Are carburettors used in modern bicycles

Adjust the nozzle needle / partial load needle

The nozzle needle sits in the middle of the carburetor and is primarily responsible for the partial load operation of the engine. That is why it is also called the partial load needle. In older carburettor engines, the nozzle needle has a significant influence on performance, fuel consumption, clean acceleration and engine stability under load in everyday life. Interestingly, this component is more important than the main jet when tuning the engine, because only very few drive at full throttle all the time. But how is a nozzle needle adjusted - and is the correct needle actually installed?

> The prehistory
> What happens with a three-quarter throttle?
> Install the appropriate nozzle needle and set it correctly
> Learnings on the subject of nozzle needles on two-stroke engines for me
> Determine the correct nozzle needle size and position by driving tests
> My carburetor setting of the BING 84


My starting position in summer 2018:

  • An almost 50-year-old MZ ES 250/2 that only came up to speed at the traffic lights with a cough on hot days
  • a built-in "modern" BING 84 carburetor from the 80s
  • an increased “summer fuel consumption” of 0.5 liters, which did not disappear even with a gentle driving style
  • a dark gray, almost black spark plug face
  • as well as a slightly oil-damp exhaust with a clearly perceptible two-stroke flag - at least with the nose

The history

In the winter the old> MZ from 1969 went very well, but on hot summer days the engine started to four-stroke when accelerating at the traffic lights and only came up with difficulty. I bought the BING carburetor new in 2010 and meticulously adjusted it for hours with various jets (main and idle jets). In the end, the Original nozzle needle exchanged for a fatter needle, because the engine rang slightly on the motorway at three-quarters of the throttle. Due to the non-original BING 84 carburettor, I had to do a lot of work before the engine ran cleanly in all power ranges after the restoration of the MZ. Since then, the carburetor has been doing its job quite unobtrusively for thousands of kilometers. 8 years so far - completely without dismantling! The fact that the MZ always ran a little cleaner in colder temperatures than in summer was, for me, reassuring proof that the engine was set a little too rich and therefore worked in a safe, jam-free area. To do this, you need some prior knowledge:


In GDR times, MZ engines were often delivered a bit too lean from the factory to score with low fuel consumption and to meet the emission values ​​of the exporting countries. Experienced MZ pilots from the east therefore immediately exchanged the main jet for a larger HD and immediately hung the partial load needle one position higher after purchasing it. After that, an MZ ran very reliably even under difficult conditions - e.g. with bad fuel or bad oil: if necessary, even with cheap four-stroke car engine oil. Many MZ engines are still considered extremely robust today because the liter output of the two-stroke engine was low compared to other brands (extrapolated from 250 ccm max. 84 hp per liter). Ignorant bikers from West Germany were amazed at the frequent piston jamming when buying from the factory via Otto-Versand or Hein Gericke. Many pilots also simply hung the nozzle needle in the carburettor one step higher after the first piston jam and then drove without any problems.


The nozzle needle can be reached in a few minutes: after loosening the two screws, the throttle slide can simply be pulled up and out.

The BING-84 carburetor is also known for its high reliability. MZ built it into the 250 and 300 two-stroke models in the last model years before bankruptcy. Originally a 28 BVF carburetor "Berliner Vergaserfabrik" from GDR production was installed in the MZ. However, this was so exhausted that the throttle slider rattled while idling, and due to a design flaw it did not idle, which was annoying at every red light, even with a vintage motorcycle. The old BVF carburetor still has a soldered brass float (also at SIMSON). In this case, the float level must be set precisely before the jets can be started. The precisely balanced float height is a mandatory basic requirement for all BVF carburettors with brass floats (SIMSON). Likewise, enough fuel must be able to flow out of the tank at full load. If the fuel filter is clogged or a tank cap with a clogged ventilation duct, the engine becomes dangerously lean, especially at full load: In the past, a tank bag alone has caused major engine damage, as it tends to close the small ventilation hole in the old MZ original tank cap airtight. In the long term, a vacuum is created in the tank, the engine becomes lean due to the lack of fuel and jams ...


The removed throttle valve with nozzle needle


So, for the last eight years, for fear of a piston jamming on faster passages, I drove around with a carburetor that was set a little too rich, precisely to rule out engine damage. Instead of going at full throttle, the experienced two-stroke connoisseur usually simply cuts off a third of the gas, thereby protecting the engine and fuel consumption on fast passages. In this gas position, the gasoline-air mixture is almost exclusively determined by the position and shape of the built-in partial load needle. That is what makes them so important!

In the last third of the throttle position, the consumption of two-stroke engines can double:

  • With an old 250 MZ from the 60s, instead of 5 liters of mixture, 8-10 liters run through the carburetor. Instead of 90-100 km / h you only drive 110-120 km / h top speed.
  • With a SIMSON-SR50 scooter, with continuous full throttle instead of 3 liters, then suddenly 4-5 liters of mixture. Instead of 55 km / h, the engine runs a good 60 km / h.
  • Interestingly, a two-stroke engine reaches more than half of its top speed on a straight line at quarter throttle.

