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#891008 09/18/22 9:15 am
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I cant get my A65L to run properly it idles and runs very roughly:
Jetting of my 1968 bsa A65L, with 930 AMAL Concentric,
according to several books:
Main jet: 190
Choke: 2½
Needle: 2 groves (2stroke)
Needle pos: 2
Jetholder: 2stroke
Pilot jet: 20
Needle jet: 106
In 1969 AMAL changes over to 4 stroke jet setting : Needle with 3 grooves, taller jet holder, pilot jet fixed pressed into carb. body: size 20
https://www.classicbritishspares.com/.../updated-amal ...
How have you been jetting your bsa 65 wit 2 x AMAL carb with the above changes?
how have you jetted your A65 Lightning with AMAL Concentric? spitfire ?
Right now my a65 with 2 x newer 32 mm Concentric is jetted as a 1967 spitfire( but converted to 4stroke specs):
Main jet 190
choke: 3
Needle: grooves (4 stroke)
Needle pos: 2
Jetholder: 4stroke
Pilot jet: 20(pressed into carb body)
Needle jet: 107
Best regards
Søren N
Denmark

Last edited by Søren N; 09/18/22 10:02 am.

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Any carbs I sell, I advise using the 69 and later jetting on a standard bike. I did as you did when I first got mine on the road and found it to be too rich. Isent my carbs back to Burlen to have the bush fitted (now I got premier carbs which means the bush isn’t needed)

Following this I made a few changes to the bike but it was a pretty stock 68 Lightning.

My own bike now prefers a 105 needle jet and bigger main jet. This is using 928 with early small port head

Depending on if I have my 2-1 with Dunstall mega or twin pipes denotes what need I have fitted (standard 2 ring for twin - standard exhausts. And 5 ring needle for the mega)

Either way I have a 3 ring pilot jet.
3.5 slide
105 needle jet
Needle type as per above, either way they are on the leanest setting (pos 1)
230 main jet.

Not sure why it prefers the 105 needle jet, can only assume it is down to the fuel or my setup. It also has an X12 mega cycle road race cam


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71’ A65 823 Thunderbolt (now rebuilt)
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If your carbs are 32 mm , "Right now my a65 with 2 x newer 32 mm Concentric is jetted as a 1967 spitfire", then 190 MJs are too small, try 220s.


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My '68 spitfire settings with 932 premier carbs, 4-stroke configuration (stock exhaust):

Pilot jet: #19
Needle jet: 106
Main jet: 250
Slide: 2.5 (could probably use a 2)
Needle clip: bottom groove (fully raised)

At this point it starts well using the choke and runs perfectly without choke once warmed up.

You don't mention at what point the bike struggles. Based on the description above, I would make sure that all passages are cleared and new jets installed. Sounds like you replaced the 930s with newer 932s; did that make any differences?

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Thank you for the answers
I have been struggling on/off for several years and have cleaned the carbs several times...
Bought the bike as project with rebuild engine and haven´t been able to get it to run proberly...
I haven´t fitted the chokes.
I have fitted electronic ignition with wasted spark 2 x 6v coils i think 1 sparkplug Champion N3 is defective,
i haven't tried bigger main jets yet but will be the next thing i will try... And 2 new NGK Bp8e sparkplug´s
My current jetting:
Main jet :190
Throttle slide cutaway: 3 - could this be a problem?
Needle: grooves (4 stroke)
Needle pos: 2
Jetholder: 4stroke
Pilot jet: 20(pressed into carb body)
Needle jet: 107

Last edited by Søren N; 10/02/22 7:58 pm.

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To be honest, your jetting doesn't sound too far off, looking at the 1969 A65 workshop manual Here, for Lightnings and Firebird's, using 930 carbs, it suggests 190 mains, pilot jet 20, slide 2 1/2, needle position 2 or 3 and needle jet .106.

If I remember correctly my A65 Firebird is jetted as above using a No 3 slide, I don't know if its using 2 stroke or 4 stroke needles, although I suspect 4 stroke.

