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Carb Tuning

by bashr52
some illustrations by JustRandy

After all the trouble I have had with the carb on my 230, I thought I would write up and article for others to follow who have questions, and perhaps a similar problem. I will try to put this in as plain of terms as possible, so even someone with little knowledge of the networking of their engine can follow it. Basically, an internal combustion engine is an inefficient air pump. Anything you do to increase the air flow in or out will effect how the engine performs. The secret to making horsepower is getting the air in and out as fast (and as efficiently) as you can. A high-flow exhaust and air filter will increase the performance/horse power of your engine. With that increased air flow, you need an increased fuel flow. That is where your carb comes in. As your piston travels down in its bore, it creates a vacuum. That vacuum is what draws the fuel from your carburetor into your engine. That fuel needs to be mixed with the incoming air in order to create a combustible mixture. The amount of fuel that is mixed into the air is determined by removable jets housed inside the float bowl. By changing these jets, you can alter the mixture going into your engine. Bigger jets will richen the mixture (add more fuel) and smaller will lean it (remove fuel). These 230 carbs are pretty simple. There is a pilot jet, which controls the mixture at low speeds, the needle valve which controls mid range, and the main jet for high RPM.  The small screw on the carb controls a needle valve which can be used to adjust the mixture at low RPM’s. The following pictures shows the passages used to create the mixture within the carb (thanks to the 230 service manual for illustrations):  

The choke circuit on these carbs is simply an air by-pass. When the choke lever is pressed down, a valve opens inside the carb throat, and air is drawn though. This mixes with gas from the starter valve to create a rich starting mixture. Running with the choke on for long periods of time will make your engine sputter and blow black smoke, while fouling your plug.


Now that I have explained the basics, here are some illustrations of an actual carb with all the parts/pieces labeled, as well as a parts diagram. 

editor's note:  Needle Valve Seat actually the needle jet.  (Thanks JustRandy)

editor's note:  Needle Valve Seat actually the needle jet.  (Thanks JustRandy)

editor's note:  Needle Valve in above picture actually Jet Needle.  (Thanks JustRandy)

Helpful Hints/suggestions:


Now that you are more familiar with your carb, I’d like to offer some suggestions and have a place to put my observations to help with tuning problems.


General info: I have found that popping/backfiring out the carb itself is a lean condition, so you need to up in jet size. Backfiring out the exhaust, fouling plugs, and black smoke are all rich conditions, go down in size.


Here’s a chart that shows what controls what with each change in RPM (thanks again to JustRandy):


Low Speed-

If your engine fires up and runs good with the choke on, but falls on its face/dies when it is turned off, you have a low-speed circuit problem. Check to make sure your idle adjustment screw is about 2 turns out to start, as well as the low speed adjustment screw. To do this, turn them in until they just bottom out, and then turn them out 2 full turns. DO NOT tighten them down to the point they won’t go any further, and then unscrew them. You risk damaging the bore of the holes, or breaking off a needle valve (neither are good things).

If that does not fix your problem, disassemble carb, and blow out/clean all passages and jets. You may have a small piece of dirt or junk stuck in a passage or end of your jet. Then re-assemble and try it again. If that doesn’t work your jet may be too small. What I found with my engine, is with the DG pipe and other modifications, I needed to go up in size on the pilot jet. You want to increase the size enough that the engine will run well, but still have the adjustment screw out about 2 turns (+- .5 turns). If you engine is making black smoke at idle however, and your adjustment screw is all the way in, your jet is too big.

Once you have found the jet that works for you, you should now tune you low-speed system. To do this, get the engine running and up to operating temperature. Slowly turn your low-speed screw in until the engine begins to run rough/stumble. You may need to turn it, wait and see what the RPM’s do, and turn it some more. Once you get the engine to stumble, turn the screw out the other way, until you get it running at the highest RPM. Go about a ¼ to ½ turn past this spot and you are done. Now adjust your idle to your desired RPM (In goes faster, out slower).



If you crack the throttle from idle and your engine hesitates, sputters, or blows black smoke, you need to adjust your mid-range circuit. To do this you need to remove your needle valve, and re-locate the E-Clip. Higher up makes it leaner, down makes it richer. You are trying to do the same thing as with the low circuit. Black smoke means rich, stumbling/popping means lean. Here’s a picture of the clips on the valve (thanks to JustRandy).


Top end-

Once the throttle is open enough, the needle jet no longer disrupts/regulates the amount of fuel passing through the main jet. The jet is now the only thing regulating the fuel. To check the condition of you main jet circuit, with the engine up to temp, bring the RPM’s up to near full, hold it there for a few seconds, and turn off the engine using the kill switch while releasing the throttle. Keeping the throttle open will just pull raw gas into the chamber and we the plug. Now, remove the spark plug (it will be hot) and check the condition of the electrode. Black/sooty means you are rich (go down a size), white means lean (go up). Ideally, what you want is for the plug to be a nice golden brown. Err on the rich side though if you have to compromise.


Float level-

Your float setting has a lot to do with your engine performance as well. To low a setting will allow the carburetor bowl to drain to fast, and the engine to stumble/die. To high will allow to much gas into the bowl, and often flood/choke out the engine. The book says to set the float height at 21.8 +-1.0 mm (.86 +- .04 in). In practice, that puts the float about parallel with the case of the carb. Just set yours like that and you should be fine. You adjust the height by bending the hook on the float that the needle valve slides onto. Here’s how it should look:


Mixture problems-


Too rich of a mixture will lead to fouled plugs, increased fuel consumption, black smoke out the exhaust, and poor performance.

Too lean will lead to burnt plug electrodes, hot engine temps, detonation, burnt valves, hot spots in the engine which can lead to dieseling, and holes in the piston top.

Your engine will run its best with the correct mixture, but if you can’t get it exact with the jet combinations offered, tune on the rich side of things.

 I think that is about all you will need to know about your carb. Hopefully this write up has shed some light on what may be a complicated process to some. Remember though, no two engines run the same, so what might be right for one engine, may not be exactly right for yours, even if everything else is the same. It is important your engine be in good shape before you start tuning, that is to say good compression, working ignition system, proper valve clearance, etc.  Any suggestions/questions can be sent to me at