Topic: XJ650 Carbs on 750A question

[tscott]

Hey friends, working on putting a set of XJ650 carbs on the 750A and am planning an entire write up of the process once it's done and running right. I've got a problem that I can't figure though...

I was told that the 750k boots work with the XJ650 carbs, but I cannot for the life of me figure out how they can. I thought they were supposed to be a direct bolt on using the K boots. I purchased a set of new boots and can fit them on the intake but they are longer than the A boots and don't match up with the carbs (neither the xj650 or the original 750A). The outside (#1 and #4) line up correctly, but #2 and #3 are way to close together. What stupid mistake am I making? Am I not putting the boots on correctly somehow? Did I somehow manage to purchase the wrong boots (and why are the 69-78K boots so much longer than the A boots?)? Any other ideas?

The intake side of the xj650 carbs and the 750A carbs line up when put next to each other so that shouldn't be an issue. I don't have a set of k carbs (round tops) to compare but assume they should be the same distance between the intakes as the A carbs and the XJ650 carbs. What am I missing? I sure hope it's something silly.
I'm having trouble uploading pics to this site but will try to do so soon.
Thanks for any insight/help on this!
Toran

[raymond10078]

Looks like your inside boots are 180° off.  Rotate the inside boot one-half turn.

[tscott]

I tried that, but the angle gets more extreme, with a 180 turn... what it seems like I need is for the boot to be swapped so the angled side is attached to the head... the problem is of course that they are not set up to do that (how they attach to the head and the carbs is not exactly the same). These are not OEM but I've looked at every kind of knock off (chinese, taiwan, japan, oem) and they all look to be the same at least in ID/OD shape (can't speak to materials of course).

I thought I had maybe put them on the wrong way at first... but that didn't work either.

Hmmm... anybody got pics of their set up? It does look the pic Raymond sent makes the intake boots look like the inners have the short angled side attached to the head and the outers have the long straight run attached to the head)
Toran

[raymond10078]

I looked at your pic again.

The bikes cylinders are numbered 1-4, from left-to-right, when sitting on the bike.

I see your have the boots in the wrong order.

In addition, I believe that the boots numbers are read when sitting on the bike, too.  For example, the bottom of the "2" is against the carb, not against the head.

Lastly, I recall buying the K1-K6 boots, not the K0 boots that you have.  I don't know what impact this will have . . . .

[tscott]

I'll check to see what year boots they are but they should fit 69-76 K models

The outer two (#1/4) and the inner two (#2/3) are the same boot, they each have both numbers 2/3 or 1/4 on them. According to the serial number on the oem's the 2/3 are the same and the 1/4 are the same (interchangeable). I'll keep messing but something seems wonky... hopefully I'll figure out some stupid thing i'm overlooking...
It's got be something I'm doing...
Thanks for the help.
T

[raymond10078]

Again - have you tried to reverse the boots (so that the numbers can be read when sitting on the bike), and swapping the center two left-to-right?  See pic for correct orientation.

Hard to tell in the pic - I wasn't sure what cylinders are shown - the center two, I'm guessing.

[tscott]

Yep, center two... gonna try to do what you suggest... prolly won't get to it until Christmas afternoon or the following day... will keep the thread posted with progress and what I was doing wrong...
Gotta be an easy fix or a  reason to stay away from the aftermarket boots I bought...
Thanks,
T

[Terry in Australia]

I'll be interested to see how this develops, I've not heard that XJ650 carbs fit CB750's, my old "Goldie" K2 from 10 years ago had Honda CBX750 CV  carbs fitted by the previous owner, a mechanical engineering lecturer at the Defence College in Canberra, and they weren't a direct fit, he had to re-space them to suit a CB750 (F2 engine) and he had a heap of headaches sorting the jetting, and making an adapter so he could use the OEM airbox.

It was a great job though, apart from a small dead spot just off idle, the acceleration was linear and the throttle was super light, compared to the OEM setup. I've had CR carbs on a CB750 and they were OK for WOT stuff, but for normal riding, the CV's blitzed them.
 

[tscott]

Thought I'd do a little update... I wasn't crazy after all, but I can't believe there isn't more frustration over this on the web that is easily found with a search.
The boots I bought were cheap Chinese/Taiwanese CB750k knock offs and quality wise don't seem entirely worse than the OEM boots that came off of my 750A. Obviously I didn't use them for any amount of time to see if they would actually stand up as well to the 40 year old boots that came off the bike. They look like exact replica's until you look closer...

The inside boots (#2 and #3) are literally manufactured upside down. The side of the boot that connects to the head has a straight run for #1/4 and then bends at the end where the boot goes over the carb. These work fine.

