Build Thread...Special to Cafe Bike

[YamadudeXS650C]

Well, good question - I only saw the LH one leaking but I'll have to check that. At that moment in time, I was mainly focussed on preventing "an earth shattering kaboom"
View attachment 103823

I am still puzzled about this vacuum line thingy? Both of my carb vac outlets are plugged with what look like factor caps.

Any chance you could post a photo and also one of the airbox installation?

"I am still puzzled about this vacuum line thingy? Both of my carb vac outlets are plugged with what look like factor caps."
One should be hooked up to your vacuum petcock. My '80 and '83 are set up like this, at least.
I will go out and look at the air box setup on my '81 out in the shed.....

 

[MaxPete]

That IS true but this was a pretty rapid flow of gasoline.

If a float needle valve was simply doing what float needles do - I would expect to see just a tiny amount of fuel.

 

[YamadudeXS650C]

Holy cow 'Dude - how the heck did those two washers get there? Was any part of Jimmy Hoffa in there as well?

OK - on to another topic:

After I finished the starter hairpin and clutch reassembly late last night, I started the bike (yeah baby - worked like a charm). I was using the newly rebuilt BS34s with NO air box or pod filters and the stock '81 Special fuel tank with the vacuum peacock set on PRIME.

After a few seconds, there was fuel dribbling out of the danged carbs (a bit scary actually ).

I guess a float needle may be stuck....but I have no experience with these petcocks. Any other suggestions about where to look for problems before I pull off the carbs <again>?

I also have a question and a request:

• To what does the vacuum barb on the petcock connect? It isn't obvious to me where the vacuum source actually is.
• Could someon please post a photo of an '81 Special showing how the airboxes are installed - including the PCV hose routing? I have the parts, but this is crowded part of the bike and the routing is not obvious to me.

Many thanks for any and all info!

Pete

AH,, I see now that you have the petcock in the PRIME position. I believe that it is functionally ON, and gas is flowing, in that position. The float needles, as I mentioned, are not expected to fullly shut off the gas. So, set your petcock back into the ON position. No gas will flow until the engine starts. Once you hook up the vac line.

 

[MaxPete]

Yup - that is what I had understood. The fact that I didn't have a vacuum line connected is the reason I was running in PRIME.

 

[YamadudeXS650C]

That IS true but this was a pretty rapid flow of gasoline.

If a float needle valve was simply doing what float needles do - I would expect to see just a tiny amount of fuel.

Maybe a speck of dirt in one.

 

[YamadudeXS650C]

Yup - that is what I had understood. The fact that I didn't have a vacuum line connected is the reason I was running in PRIME.

Before dismantling anymore, I suggest running the engine awhile, petcock in the ON position, vac line hooked up. THEN recheck for leaks.

 

[MaxPete]

Yup - good thought.

 

[YamadudeXS650C]

" and the stock '81 Special fuel tank with the vacuum peacock set on PRIME. "
..........and for Pete's Sake, make sure that you're using a petcock, not that peacock !!

 

[MaxPete]

Oh geeez - I wish you had told me THAT before. All this time.....

 

[YamadudeXS650C]

Well, good question - I only saw the LH one leaking but I'll have to check that. At that moment in time, I was mainly focussed on preventing "an earth shattering kaboom"
View attachment 103823

I am still puzzled about this vacuum line thingy? Both of my carb vac outlets are plugged with what look like factor caps.

Any chance you could post a photo and also one of the airbox installation?

Airbox:
My '80 and '83 MIGHT be the same, I dont know.
I checked my '81 barn find. Hadn't looked at it in about 4 months. It has no air boxes. No carbs either, but a member provided me a nice set.

 

[YamadudeXS650C]

Yup - good thought.

Just sent you a PM.

 

[5twins]

The parts drawing shows the right manifold capped, so I would assume the vacuum line goes on the left one. That would make sense seeing that the single petcock is on that side. Here's my '78. I have 2 vac petcocks so have a line on each manifold .....

Here, the vac line is disconnected for manometer use, petcock on "Prime" .....

 

[weekendrider]

The crankcase breather tube is a 3 part hose assembly. The longer curved hose runs over the carbs and down following the line of the backbone. A "T" is in the end. Off the "T" individual lines runs to a spigot on each airbox.

 

[Bob Kelly III]

I have to disagree with ya Dude..... the needle valve shut off in the carb IS supposed to shut off the gas flow completely...it MUST
that's what the float and needle valve DO ! other wise you'ed find a puddle of gas under your bike all the time, or worse gas in the oil !
..... set properly the float valve can and will shut off the flow of gasoline Completely !!!! even with a full tank of gas.... and that's some pressure
but it must do that .... or you have problems .
.....
.....Pete... My 83' has the vacuum petcock on it and it is plugged into the intake ports spigot with a short length of hose on the left cylinder, where the petcock is located. the hose doesn't have to bend real tight, but sense the petcocks hookup is pointed toward the left hand cylinder its a straight shot and then bends down to the cylinder intake.... getting the hose on the petcock is a pain on the bike it's much easier if you just take off the tank ! .....
if I take off the tank I usually leave the vacuum hose on the petcock and just pull off the line on the intake manifold.... that means you gott'a find it before you set the tank all the way down..... or it will be under the tank !
if I don't do it that way I have to use my bent end Needle nose pliers to get the hose on the petcock !
.....
Bob.......

 

[Bob Kelly III]

WR ... Hay no fair those are Metal !!!!!! mine are plastic.... LOL..... i see a modification coming on my bike !
......
Bob.......

 

[weekendrider]

Bob, you might as well give up on the float valve supposed to be a shut off valve. They don't recognize the connection between a fluid level in the carb and the fact the valve has to shut off to keep it from going above that.

 

[weekendrider]

No, Bob they are plastic. Probably from an '83, but maybe from an '81. They are interchangeable so I don't bother to mark them.

 

[MaxPete]

Ahhhh....I see now. Thanks WER - my airbox assembly is off the bike right now but is identical to the one in your photos. I now understand that the tee is at the bottom of the assembly. I was wondering where the heck att that tubing was supposed to go.

Dude and Bob - gotcha. I've never noticed that vacuum hose on any of the Specials I've seen but I can certainly see where it goes alright.

Finally, based on Peanut's experience in trying to get to his MOT, I'll try just giving the leaking carb a gentle little cuff with a piece of wood. It was JUST rebuilt by a really good guy who knows his stuff so I'm sure it must just be a little speck of dirt.

We're just headed out for dinner and I have a business trip to Toronto tomorrow AM - but I'll hook everything up when I return and get back to you.

Many thanks guys!

Pete

 

[Bob Kelly III]

Plastic !?!?! they looked like aluminum in the photo ! ok well, Rats ! .... mine are black plastic yours must be white or painted silver....
they look better than mine do that's for sure !....
.... well, on the float setup , you believe what ya want .... I stand by what I said ..... to each their own !
.....
Bob...........

 

[retiredgentleman]

Yamadude quote:
" The float needles, as I mentioned, are not expected to fullly shut off the gas."

Absolutely correct. That is why the petcock (or peacock ), must be 100% shut off capable.
WER.................looks like you now have Bob Kelly on your "motorcycle float valves are 100% shut off valves" team.
I believe grizld1, and 2M concurred with me when we last discussed this, but we can re-hash the topic again if needed.
That's what internet forums are good for.......................everyone gets to state their opinion of disagreement.