72 bike, 76, motor, 78 carbs. Which boots and filters

[CalsXS2]

I think everything will work from the research I have done. I just kinda need verification please.

My motor is not installed, nor is the carbs on the motor. I did a rough measurement with a tape measure,,, eyeballed it ,,, and all the center to center measurements were the same. Roughly.

So I believe that tells me I don't use the 78 linked carb boots. Right? I should just use either original 72 boots, or the ones that JBM sells for the older bikes. Right?

I'm wanting to keep my 72 side covers and not do any cutting or have it look goofy.

Hey Gary Is this your pic below. I think its supposed to be with pods behind there. Is there.

So what Uni filters do I want to use, eliminating the stock air box's. Thanks

 

[gggGary]

IIRC You need the boots to go with the carbs not engine or frame you will have some issues with where the carb inlets don't match height to the side covers using the linked BS38s Wow that's my garage all right, I don't remember doing that mock up.... But it's hard to deny it. Point me to that thread? LOL Not sure what UNIs will work best there.

 

[CalsXS2]

IIRC You need the boots to go with the carbs not engine or frame you will have some issues with where the carb inlets don't match height to the side covers using the linked BS38s Wow that's my garage all right, I don't remember doing that mock up.... But it's hard to deny it. Point me to that thread? LOL Not sure what UNIs will work best there.

I'm confused. Isn't what I described doing above exactly what your picture is.

What is the parts involved in your pic.

I have no idea what thread I found that pic in. Sorry.

 

[gggGary]

"So I believe that tells me I don't use the 78 linked carb boots. Right?"

I think those are the later linked angled carb spigots in the pic. The straight early spigots might work, I'm not sure.

 

[CalsXS2]

OK. I THINK some of you are saying the 78 linked carbs will NOT work with my 72 side covers.

Correct me if I'm just not getting this, but I think they will work unless I'm just to stupid to see what you guys are telling me.

So here's some pics comparing the 72 and 78 linked carbs.

In the construction field we use the term "from it, to it" which means the same thing as "center to center". For the pics I think this will give more accuracy rather than trying to guess at center to center.

To make thing easy I'm going to call this measurement 4.75". It's the same on everything.

78 linked carbs.

72 carbs in the boots on the motor.

Bare motor.

72 straight boots.

Air filter box.

Filter side height comparison.

Manifold side height comparison.

Like I said above. Unless I just misunderstood what you guys are saying, these 78 carbs will work on my 72 just fine unless of coarse I'm not interpreting my own pics correctly.

So if all the enter centers are the same, and the 72 and 78 heights are the same, they should work with my side covers. Right?

I truly apologise if this is a needless and stupid post.

BTW. Will I have to use a vacuum petcock. I need to buy one 72 anyway. Might as well buy two 78's. Is there anyway to make these work with out switching to a vacuum petcock. Or should I just get my mind right and spend the money. That if they are needed.

 

[kopcicle]

I truly apologise if this is a needless and stupid post.

If I stopped to apologize for every stupid post I made I'd be doing nothing else .

~kop

 

[CalsXS2]

If I stopped to apologize for every stupid post I made I'd be doing nothing else .

~kop

So you're saying it IS a stupid post.

I edited the above last sentence. Take a look about the vacuum deal please and comment. Thanks

 

[5twins]

Petcocks are a separate and unrelated item to the carbs. It doesn't matter what type of petcock gets used with what type of carb. But ...... the hole to hole centers for the petcock mounting screws changed over the years. The vacuum ones are different, wider I think, so no vacuum petcock is going to fit on your early tank anyway. You'll have to stick with the original manual petcocks. Like I said, the carbs won't care what type feeds them.

 

[CalsXS2]

Petcocks are a separate and unrelated item to the carbs. It doesn't matter what type of petcock gets used with what type of carb. But ...... the hole to hole centers for the petcock mounting screws changed over the years. The vacuum ones are different, wider I think, so no vacuum petcock is going to fit on your early tank anyway. You'll have to stick with the original manual petcocks. Like I said, the carbs won't care what type feeds them.