The inside boots (#2/3) are backwards as they are supposed to have the immediate bend on the head side and then have a long straight run into the carb. These are backwards (built same as the 1/4)

Where is gets tricky is that they are stamped correctly. If you line up the numbers of each carb boot 1-4 they look correct (bends are correct for lining up the carbs and head), but the inside of the boots are not correct anymore. The inside of head side has a flat internal face and then an indented/recessed groove for the small lip on the end of head manifold to "snap" into while the carb side has a flat internal face (not as wide as head side), then a raised ridge and then flat internal face again to match the carb. The ID are different for head and carb as well.
The knock offs (and after spending lots of time on ebay looking at various knock-offs) it looks like many of them are screwed up like this... the ones from Japan and OEM of course are not (but are pricier). I just bought a used OEM set... I can't seem to post more than one pic per reply so I'll try to show what I mean.
Here's a pic of the #1 and #2 with the bend the way it ought to be (you can see numbers line up) but if you look at the end you will notice that the internals of the manifold are different. #1 is correct (that is the head side and it fits and lines up well) but #2 is not correct... it is the carb side internals...

[tscott]

here is a pic of the knock off's all lined up correctly... what you can't see though is that the insides are wrong for #2/3 as they should be flipped (this is showing the head side down and carb side up) and this is correct for the #1/4 but not for the 2/3.
You can faintly even see that the ID is off between the #2/3 and the #1/4 boots.

[tscott]

a pic of the knock offs lined up with the internals are facing the right way (all the actual head side internals are facing down while all of the carb side internals are facing upwards). This is why the very first picture I posted would not line up with the carb. I put the boots on the head the way they were all suppose to go (to dry fit) and instantly saw a problem. Now I know why...

[raymond10078]

Good job for solving your mystery!

I do hate it when I get "caught" trying to save money, only to get screwed over, and spending more in the end!

[Terry in Australia]

I'm glad you posted that mate, I must have bought a set of the same ones recently, because I couldn't get my stock carbs to fit with them, and had to resort to using 47 year old rock hard OEM items, that, with a pot of boiling water to soften them, went straight back on, with no issues.

Can you remember where you bought them from? I'd like to send mine back with an angry note, but as I've bought hundreds of bike parts in recent months, I can't remember where I got them, except that it was from EBay? Anyway, keep going with this, I'm keen to follow your lead, if the conversion is as simple as it appears? Cheers, Terry.

[tscott]

@Raymond... so true... I told myself that before I ordered them, I deserve what I get I suppose.     

@Terry... I  found the buyer and set up a return... haven't heard back yet. It looks like there are multiple sellers that are selling the same sets... you're best off looking at your recent purchases and seeing if you can track down the seller that way.

Once I've got them on and dialed in I'll write up a proper report on all I've learned through the process. Have had good feedback from @Raymond and lots of good info from @Jon Weeks as well.

Will probably be a while before they are on and dialed in but just picked up a 76 F model camshaft to do later. Hoping to see if the XJ650 carbs make a difference before doing the cam. We'll see how it all shakes out.

Toran

[Terry in Australia]

@Raymond... so true... I told myself that before I ordered them, I deserve what I get I suppose.     

@Terry... I  found the buyer and set up a return... haven't heard back yet. It looks like there are multiple sellers that are selling the same sets... you're best off looking at your recent purchases and seeing if you can track down the seller that way.

Once I've got them on and dialed in I'll write up a proper report on all I've learned through the process. Have had good feedback from @Raymond and lots of good info from @Jon Weeks as well.

Will probably be a while before they are on and dialed in but just picked up a 76 F model camshaft to do later. Hoping to see if the XJ650 carbs make a difference before doing the cam. We'll see how it all shakes out.

Toran

Thanks Toran, is there a specific model XJ650 that I need to search for so I get the right carbs? I'm just about to reinstall an engine in my K2, so an ideal opportunity to do the carb change now.

[tscott]

Hey Terry,
Are you putting these on a K bike or an A bike? I can't speak to what kind of upgrade it would or would not be for a K/F model bike.  The K roundtops are supposed to be a great choice for upgrading the Hondamatics. The A carbs are 24mm and the K carbs are 28mm and the Maxim/Seca/XJ650 carbs are 32mm. They will bolt directly on with the K carb manifolds (assuming you get correct manifold boots! ).

Any of the Maxim/XJ/Seca 650 carbs will work... I think mine were 81's or 82's and they do seem to be cheaper than finding K roundtops but not a lot cheaper. They are Hitachi carbs... similar to any of the other CV carbs out there but these are spaced correctly for Hondamatics. Many of the XJ750 carbs will work as well, they are the same carb with some different jet sizes. Here's a really handy guide... you will be looking or the HSC32 Hitachi Carbs... you'll see they were put on a lot of XJ models. http://xjbikes.wikidot.com/carb-specs

If you go that route then feel free to reach out to me and I'll share with you what I've learned so far. The XJ forum has been helpful. Too bad you're in Australia or I'd probably have some carb internals I could share with you if you go with the XJ carbs.
Toran

[tscott]

Terry, wound up buying some 40+ year old boots myself that look to be in good shape overall... I'm sure they will be rock hard... how long did you boil them to get them pliable again? How long did they stay pliable before hardening up again (or did they regain some elasticity)? 
Expecting I'll need to do this a bit myself (warm them up somehow) but didn't think about boiling them... seems like a decent idea.
Toran

had to resort to using 47 year old rock hard OEM items, that, with a pot of boiling water to soften them, went straight back on, with no issues.
 Cheers, Terry.

