new to BS34 Carbs & putting them on a my car

[jonuts]

happened upon this community searching for information pertaining to BS34 carbs.

i have a set i just installed on my 78 toyota corolla wagon.

i was hoping to learn a bit more about these carburetors.

here's what they look like on my custom manifold:







-tapped the carb bodies to accommodate .25 npt brass fittings (being that the carbs now sit further a part)

-extended the throttle wheel levers to reach each of the carbs.

-re-jetted. hope to at least get them in the ball park.

here's what happened after installing them and trying to get the car to start:

fuel began to pour out of the smaller orifices of the front of the carbs. all 4 of them. i obviously did something wrong, didn't do enough due diligence, or just royally screwed up. i have been running and tuning dcoe style mikuni sidedraughts for a number of years and was expecting the BS34 carbs to behave similarly. i was obviously mistaken.

is my mechanical fuel pump putting too much out?

is there suppose to be a fuel return from the carbs as well?

i plugged 2 ports (on carbs 2 and 4) believing them to be vacuum ports and maybe i shouldn't have?

i'd be grateful for any insight and an education.

regards,

jonuts
hou, tx

[3Phase]

happened upon this community searching for information pertaining to BS34 carbs.

i have a set i just installed on my 78 toyota corolla wagon.

Welcome to XS11, Jonuts, that is a nice project!

Okay, the carburetors you have aren't exactly like the ones on the XS1100 but they look pretty close.

No, there's no fuel return but you could make one if it's necessary.

The upper three black hoses that run between the carburetor bodies are supposed to be the float bowl vents. At least one of those lines should have tee to allow atmospheric pressure into the float bowls.

What's the float height setting you have now? I haven't seen very many CV carburetors set up to run on a vehicle with a fuel pump but there's no real reason why they wouldn't work. You should be able to reset the floats to hold the needle valve closed under pump pressure without overfilling the float bowls.

Again: nice work!


Regards,

Scott

[DGXSER]

Yep, I agree, very nice work. I'm not certain about the floats holding against the pump pressure though. BTW, did you replace the float needle valves and seats? They tend to need replacing every 20-30 years. And 3phase has you covered on the venting issue. If it were me, I'd start by adding the vent, and making sure the float needle valves and seats are fresh.

These carbs are set to hold against gravity pressure of the fluid in the lines and tank. Depending on the curve of the pump your using, it may, at very low flows like the carbs may be using, put out alot of pressure. You may want to try putting a fuel pressure gage on a line and see what that pump generates with a restrictor valve in the line pumping back to a gas can or even the tank. Then take compressed air with a pressure regulator turned down to the pressure your pump makes and see if the floats hold against it. its alot cleaner and safer to have air pumping out of the carbs than fuel. If they wont hold against the pressure, you may want to either replace the pump, or add a pressure regulator valve to your fuel line feeding the carbs to drop the pressure.

[Ivan]

Normally those old mechanical fuel pumps struggled to put out 10psi. I don't think its the floats not holding the pressure, but a pressure check might do well anyhow, since Iam not real familiar with toyotas. If it is too much pressure, then a Tee in the line before the carbs with a restrictor that returns to the pump may be the solution.

I am betting that if you get your vent situation taken care of, your fuel squirting all over will be fixed. If there is no place for the air to go, then the fuel level can't raise enough to make the float stop the fuel, but there is pressure behind the fuel and it gets shoved through the main jet and where ever else it can find a way out. The air can't escape so essentially its a bubble caught in the bowl, if that makes any sense.

[jonuts]

thanks for all the quick responses fellas and insight.

i wanted to clarify that the upper hoses between the carbs are actually just for aesthetics. the carbs had come that way before i spaced them further and i just kept with the same "fashion" . they're just like spacers i guess to keep the carbs looking similar to other models.

i know the mechanical fuel pump on my 1.8 puts out 7 psi. i was definitely not equipped with the gravity fuel feed information. that makes a big difference in how i look at the carbs now.

i'll give the fuel pressure regulator a try. i'll pic one up of the affordable mr.gasket regulators and plumb it in line between the pump and carbs.

there's a return line already coming from the pump. with that, would it be more ideal for control to introduce just the regulator and drop the pressure to say 1 psi or would that still be too much?

i hadn't replaced the needles or seats. everything was in spec and the needles and seats looked kosher.

i'll pull the caps i put on those 2 ports and see if that helps a bit as well.

i'll take pics of them on the car and with more detail on the carbs. it may shed some more light on things. thanks ya'll.

[Ivan]

Those lines you say are for aesthetics are the bowl vents. If you had internal vents, you'd probably have another hole in the intake side of your carbs in addition to your pilot air and such.

