1982 Seca XJ650RJ

Started breaking down a spare set of carbs. Lots of info on here that's been really helpful. I couldn't however, find any info on a port in the diaphragm well. It's right beside the pilot air jet and connected to the enrichment circuit. Pic attached.

 

It’s probably plugged with a brass plug

 

Thanks for the reply. It seemed connected to the enrichment circuit...
I'm cleaning the passages. While cleaning from the top down, I plugged the hole at the bottom of the plunger well and added fluid expecting to see it exit at the small brass tube, at the bottom of the carb body. The fluid exited at the pilot fuel jet. The fluid made it's way out that port (in the pic).

Maybe it doesn't connect when the plunger is in place?

 

I’ll look at mine again—- I was thinking that kind of passage for mikunis, but I’m buried in all kinds of things so may be thinking of the wrong area at this tired moment

 

No worries...no rush...it's not going anywhere.

Hitachi HSC32

 

The fuel and air is already mixed. There is a tiny hole in the brass tube close to the carb body, that lets in air. Why would you have another air inlet?...and quite a bit larger at that.

I have a theory...
Since the enrichment circuit already has a fuel/air mixture, it must have something to do with flow. The port in the diaphragm, connected to the enrichment circuit provides an air differential to allow better flow (much like the air compensator jets).

Any thoughts?

Pics attached.

 

HSC32
Another unknown port in the carb body...couldn't find any info.
It's entry point is in the carb body above the fuel level.
It's exit point is in the bottom lip of the carb body.
When the fuel bowl is attached it sits in the same cavity as the starter circuit jet.
Pics attached.

What's its purpose?

Feels like I'm playing stump the band...

 

When you open the choke plungers ("up"), you have to have airflow (and a good bit of it) in order to create a vacuum/suction to suck up the fuel thru the brass suction tube going into the bowl (and you are correct, the tiny hole in the brass tube is the air bleed that emulsifies the fuel).

The 2nd issue seems to be a method of creating an atmospheric pressure in the well where the starter /enrichment jet is located.....otherwise, that well is sealed off from atmosphere, and a vacuum lock would occur. The only other passage into that well is the fuel inlet at the very bottom of the bowl.

Remember, for fuel to enter into ANY of the jets (and then, up into the carb throats where it can be sucked into the cylinders), there has to be two main pressure components: a lower pressure area and a higher pressure area ("high" and "low" being relative to each other, not any type of absolute pressure reading). In the case of carbs on naturally-aspirated engines, the "high" pressure is atmospheric pressure (from the airbox) which must be present in any area where fuel is located that a jet is then to "suck up" into the carb throat; the "low" pressure is created by some type of venturi effect (in / thru the carb throat; and in the case of the enrichment circuit, the air passing thru that port and into the big opening that the plunger tip covers).

 

Nice detailed explanation...thanks for posting!

 

Pity its wrong..
When the engine is on enrichment the carbs are closed, with a tiny opening for idle air when hot. The fuel comes via the idle screw and opening. There is no venturi as such.
The enrichment fuel (and mixed air) needs air for the engine - hence the big drilling. The amount of emulsifiyng air in the fuel would not be enough.

 

Just so I understand you correctly...
The passageway connecting the enrichment circuit to the diaphragm port (above the air pilot jet) is to add more air (to the already mixed fuel) ?

 

That's what I believe it does. And the air comes from? The airbox/filter..
Other carbs have a choke, which has a cam that opens the butterfly a bit.

 

Thanks for posting.
Makes sense to me that all the air jets/ports would be in the same place.
Yes, it's filtered air...but so is the air in the float bowl.
Then perhaps the small hole on the side of the brass tube is simply a drain? Can't wrap my head around 2 sources of air.

A choke just meters the air. Apples and oranges.

What do you think the purpose of the port connected to the float bowl is?

 

I measured the port above the air pilot jet. It's at least 3.5mm in diameter...way too big to be used for air/fuel mixture. Interestingly the air compensator jets are 3.6mm.

 

No quite apples and oranges - the choke strangles the airflow in so that the fuel is sucked into the airstream, and what allows that airstream? The cam on the choke spindlw that opens the throttle. So you have emulsified fuel, plus air. Emulsified fuel alone is not enough for a rich idle mixture you still need a ballanced mix.

 

Yes, that make perfect sense. So it's more like a royal gala and a granny smith. lol

 

Now then, worked this out (took some thinking without actually going out to take one apart.
This is indeed the air for mixing with fuel for combustion - how does the fuel get drawn up with such a big air passage I hear? Well you have to remember what the shape of the enrichner plunger looks like - parallel sides to close off that passage with a flat rubber disc at the bottom to close off the vacuum from the engine. So open the choke a fraction, and only a fraction of air is allowed in, the suction from the piston does the rest.

