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Guide to Dellorto Carbs

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 Topic moved: from Help, Guides & FAQs to by Korn (2 Sep 2004 - 11:21)
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 02:22 - 13 Jun 2003    Post subject: Guide to Dellorto Carbs Reply with quote

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
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random_rob
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Joined: 16 Mar 2010
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PostPosted: 13:40 - 30 Mar 2010    Post subject: hi there Reply with quote

i have an honda xr125.
It idles fine, but when i go to power away theres no power then all of sudden it comes at once.
wondered if you knew what the problem would be without having to take the carb off and taking it apart.
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 13:49 - 30 Mar 2010    Post subject: Reply with quote

Hi

Main thing I would suggest is to take it apart and clean it thoroughly (and check the jetting is standard)

All the best

Keith
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steven_191
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Joined: 31 May 2009
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PostPosted: 22:25 - 30 Mar 2010    Post subject: Reply with quote

you know you have the 2 spouts on the side that aloow air into the float bowl to equal the pressure? are they normally connected up to anything because i dont like the idea of them being left open where anything could get in.

i cant see that connecting it to the filter would help because theres a vaccum pulse in there from the engine sucking in air
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Stiffler
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Joined: 24 Sep 2006
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PostPosted: 12:30 - 24 May 2010    Post subject: Reply with quote

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
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tatters
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Joined: 05 Jan 2004
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PostPosted: 12:49 - 24 May 2010    Post subject: Reply with quote

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.
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 19:36 - 24 May 2010    Post subject: Reply with quote

Hi

Might well clear by tapping the float bowl. Could also be the float height (can be adjusted on these), but probably most likely to be wear or damage to the needle valve. These have a viton rubber tip and it has probably rotted or gummed up having sat in old fuel for 4 years.

All the best

Keith
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2-stroke madness
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PostPosted: 00:43 - 06 Oct 2011    Post subject: rs 125 Reply with quote

hi there, im running a 34mm VHSB on an aprilia rs 125 extrema 1994, with std air box, and an arrow exhaust, could you give me guidelines on what size jet you think i should have?
thanks
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