Do I have "Fixed Choke" carbs?

[TheNose]

My 1977 GS750 has VM26SS carbs. I want to use a Colortune on them, but the instructions differ for "Fixed Choke" carbs and "Variable Choke" carbs. I don't know what the difference is. Which kind do I have?

Thanks,

Terry

 

[focus frenzy]

good question, HEY EARL!!!!

I know some carbs have only off and on and often 50% settings for their chokes.
while others, like the BS cv carbs used on 80 up GS you can dial in how ever much chock you want.

I wonder if thats what the instructions are referring to.

 

[TheNose]

good question, HEY EARL!!!!

I know some carbs have only off and on and often 50% settings for their chokes.
while others, like the BS cv carbs used on 80 up GS you can dial in how ever much chock you want.

I wonder if thats what the instructions are referring to.

Yeah, where is Earl when I need him? Think he's out killing buzzards?

Terry

 

[spots25]

well, on my GS400, there is a little lever on the carb for choke. Its either on, or off. There is no inbetween.

on my old v45 Sabre, there was a lever on the handlebar that allowed me to dial in how much choke I wanted.

I assume the GS400 has a fixed choke, and the v45 had a variable

 

[KEITH KRAUSE]

Terry, Earl ran out of buzzards and is now shooting crows. No doubt Parakeets are next. They can't quite out run his 1150.
As for this "variable choke" thing, this colortune tool you want to use is an English product, correct? I think their use of the words variable choke and fixed choke is throwing you off a bit. It's making you think of the choke itself or if it's a pull knob or lever type.(?)
They use a carb over there called an SU carb. SU means Skinners Union I think. This carb operates on the same principles as the CV carb. Other names associated with SU carbs are: variable venturi, constant pressure, VARIABLE CHOKE...The CV carbs are also called similar names: constant vacuum, constant velocity...
So when you say the instructions are different, I think they're just talking about what kind of carb you have. Unless I'm missing something, you would follow the variable choke instructions if you have CV carbs. If you have VM carbs, I think those are the "fixed choke" you mention but I'm not sure about this.
The choke itself shouldn't have anything to do with the tools use of determining correct mixtures.

 

[Wingnut]

Re: Do I have "Fixed Choke" carbs?

My 1977 GS750 has VM26SS carbs. I want to use a Colortune on them, but the instructions differ for "Fixed Choke" carbs and "Variable Choke" carbs. I don't know what the difference is. Which kind do I have?

Thanks,

Terry

I am just taking a WAG here but maybe they aren't talking about the CHOKE like we think of it. Maybe they are reffering to fixed and variable venturi carbs, especially when you consider the fact that "who tunes with the choke on"? Is this thing made in England? I can never understand their stuff. (like tyre) No offense to our British friends. I just usually don't understand the language and theory.


Edit: For example, our British aircraft maintenance manual says. "Remove the fuel tank access panel, and with a torch look inside for corrosion." Now does that sound safe to you!

 

[TheNose]

Here's what the Colortune manual says -

A basic Carburettor fuel system

There are four basic elements to a simple "fixed choke" carburettor which might be fitted to garden machinery or small power plant engines.

1. A throttle to open or restrict the main passage of fuel/air mixture to the engine, this usually has an adjustable throttle stop to control idle speed.

2. A float and valve arrangement to maintain a steady fuel supply level (or a sensitive pressure regulationg diaphragm.

3. A slow speed fuel discharge and adjustment (idle jet and idle mixture screw) this is found close to the engine mounting flange and the throttle plate. The throttle uncovers extra passages (progression holes) for feeding fuel as it begins to open.

4. A high speed fuel feed and adjustment (main jet and main mixture screw) normally located closer to the inlet air cleaner mounting flange. The main fuel discharge feeds into a venturi shape, whose restriction (a fixed size choke) gives a low pressure to pull the fuel in.

Note: a cold start device is also sometimes called a choke - it also gives a restriction to draw in extra fuel.

Carburettor fuel systems as fitted to motorcycles

1. Variable choke carburettor. This usually has the same parts as described in items 1-3 for the basic fixed choke carburettor but the main discharge has fine tapered needle in a jet to meter the fuel accurately. The variable choke size is formed by a rising piston arrangement attached to a flexible diaphragm. When the throttle is opened, the engine draws in more air and the diaphragm pulls the piston and the needle further open, maintaining a constant low pressure (constant depression) in the carburetter. This gives precision control of the air and fuel over a range of conditions. Enrichment for acceleration conditions may be provided with a separate pump device or by controlling the rate at which the piston rises. Needle/jet calibration size controls mixture except at low speed/light load which is governed by the adjustable idle mixture control.

2. Slide type carburettor. The slide carburettor has been used on motorcycles and other small engines for very many years. Fuel feed is from a chamber with float and needle valve and an idle fuel system is provided with an adjustable mixture screw. There are no progression holes in the idle system as the throttle plate is replaced by the slide which gives this carburettor its name. The slide is attached to a needle running in a jet in a similar way to the variable choke carburettor but this does not give a constant depression as the slide position is governed by the throttle position selected at the time, not by a diaphragm.

Mixture is governed by the shaped bottom on the slide at lower speeds/light loads, the needle taper and position at mid speeds and the main jet size at all engine speeds when on full throttle.


