Home. Back. ...... The Gas Gas Guru. Technical Information.
Jan. 25 2000
By: Jim Snell, Pres. CEO Rising Sun Imports Inc. (USA GasGas Parts importer/distributor)

Before speaking about the aspects of the Dellorto carburetor, I wish to first mention a few safety reminders.
WARNING! Gasoline is extremely flammable and should be handled with care at all times. Many homes have open flame pilot lights on stoves, furnaces, and water heating devices. These devices can be found in the garage or workshop areas of many homes and businesses. Never have gasoline near any of these devices as an explosion of vapors is possible which can lead to damage, serious injury, or even death. Always keep a fire extinguisher nearby when working with flammable materials. You should never wear gasoline soaked clothing or gloves. Gasoline soaked rags are extremely dangerous and this practice should be avoided. Gasoline is a proven carcinogen and care should be taken to avoid contamination of your body. You should work in a ventilated area as gasoline vapors have been proven to cause cancer in laboratory animals. Gasoline is a pollutant and should not be allowed to drain upon the ground, flushed into any drain or sewer, or disposed of in any improper manner. The following article about the Dellorto carburetor is meant for educational purposes only. Only authorized licensed dealers or mechanics should attempt to diagnose or repair the Dellorto carburetor.
FURTHERMORE: The author expresses or implies no liability whatsoever arising from the interpretation of this article. The author advises anyone who suspects they have a problem due to deductions made from reading the following article to consult a competent mechanic or their dealer. The author assumes that anyone feeling competent to attempt repairs themselves, also assumes the liability of these operations to be their own and any injuries or damages caused to themselves or anyone or anything else by attempting to complete any repairs is solely the responsibility of the person attempting any repairs, changes, or modifications.

Definition of terminology:
'Lean' is the condition caused by too much air in the fuel/air mixture entering the engine.
'Rich' is when too much fuel is in the fuel/air mixture entering the engine.
Note: The terms rich and lean are also used in respect to the specific ratio of oil and gasoline used in the gasoline/oil 'premix'.
'Premix' is the fuel and oil mixture required by two cycle engines that do not have automatic oil injection systems. No trials bike currently in production uses oil injection therefore we assume that all two cycle motorcycles applicable to the following article use premix for fuel.
'Revolutions' is more commonly known as RPM. (Revolutions Per Minute).
'Jets' and 'Jetting' refers to the small brass orifices that fuel passes through in the interior of the carburetor while on its way to be mixed with air before entering the engine. A 'gas jet' regulates the flow of fuel through a small orifice. These small orifices are sized according to the diameter of the hole in the jet. On 'gas jets' the bigger the hole, the 'richer' the jet, the smaller the hole, the 'leaner' the jet. On the carburetors mentioned in this article, most jets are of the 'gas jet' type.
An 'air' jet regulates the flow of air through the orifice and is used infrequently in the Dellorto carburetors mentioned in this article. With an 'air jet' the bigger the hole, the 'leaner' the jet, the smaller the hole the 'richer' the jet.
'Float bowl' is the bottom 'reservoir' part of the carburetor that holds fuel in reserve for the engine when it requires a rapid introduction of gasoline and air due to quick acceleration.
'Gas needle valve' Is a small part with a pointed rubber tip. This rubber tip fits into the valve which when closed, shuts off the flow of fuel into the carburetor as the float bowl becomes full.
'Float' is the part which rises against the force of gravity to close the gas needle valve which in turn shuts off fuel flow into the carburetor as it comes from the fuel tank through the fuel line.