For this reason alone, the gas tap is closed again in both cases by responsible drivers, i.e. to half to three quarters of the throttle, even when driving fast. And exactly then, with this actually gentle driving style, two-stroke engines like to suffer the notorious "three-quarter throttle", caused by an incorrect geometry or setting of the nozzle needle. This phenomenon occurs in all two-stroke cubic classes, e.g. also in SIMSON small motorcycles with only 50 cubic centimeters.


What happens with a three-quarter throttle?

The engine gets much too hot in the partial load range due to the too lean setting of the nozzle needle - i.e. too much air and too little gasoline - and then rings for a few seconds (it sounds like a coffee grinder, but is often barely noticeable under the helmet at higher speeds) Suddenly solid, the rear wheel locks and in the best case you pull the clutch very quickly in a split second and with a bit of luck you can continue to drive after cooling down, albeit almost always with less power. In less mild cases, the surprisingly locking rear wheel can cause a fall from a higher speed, or a hot connecting rod bearing cancels the service by distributing its individual parts in the engine in small pieces of metal and thus causing major engine damage.

The components in the carburetor have different tasks for very specific performance ranges. The part-load needle position and the shape play a very dominant role, as this is the range that is often used in everyday life: The 1/4 to 3/4 throttle position is naturally the most frequently required. The main jet only becomes important in the last quarter to full throttle.


The engine can run too lean at three-quarters of the throttle despite a rich main jet. With the two-stroke engine, for example, this is noticeable in the fact that when 3/4 throttle is spontaneously turned up, the intake noise is significantly louder, but the vehicle does not accelerate properly because there is not enough fuel. The main jet only engages in the carburettor in the last power range, before that, the partial load needle including the installation height is almost entirely responsible. The cooling gasoline is missing at three quarters of the throttle if the partial load needle is not also set rich enough. The air then takes over, a component such as the piston becomes very hot, expands and blocks the engine. If the outcome is mild, many screwdrivers simply change the position of the nozzle needle. You hang the needle a notch higher, the engine runs fatter in the middle to higher range and does not get so hot due to the cooling gasoline. The fuel consumption increases somewhat, but the engine starts better in cold temperatures and naturally runs a little better in winter. As already described above, this simple measure alone is often completely sufficient to maintain a reliable engine - even in borderline areas.

In the photo, the nozzle needle is in the leanest position. A total of four positions are possible with the BING 84 carburettor. If that is not enough, the nozzle needle can be exchanged for another.


Personally, I didn't have a Klemmer with my oldtimer MZ in 2010, heard the engine ringing clearly on the autobahn but often at three-quarters of the throttle. At full throttle, the ringing stopped immediately and the candle image was then as it should be on the two-stroke: fawn on the electrode, black on the ring of the threaded socket. In addition a dry exhaust. It was clear to me that there was still a need for action here in order to avoid major engine damage. I really wanted a classic car that was suitable for everyday use and not always drive to the limit. My diagnosis at the time: The nozzle needle of the BING 84 simply had to be hung one position higher!


Install the appropriate nozzle needle and set it correctly

The “hanging higher” of the nozzle needle, however, had the effect for me that the 250 two-stroke engine ran much too richly and only revved up with a slow mumbling: it was a “four-stroke”. So a nozzle needle with a different geometry was needed. One that only ensured a richer mixture in the upper partial load range until the main jet took over the top performance spectrum. The carburetor manufacturer BING has something like this in its range for the> 84 BING carburettor: The partial load needles are designed differently for different engines and purposes (e.g. Siberia or southern Spain, flat land or mountains, moped or ultralight aircraft) so that the carburetor can be properly adjusted in all load ranges can. So I got a fatter BING 8E1 partial load needle from a BING expert and exchanged it for the original BING 4E1. The doorbell on the motorway was gone and I was no longer worried at three-quarters throttle. At that time I hung it in the second notch from the top, exactly in the position in which the original partial load needle of the BVF carburetor also hung. That went well for an amazing 8 years so far. Until this extremely hot summer of 2018.

The MZ engine was running too rich, the oil was dripping from the back of the exhaust pipe, the spark plug was almost black, I could barely start at the traffic lights. Idle, downshifts and full throttle were ok. Knowing that after days of trying to adjust the new carburetor on the old MZ, I might have given up too early, I hung the nozzle needle a notch lower. All you need to do is loosen two screws from the outside and, with a little fiddling, set the needle one ring groove deeper. After that, the motorcycle ran as cleanly as never before in the truest sense of the word. No more plumes of smoke after a cold start, hardly any odor, clean gas transitions in all areas. If I weren't a real two-stroke rabbit, I could have left everything like that at least until the cold season. For fear of driving a little too lean setting, I ordered the intermediate size 6O1 of the nozzle needle, which I could hang a notch higher again. Anyone who wants to understand this scheme can download a PDF from BING> BING84 nozzle needle. Even with this, the MZ still ran almost perfectly, even at 28 degrees Celsius - but with reserves for the colder season and under load.

