Has anyone got experience of removing a pilot bush and replacing it with a screw in jet? I have trouble starting my 1968 A50, and everything points to insufficient fuel. It starts fine if I put my hand over the air intake or screw the airscrew fully in. It needs full choke with no throttle to start even when warm. First thoughts are either a blocked pilot jet or air leaks, but once started it runs and idles fine, if slightly hesitant at initial pickup. What effect would there be if the 25 pilot bush was replaced with a 30 pilot jet?... or am I barking up the wrong tree?
...... I've richened the mid range already by lifting the needle a notch and fitting a 3 throttle slide, which has stopped the spitting from the carb and overheating, and prolonged use of choke.
Buy a .016" guitar string and file the end to remove any burrs.
Remove the air screw and insert it into the jet about 1 1/4" from the outside.
Spray carb cleaner into the jet. You should see jets of fluid from the tiny holes in the venturi. (flange side)
Now, remove the bowl and press lightly on the float tang, not the needle.
The float should be about .080" below the cusp, a 'stay-up' float is easy to adjust.
Use a viton tipped aluminium needle.
The air screw should be 1 1/2 turns out from bottomed.
Then tune it.
Yep, done all of that. What's next?
Do you have air filters fitted?
Yes its got an air filter which is in good condition and with which it runs normally; but I usually spin it off to start it from cold. Everything is good apart from starting. I thought it was the Boyer, but I sent it back to the factory for checking and it was given A1. The battery gives 12.7V min with little drop off through the loom, and timing is good. Hence back to the carb and my query about the pilot. The bike has always run weak, so I lifted the needle a notch and fitted a 3 throttle slide instead of the 3 1/2, which has got rid of the need to be playing with the choke whilst running. Once started it idles fine without choke. Once warmed up it will usually starts first kick but with full choke and absolutely no throttle.
It's got to be something simple. I'm pretty certain its airtight and the passageways are clean. It seems to just need a good squirt of fuel to get it started. I'm reluctant to replace the complete carb and don't want to drill out the pilot bush if I've missed something a bit less drastic.
It will run weak without the air filter fitted.
Have you checked the breather hole in the tank cap for partial blockage?
Fuel-flow check?
Has the fault just developed?
There were changes to the concentric carb during 1969 and if the bike is new to you and the carb an original 1968 you might have a mix of parts which will cause problems. There is interesting information on rebuilding the concentric and the parts changes on the Amal website:
http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter
Thanks Julian and Caulky. Yes I've checked the fuel flow which is now good having fitted an aluminium float needle instead of the brass one. By now I must have read everything ever printed about these carbs and watched all the YouTube clips.
I've just talked things through with the guys at Amal and Hitchcock's, and the agreement is that the way forward is bite the bullet and drill the pilot bush and fit a 30 jet in the early position: at the back of my mind I'm still wondering whether I'll find anything lurking behind the pilot bush. So watch this space
Hitchcock won't tell porkies and are Amal experts, I would go with what they say and please let us know how you get on.
I drilled out the pilot bush with a 2mm bit in a hand drill, so that I could feel my way through. This was followed by a 2.5mm bit. My thinking was that the hole size wasn't critical as long as petrol could flow freely. I blew out the crud, probed the fuelways with fuse wire, washed it with carb cleaner, soaked it overnight then washed the passage ways through again. Everything looked clean.
Having screwed in the 30 pilot jet and reassembled the carb back on the bike with new gaskets, it wouldn't start! Back to square one. With my hand over the air intake it sprung to life; and with the filter back on and a few tweaks of the airscrew and idle screw, I took it out for a good run. Back in the garage, I checked the turns on the pilot airscrew which was virtually in the same position as before the surgery.
It started this morning second kick, full choke and no throttle without removing the filter, so fingers crossed.
The 30 pilot jet certainly works and the airscrew is responsive and open about 1 1/4 turns.
During drilling, did I remove a partial blockage from behind the pilot bush? I don't know.
Was the original bush the correct size or had it been enlarged by probing over the years? I don't know.
Has repositioning of the jet made a difference? I don't know.
There are several thoughts on why Amal changed from a pilot jet to a bush. So far I can say that there's no hesitancy when opening the throttle and accelleration is good through the revs. Cleaning the pilot jet will certainly be easier in the future. Has anyone got any experience of the new Premier concentrics? :)