389 jetting for 1959 Super Rocket

[lawnmowerman]

Hi all

I thought I would start a new topic related to my current Spit back and Backfires topic.

I have a 389 (I believe) on my 1959 SR based on the fact that the slide has 389/3 1/2 stamped on it and the bore is nominally 1 5/32”. The only other identification on the carb is M5 stamped on the side of the flange (the inlet manifold is also 1 5/32).

I have looked through other posts and my manuals and it shows the 376 prior to 1959 and the 389 from 1960 on which is not to say that the PO fitted the newer version on the rebuild.
 
Carb spec:

Bore - 1.173" measured at the manifold end (1 5/32” ?)
Main jet - 240
Pilot jet - 30
Needle - D (The only reference I could find to D is a D2 used for alcohol)
Needle jet - 106
Slide - 389/3 ½

My BSA service sheet 216 says settings for a SR with a 1 5/32 carb is:

Main jet – 420 (250 for A10)
No mention of pilot jet size
Needle jet – 106 (what I have)
Slide – 389/3 (I have 389/3 ½)
Needle position 2 (Is that from the top or bottom?)

I looked on the Hitchcock’s site and there are no options for the needle – just the standard and the alcohol version. Also read elsewhere that the needles do not wear out and should not need replacement.

No air filter fitted – just a short bell mouth with a gauze cover.

Do these settings and jettings look OK and what notch should the needle be in – it was three from the top (do you number from the top or bottom?) The bike was sooting up plugs so I dropped it a notch and started getting the spit back and backfire problems but I think that is unrelated and may be due to slipped timing or partially blocked pilot jet after the winter layup (see related post).

It looks like the PO has fitted the wrong size main jet – a 240 for a 1 1/16” 376 for a 59 A10 which is considerably smaller that a 420. He has also fitted a 389/3 ½  slide which is used for a post 60 A10.

I am tempted to order up these parts from Hitchcocks but thought I would seek forum advice first.

Thanks

Jim

[muskrat]

Me again,
             don't know much about mono's but here's a list of settings from Roy Bacon.
Cheers

[lawnmowerman]

Just spoke to Hitchcocks and he really knows his stuff.
Looks like the 420 main jet size shown in the BSA service sheet 216 is a misprint and is probably for a GP carb - a 240 should be ok.
The D needle is ok.
The carb slide is ok if the bike pulls away alright without spitting back - if that happens then take about 1/32" of the bottom of the slide with a flat surface and some wet and dry and the 3 1/2 slide becomes a 3 3. I was very impressed with that as he could have sold me a new slide but has saved me the expense.
His advice was to strip the carb and clean all the jets and drillings with carb cleaner spray so I bought a can and a gasket set. Also bought a new pilot jet as I had been poking around in the old one trying to get the muck out.

Jim

[lawnmowerman]

Thanks Muskrat

Still looks like I need a bigger main jet according to the Roy Bacon chart although Hitchcocks cannot understand such a huge jump in jet size from A10 spec to SR spec but being as it only affects about the last 1/8 throttle is probably a bit academic to my situation. If when I have run in and find a problem at full throttle I will play around with the main jet then.
I am beginning to think that a lot of my problems are due to dissolved petseal which is coagulating in the carb jets and drillings.
I wish that real petrol was still available here - may solve a lot of our problems including some we probably don't even know about yet!

Jim

[muskrat]

Good news Jim, but if you take anything off the bottom of the slide it will sit lower, and so will the needle !!! Easy to make a 3 1/2 out of a 3 (done that) but not the other way.
Cheers

[cus]

G'day Jim,
I run the same carby, what type of fuel are you using ?, what % octane?.
I run a 370 main, No. 4 slide, top knotch of needle, 106 needle jet, b6es plugs,
as per last post, fuel 98% octane. My trouble was at slow running, plugs would
foul (black powder), but open road running would clear them, brought the 4 slide
& all is good, only cost 12 pounds & is a chromed one. You probably already know,
but spitting back thru carby can also be a too advanced problem.

regards, Cus

[lawnmowerman]

G'Day Muskrat & Col

Good point M, If I remove a bit from the bottom of the slide I will have to remember to raise the needle a bit to compensate although I am reluctant to file bits off as there is no going back - I would rather try a new slide.
Col, I was running on the best fuel (most expensive) I could find although reading other posts it may be that it means even more ethanol has been added to boost the octane rating. In the UK it is difficult to find ethanol free fuel so I have a 20L can of avgas which I plan to use - I will probably run exclusively on avgas in the future. I think it is around 102 octane but more importantly has no ethanol.
Thanks for the settings - mine are not far away apart from the main jet which is a 240. I have seen recommendations ranging from 240 to 420 for the main jet size but being as it really only affects the flat out performance I am not really worried as I am still running in.
She does spit back (see my other topic) but the timing is wrong and advanced (1/2 BTDC). Strangely enough retarding the lever does not affect things too much so I suspect there is probably something equivalent to an African storm going on inside the mag. I am resigned to getting that rebuilt anyway but I am taking the opportunity to clean out the carb and check jetting too.

Jim

[dpaddock]

Turn or abrade the slide skirt 1/16 " at a time to get the cut away you want, then raise the needle one notch for each sixteenth to compensate. I did this last year and dialed in the exact cut away I needed. Works fine.

David

[Triton Thrasher]

Turn or abrade the slide skirt 1/16 " at a time to get the cut away you want, then raise the needle one notch for each sixteenth to compensate. I did this last year and dialed in the exact cut away I needed. Works fine.

David

1/16" is a bit coarse.  Increase the cutaway in smaller increments.

Your needle procedure is flawed and wouldn't work in all bikes. 


Just by the way:  any tuner who ignores the well publicised tuning procedure for Amal carbs and then has mixture problems, gets scant sympathy from me.

[bsa- bill]

I understood that the slide numbers are sixteenths ( 3.5 would therefore be 7/32) so a sixteenth could be a bit much at a time.

thinking about this a bunch of old slides from auto jumbles might be handy to have for setting up, provided they are not to worn