-ish then turns toward the front of the carb (intercepted/interrupted by the air screw) to meet the base of the idle jet (different on K7/K8 carbs).  I'd make sure both passageways are clear.  They are more likely to get the crusty formations as you get closer to fuel -that's why the opening at the base of the emulsifier tube is usually suspect - and less suspect as you get closer to the intake side of the carb in these ports.  When I was doing mine, I was told that the emulsifier vent (one that goes to the base of the main) would cause fouling if blocked or partially blocked.  I think I had issues there.  

Also... what condition are your manifolds in?  You can definitely get differing results if you have intake leaks.  Are the clamps in pretty good shape?  I've had excellent results with the xylol/wintergreen - I'm anxious to get some (s)miles on a bike with "conditioned" rubbers to see how they stand the test of time/miles/heat etc.  

Another question. When I had the carb rack off Sunday I did not remove from stay plate and I did not touch the needle/slide; all that assembly was untouched. So putting rack back on I assumed a re-sync of the carbs was unnecessary. Would you agree?

If they were previously synced well and other variables didn't change, you should be fine.  Other variables include vacuum leaks, chamber, port or valve mods, other carb mods.  

I defer to the experts here.  If I have anything wrong, feel free to help us out.  

[Popwood]

Manifold boots good. Did the wintergreen treatment when carbs came off the first time. Picked up new intake boots then.  New set of clamps.

Yes, when I get inside those carbs this time I will pay special attention to the space where the air passages meet the emulsifier tubes.

[Popwood]

I have completed the cleaning of the air passages in the idle and main circuits so now reassembling carbs. So as I now study each one still looking for any variables, I notice this and wonder if it's normal: With the throttle valve all the way down and the main jet not in yet, the needle has a small amount of free play in the needle jet. I know, "How much is "small" amount."

So I'm wondering if this free play is normal. Or, should the needle be more seated in the needle jet? Since the needle/throttle valve is not supposed to contribute below first 25% of throttle opening, could this free play be contributing to rich condition in idle circuit? And if so, would dropping the needle down by one clip position (raising the clip) correct the condition? I realize this is all relative to where the taper on the needle begins to raise out of the needle jet. So I suppose the question goes back free play or a very tight tolerance machined fit.

Would like to sort this one out before carbs back on the stay plate and bike.

Thanks.

 

[MCRider]

I have completed the cleaning of the air passages in the idle and main circuits so now reassembling carbs. So as I now study each one still looking for any variables, I notice this and wonder if it's normal: With the throttle valve all the way down and the main jet not in yet, the needle has a small amount of free play in the needle jet. I know, "How much is "small" amount."

So I'm wondering if this free play is normal. Or, should the needle be more seated in the needle jet? Since the needle/throttle valve is not supposed to contribute below first 25% of throttle opening, could this free play be contributing to rich condition in idle circuit? And if so, would dropping the needle down by one clip position (raising the clip) correct the condition? I realize this is all relative to where the taper on the needle begins to raise out of the needle jet. So I suppose the question goes back free play or a very tight tolerance machined fit.

Would like to sort this one out before carbs back on the stay plate and bike.

Thanks.

 

My experience is they all flop around. Once the mix is flowing the needle will be centered by the fuel which will seek an equal level all around the needle as it makes its way past.

Am I right or am I wrong? Open for comments?

[GammaFlat]

My experience is they all flop around. Once the mix is flowing the needle will be centered by the fuel which will seek an equal level all around the needle as it makes its way past.
Am I right or am I wrong? Open for comments?

I believe you are correct.  If they're not a little wiggly, I think you have more potential for misalignment.  The way that they assemble seems to have some "give". 

[Popwood]

My experience is they all flop around. Once the mix is flowing the needle will be centered by the fuel which will seek an equal level all around the needle as it makes its way past.
Am I right or am I wrong? Open for comments?

I believe you are correct.  If they're not a little wiggly, I think you have more potential for misalignment.  The way that they assemble seems to have some "give". 

Makes sense, but would there be/should there be fuel flowing past the needle in the idle circuit?

[MCRider]

My experience is they all flop around. Once the mix is flowing the needle will be centered by the fuel which will seek an equal level all around the needle as it makes its way past.
Am I right or am I wrong? Open for comments?

