Carb Comparator

[Areomyst]

I haven't gotten a chance to do much with the carburetor comparator. I got a few photos of the Simons Carburetor Comparator, and I called Simons and the kind lady agreed to send me a manual if she could find one (although she's unsure if they still have any or not). I suppose we'll see if a manual arrives in the mail.

I'll take some more photos later on but wanted to share the few that I had:




I'm wondering about how the manometers are supposed to be hooked up, and why have two inclined manometers, unless it's for two separate ranges? Perhaps the large testing port is for the lower test pressure manometer and the higher test pressure is for the small carburetor port? I'm unsure - perhaps my ignorance shines now more than ever. I'm looking forward to the learning though.

~Josh

[Brucepts]

Extra manometer is probably for testing fuel signals

[Hotz]

As always I be talking nonsense, this machine is to measure motorcycle carburetor? Extra manometer for measure piston vacuum

[Brucepts]

On small engines with simplistic carb designs it's very important to have the best fuel signal you can get. Most classes run stock carbs and pretty strict rules so when the builder blueprints an engine using stock parts they look for the best of the bunch. Fuel signal is a very important part of that process.

The carbs I'm talking about have an idle circuit a transition circuit which is usually just a series of holes drilled along the carb bore as the throttle plate opens from idle to main circuit and then the main circuit. They have a simple nozzle/jet that acts as the emulsion tube and this also comes into play with the signal. Changing the height of the nozzle in the venturi area would have a serious effect on signal, it could be as small a change as a few .XXX" of an inch.

When I was playing with "mowing engines" I would hook a manometer to each circuit so I could see what was happening . . .

[larrycavan]

The carburetors you're testing are predominately going to be Mikuni or Keihin. On anything from about 1981 forward they're Constant Velocity type. On older models they will for the most part be Manual Slide type. Each type has overlapping circuits. Pilot Jet, Jet Needle / Needle Jet, Main Jet.

CV carburetors for modified high performance engines work best with the simple purchase and installation of a proper jet kit. You can screw around trying to band aid those carburetors by shimming the needles but it's an exercise in futility for the most part. Way back when they were first introduced the method most used when removing the airbox in favor of individual air filters was to shim the needles, up the mains 4 or 5 sizes and put larger pilot jets in them. They sort of half ass worked with that setup. A jet kit eliminates all that hassle.

The key element in the jet kits is a new, adjustable needle with a different taper that will deliver the proper fuel curve. You never need to increase pilot jet size. That, [larger pilot] in fact, is the wrong approach altogether. The pilot circuits are maxed out with about 3.5 turns out of the fuel screws. Needle clip position, pilot fuel screw & main jet are your tuning elements. A good accurate carb synch is a must as well.

Each slide has a lift hole that sometimes gets drilled to alter the lift response. Sometimes there are different springs with the kits. When performance cams are part of the equation you sometimes find a spot in the RPM range where the slide gets all sorts of confused by the vacuum signal. The result is a studder at some RPM [often 5K range] that you play hell trying to tune out. Sometimes it requires a custom needle.

On mildly cammed engines the simple addition of a jet kit will get you running properly in short order. On a true performance engine with more serious cams your best bet is to head straight to a Mikuni RS or Keihin FCR. On an all out drag race application Lectrons are the way to go. Their simplicity with their unique flat sided needle yields the best fuel atomization. They aren't a good choice for any other application though.

In short, what I'm saying is this. That machine you have is a wast of time for motorcycle carburetors. It may be fun to play with but if you're after quick, positive results that make HP then go with the proven method. Larger carburetors are for flowing more air. The proper carburetor for any application is the one that delivers the correct fuel curve with the proper amount of air. This wheel doesn't need reinventing...it rolls very well as things are...

On the other hand if you're trying to develop your own jet kits, then it may in fact be useful. Seems to me you'll need a dyno as well.

[Areomyst]

Hi Larry,

Thank you for the information. Though this is marketed more towards carburetor use, my plan was to use this adapted to a 50cc bore (usually 39mm with 41.4mm stroke) to check changes made to the porting and valves. I was hoping to have a somewhat useful tool in the meantime for checking airflow in these small heads while I mustered the time and money to get working on the PTS flowbench.

I don't suppose there's a good rudimentary way to calculate CFM out of this machine is there?

Instead of buying this I could have picked up some MDF and orifices...

~Josh

[Areomyst]

I found the manual for the inclined manometer:
http://www.dwyer-inst.com/PDF_files/D_59.pdf

[larrycavan]

I see. What make are the engines?

How do they figure 1 inch of water column is useful?

You could test the pilot fuel circuit with perhaps 1 inch of water column but certainly not the throttle bore.

Seems to me that by the time you get the slide fully open on a CV carburetor your inclined gauge is going to be pegged....

[Areomyst]

Thank you for taking the time to reply to my questions Larry. I myself find a hard time finding the time to get on forums (even one that I own) - I really appreciate the help! Surely you can tell by my questions that I am a novice, and your attempts to better educate me mean a lot.

I see. What make are the engines?

The engines that I want to test vary from 50cc to 250cc. Most will be 50cc though. The short list below should encompass the majority of the engines I plan to work with though:
Honda NPS50S (Ruckus) is a 2-valve 50cc
Yamaha YW50F (4-stroke Zuma) 3-valve, 50cc
Chinese QMB 139, also called a GY650 sometimes (crappy, but common)
Vespa LX50 (old ones are 2-valve, new ones are 4)

Most of these engines make around 3HP at the rear wheel according to my dyno. Some of them have aftermarket cams available, but none of the manufacturers really tell you much about the cam. I hope that with the flow bench, dyno, and by plotting the cams I can get a better idea of the relationships of the porting, cams and other engine mods to the power output. I have not done much with 4-stroke porting yet. I have been waiting on getting the flow bench done before doing a lot of 4-stroke port work. I have had a lot of fun with 2-strokes over the years though!

How do they figure 1 inch of water column is useful?

I don't know. I figured this little bench was better than no bench, but now I tend to second guess that. I was hoping for a set of instructions from Simons but as of the mail delivery today none have arrived.

You could test the pilot fuel circuit with perhaps 1 inch of water column but certainly not the throttle bore.

Seems to me that by the time you get the slide fully open on a CV carburetor your inclined gauge is going to be pegged....

Perhaps a more realistic inclined manometer will make this at least a little more useful? I got excited about this Simons machine because it was already assembled, and I thought I was going to be able to more or less "plug and play" with this thing after making a cylinder adapter. Indeed, as you suspect, when testing any parts that I've tinkered with so far the inclined gauge was pegged...

[1960FL]

Larry,

Not sure if you saw the other thread on this device but it is not to measure flow but to compare Small Briggs and Straton & Techumeh carbs for Class racing go carts. It is meant to compare gross flow and circuit signal.

My guess is these guys get a box of these carbs a 12 pack of &*&^% and comence to testing. Problem is not sure any of us know just what they were looking for but the guy who was selling the box to them did.

My guess is they used the DP across the carb to compute CFM but thats a guess.

Rick