7 psi shouldn't be a big problem for the floats and needles to handle. Especially since the hole is so small. I would bet there is only a couple ounces of actual fluid force on the needle at 7 psi.

At this point, I am still thinking vent problem. Just pull one of those upper hoses connecting the carbs loose and try it. If it doesn't settle down, then you can put it back for aesthetics.

[DGXSER]

I agree with Ivan, look at the inlet bell of the carbs. There are two holes, one is most likely the pilot air jet, the other the main air jet. If the bowls were vented internally, there would typically be one or two added holes in the inlet bell for the vent.

Seen alot of folks do some screwy things to these bikes and the carbs from lack of knowing. So do not thinkt hat because you got them a certain way they were correct. That being said, those are not the typical BS34 carbs on an XS11 mototrcycle. Maybe an earlier model from the 650 or the like? So I am purely speculating.

[tarzan]

Like has been posted, the vents need to be open to atmospheric pressure to make the fuel flow freely.I don't think they would make it flood though.I think that would more likely starve for fuel.
I would think fuel valves, however all four being bad at the same time doesn't
sound likely to me
I suspect float levels.

[psycoreefer]

I agree with everyone else, make absolutly positive that that upper hose isn't a vent. If you find that it is and vent them and they still flood, try tapping on them gently with a hammer or large wrench, sometimes a little tap will free up a stuck float and then they will work like they're supposed to.

[DGXSER]

PSi versus Gravity. I just went out and measured the height from the top of the tank to the bottom of the carbs (on a special anyway) which is approximately 1.25 feet. For a standard tank it might be another 4 inches taller perhaps? So, take the height in feet, divide by 2.31 and you get psi. So the pressure our carbs typically see is under 1 PSI, about .7 psi of pressure.

With that pressure, and no vent, I'd say it would starve for fuel. But if I am pushing the fuel into the bowl at 7 psi, it will overcome the pressure /restriction of flowing through the carb orrifices to flow out the air jets, or the inlet bell.

[DiverRay]

As someone who put flatslides on a Datsun 2000 Roadster, I would say USE A REGULATOR. I set the one on the Datsun to 1 psi, with a return line. Just put a small, about 1/16" jet into the line on the return side. I'll look and see if I have any good pictures I can post.
The vents are also a MUST, as the carbs WILL FLOOD if they are not vented properly. I would also use a single bar across ALL 4 CARBS. You need that to keep everything from moving, and in sync.
Sounds like a great project, though!

[jonuts]

DGX, psyco and tarzan i wanted to make sure we're on the same page. in the pic below, the blue arrow are what i call the "aesthetic" hoses and the purple arrow the capped port i thought was vacuum.


i didn't recall seeing any open ports in the carb bodies where i have the "aesthetic" hoses connected between all the carbs. i'll check again.

i'll pull the caps off the port at the purple arrow to see if that helps.

As someone who put flatslides on a Datsun 2000 Roadster, I would say USE A REGULATOR. I set the one on the Datsun to 1 psi, with a return line. Just put a small, about 1/16" jet into the line on the return side. I'll look and see if I have any good pictures I can post.
The vents are also a MUST, as the carbs WILL FLOOD if they are not vented properly. I would also use a single bar across ALL 4 CARBS. You need that to keep everything from moving, and in sync.
Sounds like a great project, though!

i'd love a set of flatsides but out of my budget at the moment. regarding the 1/16" jet on the return line you mentioned; which return line would that be? the mech. fuel pump return or would i need to get an fpr that has a third outlet? if so, i'll have to pass on the mr.gasket fpr then (dang it). OR can i just "T" into the feed line after the fpr (before the carbs) and rejoin the return line coming from the fuel pump?

[kirmit77]

i would love to see that set up!!! how it coming??
I live north of houston and if you need some jets i have some

[DGXSER]

DGX, psyco and tarzan i wanted to make sure we're on the same page. in the pic below, the blue arrow are what i call the "aesthetic" hoses and the purple arrow the capped port i thought was vacuum.


i didn't recall seeing any open ports in the carb bodies where i have the "aesthetic" hoses connected between all the carbs. i'll check again.

i'll pull the caps off the port at the purple arrow to see if that helps.

As I stated, I am only really familiar with the XS11 version and these are not the identical beast, but I would suspect that nipple in purple will be the beast.

[fredintoon]

happened upon this community searching for information pertaining to BS34 carbs.
i have a set i just installed on my 78 toyota corolla wagon.
i was hoping to learn a bit more about these carburetors. - - -

Hi jonuts,
first, kudos on your endeavor and hopefully the list's carb wizards can help you get yours sorted out.
But here am I wishing there was only one carb on my XS11 so I didn't have to try balancing four of the bastards.
Takes all sorts, eh?