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Author Topic: Amal Concentric Tuning  (Read 553 times)

High On Octane

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Amal Concentric Tuning
« on: April 25, 2013, 05:24:55 AM »
Bike: 1958 RE/Indian Trailblazer 700cc Twin
Carburater: Amal 930 Concentric
220 Main Jet - 3.5 Pilot Jet - 106 Needle Jet
Needle in 2nd of 3 positions - Air Screw 3 1/4 turns out
Problem: High Idle and hick up under initial throttle


Finally got my Thorspark ignition installed and the timing and spark are great now.  My problem is that the bike idles pretty high and it has a bit of a hick up upon initial throttle.  Adjusting the idle screw makes no difference.  I did get the idle to come down a bit by turning out the air screw, but if I back it out any further the bike starts to die and I just don't have it idling low enough where I want it.  After I get past an 1/8-1/4 throttle the bike takes off and hauls ass.  What changes do I need to make to rectify this problem?  Thanks guys!    :)

Scottie
Bulldog Kustoms - Denver, CO
Specializing In Kustom Paint

The Blackhawk
1958 Enfield/Indian 711cc Twin

Building the 1st EVER Supercharged RE Twin
FULL RACE motor with ACE Performance

X-file

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Re: Amal Concentric Tuning
« Reply #1 on: June 21, 2013, 03:29:35 PM »
Is that meant to be a 35cc pilot jet,or do you mean a 3.5 slide cutaway.
Most Concentrics used a pressed-in 20cc pilot jet.The new "premiere" version uses a screw-in pilot jet.The oversize pilot jet might explain the amount of air you're letting in at the air screw.If they don't run best with the mixture screw at about 1.5 turns,there's usually something wrong.Less than that can mean a blocked pilot jet;much more than that can mean the pilot was reamed oversize when it was cleaned.
If the carburettor is new,there shouldn't be much air leaking past the slide.If the idle stop screw is backed off and not touching the slide,the engine should stop.
Is the slide closing fully?

SRL790

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Re: Amal Concentric Tuning
« Reply #2 on: June 21, 2013, 05:24:36 PM »
Scottie,

To add to X-file's comments, if the idle will not come down it is probably because either the slide is not closing fully or there is an air leak bypassing the slide.  Check the mounting to the head/inlet manifold carefully.  Too loose/poor gasket can cause an air leak, overtightening can cause the slide to stick.  Also check you have sufficient free play in the throttle cable.  Of course excessive wear in the throttle slide will also cause air leaks.

Once you get the idle to come down set the idle mixture as follows.  With engine at normal operating temperature adjust the idle screw to give a fast idle.  Screw in the mixture screw until the engine starts to bog down then screw out the mixture screw until the engine starts to bog down.  Half way between these two points should be the correct setting, which as X-file said, should be about 1-1/2 half turns out.  You should then be able to set the idle to your desired setting.

Andy
Andy Wiltshire
54 350 Bullet, 62 Jaguar MK II, 68 BSA Spitfire, 69 BSA Starfire
70 Bonneville, 71 Bonneville, 71 BSA B25T, 74 Jensen Healey
74 Honda XR75, 81 Yamaha MX80, 82 Suzuki GS1100G

High On Octane

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Re: Amal Concentric Tuning
« Reply #3 on: June 21, 2013, 09:08:45 PM »
I had actually been having problems for a while.  It turns out that I had blown intake gasket and a blown head gasket.  I got all that replaced and the bike is running like a champ now.  And sorry about the bad carb info.  I honestly think I would have been better off going with a 34mm or 36mm cause at about 80mph I start running out of fuel and top out at 89mph.  BUT, the current set up does run REALLY well and has plenty of torque and power throughout up to 75mph.

Here is the current set up:

Needle in top position
3 1/2 cut away slide
106 needle jet
260 main jet
A/F screw about 2/3 of a turn out
Bulldog Kustoms - Denver, CO
Specializing In Kustom Paint

The Blackhawk
1958 Enfield/Indian 711cc Twin

Building the 1st EVER Supercharged RE Twin
FULL RACE motor with ACE Performance