If it running rough I would look at the following:-
- check the idle mixture screw is 1 1/2 turns out
- check the slides are bottoming equally against the throttle stops and open at exactly the same time, the throttle cables may need some adjustment
- check for air leaks around the carb mountings by spraying WD40 on them, any leaks will cause the engine to speed up or slow down
- check the idle circuit is completely clean, spray can carb cleaner isn't enough, they need to be poked clear with a guitar string or ting #78 drill
- check the float height is correct

Have a look at Bushmans carb tuning secrets Here

Might also be worth checking the valve clearances and doing a compression check, in-case there is a burnt valve or bad rings.

Last edited by gunner; 10/03/22 9:40 pm.

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Originally Posted by gunner
To be honest, your jetting doesn't sound too far off,

Huge difference between a 106 needle and a 107!

Soren, dont adjust a thing until you have marked the throttle grip off at 5 positions and an indicator against the housing.

you want:

0/1 Closed
1/4 Open
1/2 Open
3/4 Open
1/1 (fully open)

Only when you have diagnosed where in the throttle range the roughness is, can you make adjustment. If you have a choke lever fitted, move it over to the left side of the bars and use it to help find where mixture is too lean. - if you can close the choke down then with little or no impact, it will indicate a lean mixture. a rich one will manifest itself as being worse imediately when the choke slide drops below the cutout in the throttle slide.

Idle mixture (any mixture) wants to be checked on a hot motor (I like to ride a minimum of 10 miles on a warm day, more on a cold day), idle mixture only checked at stand still...

Slide choice is determined by blipping the throttle. If the idle mixture is correct then do not adjust it to overcome issues off idle. change the slide. continuing to run but running rough is a rich mixture. if the engine fades out and dies then it is too lean. I sometimes find a propper road test on a twisty road is a good tell. how cleanly does it pull out of a tight bend or series of bends?

a needle jet issue can be confused with a slide issue so this must be addressed indepentantly.

any lean issue will also show as an improvement in performance (like a slight boost) when you close the throttle.

Adjust/check in this order.

Float level. then

Idle mixture: - throttle closed
Main jet: - throttle 3/4 to fully open
needle position: - 1/2 to 3/4 open
needle jet size: - 1/4 to 1/2 open (the straight portion of the needle is still in the jet here)
slide cutaway: - closed to 1/4 open ( do not start leaning the needle off at this point, it is tempting and it will seem like you have succeeded - but you will still foul plugs if too rich of slide is used)

Then re-check the idle mixture.

The correct mixture is WHERE IT RUNS ITS BEST. any jetting recommendation is only a starting point. Your fuel, altitude and bike setup will change from bike to bike.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
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67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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How much difference does it make that i am using 32 mm carb. instead of 30 mm as the std Lightning uses?
I use 32 mm because i got them cheap and they were new....


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The 32 is 14% larger by throat area. For the same flow, gas velocity in venturi drops by 14%, signal on jets is lower, therefore larger jets are required.
Low speed performance will suffer, above 6 K rpm there may be a small improvement. In general rideability will be worse unless you ride at full throttle everywhere.


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Originally Posted by gavin eisler
The 32 is 14% larger by throat area. For the same flow, gas velocity in venturi drops by 14%, signal on jets is lower, therefore larger jets are required.
Low speed performance will suffer, above 6 K rpm there may be a small improvement. In general rideability will be worse unless you ride at full throttle everywhere.

+1

Originally Posted by Søren N
How much difference does it make that i am using 32 mm carb. instead of 30 mm as the std Lightning uses?
I use 32 mm because i got them cheap and they were new....

I doubt you will see much difference between the 30 and 32. You could shape the port entry to match the 32mm (or shape a tufnol spacer) but at 1mm radius change it isn’t much of a step and will create a bit of turbulence which isn’t always a bad thing.


Now let’s all have a beer beerchug

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68’ D14 trials (undergoing transformation)

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Is the larger carburettors part of my problem or not? Because i have a stock head with 30 mm port size? I also have 30 mm AMAL 930 but they er used/worn and it did not run better with them(maybe because of 2 stroke/4stroke mismatch)


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The carbs being 32mm in size is NOT your problem, I have used these carbs with the early small port head and tapered spacers (as does Adam M) they run absolutly fine with them.

you need to dial in your carb aproprately, please see what I wrote above on how to do this.

