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Kickstart |
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Kickstart The Oracle
Joined: 04 Feb 2002 Karma :
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Posted: 02:22 - 13 Jun 2003 Post subject: Guide to Dellorto Carbs |
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Dellorto Carbs
The Dellorto PHBH28 carb is used on the NSR125 and variations of the same carb are also used on the Aprilia RS125 and the Cagiva Mito.
A carb work by, in simple terms, creating a narrowing of the air passage. This narrowing causes the air to speed up at that point, which results in a drop in pressure. This means that there is lower pressure in the carb throttle than there is in the float bowl (which is just at normal air pressure) and so fuel is drawn up into the air.
The PHBH is a slide type carb. The slide is attached to the throttle cable. When you open the throttle the slide is lifted up, opening to allow more air into the engine. The needle jet is mounted onto the slide.
Fuel is allowed into the air from various jets. At low revs the throttle is basically closed and air is drawn through a small supplementary air passage with a small jet. The air to fuel ratio of through here is adjustable with a screw.
https://www.alfa-pages.co.uk/phbh_carbs/main_jet0.jpg
As the throttle is opened more fuel is supplied through the main jet, but restricted by the needle jet. The main jet sits in the large brass bolt at the bottom that holds the float bowl in place. The size of this jet determines the maximum amount of fuel that can pass up into the airflow. Fuel flows from the float bowl through small holes in the large bolt and then through the main jet. From the main jet the fuel passes through small holes into the atomiser tube and then up into airflow. The needle jet passes down into the atomiser tube and restricts the fuel flow. The needle is tapered so as it rises the amount it restricts the fuel flow changes.
https://www.alfa-pages.co.uk/phbh_carbs/needle_valve.jpg
The mount of fuel flowing into the float bowl is controlled by the needle valve. This is valve is connected to a pair of plastic floats. As the fuel level rises the floats are lifted up which in turn lifts up the needle, sealing the fuel intake into the float bowl. As fuel is used the level drops, and as the floats drop the needle drop allowing fuel back into the float bowl.
If you do any changes to an engine you will often need to change the fuel / air mixture. This is done by adjusting the various jets.
At very small throttle openings the mixture is adjustable with a screw. Depending on the exact model of Dellorto PHBH this is either an air adjustment screw or a mixture adjustment screw. Both perform the same basic function, but work in different way. An air adjustment screw works by changing the amount of air able to flow through the idle circuit. As such undoing it (turning it anticlockwise) makes the mixture leaner. A mixture adjustment screw works by changing the amount of fuel entering the idle circuit. As such undoing it (turning it anticlockwise) makes the mixture richer. On the PHBH it is obvious which kind of idle adjustment the carb is equipped with. If the adjustment screw is on the engine side of the carb slide then it is a mixture screw, but if it is on the airbox side of the carb then it is an airscrew.
https://www.alfa-pages.co.uk/phbh_carbs/2_carbs.jpg
In the picture above the carb on the left is one from an Aprilia AF1, and the idle airscrew is visible towards the left of it. The adjuster in the middle of the carb is for the idle speed, and merely works by preventing the throttle slide from descending all the way down.
https://www.alfa-pages.co.uk/phbh_carbs/carb_slide.jpg
Also affecting the mixture is the cutout at the bottom of the throttle slide. This can be changed by swapping to a different slide. There is also a jet in the float bowl, which is marked with its size (in 100ths of a mm) which controls the maximum amount of fuel entering through the idle circuit.
https://www.alfa-pages.co.uk/phbh_carbs/needle_jet.jpg
For adjustments to the fuelling at middle throttle openings the needle jet is adjusted. For simple adjustments the needle can be moved. The needle is secured in the carb slide with a small circlip in one of several small grooves. If the circlip is moved to a groove closer to the sharp end of the needle then the needle will sit slightly higher in the slide which will richen the mixture. For fine-tuning a thin washer can be placed under the circlip to lift the needle less than by moving the circlip by one groove. For major changes then an alternative needle can be used. The needle is marked with a code number. For example an X1 needle jet has a 2.48mm diameter at the top, a 1.2mm diameter at the bottom with a distance of 26mm from the narrow section to the wide section. An X58 needle jet has a 2.50mm diameter at the top, a 0.6mm diameter at the bottom with a distance of 30mm from the narrow section to the wide section
https://www.alfa-pages.co.uk/phbh_carbs/main_jet.jpg
At large throttle openings the fuelling is adjusted by changing the main jet. Main jets are marked with their size, which is their diameter measured in 100ths of a mm. Although it is possible to increase the size of a main jet by drilling it out this may not give the expected result as they need to be very accurately controlled. Furthermore some replacement jets are available made out of different materials. Although these may be the correct size as they are very small the change of material alone may affect the flow of fuel.
All the best
Keith ____________________ Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing |
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random_rob |
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random_rob Two Stroke Sniffer
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Kickstart |
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Kickstart The Oracle
Joined: 04 Feb 2002 Karma :
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steven_191 |
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steven_191 Nearly there...
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Stiffler |
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Stiffler World Chat Champion
Joined: 24 Sep 2006 Karma :
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Posted: 12:30 - 24 May 2010 Post subject: |
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Hi,
Sorry to dig up this thread but this query seemed very appropriate. I'm currently trying to fix up a Cagiva Planet which has the carb in question.
The bike had been stood for almost 4 years the fuel line had rotted away and the carb was clogged up. I've replaced the line, cleaned out the main jet and a few of the other components and now the bike starts. The idle is all wrong but thats by the by. Anyway the carb is now spewing petrol from the overflow so I guess the float's aren't operating correctly. Any hints on what I can do to sort it?
Thanks
Tim ____________________ Current Bikes - Kawasaki ZX-6R (636) | BMW G650GS |
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tatters |
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tatters Exxon Valdez
Joined: 05 Jan 2004 Karma :
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Posted: 12:49 - 24 May 2010 Post subject: |
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The float is stuck in the down postion they normaly free them selfs while riding from the vibration or by tapping the float bowel to dislog the float. other option is to take the bowl off and check the movement of the float.
l would also say check the float high is set correctly but l dont thing you can change it on theses carbs. ____________________ Past:NRG50,AF1125(x2),NSR125RR,ZZR250,CX500,VFR400,KR1S,ZZR600(x2),CB400N,YZF1000(x2),KH125,Z200,FX400R,CBR954RR(x2)GPZ500S,GT550,VFR750F(x2),RD350N,XR650R,CBR600F,CB250,KDX250,YZF750R,CRM250,400EXC,KLR650,TTR600RE,DR350S,R100GSPD,RGV250,VMAX1200,DL650 Present:G650XC,C12,KZ750,1190ADV |
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Kickstart |
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Kickstart The Oracle
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2-stroke madness |
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2-stroke madness Two Stroke Sniffer
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