 

One (actually several) fully disassembled and sectioned.
Inside your Carbs

 

Yes, thought it through a number of times myself...after initially dismissing the idea of relating it to the choke, the more I think it through, the more it makes sense.

The choke plate controls the amount of air entering the carburetor. That happens before the butterfly valve.

The enrichment circuit opening is after the butterfly valve, which would be almost closed at idle. Only a small amount of air comes through to mix with the emulsified fuel from the pilot circuit. As you introduce more emulsified fuel (the enrichment circuit) you must introduce more air.

The enrichment circuit would need it's own source of air...hence the port above the pilot air jet.

Vacuum from the engine draws the fuel in...no doubt.

Plungers seated with help from the springs and engine vacuum closes the circuit.
(mushroom headed tip- no flat rubber disk HSC32)

Thanks again!

 

Cheers.
I did have a look at that...many times. However, I couldn't find the port I had questions about. While the last section (enrichment circuit) were pictures of a Mikuni.
Perhaps the Mikuni and Hitachi differ that way?

 

Excellent pictures, and mostly correct descriptions. It misses the air feed to the enrichner (the discussion above), and also says the progression holes are for accelleration fuel. They are not. They do supply fuel during this time, but they do it at constant throttle as well, so where does the extra fuel come from for axcel? Simple, it's the main jet - the hole in the bottom of the slide feeding vacuum to the top of the diaphragm is callibrated such that it restricts the rate it lifts. This increases temporarily the vacuum at the jet, thus drawing up accelleration fuel.
Anyone used to setting up an SU carb will know they don't have progression holes, or idle jets. So the rate of rise of the piston is critical, and you can't mask lack of axcel fuel with idle jet changes.

 

I guess I don't understand..........the enricher circuit air port (next to the pilot air jet) which opens into the enrichment plunger valve "well" exits into the carb throat PAST the throttle blade, so even with the butterfly valve "fully closed" (I realize that it never really fully closes, but you know what I mean), the air will flow thru that tunnel (starting below the vac piston diaphram) over/past the open choke plunger valve opening, and exits in front of the butterfly valve (cylinder head side of the butterfly valve). Thus you have airflow from the airbox directly into the cylinder head, and the butterfly valve positions provides no obstruction (at all) to this airflow path, no matter what the butterfly valve position.

I believe the expansion in air space volume, where the "tunnel" ends (in the aforementioned plunger valve "well") creates the low-pressure area that allows the enrichment circuit fuel supply to be "pushed up" the brass tube and into the well (and further into the carb throat). There has to be a motive force for a liquid to defy gravity (i.e. move upwards); that motive force (absent a pump) is the presence of a lower-pressure area compared to a higher-pressured area acting on the fuel supply. Air which is moving quickly, or air which enters from a constricted space into a greater volume of space, will experience a pressure drop, and that is the motive force.

 

Of course they supply fuel during this time, and of course they also supply it at constant throttle (although, if the butterflies are that wide open during "constant throttle" you'd be running some pretty steep rpms already). Those "progression holes" do provide a good bit of fuel during accelleration, especially (perhaps mainly) during the transition from idle/off-idle to accelleration. Without those pilot circuit holes there, the engine would fall flat on its face when trying to do anything other than "feathering" the throttle past idle. They accomplish the same thing that an "accelleration pump" does: they supply the transitional fuel needs of the engine when coming off idle, before the main jets have a chance to start working. The vac piston rises somewhat slowly from an idle/off-idle situation when hard accelleration is called for, and these pilot circuit progressive holes provide that extra dose of fuel to make the transition smooth and somewhat seamless during the fractions of a second that it takes for the main fuel circuit to become active.

 

This bit - well actually the pump is the engine, sucking hard every time the piston goes down with the inlet valve open. This is your motive force. The metered holes (one in the well in the float, the other in the side of the enricher tube) allow fuel and air to mix as they are sucked into the enricher plunger chamber. This chamber is where this emulsified fuel is mixed with the roughly correct ammount of air for the engine to run. I believe it all makes sense.

 

Well you can't have it both ways - do they supply fuel during the progression phase? Yes all of it, with some help from the idle screw hole. So if they are callibrated correctly for a hot engine progression, they cannot then provide the additional fuel for snap opening the throttle. So in a way you are correct, they do provide fuel in snap opening, but not the amount you need. This is really not hard - have a look in the end of one of these carbs when you snap open the throttle - you will see fuel pouring out of the main jet.