I believe you are correct.  If they're not a little wiggly, I think you have more potential for misalignment.  The way that they assemble seems to have some "give". 

Makes sense, but would there be/should there be fuel flowing past the needle in the idle circuit?

maybe not. Point is, on the bench, the needel wobbles in the jet holder/emulsion tube.

[TwoTired]

As long as there is negative pressure across a metering circuit, fluid will flow though it.  The jets work just like a straw in a beverage.

However, look at where the exit ports are in the carb throat.  With the slide near closed, most of the deepest vacuum is right at the pilot circuit exit.  Yes, there is still some vacuum at the needle exit, so it never completely shuts off.  But, the depth off the vacuum at that point is significantly less than at the pilot exit port.  As the slide gets open, the vacuum moves closer to the venturi, which is where the slide needle exit is located.  Further, as the velocity increases, the venturi creates a pressure drop independent of what the engine cylinder provides.  And this makes the slide needle and main jet more dominant at determining mixture as the slide is further opened.

The slide needle does wobble a bit in every mechanical slide carb I've examined, btw.

Cheers,

[MCRider]

Thanks TT. I learned something. 

[GammaFlat]

You gotta love this place. Whether you're a little familiar with these nuances or completely new to them, it is so nice to have the stuff spelled out.  Thanks TT and MC

[Popwood]

Thank you, TT, and yes, I do love this place. Sure glad I read that carb manual a few times or TT's explanation would have been just as effective were it written in a foreign language. I've learned so much working on this problem.

[GammaFlat]

How did this turn out? 

[Don R]

On my 75 F carbs I ended up with all new jets including the needle jets and needles, I still had to drop the needles one notch. But my perfect looking filter is probably old so it gets replaced soon. The original problem was some fool reamed all the jets presumably while cleaning them. I used a set of pin drills and inserted the solid end to determine if they were drilled and they checked looser than another set of the same number.
 Why I read here a lot, many times a similar issue will get varied responses and yeild a new idea.

[Popwood]

How did this turn out? 

Upon getting carbs back on and testing, continued disappointment. Oh, yes, I also raised the needle clip one notch to mid-position. Frustrated, I felt I needed to go back to where I started (at least without taking the carbs off) and try again. That meant putting the smaller main jets in I found when tearing down; those were three 95s and one 90. I also cleaned up and put the D7EA plugs back in #1 and $. Slight improvement but still not even close to acceptable; plugs sooty on 1 and 4, not bad at all on 2 and half decent on 3. What's the variable, what's the variable? Read some more yesterday and started wondering about float level. Yes, as far as I was concerned, set at 26mm and double checked.

So then last night I decided to remove the float bowls and measure how much fuel was in each one. The fuel level, measured from the deepest, center portion of the bowl varied from 7/8" deep in #2, the best working, to 1" in # 4, with #1 at 13/16+ and #3 at 13/16.  This does correspond to plug reading pretty closely. Fuel level too high in bowl causing overly rich condition? That's a real possibility and one thing I had not touched throughout the whole trial/error process. Only had time for a 3 to 4  mile ride after adjusting floats to a slighly lower fuel level in bowl. Found notable improvement on #1 and 4. I've ridden to work today and will read plugs after work. Fingers/Toes and everything else crossed.

[Popwood]

I should add that I also had a correspondence with Hondaman to clear up the air screw/rich/lean question. Turning in leans (yes leans) and turning out enriches the fuel air mixture.

[TwoTired]

I should add that I also had a correspondence with Hondaman to clear up the air screw/rich/lean question. Turning in leans (yes leans) and turning out enriches the fuel air mixture.

This is, in fact, impossible, both physically and practically.

The 750 carbs use the same circuit parameters/arrangement as the CB550 non-PD carbs.  An exhaust gas analyzer proves that turning in the air screws creates a richer exhaust fuel mix, and turning them out makes the exhaust gas leaner.  There are no physics or tests that will prove other wise.

Also, the problem of measuring drainage quantity from the fuel bowls to determine fuel level is complicated with the fact that the first carb drained also includes the fuel in the supply tubing from the fuel valve.  If the carbs have two supply tubes, the first carb drained from that tube will also include the fuel in the second supply tube.
 Measuring the released quantity must be done twice with this in mind, so the alternate bowls are drained first.

