Author Topic: A65 Spitfire mkiv needs choke ?  (Read 643 times)

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Mickygee

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A65 Spitfire mkiv needs choke ?
« on: 04 May, 2024, 20:15:35 »
Hi just put a set of new Amal Premiers on the Spitfire as had odd carbs previously and didnít idle well (a 930 and a 932 !)

When starting cold have to really flood the carbs and choke for it to start, but then needs choke on even when cold or warm to rev cleanly from idle.

Tick over with or without choke is fine. Itís the initial throttle response that is poor if start to rev with out choke. It sort of fluffs then picks up.

Iím guessing itís too lean on the pilot jet ? Will need to check whatís fitted. Has 190 main jet and needles are on the middle clip- am I on the right track ?
1968 BSA A65 Spitfire IV hybrid

peasy

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Re: A65 Spitfire mkiv needs choke ?
« Reply #1 on: 04 May, 2024, 20:28:44 »
How many turns out are the mixtures screws from fully in?
 I think 1.5 turns out is the base setting then turn in for richer and out for leaner on Amal.
Iím sure someone will correct me if wrong :-)

Also the throttle stops need to be balanced up first otherwise mixture screw settings and throttle stop setting would be all over the place.

Some good video on YouTube on balancing Amal carbs

Mickygee

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Re: A65 Spitfire mkiv needs choke ?
« Reply #2 on: 04 May, 2024, 20:41:22 »
Pilot Jet set at 1.5 turns have tried between 1and 2.5 turns doesnít seem any better ??
Have set both throttle stops both to just off bottom and slides look synchronised. Will have a look on YouTube - thx
1968 BSA A65 Spitfire IV hybrid

EDDIE SIMPSON

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Re: A65 Spitfire mkiv needs choke ?
« Reply #3 on: 04 May, 2024, 22:39:39 »
i have had a similar problem on my thunderbolt with a new wassel carb.
choke and tick over o k ish but jerky pulling away unless choke on. i raised the needle 2 notches from standard settings but now a lot better.

DAVE BRADY

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Re: A65 Spitfire mkiv needs choke ?
« Reply #4 on: 05 May, 2024, 08:11:35 »
Hi,

I would just check that the carbs are clean.  Even with Amal Prems. it is worth giving them the once over.  Have you got the correct slide cut out as this will affect the transition from idle?  Also, are air filters fitted?  Standard carb. settings and jetting etc. are for air filters being fitted.  If they are not on then this will in effect weaken the mixture.
I noticed after fitting Prems. that two bikes that only needed a tickle with the old Concentrics and were not fitted with a choke, needed to have the chokes fitted and used as well on the Prems.

Dave.
« Last Edit: 05 May, 2024, 08:15:34 by DAVE BRADY »

Steve.S

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Re: A65 Spitfire mkiv needs choke ?
« Reply #5 on: 05 May, 2024, 12:55:13 »
I have heard these carburettors sometimes need cleaning thoroughly before fitting, so I agree with Dave Brady. Also, sometimes jets are loose.
Are you saying that adjusting the air screws don't make make much difference to tickover speed? If all is well, they should be quite sensitive.
I wonder if you might have a little bit of debris in your Pilot By Pass orifice?
A friend of mine once found a dead fly in his float chamber!
You are correct. The mixture adjustment screw is an air screw.

Mickygee

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Re: A65 Spitfire mkiv needs choke ?
« Reply #6 on: 05 May, 2024, 16:19:33 »
Itís on standard pancake filters. Will take them off and check what slide jets fitted. Ill pull the the jets and make sure alls clean.

Will just recheck carb balancing first.

Thanks allÖ
1968 BSA A65 Spitfire IV hybrid

Mickygee

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Re: A65 Spitfire mkiv needs choke ?
« Reply #7 on: 05 May, 2024, 21:26:47 »
Carb balance was all good I had used one of the guides on synchronising previously and still seemed fine.

I dropped the bowls and took out the pilot jets and gave the carbs a blast of brake cleaner both ways, one of the carbs seemed a bit restricted at first but went clear.

I reassembled and gave a tickle and a little choke. It started fine still needed a little choke but would now hunt if you gave it too much.

After warming it up set the pilot jets (which now work) and the idle. No choke required 👍

Bike now runs great, pulls strongly and responsively with a nice slow tick over.

Sorted ÖÖso yes it was just a bit of crap in the pilot orifice.

Thanks for all your help !

 😁😁😁
« Last Edit: 05 May, 2024, 22:21:31 by Mickygee »
1968 BSA A65 Spitfire IV hybrid

peasy

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Re: A65 Spitfire mkiv needs choke ?
« Reply #8 on: 07 May, 2024, 20:25:01 »
Good news!