(End of Colortune instructions.)

I have the slide type carburetor, but I still don't know from their definition if I have a fixed choke or not. Evidently, the Brits are referring to something else when they say "choke".


Maybe we should get Simon to translate this into English. :wink:

Terry :?

 

[KEITH KRAUSE]

I don't know if you read my first reply, but their instructions are for CV carbs or VM carbs.
Variable choke carbs ARE CV carbs. So I figured fixed choke carbs are VM's. Their description of the slide carb tells you the VM carb slide position is governed by the throttle position (fixed choke) and there is no constant pressure as I said earlier or constant depression as found in CV carbs.
Constant pressure, constant depression, variable choke... are describing CV carbs. The diaphragm assembly creates a constant depression that forms the variable choke.
Use the fixed choke instructions with VM (slide) carbs.
If you still have doubts, I'm sure you could ask the Colortune folks.

 

[saaz]

Keith nearly had it right. I have a Colortune as well, have done for more than 20 years.

The fixed or variable choke refers to whether the venturi is fixed (ie no slide going up and down inside) or variable (a slide goes up and down varying the venturi). A variable venturi carb can be a CV/SU type or a slide carb.

Fixed choke carbs typically only have a throttle plate to control the intake. Think of a Holley carb off a car, or a weber etc. They have various jets inside of them, with an accelerator pump usually.

CV type carbs have a variable choke and a throttle plate.

A true choke for starting when cold does just that, chokes of the intake, rather than enriches the mixture like most bikes. A few cars I had with fixed choke carbs had a real choke plate for starting.

My colortune instructiosn specifically say that slide carbs common on motorcycles are variable choke carbs. maybe that is showing the age of my kit!!

 

[TheNose]

Thanks, guys! It's not that the instructions are different for a fixed choke versus a variable choke carb, it's the fault remedy that is different.

Terry

 

[KEITH KRAUSE]

Yeah. The description Terry showed of "the 4 basic elements" didn't perfectly fit either style of carb. I got tired of reading the whole thing. :lol:
Seems to me they're saying CV's are variable choke though. Since the Colortune info lists two kinds of carbs, I figured the fixed choke had to mean the slide or VM carbs.
Terry, why do you have to ask such PITA questions?? :?
And in the words of an infamous Mexican general...Colortune? We don't need no stinking Colortune! :lol:

 

[TheNose]

Yeah. The description Terry showed of "the 4 basic elements" didn't perfectly fit either style of carb. I got tired of reading the whole thing. :lol:
Seems to me they're saying CV's are variable choke though. Since the Colortune info lists two kinds of carbs, I figured the fixed choke had to mean the slide or VM carbs.
Terry, why do you have to ask such PITA questions?? :?
And in the words of an infamous Mexican general...Colortune? We don't need no stinking Colortune! :lol:

You got tired of reading it, and I really got tired of typing it. But, I got the info I needed. Once again, thanks.

I don't really have a good feel for how carbs work, so it was confusing for me to try to convert ENGLISH to AMERICAN when I didn't really understand the principle. :? That's why I decided to be a PITA. Sorry. :wink:

Terry

 

[Hoomgar]

OK all that is well and good but I was wondering how the choke works on the vm26ss carbs like Terry and I have? It looks like some plunger and there is no actual restriction of the main air flow that I can see? Can someone explain to me how the choke works on the vm26ss carbs or any that use a choke like it?

 

[TheNose]

OK all that is well and good but I was wondering how the choke works on the vm26ss carbs like Terry and I have? It looks like some plunger and there is no actual restriction of the main air flow that I can see? Can someone explain to me how the choke works on the vm26ss carbs or any that use a choke like it?

Good question. I'd like to know the answer too. Soooooooo--

ker-bump

Terry

 

[KEITH KRAUSE]

One last word about the Colortune thingy.
Why do they even have seperate instructions for "fixed choke" carbs, when this design of carb wouldn't be used on our bikes?
Now I'm off to help Earl with them crows.

 

[KEITH KRAUSE]

And about the VM choke. There's 3 parts to the choke. A starter jet, starter tube, and the plunger.
The throttle must be closed for the negative pressure to draw fuel. Open the lever/pull knob to lift the choke plunger.
The fuel goes up through the starter jet, then up the starter tube where it mixes with air coming from the float chamber. From there it goes into the plunger chamber and mixes with the starters primary air and then enters the main bore.
This very rich and well atomized mixture is added to mixture being supplied by the pilot circuit.

 

[TheNose]

One last word about the Colortune thingy.
Why do they even have seperate instructions for "fixed choke" carbs, when this design of carb wouldn't be used on our bikes?
Now I'm off to help Earl with them crows.

Evidently it can be used on any type of carb. I just wasn't smart enough to figure out which kind I had. :?

I must report that after using it, I checked my plugs and all four are a nice tan/brownish color for the first time since I've had this bike. I don't hear well and could not set the carbs by sound, and my tach is slow to react, so I had a hard time getting max idle out of it.

I could not have done it without your help, Keith and I really appreciate you taking time to use your vast knowledge of carbs to help us "carburetor challenged" members.

Thanks!

Terry

 

[KEITH KRAUSE]

Thank you too Terry! I learned something through this topic too.