The vac piston rises somewhat slowly from an idle/off-idle situation when hard accelleration is called for, and these pilot circuit progressive holes provide that extra dose of fuel to make the transition smooth and somewhat seamless during the fractions of a second that it takes for the main fuel circuit to become active.[/QUOTE]
This is almost correct, but you miss out the bit where the rider has the throttle at progression constantly - actually a lot of the time. So they work well here. Snap open the throttle and what happens? They carry on the same that's what, well actually they will supply more fuel, but they are tiny, and they only supply more because the piston hasn't risen, making the vacuum in the inlet increase (remember, constant vacuum?). So this poor misunderstood piston is now trying to balance its position based on the airbox (if you have one) air pressure and the vacuum underneath it, but the hole allowing the vacuum underneath it is callibrated to allow it to rise at a constant rate...
So it rises slowly, and in this time the main jet delivers a massive (relatively) squirt of fuel into the air going into the airpump.
Crikey, a lot going on eh?
If you want to prove this, open up the hole in the bottom of the piston - it will jump up immediately, and for steady state riding this would be fine - why wouldn't it? But your engine will fall flat on its face when you snap open the throttle.

 

The "progression holes" certainly add fuel on excel but more importantly...I think, their purpose is more to do with decel...or when the butterfly valves are snapped shut.
If your at cruise and close the throttle they add fuel so that there is no overly lean condition...coasting enrichener.

 

No, not at all. The throttles are closed, the progression holes see ne vacuum, so no fuel comes out.
And why would you want coasting enrichning? Fuel injection systems have DCFSO - decelleration fuel shutoff, to save fuel. I normally set this to anything over 1800rpm and zero throttle position.
They are progression holes - clue is in the name. They would also work in the gradual closing of throttle - note gradual, not closed or snapped shut.

Edit to add, I'm going to give up on this now, other than to say that these carbs are relatively simple, and once you accept that they are constant vacuum instruments you begin to understand how they work. The very name tells you most of the story, sure you can call them constant velocity, and one begets the other, but constant velocity doesn't help you understand...

 

I'll give it a shot. Cheers.

 

Constant velocity helped me understand.

The idle circuit sees vacuum at idle. The progession holes gradually see a local change in vacuum as the throttle plate opens, and when opened rapidly they do flow additional fuel because while vaccum in the system is constant overall, how and where the greatest pressure diffrence is within the carb can be manipulated. Otherwise fueling would be at a constant ratio and either an accelerator pump, or a dashpot to slow the initial rise of the slide, would be required for smooth acceleration.


Also the pump that Chacal refrences is the accelerator pump ( a literal piston pump that's connected to the throttle linkage) that squirts additional fuel on the majority of carburetors that are fitted to cars and trucks in the U.S.

 

...

 

In short, we are all correct. But the Devil is in the details when looking at any specific carburetor.

 

After some more reading, I've found this to be incorrect. Looking into coasting enrichener systems (on other carbs) more carefully, they utilize a circuit that reduces the amount of air (pilot air) that is emulsified in the pilot circuit. This process allows the emulsified fuel to be more rich when entering the engine during closed throttle higher rpm situations. The Hitachi has no such circuit.

Yes, devil in the details.

 

Correct. The purpose on these carbs is to increase fuel during sudden throttle plate opening.

 

Hardly correct:-
Ok, without resorting to looking up my fuel injection data, which might or might not tell me anything conclusive, I looked up my Dellorto jetting charts. In each case the accell fuel jet is at least 2/3 the size of the idle jet. That is, for a given vacuum it would give 2/3 as much fuel as the idle jet. But Dellorto accell jets are pumped, so they provide more fuel. Anyway, be that as it may, where in our Mikunis or Hitachis would this extra fuel come from? One thing is pretty certain, not from the idle jet...

 

i think your extra gas is coming out the emulsion tube. if the throttle is snapped open the butterfly is out of the picture and the fastest air flow is right over the top of the emulsion tube, that's the fastest air and lowest pressure point. there's a fit in the needle and the tube for this, that's why a ovaled out tube runs so bad. when the butterfly is open the idle feed hole is out in the big part of the bore under higher pressure not doing a lot. it just needs to supply mixture until that magic starts
that's my story and i'm sticking to it

 

Had another think and some reading on this gents...

Relating it to the carb on my car...
It has both transfer slots (surely they work the same way as transition ports) and an accelerator pump. The transfer slots (metered by the idle circuit) adds more fuel and prevents a stumble under light acceleration (that's from Holley)...making the transition to the main circuit smoothly. The accelerator pump adds extra fuel on large throttle changes.

From the tuning pages I've read...tuning the jet needle has the most impact on large throttle changes....particularly the tip and taper together with the float height.

Note...nothing I've read is particular to the Hitachi HSC32. I don't think it's apples and oranges tho...

 

Aand - full marks go to @Polock . Well done. The piston takes a second or two to rise (due to the restriction in the hole at its base), the vacuum above the main jet goes up, fuel spews out, bike accellerates, Huw smiles, piston rises, mixture weakens out, steady state achieved...

 

So how does the engine not stumble (lean out/starve for fuel) during this "second or two" (actually, it's doubtful that it takes that long). If the piston hasn't risen, and the needle is still plunged fully onto the jet, how does this fuel "spew out"?

