Carb Needles Used In Non-American Flat Tops

HS30-H · October 27, 2022

15 hours ago, RIP260Z said:

I am unsure if any JDM market had the HMB46W fitted (HS30-H?), apart from their recalled Fairlady Z with the 2.6L fitted.

Hi Ian,

Very late 1972 and then 1973 year L24-engined Fairlady 240Z, Fairlady 240Z-L and Fairlady 240ZG got the HMB46W-1Bs as part of Nissan's drive towards reducing emissions. Specs here, in bottom right box:

...which says the jet needles were 'N-66'.

RIP260Z · October 27, 2022

Thanks Alan,

So, slightly off topic, but connected, the "flat top carb" HJG38W in your JDM scan also has the suffix, -3A in the top, with -8 and -9 (A/T), which would help support the theory of slight changes due to models/engines, where the HJG36W I have is -11. Certainly confirms different needles in the JDM Fairlady 2.4L and the European L26 (which would be the case). I take it the top box is the earlier model, the 48 year (in kanji) is later models? So, a needle change from N-54 to N-66. Therefore do Nissan's needle series go by letter, M the N...?

HS30-H · October 27, 2022

2 hours ago, RIP260Z said:

So, slightly off topic, but connected, the "flat top carb" HJG38W in your JDM scan also has the suffix, -3A in the top, with -8 and -9 (A/T), which would help support the theory of slight changes due to models/engines, where the HJG36W I have is -11.

That seems likely, and logical, to me. I have no idea what these minor differences were, and Nissan doesn't seem to have been too forthcoming with data to explain them either (possibly in line with their 'Do Not Fiddle!' stance?). That difference of '-8' suffix on manual trans '73 L20 and '-9' suffix for the Auto trans version does indeed seem to point to differences being minor.

2 hours ago, RIP260Z said:

I take it the top box is the earlier model, the 48 year (in kanji) is later models?

Yes. Pretty much. The top boxes are up to very late '72 whilst the bottom two are the 'Showa 48' (1973) versions.

2 hours ago, RIP260Z said:

Therefore do Nissan's needle series go by letter, M the N...?

I don't know enough about that to comment, except that it seems - again - likely and logical. I've been sifting through factory parts lists to see if I can see any corresponding (full) part numbers, but no dice so far. I wonder if that too is part of a 'Leave Them Alone!' thing?

Captain Obvious · October 28, 2022

Thanks for the thoughts Ian.

The parts breakdowns (for the US) list the needle numbers for the later (three screw) version of the round tops as N-54 and N-58. It isn't explicit on the breakdowns, but my assumption is that those two numbers were determined by whether the car had a manual or an auto transmission. But as far as I can tell, neither of those numbers were ever used in a flat top. At least not in the US.

Then in June 72 they changed to the flat tops, and while they list the needle part number as 16354-N3310, they no longer listed the needle profile number (N-27, N-54, etc) on the parts breakdown. And there does not seem to be any variation based on any car configuration (like whether it's a manual or auto).

That said, my experience is that the needle number used, at least in the US, is N-62:

I've messed with a couple distinctly different versions of the flat top carbs, but there's no indication that those versions correspond to any of the "first type, second type" etc notations on the Fuel System Modification Plus literature. In fact I believe that they do not. It is clear, however, that Nissan was "flailing" during the early flat top years trying to get things right.

Some additional thoughts about your thoughts :geek: :

Yes, you do remember rightly about the shared idle mixture control and I don't think that's the best system either. I have modified a couple flat top systems to allow independent adjustment of the idle mixture instead of one shared control and that seems to work well.

With regard to some of the other things you mentioned, the flat tops shouldn't really be any different than the round tops. For example, the round tops have a very similar inlet filter which needs to be kept clean, and the flat tops should not be any more or less susceptible to nozzle wear due to needle rubbing than the round tops are. In fact, the dome alignment scheme used on the three screw round tops is identical to what they used on the flat tops.

I find your mention about small bowl capacity a little curious. The flat tops have a larger bowl capacity than the round tops, so if the round top capacity is "enough", then the flat tops (which have about one and-a-half times the capacity of the round tops) shouldn't be an issue?

Thanks again for the input.

RIP260Z · October 29, 2022

Thanks Captain Obvious, always good to have others input.

With regards fuel bowl volumes, I have only looked at the flat tops HMB46W, so I bow to your knowledge with this comparing to the HJG46's. Though my understanding is fuel vapour /insufficient fuel was a problem (for the HMB46W's), so I was thinking if this was larger than it is, this would have more fuel in the tank so to speak, and maybe help a little with cooling, as there is more volume. But as I said, it was an idea that I never looked at fully to whether it was worth moving forward with.

Having the fuel bowl below the base of the carb doesn't seem the best idea above a hot exhaust manifold. Nissan thought that moving from the HJG36's (the other flat tops) with the side fuel bowls like the HJG46's to the HMB46W was the way forward.

Both pairs of HMB46W's had nozzle wear due to the needle rubbing that I stripped (one partially). If Nissan US had this mentioned in the 1974 (?) published modification booklet, I presume it was already noticed on the cars that were in for remedial work with only a low number of years under their belt...?

Captain Obvious · October 30, 2022

Thanks again for the info.

it's been too long ago for me to remember why I was measuring the bowl capacity, but I show the results mentioned above. And I agree about the bowl placement down by the exhaust manifold with no thermal break isn't ideal. The round top bowls were not only located farther away from the exhaust, but they also had the rubber vibration and thermal break mounting. Both of those probably helped keep the fuel in the bowls cooler than the that tops.

And I'm not saying that the flat tops can't rub the needle against the nozzle. I'm just saying they are not more prone to it than the 72 round tops. I wonder if they were having some sort of alignment issue at the factory. It just seems weird that with the exact same design, nobody seems to complain that the 72's rub.

So did you happen to take notice of the needle number on the flat tops you stripped down?

RIP260Z · November 1, 2022

Off the top of my head, I seem to remember not seeing anything, but the needles are kept with the stripped down carbs. Will be a while until I get a chance to look, now in France for work....

Captain Obvious · November 2, 2022

Thanks. I've found the inked on numbers on the needles are not very durable. Wouldn't surprise me at all if they were illegible or even gone completely. That pic I posted above is the best pic I have of the number. I've got other pics, but that is the most readable I've found to date.

And one wipe with a strong solvent (like carb cleaner), and poof... It's gone. :pufff:

dutchzcarguy · November 3, 2022

I just looked in my garage for a set of needles i saved from a set of hmb carbs but i can't find them.. (I'm sure i saved them in a little box like were you store some thread making tools in..)

BUT.... I do have here 1 HBM-46W carburetor from a '73 DUTCH car i'm 100 eh.. 99,90% sure.. It's in a very dirty state, comes from under a pile of junk that was cleaned away after the junkyard was cleaned up.. actually there was a complete 240z under a enormous pile of steel junk and... they scrapped it!! They only bothered to take out the engine and gearbox.. wich i bought in the 2010's or so..

Let me know if your interested in this carb.. not expensive just a nice price.. (I'm sure it has never been opened up.. or messed with!)