XJ750P Emulsion tubes

Hey Guys,
As you might know I rebuilt my engine a while back and made several mods. Started as an XJ650, now bored and stroked with XJ750 crank to make 820cc, its running XJ750P carbs.

Anyway, it runs well but is still a bit rich, the carbs are from an XJ750P (Australian Police Special). The main needles are Y14 but wondering if anyone can confirm if the XJ750P has the same emulsion tubes as a stock XJ650 or XJ750?
I know there are different profiles for the main needles but have been yet to confirm if all main needle profiles fit into one profile emulsion tube?

Cheers,
Simon

 

Morning Simon.
I can't answer your question, but I will say that emulsion tubes are not just different orifice (where the needle sits) sizes - the size and placing of the air holes in the sides can also be changed, and have a bearing on mixture mid range up. I'm not saying Yamaha did use this to "tune" the mixture, just that you might want to check. Obviously if the part numbers are the same, thats that.
Huw

 

FWIW,
Simply the tube orientation could have an effect, too.

I always orient mine so that the “4 holes” are front and back, the “3 holes” are side to side.

dfox

 

That's interesting, never thought of that.
Do you also orientate the orifice at the bottom of the slider too? I always turn mine on the diaphragm so it sits next to, rather than before or after the jet. I'm not sure if this is correct but I could see this posion having an effect on the level of the slides.

 

The slide is indexed by the bump on the edge of the diaphragm when it it correctly lined up in the detent in the groove under the hat. The mikuni also have the pin that indexes the emulsion tube automatically, and the slide needle has a small pin which indexes IT, as well, for proper alignment and seating pressure at slide-drop.

 

I think my memory was playing tricks on me - the last set I rebuilt were from my Vmax, so perhaps my comments werent applicable. The slides were free to turn on the diaphragm though, so no reason to suspect the Mikunis on the xj would be different. So as built from the factory, where does the port in the bottom sit?

 

Thanks guys. I never thought about the emulsion tube orientation but as hogfiddles said I agree there's a pin there to locate it. I will check that though.

Unfortunately I can't find the part number for the emulsion tube from an XJ750P (police version) to confirm if its the same. The part numbers for other 750's are the same for all models including XJ650. I'll take the carbs out again soon and will swap the emulsion tubes as I currently have my original 650 tubes in there now but with the XJ750P main needles which are Y14.
The odd thing is the mixture looks good at idle but is rich at mid to high end. I've reduced the main jet size which has helped but its still rich which is whats got me wondering about these tubes as I kept my 650 tubes thinking they were the same given the side holes are identical when I checked, just couldn't check the inside bore.

Anyways, I might add a video of what I do as it might help others. Thanks for the ideas....

 

You might want to change the needles ..... they have different “ramps” or “shapes/contours”

FWIW....
- the Y-14 needles (non-USA XJ750) are "richer" than the Y-13 needles (USA market XJ750) at 3/4-throttle positions and beyond; otherwise, almost identical to the Y-13 needle.

dfox

 

Slides should not be free to turn on the diaphragm

 

do you know if the Y13 and Y14 emulsion tubes are identical?

 

As far as I know, there are only 2 different emulsion tubes for the Hitachi carbs: those for the HSC32 series (which is what the all XJ750P uses) and a different version for the HSC33 carbs. The 37H model (Western Australia) and the 24L (New Zealand) Police bikes both used HSC32 series carbies.

 

Thanks Chacal, I have the WA version, 37H HSC32 carbs, so can I assume they are like a stock XJ750 which I also understand is the same as an XJ650 for the emulsion tubes? Were the HSC33 used on the XJ650 or XJ750?

 

As I understand it, the xj650 and xj750 hitachi carbs are all the same core rack— the jetting/slide needles are the variables.

 

Correct!

Never on the 650's; the HSC33's were used on all of the XJ700 air-cooled engines, and on the 1984 XJ750RL (the "baby" XJ900 models) engines. The 33's have a wider overall width, and different carb throat center-to-center spacing (besides a larger throat diameter and outer flange size) and require the matching boots to accommodate them. Although I haven't tried it, theoretically you could use the 29R (HSC33) intake manifolds on any XJ650 or XJ750 engine and they would work (fit), but be aware that they are jetted much leaner than any HSC32 carbs ever were...........of course, that could be changed, but now you're into some serious trial-and-erroring. The slide (vac piston) springs are also different from the HSC32 series carbs, as well as some other minor differences (mainly related to their greater width and different spacing between carbs): different fuel tee-fitting and fuel supply tubes, different shim bushings on the choke shaft, etc. and of course they have the overflow ports relocated, and some versions have additional brass vacuum ports sprouting hither-and-yon all over the carb body....

