Topic: 750 Needle Clip Position-- Please Clarify

[Popwood]

I've searched the forums and FAQs and can't find a clear answer to a simple question. And oh, yeah, there's no reference in the manual.

It's a stock '75 750K. What is the stock main jet needle clip position and please be specific about how to count the specific clip position options. Let's call the needle blunt end near the clip positions up and the pointed end lower in the carb near the jet down.

I'm sort of at wits-end on what to do next (after almost, if not everything else done). I'm in Ohio so no special altitude problems. I'm just running rich with too much carbon on plugs. Currently, the needle clips are in the second position from the bottom. I have not changed these but found them in this position. I'm on the verge of removing the carbs once again and moving the clip up a notch or two to drop the needle a bit and lean out the fuel.

They are the correct needles and the main jets are 105s. I have not done a plug chop yet, but if the clips should be changed to drop the needles (one notch or two?) I'd like to get on with that.

Thanks.

[TwoTired]

The notch positions are counted from the top (non-pointy end).

See :
http://www.motorcycleproject.com/motorcycle/text/specs.html
Find your carb number stamp and look in the JN (jet Needle) column.

[MCRider]

What a great website! My dang favorite places menu is 3 screens long.

To answer your question the stock setting is always the middle, or #3 in this case.

[Popwood]

Thanks, TT and MC, and I'll double check with the link TT against my carb model.

And yes, this is an awesome sight that never ceases to amaze me. Of course, much of the credit goes to the stalwart contributors, of which I consider you both, and a host of others.

And once in a while I'm able to help someone who is trying to figure something out. And very few things give me more pleasure than being able to give back to the place that's given me so much.

[Popwood]

Hmmmm, my carb model is 657B and JN is 4 according to the spec chart from TT's link. That's one down from middle. At the risk of a scolding from TT, would there be harm in going to middle or 3 to determine if there's an improvement? I swear I've tried everything else.

From my searches on the site, Hondaman contends Honda specs were a bit rich to begin with except for the 550, if I remember correctly.

[MCRider]

What a great website! My dang favorite places menu is 3 screens long.

To answer your question the stock setting is always the middle, or #3 in this case.

I should hasten to amend this by saying, the carbs I am familiar with, the needle was always set in the middle. The chart is better than me for sure.

[TwoTired]

Hmmmm, my carb model is 657B and JN is 4 according to the spec chart from TT's link. That's one down from middle. At the risk of a scolding from TT, would there be harm in going to middle or 3 to determine if there's an improvement? I swear I've tried everything else.

From my searches on the site, Hondaman contends Honda specs were a bit rich to begin with except for the 550, if I remember correctly.

If your bike is in stock condition induction and exhaust, the stock settings resulted in a great performing bike right off the showroom floor.

If induction and exhaust have been changed from stock.  There are doubts about ANY of the stock settings still applying to your non stock machine.

FYI, the JN middle position is a leaner setting than clip in slot 4.  What do your plug deposits say with cruise throttle chop?

Remember the Jet Needle has a taper so it can vary the mixture at all throttle positions (mostly effective between 1/4 and 3/4 positions).

If the taper angle no longer applies to to your bike's changes, then changing the needle may improve, say, 1/3 throttle and make 2/3 throttle outside of ideal mixture.

Ain't carb tuning fun?

Cheers,

[Popwood]

All stock, TT, and because you might ask, the paper air filter (honda product) is just a year old. So, OK, I'm in for a plug chop. That's certainly easier than pulling the carbs to reset the needle clip.

So being new to plug chop (although I know you need to run within the throttle opening range for awhile then kill the engine) do I need to start with virgin plugs? Or can I simply use the nearly new ones after cleaning off the carbon soot before heading out for the test?

And yes, carb tuning is fun. Sure glad I like this stuff because otherwise this bike would have been on Craigs List a few months ago.

[TwoTired]

The replacement schedule for the paper air filter is one year.  So, yours is due for replacement.  (It's not just use.  the paper fibers collapse and close up the filter pores.  This creates a deeper pressure drop across the filter membrane.
For certain your used filter is not going to flow or have the same pressure drop across it like a new one does, and this will effect mixtures that carb provides.

Further, if you are going to tune carbs, do it with a new baseline reference, not an old unknown baseline.
If, after tuning to the new filter, you may wish to try the used one to see it's effect.  But, it will probably cause the carbs to go richer.