The popular Dellorto carburetors (models PHBL and PHBH) used on many different trials machines have proven to be very reliable and deliver good performance. Recently, I have noticed that older Trial bikes we have been servicing appear to be having problems with the floats in these Dellorto carburetors. The model of Dellorto carburetor discussed in this article feature the two black colored floats that are used side by side in the float bowl. The older model Dellorto with the white plastic floats do not seem to be affected. The black floats used in most trial bikes since the 1994 model year are marked '2.8 grams'. This marking on the side of the float is the actual weight when new. On a recent repair and service of a 1994 model GasGas JT25, I noticed that the floats seemed heavy when I held them in my hand. There are two ways to check a float. The easiest way is to put the float in a small jar with premix. The float if it is good, will be about 50% above the surface of the gasoline. There is a small brass pin on the side of the float, which will cause the float to tilt when it is in the test liquid. If the float you are testing is almost submerged, or sinks, it is bad. I have found that the float on the side of the carburetor that tilts toward the kickstand is usually worse than the one opposite the kickstand. The reason for this is because when the motorcycle is parked for long periods, the gasoline tends to be surrounding the float on the kickstand side, as this is the way the machine leans when parked. For example, this 1994 model motorcycle in question had one float that actually sunk to the bottom of the jar of premix and the opposite one barely stayed above the surface. The second way to test a float is to weigh it. This may not be practical, as a precise scale is required for this procedure. The float that sank actually weighed 7.1 grams. The one that barely floated weighed 5.4 grams. As previously mentioned, 2.8 grams is the proper weight. Obviously these are bad and need to be replaced. This explains why they felt heavy when held in my hand.
The reason a float must be in good working order is to effectively turn off the flow of gasoline from the fuel tank into the carburetor float bowl. A brass lever that pivots on a steel pin actuates the small 'gas needle valve' on the underside of the carburetor. The floats raise this lever as the float bowl begins to fill with gasoline. As the gasoline rises, the lever raises the needle valve shutting off the flow of gasoline from the fuel tank, which prevents the carburetor from becoming flooded with excess fuel. As the engine runs and uses fuel, the floats and the needle valve supply a constant supply of fuel to the float bowl chamber keeping the motorcycle running correctly over its entire operating range. When the floats are not working correctly, the float bowl chamber becomes flooded and may overflow through the vents on the sides of the carburetor body or cause a rich running condition of the engine. Most Trials applications have the carburetor slightly tilted. This tilt may be side to side, downward, or both depending on bike manufacturer, application, and/or model. This in itself is not a problem as the manufacturer of the carburetor has designed a certain degree of acceptable tilt into their product. However when the floats are not operating properly, this tilt can cause the carburetor to flow excess gasoline directly into the intake tract of the engine when it is not being drawn in by intake vacuum as it does under normal operation. If the operator of the machine leaves the fuel petcock turned on, the bike may fill the lower crankcase of the engine while the machine is parked. If the machine is left running at idle speed for a period the engine may begin to 'load up' with excess gasoline and become very rich and even foul the spark plug. In addition to loading up of the engine, other common symptoms of this are difficulty starting the engine (cold and or hot). Excessive black oily deposits coming from the tail pipe and soiling the rear fender or the area around the end of the silencer. Note: Some oil and carbon is always found in the silencer area. Spilling of gasoline onto the engine cases from the overflow taps on the sides of the carburetor body when the machine has been kept upright. Note: It is normal for the carburetor overflow taps to spill when the machine is tipped over in a fall or placed on it's side for repairs etc.
The reasons for a float going bad are varied, but I will mention a few of the common problems. Trials machines often sit for long periods and gasoline will eventually permeate the float making it unable to lift the gas needle valve. Some fuels appear to cause premature failure of the float materials. I suspect that the grain alcohol found in some gasoline brands is a common culprit. If a machine is going to be stored for a long period, it is best to drain the fuel from the tank as well as the float bowl. Note: Simply allowing a float to dry may return it to it's proper weight, however the float remains permeable and must be replaced. If a float (or floats) is bad, the richer mixture coming into the engine will cause poor running. The operator may mistake this problem as being caused by the gas jets. The operator or his/her mechanic may change the gas jets to ones that are 'leaner' to compensate for the rich condition. This is not a good solution as the correction only applies to the engine when it is running at low revolutions. As revolutions rise, the carburetor requires more fuel and the engine will quickly use all of the excess gasoline that is in the float bowl. If higher engine revolutions are sustained for a long period (such as riding down a road in higher gears, or climbing a long incline under heavy load at higher speeds) the engine will become lean due to the improper jets that were installed. This lean condition can severely damage the engine due to overheating and/or improper lubrication. If floats are replaced, the jetting should be returned to suggested settings. Never attempt to correct bad floats with changes to jetting. If you have changed jets from the normal settings, you should suspect another problem such as the floats or a leaking gas needle valve.
Jim Snell of HEBO USA (Rising Sun Imports) has provided a wealth of information for this site. Many Trials bike owners find it difficult to find 'definitive' information on what's good and bad for their machines. Jim has given us the official Gas Gas recomendations.
This article originally appeared in 'Trials Competition', and was reproduced with permission.
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