carb diaphragm assembly

81 xj650

so how can you check to see if the diarpham assembly is working and lifting all the way up.

besides using a mirror to see what the diaphrams are doing. but i don't think you'll get a full rise by wrapping the throttle at stand still. as it don't take much to wrap the motor out.

seems to me that those throttles would rise more under driving conditions.

if i pull the carbs off. how can i test those diaphrams. i can't see throttle vacuum lifting those things up as there's no way for vacuum to to reach the top of the diaphram. so i'm assuming there is some type of throttle pressure pushing up from the under side of the diaphram.

if i'm incorrect. then tell me what makes those things rise and how do i test them. cuz i have a feeling those babies aren't lifting up all the way. and that's what's blocking me from acheiving maximum freeway speed.

i'm talking about those throttle slides. and not the actual throttle plates. the throttle plates are rotating a full 90 degrees so those babies are working.

 

The space under the diaphragm is open to the atmosphere.

The slide has a small hole at the bottom that is open to the throat of the carburetor. Because the slide is hollow the air pressure on the top of the diaphragm is the same as the pressure in the carburetor throat.

As you open the throttle the air moves through the carburetor faster, lowering it's pressure - also lowering the pressure on top of the diaphragm.

The diaphragm lifts, pulling the slide with it. As the slide moves up it enlarges the cross section of the carburetor throat. As the cross section enlarges the air speed slows down, causing the pressure to rise.

When everything is in equilibrium the slide stops moving.

This type of carburetor is called "constant velocity". It keeps the airflow through the carb throat nearly constant (the force of the spring increases a bit as the slide rises keeping it from being truly "constant").

So long as the slide moves freely in the bore, the hole at the bottom isn't plugged and there are no holes in the diaphragm you can be confident that the slide will function properly.

 

Do the Clunk Test.

Search: Clunk Test.

Polish the Bores.

To get the Diaphragm Pistons rising and falling for Performance riding, ... Polish the Bores.

 

since the diaphram completely covers the top. i can't see any type of atmospheric pressure or vacuum changing from the topside.

i was starting to think that maybe air comes through that jet on the input side of the carb. (or backside). the more throttle. the more air and atmospheric pressure going through that jet to push the diaphram up from the bottom side.

if i remember right. it seems like i sprayed carb cleaner through those top jets and the big one squirted out the back jet.

who knows.

i didn't want to ride this thing anyways. so i guess i'm pulling the carbs off one more time. polishing the bores and seeing what else i can find to fix on the damn things.

i really think this thing has a limiter on it though. although where is beyond me?

prolly want to polish the sliders too. correct?

 

A Limiter would have to be wired to the Coils.
You know when you hit the limit when a Rev-Limiter kicks-in.

The Coils start getting shorted-out to hold the rpm's at the set limit.

The decrease in air pressure on the open port at the bottom of the Piston lets the Air within the Diaphragm Assy rush out into the Intake Stream. The subsequent Vacuum Raises the Piston allowing the Needle Valve to supply Main Jet Fuel to the Engine.

The Spring applies Pressure to descend the Piston when the The Manifold Pressure is normalized.

A delay or any impediment to allowing the Piston to Rise and Fall adversely affects Performance.

 

thanks rick.

there's also another way to have a limiter. and i'm wondering if this is my problem.

i'm gonna pull them off after work today.

but here's a question if anyone can help me out here. i was getting ready for work this morning and i had an idea i'm going to be checking when i get home tonight.

i think the springs are the wrong springs. cuz they seem kinda tall. THYUS, stopping the throttles from going up.

can anyone tell me the sprint height on these things? standing tall. AND compressed.

it's just an idea. but something i'm defenitly going to be checking out.

 

pulled the rack off. i could feel #3 binding on the spring. it was rising but not much. pull the cap off and put back on. used compressed air to blow through the top ovalish ports on the backside. all 4 now go up and come back down like they are supposed too.

looks like the throttles rise by forced air intake into the carbs.
pressure from the underside.
springs force them back down.

can't count how many times i've pulled those things completely apart.
kicking myself in the ass for not doing quality control the last time round.


LOLOL.

might just get this beotch running 100 percent after all. waiting on mufflers now and hopefully some serious riding now.

 

The Vacuum Diaphragm Piston Assembly, ... is dependent upon Vacuum.

When the Intake Air moves rapidly through the Venturi pressure is reduced at the Top of the Emulsion tubes opening ...
Fuel is drawn to the Vacuum created.

Likewise, ...

When the Intake Air stream flows rapidly below the Diaphragm Piston, ... a Vacuum is created at the hole in the bottom of the Piston.
The Piston rises because the Diaphragm is Air tight and the air in the space is drawn-out into the Intake Stream.

Some Jet Kit Suppliers think adding another hole aids in the more rapid rise of the Piston.

 

don't mean to hijack but if the diaphram plunger has a small whole init would that stop a cylinder fron fireing. my number two will not fire but i have good spark compression is a low 125. plug is dry

 

Yes, then the slider would tend to not lift, but the cylinder would still idle and run at low RPM's

Another intrusive thought- - if a person were to throw 4 little fishing weights in there, you would get a little better gas mileage !

 

i'm not worried about rapid rise. LOL

at this point and time i'd just be happy to have the damn thing out on the highway instead of in the garage. this thing has been one pita after another.

gonna have to take her out tomorrow so i can find another thing wrong with the damned thing. mufflers aren't coming untill saturday and since fedex don't work saturday. i'm stuck waiting till monday. gonna have to stick with the home made pipes. and my big ride saturday with lots of patrol. this outta be fun.

 

if there is a small hole in the diaphram could i
lift the plunger manually"air box removed of course"
And get it to fire on that cylinder?

 

Sure.

Break the Point off a Pencil and lift the Piston.

You don't want your finger in there getting sprayed with Main Jet Fuel and having the Bike backfire on you, ... burning the Hell out of you in less than instant.