Cheers,

[Popwood]

Did my plug chop today and was pleasantly surprised to see nice tan colors on the plug insulators. I did two chops but I do wonder if my throttle positions where what I thought. To measure throttle opening on the grip, I used masking tape and a marker pen to align with a tape/pen mark on the kill switch. I marked the grip at idle, then at the furthest place the grip would turn. Found a half-way point for half open, then marked 1/4 and 3/4 position on the tape on the grip. I was doing this test with the air filter out but the air box assembled.

On the highway, I was running 7000 rpm doing 80MPH in 4th (maybe 3rd?) gear and my throttle marking was only at the 1/2 open mark. So I don't see how it would be possible to do a totally WOT for any period of time unless there was a very long, very steep hill. Maybe I'm missing something? Wouldn't be the first time.

Anyway, at that chop, plugs looked real good. On return, I was running closer to 1/4 open to maybe 3/8 open running about 60-65 mph at about 4500 rpm. Did another chop and again found good looking plugs. Exited highway, did normal street ride home for a few miles and find plugs carbon fouled. Air screws are set at 1 turn out (mid place of spec which is +- 1/8 turn.

So, a dumb question. Is it possible everything is fine and this fouling is just a by-product of slow speed, slower rpm (between 3000 and 4000) street riding. And does this fouling burn off when running highway speeds? Is there a cycle to this plug condition ranging between street and highway riding? 

[TwoTired]

No.  Honda didn't make a bike that would foul plugs normally.
No filter membrane and it still fouls plugs at low speed?  Seems like a pilot circuit issue.

Do all four spark plugs show the same deposits?

If so, then I would check the air bleed jets for the pilot circuit, and also make sure the little emulsion tubes are clear.
I would also verify that all the pilot circuit passageways are clear and not corroded.

Then I'd see what opening the air screws more does with low speed plug fouling.

Did you ever verify that the throttle slide cutaways face the inlet of the carb throat?

Cheers,

[Popwood]

No.  Honda didn't make a bike that would foul plugs normally.
No filter membrane and it still fouls plugs at low speed?  Seems like a pilot circuit issue.

Do all four spark plugs show the same deposits?

If so, then I would check the air bleed jets for the pilot circuit, and also make sure the little emulsion tubes are clear.
I would also verify that all the pilot circuit passageways are clear and not corroded.

Then I'd see what opening the air screws more does with low speed plug fouling.

Did you ever verify that the throttle slide cutaways face the inlet of the carb throat?

Cheers,

Thanks, TT. I'd agree it seems the pilot circuit is the issue, so identifying that seems like some progress. So for now, will assume clip position and main jet are OK-- they are stock, and clip set to recommended position.

As for all four plugs, No. 2 has always looked fine and NO. 3 not perfect but much better than No. 1 & 4. I am certain the pilot jets are clear both through their length and the cross drilled emulsion tube holes. I checked these carefully, twice.

Next, TT, you mention "all pilot circuit passageways. Maybe there's something here I've overlooked. Obviously, there's a passage for the air screws. I know these were open and clean. I found other passageways, like between the top of the bowls and was able to squirt carb cleaner through them. There are passageways between Carbs 1 & 2 and 3 & 4 that are connected by a short rubber tube. These were clean shooting carb cleaner through. There was another external opening coming off a short arm that was clean. Sorry I can't describe better. But how many air passages on the pilot circuit are there?

Yes, the throttle slides are in correctly.

[TwoTired]

Each carb has a pilot circuit drilled into the carb bodies.  There are four holes to the outside world.  One hole has the air jet (near the carn mouth inlet, one hole has the air screw, one hole has the pilot jet, and one hole exits into the carb throat.

I don't know why, but lore has it that these passageways can get corrosion buildup and impede air passage.  I've not experienced it personally.  But, it just might explain why your pilots are delivering too rich.

Might be worthwhile to eliminate that as a possibility.

Cheers,

[Popwood]

Each carb has a pilot circuit drilled into the carb bodies.  There are four holes to the outside world.  One hole has the air jet (near the carn mouth inlet, one hole has the air screw, one hole has the pilot jet, and one hole exits into the carb throat.

I don't know why, but lore has it that these passageways can get corrosion buildup and impede air passage.  I've not experienced it personally.  But, it just might explain why your pilots are delivering too rich.

Might be worthwhile to eliminate that as a possibility.