 

This discussion is going nowhere.
Until and unless a Hitachi or Mikuni factory service manual becomes available (highly unlikely as they were not supposed to be sold on the open market) then this will never be settled.

I suggest that we move on to more productive endeavours.

Or at least keep it in a thread about that specific topic insead of derailing existing threads.

 

Simple, since the piston is slow to rise, a temporary low pressure area develops below the piston, right where the jet is. Oh, the jet is not fully into the jet, there is a space, handilly. And yes, it takes more like half a second to suck this fuel out.

I'm dissapointed with you @k-moe , you at least I would have expected to stay with this to it's logical conclusion, but you are right, it's all over others posts - but isn't it to the good that we all understand the least understood bit of our bikes better?

 

Don’t know if this will help or not——
https://www.mazdabg.com/ftp-uploads/Mazda/--Engines--/Mazda Carburetor Training Manual.pdf

 

I'm not disappointed with me at all. Again, until there is a definitive explanation from Hitachi or Mikuni as to the purpose of the passages in question we will be running in circles of conjecture. On this topic we are all laypersons at best. Unless one among us has a degree in fluid dynamics, and experience with CV carb design.

As a practical matter all we really need to know is to keep the carbs clean, replace the wear items as needed, and set the adjustabe bits to spec.

 

Thanks to all for posting your thoughts...

The conversation continues here. https://xjbikes.com/forums/threads/cv-carburettors-how-do-they-really-work.129527/

 

So I measured the "progression holes" in the both a Mikuni (BS28 series from an XJ550----3 holes + the mixture screw hole) and an HSC32 (XJ650/750----2 holes + mixture screw hole). I can't tell for sure whether all the holes in each (or between) series carbs are the same size....I don't have a set of pins that small and didn't want to go dig out the fractional size bits......but a quick measurement with an idle jet screw tip shows that:

a) each hole is even larger than the largest part of the mixture screw shaft "taper" section; i.e. the entire taper tip will fully pass thru the hole, and even then there's some wiggle room. The widest part of the tapered tip on the mixture screw shaft (this was an HSC mixture screw) is about .75mm OD, so the holes are at least that large and probably larger by perhaps .10mm or so.

b) EACH of these "progression" holes is thus much larger than the pilot fuel jet that feeds them; of course, the forward-most hole (that the mixture screw would fit into during normal operation) is restricted by the taper of the screw tip.


I found this arrangement to be surprising, and don't quite understand why you would have such a large "total" opening dimension, that could theoretically flow much more fuel than the pilot circuit could provide (although, it seems that fuel would only flow thru the "nearest" exposed hole; but then, why have multiple holes? Once you uncover the "first" (not the mixture screw hole, but the one closest, linearly, to it), what is the purpose of the rest of them?

 

I would think the more progression holes there are...the smoother the flow. Assuming the 2 transition holes would be larger individually than 3 smaller holes individually. Collectively, again assuming (do not have Mikunis to measure) the total area of all the holes would be very close. Too rich too soon is difficult to tune out with fixed passages....the Holley guys drill small holes in the butterfly plates to cure off idle stumble when using up too much of the transition slot during idle. Big dollar Holleys have a tunable air bleed for the transfer slots (no need to drill holes).

Re: progression holes during idle. When the butterfly valves are closed (I know they are not really closed completely), air is entering the progression holes, mixing with the fuel and exiting the pilot screw. When the throttle is opened this flow reverses and fuel flows out of the transition holes. This, I believe, would account for the size of the holes...for air entering, not exiting.

Relating the Holley to the Hitach/Mikuni may not be correct, but it's the only way I can relate...surely the theory would be similar. This is where I struggle.

 

It's more correct than relating them to the original SU design and its later iterations.
Hitachi and Mikuni both did quite a bit of engineering to improve (or at least try to improve) performance after having learned the basics by making SU carbs under license for several years.

 

Now why would you say this @k-moe ? You know that Holleys are variable vacuum carburrettors, and while the primary circuit of a Mitachi uses this principle, they don't have an accelleration pump, and the secondary or main circuit isuses the constant vacuum principle. Sort of a bastardised design, to reap the benefits of both types. Mind closed still?
Lucky we're not talking about certain Keihin's, they have two primary jets as well as the secondary circuit.

 

The Hitachi and Mikuni also don't have dashpots.
My mind is rarely closed. It does however require citations from the original sources, which we still lack (though I've been looking for a number of years).
Again, everything else is conjecture.

 

The dashpot is merely a damper, it stops fluctuations and increases the resistance to piston lift. The hole under the xj pistons do a similar thing - ask yourself why it's so small?
And conjecture? Are you not capable of reason?

 

This is not a politics discussion. Maybe tone it down a bit?