POST 12: The orange bike- carburettor PA11
Carburettor
The carb on this bike is a PA11- it is similar, but not identical, to that on the blue bike which was a PA25. In particular it seems to be a more primitive design. The throttle needle jet isn't removeable and the float needle seat is coarsely moulded into the carb body- there's no brass insert. The Chinese replacement looks a much better engineered alternative with pressed-in brass jets and seats. However, that carb doesn't have the self-mix option for 2-stroke oil and so has to be run on pre-mix. It may come to that, but I would like to use the self mixing facility if it works so in the meantime I'll try to fix the original carb. There are a few differences in the connections to the carb where the junction unions look more complex with cubic facets and the oil pump connection to the throttle cable is now contained in a separate trapezoidally-shaped junction box. I photographed the hose connections before easing them off with a screwdriver.
 |
| Carb connections- note the cubic facets on the 2 oil hoses. |
 |
| Carburettor connections - oil inlet at front and auto choke at rear. Fuel inlet stub is visible at the rear but disconnected. |
 |
| Carburettor auto-choke connections to bystarter and carb via the corrugated hose. Auto choke hose "1" connects to the base of the inlet manifold |
Rear brake cable passes beneath the carb on the LHS
General view of hoses- oil and fuel
 |
| Oil self mix inlet hose, fuel inlet stub to rear (no hose) and carb overflow at front, reinforced hose from autochoke visible to left. |
Both eased off with a screwdriver
 |
| Self mix inlet and overflow hoses disconnected |
Auto choke hoses detached as well
 |
| Disconnecting the auto choke reinforced hose. |
The carb is held in place by 2 bolts threaded into the air filter holding plate as before although in this case there is a separate bracket holding the rear mudguard. I'm hoping this might make fitting the Chinese carburettor easier if I have to do that. The bolts were unscrewed to release the carb, note the drain hoses had to be detached from the carb base (not shown).
 |
| Bystarter connection hose disconnected, both carb mounting bolts are visible |
I immersed the entire carb in the sonicator to clean it externally before opening it. I used san alloy-friendly recommended carb cleaner but its worth noting that this has also removed much of the gold passivating coating.
 |
| Carb type PA11 |
As before the float bowl is held by 2 screws
Opening the carb revealed a nasty soup of oil, fuel and water. The float and needle were completely gummed up.
The float is detached by removing the screw, but owing to the gumming up, it was stuck and needed to be carefully eased up with a screwdriver.
The individual carb sections were then returned to the sonicator in fresh fluid.
Unfortunately one of the oil hose unions turned out to be loose. It will need securing with Loctite on reassembly once I've worked out which direction it should face.
 |
| Loose oil mix inlet |
I cleaned out all the passages with carb cleaner and a guitar string. I cleaned the needle valve socket with a cotton wool bud and toothpaste in a Dremel.
I could not remove either the needle jet or its emulsion tube from this carb- the jet needs to come out downwards before the tube can be tapped out upwards... However, as the jet doesn't unscrew, neither operation is possible. The jet I can see is clear but 'm equally sure that the emulsion tube isn't! However, I don't think I can do more than squirting it through with carb cleaner in copious quantities.
I was able to remove the idle speed circuit emulsion tube using a 1.5mm drill bit to prevent crushing as I used a pair of small grips to pull it out. This was partially blocked and needed cleaning.
Finally, I reassembled the carb, setting mixture and throttle stop screws to fully-in minus 2.5 turns. However, on reassembly the carb leaked from the float bowl/body junction even with a new seal exchanged with the Chinese replacement. I'm not sure of the reason for this as there doesn't seem to be a specific leakage point- its more of a seepage. I decided to check the actual fuel level reached in the carb by using a section of clear tubing as a U-tube to show fuel height in the carb. This is attached to the bowl drain stub and held along-side the carb whilst the drain screw is opened.
 |
| Fuel level indicated by U-tube. The level appears to reach the height of the seal below the carb body flange. |
The miniscus shows that fuel level is indeed at the bowl junction which seems rather high. I did the same for a new Chinese carb, and the level does seem to be lower. This is closer to the base of the float which seems more appropriate.
I cant find a statement of fuel level for the PA11 carb- the only setting given is float height set when the carb is dismantled. However, there is no adjustment for float height/fuel level in this carb anyway- the only fix is to change the float and/or needle. I attempted to measure the float height and it did seem to be a little high at 14mm instead of 10-11mm. However this would lead to a lower fuel level rather than a higher one! I swapped both needle and float from the Chinese carb into the original but it didn't change the fuel height reached. However, the carburettor doesn't overflow through its inbuilt overflow drain with either float, probably because it leaks out past the sealing ring first! I needed to check whether the float and needle system is actually regulating the level but leaving it simply too high. I confirmed this by immersing the carb top in a bucket of water and blowing into the fuel inlet pipe. Air passed freely if the carb top was upside down (float upwards), but was shut off completely when it was inverted (float downwards) the float is pushing the needle into its seat. Consequently, the float valve is clearly working, but perhaps it requires more pressure to seal the needle into its seat than it should. This could only be provided by immersing more of the float- ie a higher fuel level (?) and its possible that a PO has managed to adjust the "non-adjustable" float tab somehow leading to the higher than expected float height. All of this seems to me to indicate that the needle valve seat although functional, is either damaged or dirty. I will try to clean the seat again for a better seal and also check that there isnt a brass seat missing. Finally if all else fails I'll need to explore fitting a new seat and needle from another carb.
I tried cleaning the seat using the cotton bud and toothpaste method- no success. I changed the needle for a new one- no success. Finally I cleaned the seat by rotating a 2mm centre drill in it lightly by hand. This did remove a little metal but resulted in much firmer seating using the blow-in-a-bucket-of-water test as above. (note I must get one of those neat carb seat pressure testers!). I fitted a new seal and the carb had at last stopped leaking. However running the bike was still impossible using fuel- it would only run on starter fluid. I think the float level is simply too high- the spark plug wets up excessively but there is no sign of firing.
There is no adjustment in the float to regulate fuel level, but I attempted to adjust it using a hot air gun to soften the plastic tab for bending. I held it in position as it cooled, to "fix" it. After this I had to clear the pivot pin holes by running a 2mm drill through them. Reassembly stopped the leak. Float height is obviously irrelevant now so I checked the fuel height as before.
Fuel level was now well below the carb bowl flange although I have no way of knowing if its correct or not. The gasket was now not put under pressure (even though it would probably leak) and fuel no longer poured out of the overflow.
I refitted the carb and attempted to start the bike.... Success!!... but sadly short lived. The bike starts easily and idles- but on opening up the throttle, the motor immediately dies. If you open it slowly but steadily you can get the motor to rev but clearly not in normal throttle use. The second observation was that the air screw seemed to have no effect on idle speed, so combining these effects I think the bike has an air leak somewhere in the induction system.
I searched for the leak by spraying carb cleaner around the intake mechanism- I found the bike would rev if cleaner was applied around the base of the intake manifold /carburettor mount.
I stripped the carb and manifold down (difficult one screw was jammed and broke off in the hole. It had to be drilled out and retapped). I found that there was no gasket or visible sealant around the crankcase to reed valve- or reedvalve to manifold junctions ( or for that matter between the cases!- Im assuming this is a film of hondabond and invisible). I refitted the reed valve plate using RTV "Gemlock" on both sides. I'd expected success - and so was very disappointed when the bike behaved in the same way- it ran but does on throttle- clearly I have more leaks!
Comments
Post a Comment