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Discussion Starter #1
Hey guys, what's a good starting point for tuning an Amal 930 carb? I've tried to mess with it but I've never really dealt with carbs before. The shop manual says 2.5 turns from closed for the pilot screw, but that doesn't seem to work. Also, what's a good starting position for the throttle stop? What's rpm is ideal for idle? Also, when tuning the carb, do you just lean it out until it sounds like its running poorly then back a little to where it runs strong? I'm at a higher altitude (~4500ft) so I heard the higher altitude makes carbs run rich. The bike is a '70 TR6R. Thanks.
 

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Fiirst make sure that the tappets and timing are set right.

It's best to start with the carb set like in the book- slide, needle position, needle jet, main jet etc, plus the air filter.

Check the float height is 80 thou below the level of the float bowl top by removing it and pressing down on the float tang that fits on the float needle and not the top of the float needle itself.

Start the engine and just set the throttle stop so that the engine keeps running with the air screw about 1 1/2 turns out.

Turn the air screw in or out until the engine speeds up. Drop the throttle stop until the engine begins to falter and then turn the air screw in or out again until the engine speeds up again. Drop the throttle stop again until the engine idles at the speed you want- not lower than 750 rpm. If you set it too high you will get a grind when you select first gear and the engine may overheat at idle.

When the bike is idling open the throttle very slightly and the engine should pick up to about 2000- 2500 rpm without spitting back through the carb. If it does spit back first try turning the air screw in slightly. If this doesn't help put the air screw back and try a bigger throttle slide. If it runs heaily try a bigger slide. Open the throttle slowly a little further and it should just run smoothly. Don't just crack it open as the best set carb will just make the engine die momentarily.

Best to go for a ride and see how it goes at various throttle openings. Small opening is the arscrew, up to 1/4 is the slide, then up to about three quarters is the needle and full is the main jet. There is some overlap between the settings. If at full throttle it feels like it has more power if you roll the throttle back a bit the main jet is too small. If it runs heavily or eight strokes it is too rich. Set the needle where it runs smoothly but don't let it get too weak. Run the bike for about two minutes at various throttle openingsand cut the engine without letting it idle using the cutout switch and check the plug colour.
 

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Sorry rivera, dont want to hi-jack your thread but I have question about setting up twin carbs,
With the points/contact-breaker system its fine to whip off one of the plug leads and set-up the opposite carb. (careful, high voltage)
But not with electronic ignitions...the spark energy has to go somewhere and its dissipated as heat in the box, which could destroy the unit.
I`ve yet to find an answer for this problem.

I suppose it is possible to start the engine on one cylinder with the detatched plug-cap fitted with another plug.
Usually, there`s already set-up/starting/tickover problems, so starting is already difficult.
Any suggestions?
 

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I have question about setting up twin carbs,
With the points/contact-breaker system its fine to whip off one of the plug leads and set-up the opposite carb. (careful, high voltage)
But not with electronic ignitions...the spark energy has to go somewhere and its dissipated as heat in the box, which could destroy the unit.
1- I`ve yet to find an answer for this problem.
I suppose it is possible to start the engine on one cylinder with the detatched plug-cap fitted with another plug.
Usually, there`s already set-up/starting/tickover problems, so starting is already difficult.
Any suggestions?
I have never been one for running on 1 pot to set tickover (its just not natural)
if you start it with any tickover you can get, then put your hards over the exhausts you can feel the pulses.
If say, the right hand pulse is cooler, this carb needs turning up a bit and the left down.
Adjust the slides until both exhausts feel the same in heat and pulse rate,
as tickover gets lower you may need a teeny twiddle on the air screws
to get it nice and smooth.
I aim for around 7-800 rpm for tickover, but up to 1000rpm is ok.
Make sure the engine is nice and hot, and take your time :cool:

1- if you must take a plug lead off, put a plug in the cap
and make sure it goes to earth. You could strap the plug to exhaust with jubilee clip.

Starting with a lead already off, relies on both carbs being set near enough, to work anyway.
You will also have to raise the tickover to around 1500-2000 rpm before you remove the lead.
 

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Discussion Starter #8
Tried to mess with the carb again today.. The bike is kicking back very badly. I have the idle screw set to less than one turn out and the pilot about 1 and a 1/4 turn out. If it doesn't kick back it starts and idles really high, around 2500 rpm. I try to get down and adjust the screws but the bike vibrates and moves to badly I can't get it to stay in place for adjustment. If it doesn't kick back it causes a backfire through the carb. What am I doing wrong?
 

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Tried to mess with the carb again today.. The bike is kicking back very badly. I have the idle screw set to less than one turn out and the pilot about 1 and a 1/4 turn out. If it doesn't kick back it starts and idles really high, around 2500 rpm. I try to get down and adjust the screws but the bike vibrates and moves to badly I can't get it to stay in place for adjustment. If it doesn't kick back it causes a backfire through the carb. What am I doing wrong?
Hi rivera :)
It sounds to me like you could have an ignition timing problem,
have you checked the timing and are you on points or electronic ignition?
 

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Discussion Starter #10
Made another attempt at tuning today with somewhat of a success, although the bike is still kicking back (how do I fix that?). The idle is back to normal, (somewhat), either my rpm gauge is off or the bike just doesn't like to idle below 1000 rpms. Anyways, I got it idling to where it wouldn't grind when I went into first so I was happy with that. The bike still sounds fairly weak at idle, like it wants to die. I played with the rpms a bit and there wasn't any hesitation or backfiring so I took it out for a short plug test. I took it up to 50 mph in 3rd and switched off the ignition. Pulled the plugs and this is what I got for the right cylinder..





Looks better than lean I suppose, but then I pulled the left..





Clean as a whistle! Granted it wasn't white, it did puzzle me a bit. What could it be? Both cylinders are running, any ideas?
 

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Discussion Starter #11
Oh and Plewsy, I suppose the kicking back could be from the timing. I'm on a Boyer right now..
 

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The kicking back could be Boyer and bad battery syndrome. What voltage do you have at the battery? Leave the lights on for 15 seconds and then test immediately after turning the lights off. Charge the battery fully and see if the kick back stops. The plugs are quite light- have you got air filters and the standard exhaust on there? You can set the airscrews where you want. If it is too weak on the idle then screw them in and see if it makes it better. The air screws are the horizontal ones and the throttle stops point up, by the way. What size slide cutaways have you got? A change of throttle cutaway is rarely needed. What needle position are you on?
 

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Discussion Starter #14
The battery is at 12 volts although it wasn't tested under a load. I hear the correct way to test it is to leave all the lights on and if the voltage drops within ten minutes the battery is no good. is this correct? Do I have the engine on or off when doing this? I do have an air filter and the standard exhaust.
 

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Discussion Starter #15
Bump for some answers.. I need to know what a good test is for the battery still, any ideas? Also, any advice for setting the idle? What's the engine supposed to sound like?
 

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Just had a look at mine...charged a couple of days ago, still in the shed.
12.56V, drops a little (12.4-ish) with the ignition and headlight on.
I suggest testing without the engine running...defeats the object really!
Try with just the lights on and see if the charge drops rapidly after a few minutes.
Dont forget to charge it up again;)
 

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With the engine off turn the headlight on high beam for about 1 minuite and take a reading across the terminals. Anything below 12 volts will give you trouble with a boyer.

HTH,

TD
 
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