Carburetor jet measurment

I'm trying to figure out what the measurment scale for carburetor jets is based off of. I want to be able to check the orfice size with a gage pin to see if it's worn out. The jet is stamped 32.5, other options in the parts book are #30 and #35. What is the orfice size of these jets in thousands? Are furnace and hot water tank gas jets measured the same way? Is there a formula for converting to a decimal diameter? Thanks, Don

in the MC world, jets are (I believe) manufacturer-specific (IOW they use what numbering system suits them), and not based on any one gage method.

Ironhead is right. It is an arbitrary number. Some manufacturers go up in number when going smaller and some go down.

As far as worn out jets go, personally I think jets wearing out is a myth. I am sure there are cases where it has happened, but I have never seen it. I spent about 5 years rebuilding carbs of all types. Updraft, downdraft, side draft, etc etc.

In the 70's we would take 2 bbl Ford 302 v-8 carbs and drill out the jets one number size larger. This would improve power and mileage.

The number should reflect the "Flow" of the jet.
Since there are several thngs that can affect that, the hole size won't necesarily track what you think.
You could take a new jet, and measure it, and compare it to the worn jet, (but only if from the same manufacturere, not another vendor of "equivalent" jets) or just swap them, and compare results.

Chances are good to ruin the jet by using a gage pin. Jets are tested for the amount of flow in the flow direction (read this again until you understand it). Comparing jets can be done with a fixed amount of fuel and a stopwatch.
Mikunis, Keihins and European makes are most of the time dead nuts the same out of the box, so the stamping ist right. Other asian stuff is sometimes at least inconsistent...

During the years, jets tend to grow smaller since oxides and dirt stick or form on the inside, especialy if water and/or alcohol is present. Get an ultrasonic cleaner with the right cleaning solution and throw in the jets. It´s amazing what amount of dirt will com loose from the jets. Same thing for the carbs, most carb problems are not from the jets but from clogged or messy holes that hinder the airflow to certain parts of the carb or disturb fuelflow at higher rpm.
If the problem persists pull the pet cock and clean both fuelfilter and the tank itself.

And if you tune a bike yourself, buy the handful of jets you may need new from a trusted dealer. Jets are cheap, motors are not.

Greetings,
Johann

I was wondering about jet sizes myself sometime back. My question was specifically for a GM model 4G carburetor. Upon checking the jets with pin gauges, I found that the jet size was just the diameter of the orifice in thousandths in this particular case.

Dennis

Some jets are actually marked in relation to their size, others have arbitrarily selected numbers related to their flow in proportion to others. I'm told Mikuni does it both ways for different types of jets. I do know that Holley jet numbers do not relate to their diameter, there are several sets with different numbers that have the same minor diameter, the different flow is caused by the entry and exit to the minor diameter. Clear?

To answer your question, jets don't wear out, unless someone has been poking wires or tools through them. +1 for Johann's advice.

On Keihin carbs the jet# is the area of the hole, not the diameter. So a #60 jet has twice the area of a #30 jet. while the diameter is increased by 141.42%

+1 for the reply by Johann!

Very slight differences in the entry and exit profiles of the orifice can vary flow dramatically therefore the diameter or area of the hole is not sufficient to predict flow rates.

While it is true that carburettor jets, in general, do not "wear out" they do get crud built up in them. Gums/tars, oxidation build up, soot... and just plain old debris can clog or restrict the flow. There is also the case, especially in motorcycle carbs and other CV carbs (I am thinking of SU, Stromberg and Amal carburettors) where the main metering needle moves up and down inside the main jet and can change the area of the jet quite a bit. Often this can be avoided by proper centering of the needle. It can sometimes be adjusted out by lowering the needle further into the jet (or raising the jet). Some SUs were designed so that the needle was spring loaded to one side of the jet. Many were made with jets that were far harder than the needles and depending on age (time in service) one could wear out many needles in a given carb before having to replace a worn jet.

Then there are the idle air, idle, and mixture adjustment screws AND their seats that often get buggered beyond use by hamhanded mechanics.

The number one problem I used to see with carburettors was worn throttle shaft bores and/or throttle shafts. There is NO way to get an engine to idle correctly when it is sucking air past the throttle shafts. It is a simple problem to diagnose but so often overlooked. It can be much more complicated to fix.

First thing on tuning of an engine is to check valve adjustment (if applicable), then check compression, then check for any induction system vacuum leaks, then check that ignition system is 100% to spec, then, and only then, proceed to fuel system. Unless you have some obvious fuel system problem like black smoke at the exhaust or no fuel even getting to the carb.

As far as jet sizing is concerned the only manufacturer I know of that has a consistent system across all their products is Webber. All their jets, venturis, bores, strokes, etc... are sized in millimeters. Their DCOE and IDF carbs also have ball bearings for the throttle shafts and easily replaceable seals. They are some of the nicest carbs I have ever worked with.

Tip: Never pick up a Webber DCOE and blip the throttle while looking at the rear of the carburettor.... you will almost certainly get two nice well metered streams of gasoline squirted directly into your eyes.

-DU-

Well, some jets are measured in thousandths of an inch.

Well, actually sometimes the number means something that relates to a real number. I'm not sure if it was an AFB/edelbrock, or a quadrapuke, but they had jet sizes which were thousandths of an inch. Another carb I took apart had a numbering system similar to yours, and it was actually 30 = 3.0mm Does that sound farmilliar, or look close??? Sometimes, it's simply an arbitrary number. Mose carbs use the same basic shape of the jet, and the only thing they change between jets is the diameter. Granted, there are a lot of things that affect flow through a jet, and one of those IS diameter. I've never seen different jets for the same carb that changed anything other than diameter. SO, I'd say it's safe to assume the number on the jet corresponds to a diameter. The question is whether it's founded on a real scale of measurement, or if it's a number they assigned to it. Sometimes it's based on a volume of fuel that flows through it, which I've also seen BUT, they still changed the diameter to change the flow. I've also seen jets that were based on the number drill size from 1-60.