Cheers,

TT, would the air jet be located in the "ceiling" ok'd the carb bowl and be a pressed in brass fitting/jet? The air screw hole is obvious as well as the pilot jet. That leaves "one hole exits into the carb throat". There was another hole opposite side of the brass fitting/jet, but I thought this one exited to the carb exterior at the arm I referenced above. In any event, if my interpretation is correct, all these passages were clear. I poked a fine guitar string as best I could then made sure carb cleaner would spray through, then blew compressed air through. Do you think I have the openings identified correctly?

[TwoTired]

I could probably tell from a picture.  But, although they are functionally equivalent to the 550 carbs, there are probably specific location details I don't know about for the 750 carbs.

You should have two air jets, one feeds the main jet/slide needle jet cavity and one feeds the pilot circuit.  They should both be at/near the entrance to the carb throat facing the air box.  NOT in the roof of the carb bowl cavity.

I can't find any pictures in the shop manual that depicts the air jets.

I can usually determine which air jet feeds which by squirting carb cleaner under pressure into the air jet inlets and noting where it comes out.
Either post a picture of the carb mouth entrance and I can point them out.  Or tomorrow, I can post a pic of the Cb550 carb's air jets to give you an idea where to hunt for them on yours.

Cheers,

[Popwood]

I could probably tell from a picture.  But, although they are functionally equivalent to the 550 carbs, there are probably specific location details I don't know about for the 750 carbs.

You should have two air jets, one feeds the main jet/slide needle jet cavity and one feeds the pilot circuit.  They should both be at/near the entrance to the carb throat facing the air box.  NOT in the roof of the carb bowl cavity.

I can't find any pictures in the shop manual that depicts the air jets.

I can usually determine which air jet feeds which by squirting carb cleaner under pressure into the air jet inlets and noting where it comes out.
Either post a picture of the carb mouth entrance and I can point them out.  Or tomorrow, I can post a pic of the Cb550 carb's air jets to give you an idea where to hunt for them on yours.

Cheers,

Thanks again for the help. The carbs are still on, but I'm attaching to illustrations from the Honda Carb Manual. These show the "air bleeds" for main and pilot circuits. This is what we are trying to identify, yes?

I have a  chore to tend to today but I'll try and get the carbs back off later this afternoon and get some actual pix of the carbs to post.

[TwoTired]

Yes, that is the function (s) we are trying to be certain is working properly.

Cheers,

[Popwood]

Yes, that is the function (s) we are trying to be certain is working properly.

Cheers,

Carbs off. Have identified the slow and main air passages. Have spray can carb cleaner flowing through the slow circuit -- from air screw port to passageway at mouth of carb throat on intake side and through slow jet. In photos of carb rack, the pilot air screw at the mouth of the carb throat is the one without the pressed in brass jet. The one with the brass jet feeds the main jet/emulsion tube.

In the second photo is the "ceiling" of the carb bowl area. Carb fluid went straight into the carb throat and on others either vented straight to the outside of the carb or was connected to the adjacent carb by the rubber tube. My guess is these control atmospheric pressure inside the carb by venting to the outside?

Also cleaned the slow jets again, but they were in good shape. Is it time to put back together and remount then test?

[TwoTired]

Given the lore about corrosion flakes in the carb bodies, I'd want to check the flow rates of the those air passages.
The theory being that if those passages were closed more than the air jet orifice opening, the flow would be restricted and starve the pilots for air.
I'm not exactly sure how to test for flow rate.  I guess I would use safety wire smaller that the air jet orifices and try to rod out the carb passage.  Alternately, you could put lemon juice or vinegar through the passage to try and dissolve corrosive flakes.
It would be cool to have a tiny camera to look inside.

I'm in inexperienced territory here.  But, I am trying to reason this out with you.

Cheers,

[GammaFlat]

If you guys are talking about the "vent" at the base of the emulsifier tubes, I've had success cleaning those ports with my fishing equipment (leader with plastic coating - kinda stiff but wouldn't gouge or damage openings, yet still has a "scrubbing", uneven surface to it).    Guitar strings are interesting too but I'd be cautious about a "D" string or larger due to the abrasive nature it could bring to the table (wound steel as opposed to smooth straight steel).  Others have expressed concerns about using guitar strings because they could change the shape of the softer metals you are rubbing them against. 

The problem is that in the K6 and prior 750 carbs (not so in K7 and K8.. more of a straight shot), there is a goofy bend in the vent hole, so when you approach the carb from the air intake side, you poke something in that hole and it needs to take a 20-something degree bend to get to the base of the emulsifier tube.  What I like to do is similar to what kids do with spaghetti... in their mouth and then out through a nostril.. pull on one end and let the other end go in..

