POST 7B: Blue bike; The Carburettor PA25

I have already removed the carb cover and detached the carb from the inlet manifold in the process of removing the motor. The air filter was missing on my bike anyway.The carb was now held only by the myriad of hose connections and the throttle cable.

The carb has 5 connections and a drain hose. These are:

Throttle cable

Fuel in from tank

Oil in from pump

Mix out via autochoke

Bystarter chamber hose

Ive tried to show these below but access was tricky. View of carb connections. Breather hose- black rubber hose nearest at bottom of picture; Bystarter chamber hose connector... black plastic connector at top of carb; Fuel line in, brass angled connector; Oil line connector, metal elbow adjacent to breather tube

Ive indicated some of these individually below.

Breather hose

Throttle cable and carb cap.

Fuel mixture to bystarter chamber

Mix out to cylinder via autochoke

Oil in from pump

Routing and disposition of all hoses shown in next 2 pics as a record 

I disconnected all hoses- the clips are not reusable, and unscrewed the carb cap to release the throttle cable and slide which pulled out of the carb body.

Throttle needle and slide removed but still attached to throttle cable.

Disconnecting all cables and unscrewing the carb cap to withdraw the slide allowed me to remove the carb- so here are some general views before I take it apart.

Front of carb showing Fuel mixture to autochoke connector, brass hose union above adjustment screws. Adjustment screws; upper, Throttle stop; lower, Airscrew.

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I immersed the whole, intact carb into the sonicator before opening it to clean. This improved matters. Id used special sensitive metals cleaner but even so it appears to have removed some of the passivating coating

I opened the float chamber by removing the 2 fixing screws. Inside it was a house of horrors. The carb float chamber was particularly gummed up with a toffee like deposit. I removed the float securing screw, float and pivot pin. This allowed the needle valve to fall out. 

I removed all the regulating screws noting that both air and throttle stop screws were 2.25-2.5 turns from fully in.

Airscrew

throttle stop screw

float chamber drain screw.

I unscrewed the needle jet from its home in the centre tube.

Note main fuel tube in centre of carb body inside the float. You can see the needle jet inside (just peeking through the crud). I cleaned the carb body with some cleaner spray and  flushed all the passages (of which there are many- invluding those nomally covered by the air screw) through with hot sonicator fluid using a plastic squeezy dropper before trying to dismantle further the carb.

I removed the float assembly (screw float pivot pin and needle valve).

The foat chamber had cleaned up quite nicely.

Note the small hole located to  the right (appx 3 O'clock) at the side of the chamber. It should penetrate to  the bore behind, but in my case it was filled with waxy crud. I cleaned it with guitar string and cleaned the bore itself with a cotton-bud.

I could then turn to the upper part of the carb body. I removed the needle jet with a wide but thin-bladed screwdriver.

screwing out the needle jet.

The emulsifier tube is fitted in the carb body above the needle jet. Its a tight push fit but can be tapped out from the top (throttle slide end) using a soft drift. The tube then pops out through the base of the carb body.

Once removed its clear that the tube has a series of holes bored in the sides communicating with an air space surrounding the tube. I found all were blocked to some extent and a few completely obscured.

I cleaned these with guitar string and polished the tube.

I could then tap the tube back into position from the base (jet end).

The tube seen here (below) next to the mainjet is the idle speed fuel circuit. It opens to the tiny hole in the carb throat. The tube itself also narrows and its virtually impossible to clean without removal. In fact cleaning from the tube end risks ramming debis into the narrowest parts of the system. Given the mess I'd found in the needle jet emulsifer I decided that I needed to remove this tube to clean it thoroughly.

The tube is removeable with care. I found a 1.5mm drill bit is just right to fit closely inside this tube. 

You can then grab the tube with grips whilst the drill prevents it compressing the tube. You can then pull or tap the tube out of the carb body.

The lower section of this tube (covered in white corrosion in the pic) has 4 pairs of small holes which in my case were largely blocked.

I cleaned the holes with guitar string and brushed the white corrosion off.

I cleaned the socket the tube had vacated and ensured that the tiny hole a the bottom was clear. The tube could then be tapped back into place.

Once flushed and cleaned, the carb body was ready to be reassembled

Cleaned carb body ready for reassembly. Note central tube ready to receive the cleaned needle jet. I did change the O ring seal at a later date (see on)

I started the reassembly by fitting the float and its needle valve

I refitted the throttle needle jet and considered the air and throttle stop screws- as show below.

There is an O ring benath the air screw- I hadnt found that when I stripped the carb, but looking in the screw hole I saw one in position. The screw should also have a second structure which I think is a thrust washer of sorts. This was missing in my carb and as a result the spring has worn a groove into the O ring. This ring appears to be a 3 (or 3.5) mm id and 5m od. I didnt have a spare but I had a 4mm id (5mm od) nitrile ring which I used as the thrust washer.

 I then inserted all the regulatory screws,  positioning the air and throttle at about 2 ¼ turns from fully home. I screwed the drain screw fully in.

Reassembled carb ready to be refitted.

Of course I still need to check and clean the needle and carburettor cap which are still attached to the throttle cable but I'll do that when I've refitted the motor.  Secondly Ive not been able to trace a source of replacement gaskets including the complex O ring which was in poor condition. I dont expect to get away with this and may have to look into making something to ensure a good fuel  seal.

Notes added 

 Following reassembly I discovered that the carb overflowed due to poor seating of the needle valve. The float needle housing was cleaned using a cotton bud held in a drill and a dab of toothpaste. This polished the recess and the sealing surfaces and restored valve closure which now sealed and required a 1m pressure head to overcome.

However, I now had a fuel starvation problem which I traced to an incorrect float height (13.9mm instead of 10.3mm). Unfortunately the float cannot be adjusted and simply required replacement. This raised again thecwhole spectre of spares for the carb which are hard to come by in Europe and v expensive from the States. However the Chinese carb clone replacement is very cheap and readily available viavBay... although postage is slow and it lacks the oil injection and autochoke connections of the later models.

However, most of the service parts are interchangeable and I was able to cannibalise the sealing O rings (manifold and float chamber), float and needle. This restored the float height to 10.3mm. The cost of this whole carb was less than just a gasket kit from the USA!

Loads of info about all the carbs used in these bikes on the excellent Myronsmopeds site here

https://www.myronsmopeds.com/2021/04/other-moped-carburetors/