The HSC33 bowls are different, only because of the size of the starter jet, but otherwise they will interchange with the HSC32 bowls (and the difference in starter jet size has no real effect on operating performance, as the choke isn't used very often, and its variable settings can compensate for the smaller starter jet size in the HSC33 bowls).

Fun fact: 1980 XJ650 bowls had removable (threaded) starter jets in the bowls, unlike all other models and years, which had pressed-in, non-removable jets. This change (to non-removable jets) is one that should not have happened, as the removable jets makes cleaning out those jets so much easier.

This is correct, although as Dave pointed out to me a while back (which I hadn't noticed before), some of the early 650 carb bodies had a different system of mounting the upper rack bracket to the outer carb bodies (only one threaded hole and one locating peg versus 2 threaded holes as is typical) and of course the coarse-threaded vs. fine-threaded mixture screws issue. "Later" XJ650 and all XJ750 carb bodies (the bare bodies, never mind the internal jetting) are all identical to each other. There is also a difference in the in the carb BOWLS, as there is a 1 vs. 2 outlet port difference between early vs. later versions.

 

Interesting stuff. I also notice my XJ750P mixture screws are fine thread and XJ650 are course, so the whole 2.5 turns goes out the window. Other thing is my XJ650 carbs had a cover internally over the air jets whereas the XJ750P doesn't have these. Getting my modified engine tuned perfectly has taken a lot of trial and error so far. I seem to have a mix of parts that I haven't heard of others doing so it just takes a bit longer to figure out.
I'm going to remove the carbs again this weekend and try to measure the internals of both the XJ650 and XJ750P emulsion tubes to check if they are identical as well as drop down another main jet size. Hopefully that will fix my mixture issue.

Also, I've fitted an XJ750 11M head to by 650 casing. I might be wrong but I also think the cam profiles between the XJ750 11M are a bit different to the cams on the XJ750P....

 

"later" (yics -engined) 650'salso used the fine mixture threads, which allow you to more easily make "finer" adjustments to the mixture settings, but in theory it makes no difference, you just need to turn a coarse threaded mixture screw much less than a fine-threaded screw to affect the same adjustment.

Some carbs had them (earlier models) and then it was dropped by Hitachi. No idea what difference it makes (or what the purpose was).

37H models used the same cams as basically every other XJ750 engine (and thesame intake cam was also the XJ700 air-cooled engines; exhaust was different). Now, I don't have the specs (or part numbers) for the 11M models, but I think it would be the same; if you can provide that info then we can do a further analysis.

 

That's quite true, but my point was the wren't all alligned in the same place, so I turned them - didn't take much effort.

 

Thanks Chacal. With regards to my current cams, what I did was purchase a very low kilometre (7000) 11M head to install on my XJ650 engine case. It didn't come with cams.

I also had a complete XJ750P engine however the cams were quite worn so I purchased another set of XJ750 cams. Hence why I say the combination of parts I've used has made tuning a bit tricky and with it running rich I have so many changes that its taken some time to figure it all out. One day I'll dig out the original XJ750P cams and see if there are identical. Its not urgent but more for interest. I wouldn't have thought the XJ750P was that "special" to have different cams...

Anyways, I'm very happy with my engine mods and now just need to get these carbs fully tuned so I can get out on some cruises...

 

Hey guys, I pulled my carbs apart today. I measured the emulsion tubes and can confirm all external dimensions and side holes are identical. I tried measuring the internal hole, there's a step at the end and hard to tell if its a straight or tapered bore. They are very close in size, I might be wrong but felt like the XJ650 tubes were slightly larger towards the top compared with the XJ750P tubes. I swapped them to have the XJ750P tubes in there as I probably should have done in the first place to be sure. Also can confirm the tube rotation is not fixed by any pin, so I installed them all the same with the 4 holes pointing towards the inlet, not sure it will make any difference though.

Also changed the main jet size down from 125 to 120. Stock on an XJ750P is 118. Only other thing I did was set the mixture screws from 2.5 turns out to 1.5 turns. I'll check idle with colortune and sync carbs shortly to hopefully finish this tuning off.

Started no problem, idle good. Need to go for a ride but weather is crap at the moment. I've taken some video so will upload in the next week or two to my YouTube channel for anyone interested. I still haven't posted a ride video, so well overdue now...