Amal 225 Carb Throttle Setup

[vhgmcbuddy]

Need some advice on how to setup the throttle control on a Amal 225/3 carb.
In the PDF file attached, I have shown 3 positions for the throttle butterfly valve inside the carb. First is obvious, being fully closed. If I then rotate the butterfly valve by 45 degrees clockwise, this puts it parallel with the air flow, which I assume is the full throttle position. What is puzzling me is that I can turn the butterfly valve through a further 45 degrees clockwise (giving a total of 90 degrees), so it is fully closed again. Is this maybe a choke setting, or should the butterfly valve only be turned through 45 degrees (i.e. the top and middle diagrams on the attached PDF document)?

Thanks,

Sean

Attachments:

1. Throttle-Positions.pdf

[charlie]

What machine is this fitted to? Does it have a throttle cable or is engine governed only?

[vhgmcbuddy]

It is currently fitted to my Norton engined Trusty, as it was the closest match I could obtain to the original 225/7 model carb. I am making the throttle linkage that joins to the twist grip control on the handlebars from scratch. There is no governor in the engine.

[trusty220]

I’m sorry that I didn’t see this at first, Sean, I’ve been a bit busy building the Ploughing section and the Garner Registers- I hope you like them!
Anyway, throttle settings on the Amal fitted to a Norton Big 4, here goes.
Use the closed butterfly as your starting point; set the twist grip throttle to the fully anticlockwise position, then you need to adjust the dog-leg at the end of the throttle rod so that it is offset towards the carburettor (you do this by undoing the clamp in the throttle rod joiner, adjust the dog-leg at the end, then tighten it all back up).
The carburettor link should now be pushed towards the carb by the dog leg, so that now all of the linkage should be in the tick-over position. After double-checking that the butterfly is in the fully closed position, tighten the clamp on the arm around the throttle spindle so that the whole lot is now tight and set at tick over.
The engine will not work at this setting with the butterfly fully closed because no air can be admitted, so we now need to open the butterfly slightly using the tick-over screw clamped to the throttle spindle- I use about 1/16″ gap as a starting point.
The main jet needs to be screwed in fully (gently!) then backed out by one turn. The tick-over mixture screw also needs to be turned fully home, then backed off by one turn. This should at least make the engine run so that you can tune it by ear.
Somewhere near 95% of the fuel goes through the main jet, so we set the main jet up by running the engine at about 1500 revs and turning the main jet in and out until smooth running is attained with no misfiring or banging, or excessive black smoke.
Turn the tick-over speed screw in a few turns so that when you drop the revs to about 1,000rpm the engine will be running on the tick-over screw; this will allow you a lot more control when you come to set up the slow-running jet.
You should now have the engine running at 1,000rpm with the screw touching the carb. barrel. Next you can slowly turn the revs down by unscrewing the tick-over screw slowly; as the speed drops then the slow-running jets alongside the butterfly start to take over and become more important. By using your ear to listen to the engine’s behaviour you should be able to adjust the slow-running jet at the front of the carb to achieve an even, balanced tick-over. Just remember that turning this jet outwards will admit more air to the tick-over mixture and thus weaken it down, turning it in richens it up; at this point do not touch the main jet, however tempting it may seem! If you do then you will have to start all over again.
When you get near to the correct speed for tick-over you will hear the clutch drum clanging like a bell; this is caused by the clutch shoes becoming disengaged. Adjust a little slower than this until you can see the end of the countershaft stop spinning.
You’ve done it!
It takes a lot longer to describe than it does to actually do the job, but be prepared to get it wrong a few times before you reach a setting that suits you and the machine.
Best of luck.

[vhgmcbuddy]

Thanks for the very detailed instructions Geoff.

On your Norton engined Trusty, when you turn the twist grip on the handlebars fully clockwise, how far does the throttle butterfly on the carb rotate from the idle position when pulled by the linkage? In other words, when the twist grip is in the fully clockwise position, is the carb butterfly aligned the same as the middle diagram on the document attached in post #1 or is it the same as the lower diagram?

Cheers,

Sean

[trusty220]

When it is fully clockwise the butterfly is in position 2 (i.e. it is in line with the airflow). The choke position does not exist as such- mine starts with a tiny bit of throttle and the cold starting aid is the primer on the float chamber. If you depress this until a stream of petrol comes out of the two air vents on the float chamber lid then pull the strap to start it you are normally rewarded with a nicely ticking-over engine.
A cautionary word, though- don’t let Steve Woollas anywhere near it, it’ll never work properly again!
Best of luck with it, I think you’re nearly there.

[vhgmcbuddy]

Thanks again Geoff. The Trusty is about 90% complete. Just have to make the throttle linkages, the countershaft brake linkage and some of the missing bits on the drawbar. Oh and put some decals on!!
I recently visited the Woollas residence to collect a miller cover for my Simar 56, which Steve had so generously offered to me. I did wonder at the time why his good lady looked so pleased to see me, that was until I saw the garden and then realised that she was just glad that I was there to take something away!!!

[trusty220]

A couple of years ago I took my Norton Trusty to Holbeach working weekend for Steve to push around whilst I was ploughing with another one. When I arrived I unloaded it, started it up (first time as always) and parked it up.
“Let me put a drop of the good stuff in”, says Steve, so (like a fool) I let him. I started it again for him (first time again) and drove it about ten yards, just long enough for the engine to pull “the good stuff” through then it stopped and refused to start for the rest of the day.
7a.m. Sunday morning saw me dismantling and cleaning it so that I could put some fresh petrol in to use it myself. With fresh petrol it worked!
No doubt Steve will have a different angle on this story, probably involving ladies of the night and Fenland Mountain Rescue, but it’s all lies!

[Author]

Posts