Thanks. I didn't really want to put out more $.


What pod filter do you recommend.

 

[5twins]

The UNI foam pods, they work the best. Any K&N pleated style pod won't work as well. They will be difficult to tune for and you won't get the most and best performance possible. For the BS38s, the UP4200 or UP4200ST (dual layer) are the size you want (2" mounting flange I.D. x 4" long). You don't need or want the special foam filter oil either. It's too thick and will clog them. Just use a light coating of a 50-50 mix of motor oil and gas. I bought a quart of the cheapest straight 30 wt. from Walmart (their house brand) to use just for this.

 

[XSLeo]

Ok, the early bikes used straight holders. The carbs when in the holders angle out because the flat area on the engine angle out. Most unlinked carbs can work in these holders. They can even work with angled holders. 74/75 used unlinked carbs with angled holders.
76 was the first year of linked carbs.
Once they linked the carb the carbs point straight back. This requires angled holders.
So if you want to use 78 carbs you need angled holders.
almost forgot, your 72 side covers should fit fine. The stock 72 air box won't.
Leo

 

[CalsXS2]

Again. I apologise for being so stupid about this. But I'm just not seeing what your saying.

Hopefully you can see in the pic my flat areas on my engine do not angle out. There is a straight edge across there and all sides are touching.

What the hell am I missing here.

I don't understand why I would need angled holders since the linked carbs are pointing straight back in the straight boots. And the center to center is the same 4.75" as my air box. Which makes me believe I could use it if I wanted to.

Maybe I'll get it when I put the motor in and things just won't line up.

What was that line in a movie. Maybe Denzel Washington said,,, "Explain it to me like I was an 8 year old".

God I feel stupid and like I'm just pissin you guys off. Sorry.

 

[XSLeo]

I just pulled an 82 head out and looked at it. Your right the flat area is level across the head.
I guess I was wrong on that. I assumed they were at an angle, can't say why.
Try the straight holders on the engine and see if the linked carbs fit.
Leo

 

[CalsXS2]

Why yes they do fit. This pic is straight boots, 78 linked carbs.

Does they motor tilt forward in the frame and maybe the angled boots point down.


Cause all I'm seeing is EVERTHING is lining up. But again. I don't have my motor and then as they say."The proof is in the pudin". Guess it will be awhile before I do.

 

[arcticXS]

I have a 650D plus an XS2 spare/parts engine. Both have perfectly flat/parallell inlet flanges on the cylinder head. OEM and reproduction intake rubbers for both BS34 and BS38 are angled slightly downwards. If they angle down the same amount, I'm not sure, but can check later, as I have a stock BS38 set, and a JBM set for BS34 (that unfortunately are a little bit too big for the 40 mm spigots on my RS/TM34 carbs)
I ended up getting some straight 40 mm ID intake rubbers from Allens Performance in the UK. These fit the carbs like a glove, but tilted them up a little bit. Maybe good for flow, but unmodified side covers no longer fit.........

 

[XSLeo]

I know the early carb holders were straight and later angled. On thinking about this, yes the engine tilts forward. The straight holders hold the carbs straight out from engine. With the engine tilted forward this tips the back of the carbs higher than the front.
On the later bikes with angled carb holders the angle holds the carbs level.
Using angled holders moves the carbs a bit and they wont line up with stock air boxes. Not an issue if your using pod filters.
On the side covers this change may move the carbs enough so the sides covers touch the holders or carbs a bit. This may require a bit of modification to the covers.
If you have both a set of straight and angled holders I would try your carbs using both sets. This way you can see which set holds the carbs in the right place so the covers clear the carbs.
This change in tilt on the carbs probably won't be an issue. Just be aware it might have an effect on float height adjustment.
Leo

 

[gggGary]

Level carb reduces chance of filling crank with gas on a leaky float?