Post from above


Now let’s all have a beer beerchug

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BSA could have fitted 32 Concs, the bodies/costs are the same as 30s ,and yet they chose not to. Apart from a one year deal with the last Spitfire models which came with ports to suit. My own bike is a 732 cc, I use 30 mm carbs, unless you are going for land speed records 32s are way too big.

Whether this is your problem or not is very hard/ nearly impossible to say. if your 32s use the same jetting and slides as the 30s then the mixture will be weak through the range of throttle openings, I explained why in my previous post, expect symptoms of spitting back and back fires in the exhaust .

90% of carb faults are electrical, any bad connection, dirty switch contact, failing HT lead will give symptoms suggesting a carb fault.

"I cant get my A65L to run properly it idles and runs very roughly: "

Most idling problems on Concs can be cured if the pilot jet is cleaned properly with a #78 drill , no amount of solvents , torch tip cleaners ,poking guitar strings and air blast will give the same results as spinning a #78 drill in the metering jet. This throttle position, idling, closed throttle is entirely governed by the pilot jet and its fuel / air passages, it has nothing at all to do with 2/4 stroke or main jets or slides. As well as the pilot metering jet there are two small mix delivery jets in the floor of the carb throat/venturi, these must also be clean.
The pilot system has a massive influence on normal riding where the throttles are seldom opened past 1/4 of full .

All this comes with a huge caveat, for all I know , you may have any number of reasons apart from carbs causing bad running, poor ignition timing ( has it been checked on a strobe), faulty ignition switch ( check closed contacts with an Ohm meter),Carbs out of synchronisation , the possibilities are many.

Theres nothing wrong with being economical, my own carbs are the originals which were fitted in 1971, apart from renewing the bodies twice,float bowls once, slides 6 times and needle jets 6 times, fuel needle valves twice, all that remains of the originals are the banjos,jet holders, carb tops, floats and float spindles

Ignition switches are very prone to intermittent failure, I have replaced this item twice, contacts come loose in the body and contacts get oxidised, no matter how good the electrics they are limited by this one item.

Nowadays new carbs are available in three flavours, cheap copy, original, and superior. Fitting old 32s is like buying 2nd hand shoes which are 3 sizes too big. Unless you live in a cardboard box in a subway I cant think of a good reason to do this.


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I just noticed that you said the pilot jet is #20, this sounds like its a bit on the small side especially as the carbs are 32mm and presumably have less vacuum at idle and may not be able to suck enough fuel through the pilot jet? As they are the pressed in type, I'm not sure if they are replaceable.

There's really good guide to tuning AMAL carbs written by John Healey, see This Link, the base setup for 4 stroke 932 Concentrics is listed as 220 main, #25 pilot, slide no 3, needle position 2, needle .106. This is just a basic setting so might need some adjustments.

I agree with Allen, you should mark the throttle and then tune up each stage, usually you would check the plugs at each throttle opening to ensure the mixture is correct.

The rough running could be caused by a lot of other things, so I would check the ignition circuit, compression, valve clearances etc.


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the pilot circuit should not really be influenced by the size of the choke (the carb size), if the idle circuit is tuned correctly. So little air will pass under the slide it should not influence it as much a float setting or the mixture screw. the port size is still the same so gas speed down the port is relatively unchanged.

An indication to a weak pilot jet would be that you are less than 1 turn out on the air mixture screw, the beauty with the later premier is you can change that jet easily.

Thankyou for posting the article though, It is one which is well worth refrencing!


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"Unless you live in a cardboard box in a subway" - A cardboard box? You were lucky. We lived for three months in a rolled up newspaper in a septic tank. We had to get up every morning at six o'clock and clean the newspaper, go to work down mill fourteen hours a day, week in week out, for six pence a week and when we got home our dad would thrash us to sleep with his belt.

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But you try and tell the young people today that... and they won't believe ya'!


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