[Terry in Australia]

G'Day Toran, thanks for all the info mate, much appreciated! I've got an old electric kettle out in my garage just for that purpose, so I dropped them in the kettle, boiled it (carefully) pulled them out, and installed the carbs immediately. They only stay pliable for a very small window, so you need to work quickly, install them on the head first, then shove the carbs in.

Don't be tempted to whack the carbs with anything to insert them, grease the rubbers with some vaseline, silicon spray or grease, and if necessary, use SteveO's method of levering them back on with a fence paling or other suitable piece of timber. I'll have to pull out my CBX750 carbs and see how far off fitting they are, or whether I just buy some XJ carbs. I did love the super-light throttle and excellent fuel consumption on my gold bike, riding modern bikes it's always a shock when I hop on one of my CB750's and experience that super heavy throttle again.    

[tscott]

One more pic of the OEM 750K boots (these came off a '74) and the backwards Chinese 750K boots. The mistake is obvious (and egregious... how could you mess this up so badly??)

[Terry in Australia]

Yep, that's awful. I reinstalled my carbs on my K2 "Bitsa" yesterday, and using the boiling water and some silicon spray, the carbs just popped straight on. One thing of importance though is that once the carbs are on, if anyone is using the stock airbox, you need to slightly rotate the rubbers to achieve the correct carb angle. It had me stumped for a few minutes as to why I couldn't get the airbox to fit until I realised that just by turning the rubbers, the carbs moved up and down substantially.

Terry's K2 Bitsa Sat 28 Dec 20195 by Terry Prendergast, on Flickr

[tscott]

Great reminder about fitting the airbox Terry!

[jukku]

A little bit off topic!
Theese are better than cracked originals! Made of 2 silicone hose 90 degree bends.
http://forums.sohc4.net/index.php/topic,49393.msg636519.html#msg636519

[tscott]

A little bit off topic!
Theese are better than cracked originals! Made of 2 silicone hose 90 degree bends.
http://forums.sohc4.net/index.php/topic,49393.msg636519.html#msg636519

This is actually a great thread to read through... How are the boots holding up still Jukku? Also, gonna keep reading but the xethanol being used to soften old carb boots sounds promising... I have been using a heat gun to warm them and soften them enough to get on but it would be great to have an option that worked longer term...

[tscott]

As far as softening old hard rubber boots (or other parts) here's an interesting video:

 

[jukku]

A little bit off topic!
Theese are better than cracked originals! Made of 2 silicone hose 90 degree bends.
http://forums.sohc4.net/index.php/topic,49393.msg636519.html#msg636519

This is actually a great thread to read through... How are the boots holding up still Jukku? Also, gonna keep reading but the xethanol being used to soften old carb boots sounds promising... I have been using a heat gun to warm them and soften them enough to get on but it would be great to have an option that worked longer term...

After 10 seasons still working better than the rock hard cracked originals! Fresh originals would of course be far better than theese self made ones!

[tscott]

Jukku, curious why you think fresh originals would be "far better" than your hack? It sounds like your hack is performing quite well (as best as you can tell)... even after 10 seasons. Where did you get the silicone and how much did it run you?
T

[jukku]

I can´t say if fresh originals are 5 or 25% better but who cares when the bike is running and not garaged.
Theese silicon ones are a little bit soft and flexing so you can count the heartbeats.
The silicon 90 degree bends are availeble in almost any tuning shop.

[Terry in Australia]

Yep, I think the silicon bends are a great "Redneck" fix, but original insulators are designed to give a smooth transition from carb to inlet without the "steps" created by just having a tube that fits the OD of the carb and inlet stub. As has been said above, maybe there's some value in the turbulence produced by this, but if there was, Honda wouldn't have gone to the trouble of engineering them as they did, if rubber hoses would have sufficed.

[Terry in Australia]

I found a pic of my old CV carb equipped K2 today when looking for something else, so i thought I'd post it here as inspiration. Another bike I wish I'd never sold.......

 

[bear]

It was a great job though, apart from a small dead spot just off idle, the acceleration was linear and the throttle was super light, compared to the OEM setup. I've had CR carbs on a CB750 and they were OK for WOT stuff, but for normal riding, the CV's blitzed them.
 

Yep I agree. I could never quite understand why anybody would want to put a set of CR's on a road bike.

[Terry in Australia]

It was a great job though, apart from a small dead spot just off idle, the acceleration was linear and the throttle was super light, compared to the OEM setup. I've had CR carbs on a CB750 and they were OK for WOT stuff, but for normal riding, the CV's blitzed them.
 

Yep I agree. I could never quite understand why anybody would want to put a set of CR's on a road bike.

Thanks Brian, I think that a lot of folks enjoy the "form" as much as the function. My current project (Kawasaki Z1000 drag bike) that I'm slowly putting back on the road has an old set of Suzuki GSXR CV carbs on it, and I'd thought about replacing them with new Mikuni's, but I was surprised at how clean they are when I pulled them apart yesterday, all the diaphragms are still fine and no other damage that I can see, so apart from cleaning out the old Avgas and blowing out the various circuits, they were good to go. I love the lighter throttle of CV carbs, so it should be a nice thing to ride when finished.