You would think there would be some sort of manufacturing standard for sizing jets. What if nut and bolt manufacturers used whatever system they wanted for there parts, what a nightmare that would be!

The jet I had was stamped 32.5, it pinned at .026. I went ahead and reamed it to .032. This is out of my sons dirt bike, after putting things back together the bike ran, but very poorly. We took jet back out and filled it with silver solder and drilled orfice out to .027 (smallest drill I had). At this point the bike runs O.K. but the throttle response still isn't what it should be. I realize the orfice may not be smooth enough for proper flow. I will order a new jet and see how that works out.

I did call the local bike shop and asked the parts guy what the jet numbers represented and he said the size of the hole but didn't know what it should actually measure. He said for that carb. I could get a #30, #32.5 or a #35.

Thanks for the input, Don

The number one problem I used to see with carburettors was worn throttle shaft bores and/or throttle shafts. There is NO way to get an engine to idle correctly when it is sucking air past the throttle shafts. It is a simple problem to diagnose but so often overlooked. It can be much more complicated to fix.

This is very good advice! On old mikunis, the seals for the choke (rubber cover or O-Ring) tend to leak air and therefore idle setup can be a pain in the neck.

32,5 sounds like the idle jet to me. Play around with the idle mixing needle, sometimes they have a small plastic cover to discourage the owner to tweak around and the bike is set very lean to fulfill regulations.
On twostrokes it´s the opposite, they are jetted very fat so the user can´t kill the engine by leaning it to the point where it will eat a piston.

Greetings,
Johann

.026 = .66mm according to my digi caliper.

Well, it COULD be that the jet size is half the diameter, or radius in milimeters. That makes sense accoring to your pin size, reaming/drilling probably screwed it up. Anyways, tinkering witht he mix screws might sort things out if your havent done that allready.

This project started out because the bike wont idle consistently and has poor throttle response. We just bought this bike and knew it had some issues so it will be a good learning project for my son,(and me). Adjusting the idle screw and air mixture screw didn't fix anything. I looked at the reed valve and thought it looked O.K. so I started tinkering with the carb. When I order new jets I will replace the reed valve and see what happens. I don't have a way to check compression except to push down on the kick starter, but compression feels O.K.



Johann, how can I check the throttle valve and throttle shaft to see if their worn?

Thanks

You did not mention what brand of bike this is. Most dirt bikes do not have a throttle shaft at all. The throttle cable enters the carb from the top, and is hooked directly to the slide valve, no throttle plate. The slide holds the main needle.

Theres a fair number of riders on this site. But Milacron does not want this do be a bike forum.

Toms Wheels, well my question started off as a dimensional question about a carberator jet and kind of evolved from there. I don't want to turn this into a bike thread so I won't mention the bike is a 1999 YZ80.
I say let's put an end to this thread before I get us all in trouble!

Thanks to all,
Don

This goes off topic now...

Hy,
I guess it has a mikuni TM carb then. Buy a Keyster Carb set for it, all jets and gaskets are included, costs 20 bucks. Or buy the original jets at a yamaha dealer.The leaking shafts we talked about are a malaise of constant velocity carbs that are used only on fourstrokes.
Clean the outside of the carb , don´t go into detail here, just get it clean enough to touch. Find a speedshop and let them clean the carb with an ultrasonic cleaner, all jets are screwed out, otherwise you shake loose the dirt but it won´t leave the cavity it´s hidden in. Again 20 bucks. No compressed air, no matter what they say, ultrasonic is the word here.
Fill the tank half full and shake like a madman for a few minutes. Pull the petcock, drain the fuel into a container. Clean out the petcock, too. Now, get a funnel, place some kitchen roll in it and drain the container back into the tank. You´ll be amazed by the amount of dirt you´ll find.
Also, drain the oiltank both in and outside, especially the filling plug and its rim next to the and spray it clean with brakecleaner before you refill.

The ultrasonic cleaner will shake loose dirt in places you cannot reach or cannot blow clean 100% with compressed air.

Other places to look for bad midrange and idle: spark plug (new? gap? right type?), high voltage connections from coil to cable and from cable to spark plug connector. Also check the ground connection of the coil to the frame. check intake for airtightness. The reeds should be o.k. if they lay flat on the frame and seal it without a gap and aren´t burnt so they bend open.

If in doubt, drop me a mail with a picture.
This should be the beast, or?

http://www.beedspeed.com/html-pages/Mikuni-TM.jpg

Cheers,
Johann

Mudflap said:

Some jets are actually marked in relation to their size, others have arbitrarily selected numbers related to their flow in proportion to others. I'm told Mikuni does it both ways for different types of jets. I do know that Holley jet numbers do not relate to their diameter, there are several sets with different numbers that have the same minor diameter, the different flow is caused by the entry and exit to the minor diameter. Clear?

To answer your question, jets don't wear out, unless someone has been poking wires or tools through them. +1 for Johann's advice.

Click to expand...

Some jets have things poked in them by design, these things move in and out of the jet as the throttle is opened or closes, or the manifold vacuum changes also in some designs. Those kinds of jets COULD have wear on them.

Bill

Jep,
needlejets can wear out by vibration. Here, a conical hardened steel needle attacehed to the throttle slide slides in and out of a sleeve made of brass with constant diameter. Changing these against new parts on old bikes can get the fuel consumption down by one litre per 100km and make the bike behave far better.

Greetings,
Johann