Cheers,

[Popwood]

I should add that I also had a correspondence with Hondaman to clear up the air screw/rich/lean question. Turning in leans (yes leans) and turning out enriches the fuel air mixture.

This is, in fact, impossible, both physically and practically.

The 750 carbs use the same circuit parameters/arrangement as the CB550 non-PD carbs.  An exhaust gas analyzer proves that turning in the air screws creates a richer exhaust fuel mix, and turning them out makes the exhaust gas leaner.  There are no physics or tests that will prove other wise.

Also, the problem of measuring drainage quantity from the fuel bowls to determine fuel level is complicated with the fact that the first carb drained also includes the fuel in the supply tubing from the fuel valve.  If the carbs have two supply tubes, the first carb drained from that tube will also include the fuel in the second supply tube.
 Measuring the released quantity must be done twice with this in mind, so the alternate bowls are drained first.

Cheers,

Below is what Mark sent me regarding air screw position and lean/rich fuel mixtures:

"Yeah, it seems like an uphill battle about the air screw action: I've given up on trying to make some folks understand it. Final definition: 3/4 turn on the 750 carbs is a 16:1 air/fuel mix, while 1-1/4 turn is 10:1 A/F mixture, per Keihin's spec on the SOHC4 carbs (CB500K/550K/750K). Due to the specific volumetric inefficiencies of the SOHC4 engines, the idle mixture must be in the 11.5:1 to 12.5:1 A/F mix range (at the carb) for smooth idle. This "spreads out" to become about 14:1 in the chamber itself, due mostly to the overlap valve timing that is used.

I spent almost 2 pages in my book trying to explain how these carbs' idle circuits work, and some folks have even tried to take me to task over it!  Shocked  But, physics is physics, and that's how they work, despite folk's opinions. Probably the biggest 'confuser' of all: automotive type carbs (and lawn mowers, weed-eaters) are exactly opposite, getting richer as the screw is turned out. I suppose that's why it is stuck in so people's many minds that way. There are 2 exceptions in the Honda line that I know of: the CB350 and CB450 carbs. Both of those have fuel-metering idle screws, instead of air-metering screws, so out is richer on those. I'm sure that doesn't help anyone's confusion either..."

If I have misunderstood Mark's explanation and misspoke in my post above, apologies. Please correct me if that's the case.
If not, I would certainly hope there could be a meeting of the minds on this. As you both say, it's physics and not a subjective matter of opinion.

[MCRider]

I've read this a couple of times now and it seems to me both are saying the same thing in different ways. Out is leaner, in is richer.

Popwood has it wrong (maybe). HM says 3/4 turn (which is more out than 1.25 turns) is lean and 1.25 is rich. So PW, turning IN from 3/4 to 1.25 makes it richer.

But before i stick my foot in my mouth we need clarity. Is HM 3/4 turn from the top going down (which i assume), or from the bottom coming up? Most of us were taught to bottom the screw first then back it out in which case, 3/4 would be rciher than 1 1/4. But I don't think that's HMs perspective here.

In your quote HM says "automotive type carbs (and lawn mowers, weed-eaters) are exactly opposite, getting richer as the screw is turned out." So the opposite is getting leaner as the screw comes OUT, which is what both HM and TT are saying.

OCICBW

[Popwood]

I've read this a couple of times now and it seems to me both are saying the same thing in different ways. Out is leaner, in is richer.

Popwood has it wrong (maybe). HM says 3/4 turn (which is more out than 1.25 turns) is lean and 1.25 is rich. So PW, turning IN from 3/4 to 1.25 makes it richer.

But before i stick my foot in my mouth we need clarity. Is HM 3/4 turn from the top going down (which i assume), or from the bottom coming up? Most of us were taught to bottom the screw first then back it out in which case, 3/4 would be rciher than 1 1/4. But I don't think that's HMs perspective here.

In your quote HM says "automotive type carbs (and lawn mowers, weed-eaters) are exactly opposite, getting richer as the screw is turned out." So the opposite is getting leaner as the screw comes OUT, which is what both HM and TT are saying.

OCICBW

Thanks, MCR. When I refer to "turns of the screw," I am measuring from lightly seated IN to full screw rotations out, so 3/4 turn would be 1/4 less than 1 full turn out from lightly seated. I'm confused by your saying "turning IN from 3/4 to 1 1/4 . . ." how could you turn "in" to a number greater than where you started?