Author Topic: main jet for 389/46 with new amal cone filter  (Read 461 times)

Online mikeb

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main jet for 389/46 with new amal cone filter
« on: 15.08. 2019 11:15 »
for my 61 super rocket i've just ordered a new burlen 389/46 coz i'm sick of the old one that has oval slides, distorted flange and appears to be getting rich by wearing fast on the inside. i noticed that amal now sells a screw-on cone filter for the 389, as per the pic below. as i've spend so much money on valve gear, i've just ordered a filter too.
here's the link: http://amalcarb.co.uk/monobloc-series/389-series/air-intakes/threaded-cone-filter-389-series-monobloc.html

but how to jet for it? the standard bell-mouth / no filter setup is 420 main. the us export with the bsa filter is jetted at 290.
i would have thought 290 is way too low for the new 389-filter combo and that a 400 or 390 would be a place to start.

any thoughts on what's right?
thanks
Mike
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Offline RogerSB

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Re: main jet for 389/46 with new amal cone filter
« Reply #1 on: 15.08. 2019 11:48 »
Hi Mike, In BSA Twin Restoration book by Roy Bacon it says this:-

A10SR, 1960 - 63, type 389/46, size 1 5/32", main 420, pilot 25, slide 3, needle position 2, jet .106.
A10SR (USA), 1960 - 63, type 389/47, size 1 5/32", main 290, pilot 25, slide 3, needle position 2, jet .106.

So it seems that is the only difference in the carburettor between home and USA models. Not saying this with any first-hand experience, just copying from the book.
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Offline Topdad

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Re: main jet for 389/46 with new amal cone filter
« Reply #2 on: 15.08. 2019 16:52 »
Always you dropped the jet by 10% as a starting point .
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Offline Scott and Jay

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Re: main jet for 389/46 with new amal cone filter
« Reply #3 on: 17.08. 2019 00:10 »
Hi Mike, I have taken a similar pathway. I actually got a 389/47. I ordered the hard anodized slide, with it. My experience recently has verified the 3 1/2 slide, no 3 needle position, 106 needle jet, 25 pilot. Anyway, I got a range of main jets. When I started with the 280 it missed going up the Wainui hill in 3rd. I went up in increments of 10 (but with monoblocs you can make bigger leaps, apparently). I ended up with a 310 and she flew up the hill, nicely. I have kept the choke as a tuning aid. I mark the accelerator - into the ranges (main jet is top 1/4 of the travel). If you put the choke on in full throttle range and it's got more power, you can go to a bigger main jet. I might try this again, when I get my head back on. Oh, importantly - I have a similar air filter, a KNN. The pipes have an influence too. My mufflers are cheap (but good) Emgo reverse cones- maybe a less restrictive choice. If it's really baffled you go down in main jet, for this, too..
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Online mikeb

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Re: main jet for 389/46 with new amal cone filter
« Reply #4 on: 17.08. 2019 11:03 »
thanks for the comments guys. that should get me started
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Re: main jet for 389/46 with new amal cone filter
« Reply #5 on: 20.08. 2019 06:31 »
Mike if they (the filters) are too small they will suck power and you may get passed by plunger framed iron singles.
I had similar setup on an atlas until I put it on a dyno! Size sadly matters and I know you like twisting that throttle.
you dont want to restrict airflow. whats a bit of valve wear for that Super Rocket let it breath!
 *yeah*
I dont run a monoblock (steve does on his SR!) but with modern fuels you're typically running smaller than quoted jets
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A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Offline SteveH

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Re: main jet for 389/46 with new amal cone filter
« Reply #6 on: 31.08. 2020 07:58 »
Mike, what main jet did you end up using: 290 or 420 or other?
I want to order a new carb for the Super Rocket and am not sure whether to get a 389/46 or 389/47
Did you end up fitting the pod filter?
It would be great to know exactly what setup you used.
Thanks, Steve
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Online mikeb

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Re: main jet for 389/46 with new amal cone filter
« Reply #7 on: 31.08. 2020 10:08 »
Hi Steve
i ordered (direct from burlen) the 389/46 with a filter (as per the pic) and a 400 main, thinking that smaller main would accommodate the modern filter. it was sooo lean, especially with wrist open. i raised the needle, went up on the main by steps, even tried cooler plugs (was ngk 6 so tried 7's) as couldn't quite believe it was so white/lean.  WITH the filter, I've ended up with a 440 main  *eek* and back to ngk 6's (on the 7's it would not pull). I think the needle is back on '2' but can't quite remember. weirdly, i also dropped the 25 pilot to a 20 as the idle was very rich. Now its running well and pulling well. One day I need to do another plug chop just to be sure.

I've wondered if there is something wrong or random with their jetting. in fairness, I didn't check the float height as assumed I didn't need to. from memory, some threads on this forum talk of potentially the wrong height due to the absence of a seal (washer?) under the banjo on the new 389s, but I'm not sure of the details.

If you are going for the new 389 with a filter I'd suggest you start WITHOUT the filter and use the standard 389/46 jetting (420 main, '3' cutaway, 106 needle on '2', 25 pilot). then when all running ok do a plug chop, fit the filter and do another plug chop. adjust from there. the standard amal tuning guide is an easy workflow. British spares in nelson has jets in NZ or get burlen to throw a few extras in the box.

is it worth it? Absolutely. With the new carb and after the fettling the SR is easy to start and runs much better. love it. When i fitted a new 376 to the b33 the transformation was even more remarkable.  *smile*

cheers
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Offline SteveH

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Re: main jet for 389/46 with new amal cone filter
« Reply #8 on: 01.09. 2020 00:07 »
Great thanks for that Mike.
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Offline gt6uk

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Re: main jet for 389/46 with new amal cone filter
« Reply #9 on: 01.09. 2020 18:16 »
I bought a new 389 for my RGS last year and the float height was too high. I put the needle seat in the lathe and removed about 0.5mm to get the height correct and it runs a lot better.

Steve.
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