So... poke something in through the brass fitting on the air intake side.. get past the 20-something degree bend.. then make a 90 degree bend out the bottom of the float bowl area through the main jet holder (definitely the hard part).  Then operate like floss - pull on one end then the other.  Do this every day   Okay, don't get excited, not every day but if you are cleaning your carbs, this is a must and I believe it is often overlooked (mostly because you can almost always spray carb cleaner through the vent and be gratified by "seeing" the results as the cleaner comes out the emulsifier tube hole, yet it is not completely clean - you can't visually inspect). 

I believe there were periods where particularly formulated gasoline particularly formed "crusties" in this vent and in the emulsifier tubes (both down the barrel and in the holes).  Look for posts on this but you may also want to enlarge the emulsifier tube holes some.  I did on my K6 and it seemed to run a lot (not kidding) better.  Unfortunately, I don't have real good details on how much better because it (my engine) started making a loud mechanical "ruckus" (can you describe the ruckus?) after I was able to pull the front wheel off the ground (no clutch popping) in first gear (okay, I did have a 17t front sprocket as well).  I just got it running great and now I face splitting the cases to see what the crap is going on.  dang it!



I was skeptical about the "even a 25% blockage in the holes of the emulsifier tubes..." argument until a "cleaned mine up". 

[Popwood]

Thanks GF and TT. I was feeling confident yesterday evening that the passages were clear given the free flow of carb fluid I was getting in all passages in both directions. I also did some spraying with one finger of the air screw opening to get a better read on what was flowing through deeper in. Seemed good. I also used some nylon line to push through the passages as best I could but did get hung up at the turn as mentioned. Put the carbs back on.

I rode to work today, mix of highway and street. Will be 12 miles round trip and will check plug condition after work. I'm really hoping this clears up the problem. But for all the frustration, I do have a much better understanding of how the carbs work, so a worthwhile journey.

[Popwood]

Would be an understatement to say I wish I had better news. Basically, I'd say no improvement after recheck, reclean of air passageways for both pilot and main air circuit. Photo shows plugs after today's ride. Plugs arranged in order left to right, and show only acceptable result is No. 2 which was never a problem to begin with.

TT, you said it would be good to eliminate blockage or restriction of airways as the culprit before moving on. While I did not get a wire all the way past the turn as a potential issue or "floss" the passageways as Gammaflat describes, what would be the next steps toward getting rid of this carbon fouling problem? I'm not averse to pulling carbs off again and "flossing", but if that's next, what else should I be investigating if the carbs are off again? And what else besides carbs should I be looking at?

[Popwood]

Another update. A couple developments. I noticed on my ride today that the turn signal beeper was weak at low rpm then would not sound at all at idle. Then checked headlight and nothing. Checked battery and 12.4v when not running. While running slight improvement at 2000 rpm and read 3.55 at 3000+ rpm. Checked water level and it was low but not below the plates.

Cleaned plugs 1 & 4, added water to battery, leaned air screws 1/4 turn (that's 1/8th more than manual hoping to find a lean running condition) and went for 3 or 4 mile ride. Signal beeper ok at higher rpm but weak at lower and no sound at idle. No headlight. On return, only checked plug 4 and it was sooty, with perhaps a very minor improvement as there just a bit of tan at the very very top edge of the insulator.

Checked the headlight fuse and it's fine. I suspect there's a loose ground for the headlight and blinker beeper but maybe a compromised battery? And would a compromised battery affect spark quality at idle to low rpm resulting in fouled plugs?

I have the battery on the charger for the night.

[TwoTired]

I don't understand why all plugs don't have the same deposit conditions.

Were the carbs synced?
Were the valve all adjusted the same?
Do all the cylinders have the same compression numbers?

Are you certain that the choke plates open completely?

If you are satisfied that the air channels can't have restriction, it would seem the next step is to reduce the pilot jet size.  Though I have no explanation why your bike should differ from thousands of others made by Honda.

Pity you can't try another set of carbs from a bike that runs as expected.

Cheers,

[Popwood]

Carb sync, check
Valve adjust, check
Compression on all between 140 and 150
Choke plates fully retract,

I'll have another go at the air passages and make sure I can get a probe all the way through and try to floss, as Gammaflat would say. As to smaller pilot jet, I'd agree there's little reason to think my bike is "special".

Just to ask again, no reason to suspect spark quality at low rpm if battery is possibly deficient?