Incidentally, I could hardly judge anything on the candle image of the brand-new spark plug. Unfortunately, the addition of alcohol and the fully synthetic oil of modern unleaded fuels leave almost no traces at all, even after driving 100 kilometers. But the exhaust is now dry and the engine pulls away cleanly even on hot days without gas orgies at the traffic lights. Consumption will probably also decrease somewhat in the long run.


Learnings about the subject of nozzle needles on the two-stroke for me

In retrospect, I was amazed at how drastically the jet needle alone can affect the entire engine characteristics. The shape and position of the nozzle needle often has the most important influence on the engine characteristics in everyday traffic:

  • the very few kilometers you really drive at full throttle, but almost always between eighth and three-quarters of the throttle
  • The influence of the nozzle needle can also be clearly felt in the lower performance range
  • Fine-tuning and / or your own summer and winter setting is therefore worthwhile, at least with old two-stroke engines
  • A well-adjusted carburettor makes the exhaust gases much cleaner, even on vintage engines, and it smells better
  • The exhaust is no longer clogged because the mixture burns cleanly
  • The amount of oil included can probably be reduced a little, as the engine is well cooled by the gasoline even at part load. With my two-stroke mixture, I will go from currently 1:60 with good, fully synthetic oil to 1:70 (no guarantee for imitators!)


The photo shows both nozzle needles in comparison: The new nozzle needle is exactly the same length but slightly thicker. This means that the engine runs leaner, as less fuel can flow between the needle and the nozzle assembly.



Old school instructions: Determine the correct nozzle needle size and position by driving tests

According to the cult book “Make Better” by the post-war two-stroke guru Carl Hertwick, the correct main jet size must first be determined for each carburetor setting. A new spark plug, unprocessed original nozzles, the original air filter with unchanged intake tract, an original exhaust and a> halfway set ignition are required in order to be able to adjust a two-stroke carburetor from scratch. Unfortunately, the process is quite complex. To do this, you drive off full throttle (on a test track) with the fattest main jet and, as the main jets get smaller and smaller, determine the one with the best top speed. All measured values ​​are meticulously recorded so that the tests do not have to be carried out multiple times. Then the next larger nozzle is used again so that the engine is 100% fully throttle-proof even in poorer conditions.

For the partial load range - i.e. for our nozzle needle - the gas slide is blocked with a pipe exactly on half of its normal slide travel. A 28 mm carburettor is blocked by a stop at exactly 14mm, so that the gas tap can only be opened halfway. Again, the optimal nozzle needle (of course there are several partial load needles to choose from ...) and the best position are determined by the maximum speed reached. The same thing happens with a tube at quarter or eighth throttle position. Even after that, the nozzle needle is hung one position fatter or fatter, so that no three-quarter throttle jams can occur. If there are poor gas transitions in the lowest power ranges between idle and quarter throttle are still different needle nozzles tried. In connection with the part-load needle, the needle nozzle is responsible for the lower part-load range. The same procedure also applies to the idle jet and idle air screw, which are initially set according to the manual. Here the throttle valve has to be artificially blocked at a tenth of its normal stop. Don't worry, the moped really still drives with it.


Then you have a fully throttle-proof moped with maximum performance and the necessary reserves. It is easy to imagine how much time this type of carburetor set-up costs: without a dynamometer and experienced engine experts, it can often be several days. Incidentally, a two-stroke engine has the highest performance if the set-up is a little too poor. Unfortunately, this makes it very hot and has no reserves left in the mountains, on cold days, with a pillion passenger, full throttle downhill, in the slipstream of trucks ... it's playing with fire. That's why I advise everyone rather to forego the last PS and deliberately let a two-stroke engine run a little fatter than absolutely necessary.

Modern engines today work with a lambda probe that constantly analyzes the current gasoline-air mixture and regulates it accordingly. The best ratio in which the fuel burns completely is achieved with a ratio of 14.7 kilograms of air to 1 kilogram of fuel. Here λ = 1. This requires complex control mechanisms that can usually only be implemented with electronic injection. In times of 3-way catalytic converters, AdBlue and chip tuning, it is no longer the nozzle needle that controls the mixture preparation in the engine, but the software and its developers or corporate managers.


My carburetor setting of the BING 84 on the MZ ES250 / 2 built in 1969, 19 hp

  • Main jet 125 (a 124 would probably be even better for optimal top speed)
  • Partial load needle “BING 6O1” hung in the 2nd notch from above, so the second leanest setting (original equipment was BING 4E1)
  • On hot summer days, even the leanest, first notch from above is sufficient (I'm currently testing in midsummer), so the engine revs up even cleaner in the lower speed range
  • 45 mm idle jet, idle air screw open approx. 1.5 turns
  • Fully synthetic two-stroke oil 1:60 to 1:75 (no guarantee for all imitators!) - In 1969, simple two-stroke oil 1:50 was recommended
  • Electronic VAPE 12V ignition with fixed ignition timing> details at
  • Details on> installing the BING 84 carburetor in the MZ ES 250/2