Amal 930 questions.

Josh Cox · May 10, 2011

Hello,

Have a pair of Amal 930's fitted to a 750 Commando and have a couple of questions:

* Pilot screw - wound all the way in is full rich ?,

* Talking to a local vintage bike guy, he was of the opinion that the air line between the two manifolds is best blocked and left that way, thoughts ? ( something along the lines of, if one carb is causing a problem it will mess with the other one ),

* Air lever / Choke - Do you use them ?,

* Idle RPM - have done a search on here, some claim as low as 350 RPM, what is the norm ?,

* Using Vacuum Gauges to tune - what normal idle Manifold Pressure (IN HG) ?, 25-27" for full throttle ?.

Thanks,

Josh

comnoz · May 10, 2011

Josh Cox said:
Hello,

Have a pair of Amal 930's fitted to a 750 Commando and have a couple of questions:

* Pilot screw - wound all the way in is full rich ?,>>> That is correct.

* Talking to a local vintage bike guy, he was of the opinion that the air line between the two manifolds is best blocked and left that way, thoughts ? ( something along the lines of, if one carb is causing a problem it will mess with the other one ),>>> Leave the balance tube- it affects the crossover point from idle to needle jet.

* Air lever / Choke - Do you use them ?, >>>Many don't but if the carb is jetted lean and clean you will need them.

* Idle RPM - have done a search on here, some claim as low as 350 RPM, what is the norm ?,>>> 900 is ok- 1100 is better for the engine

* Using Vacuum Gauges to tune - what normal idle Manifold Pressure (IN HG) ?, 25-27" for full throttle ?>>> depends on altitude and a lot of other things. Jim

Thanks,

Josh

pete.v · May 10, 2011

Josh Cox said:
Hello,

Have a pair of Amal 930's fitted to a 750 Commando and have a couple of questions:

* Pilot screw - wound all the way in is full rich ?,

* Talking to a local vintage bike guy, he was of the opinion that the air line between the two manifolds is best blocked and left that way, thoughts ? ( something along the lines of, if one carb is causing a problem it will mess with the other one ),

* Air lever / Choke - Do you use them ?,
* Idle RPM - have done a search on here, some claim as low as 350 RPM, what is the norm ?,
* Using Vacuum Gauges to tune - what normal idle Manifold Pressure (IN HG) ?, 25-27" for full throttle ?.
Thanks,

Josh

Pilot/Air screw in = full rich
Balance tube. you may block to get as close to tuned as possible then unblock to finish.
I dont use the choke and yank them and blank the hole. It's truly the choice of the owner. FWIW I live near the 45th parallel.

Idle RPM should be higher than suggested to keep from oil starvation. 800 to 1000 rpm. Below 600 is bad.
Don't know about vacuum gauge.

stockie2 · May 11, 2011

Hello Josh,

Up in your neck of he woods, you probably will never need the choke, just tickle up and flood. Pretty rare I need the choke in Sydney, even in the depth of winter.
The balance tube should be left un-blocked in normal running. It actually helps smooth out carbs on acceleration and idle. Vacuum gauges don't seem to work for syncronising, they flutter too much. Just ensure your slides are accurately set up and cables are in good nick. I use 1/4 ball bearings, to ensure slides are spot on, rest the balls against the cutaways, then carefully open, when both balls fall through at exactly the same time, your in the zone. But don't let em go any further than the back of the slide! Before I get to this stage I slacken off cables and set the slides up on the on the idle adjusting screws. Then turn them down equal amounts, you can even check the result with 3/16 balls to ensure it's all good, then carefully adjust up the cables. Float heights will effect idle mixture screw settings. These should be around or near 1.5 turns out when the float heights are set up right, do a search for float setting up. Oh and if the bike is new to you, worth changing out the needles and needle jets, if you are unsure of their condition. Or if part throttle riding is lumpy. These parts wear.

Cheers Richard

L.A.B. · May 11, 2011

Josh Cox said:
Idle RPM - have done a search on here, some claim as low as 350 RPM, what is the norm ?,

If you try to set it too low, with even slightly worn Amals then the engine will probably develop a tendency to stall unexpectedly (usually at the most inopportune of moments) I would usually set the idle to about 900-950 RPM-engine fully warmed up.

Josh Cox said:
Using Vacuum Gauges to tune - what normal idle Manifold Pressure (IN HG) ?

There are no specific intake vacuum settings as far as I'm aware (you could do a cylinder compression test first to check the cylinders are within 10% of each other and both have a reasonable amount of compression?) so, set the throttle stops to give equal vacuum readings and adjust both throttle stops equally to set to the desired idle RPM.

ILLF8ED · May 11, 2011

Also make sure there is slack of equal amount in the cables so that the slides are stopped by the throttle idle screw rather than the cable length.

possm · May 11, 2011

Probably a dumb question, if you are running a Combat Head and inlet manifolds which are both 32mm inlet ports should you not run a pair of 932's to match?
Al

comnoz · May 11, 2011

32s will give slightly more power but will run the same otherwise. Jim

Josh Cox · May 11, 2011

Thanks gents.

LAB - LH 7.75, RH 8.25.

Possm, of course you are correct, but I got the 30mm for the right price and when I applied Bernoulli's principle to what I was doing the only downside I could see was the loss of a small amount of top end power, not going to be racing the bike.

My question relating to vacuum gauges, piston aircraft use vacuum gauges to manage the engines power in all respects, a pretty simple system. the average aircraft will idle at 11 In Hg for a good engine, up to 17 for a not so good one. full power, at sea level, on a 15 C day, about 27-8 In Hg. Cruising about 22-23 In Hg.

L.A.B. · May 11, 2011

Josh Cox said:
My question relating to vacuum gauges, piston aircraft use vacuum gauges to manage the engines power in all respects, a pretty simple system. the average aircraft will idle at 11 In Hg for a good engine, up to 17 for a not so good one. full power, at sea level, on a 15 C day, about 27-8 In Hg. Cruising about 22-23 In Hg.

I don't honestly know how the vacuum readings relate to aircraft engine performance? But for normally aspirated land vehicle engines, the highest vacuum reading is usually registered at idle speed-when the throttle is almost fully closed or when the throttle is snapped shut. The vacuum reading drops as the throttle is opened until around half throttle or more, when the vacuum reading will be down to practically zero.

Josh Cox · May 11, 2011

Thanks LAB,

Standard atmosphere is about 29.53 In Hg, so that would agree with your statement.

I've ordered a pair of vacuum gauges and will report back when I have them.

Amal 930 questions.

Josh Cox

comnoz

pete.v

stockie2

L.A.B.

Moderator

ILLF8ED

possm

comnoz

Josh Cox

L.A.B.

Moderator

Josh Cox

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