Author Topic: advice please on fettling new 389  (Read 152 times)

Online mikeb

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advice please on fettling new 389
« on: 27.10. 2019 03:21 »
hello
can you offer advice on the plug chop pics below please - especially the second pic.
i fitted a new 389 from burlen to my super rocket (1961, 356 cam, .080" oversize). out the tin its 420 main, 3 slide, 106 needle at 2nd from top, 25 pilot.
Plugs are BP7ES - which appeared right with the old carb. 98 octane unleaded fuel.

it ran well, so i fitted the new screw-on conical air filter that goes with it from burlen, changed the main to 400 and did a plug chop (wide open up slight incline etc...) - thats the first pics below.
it looked way too lean so i went back to the 420 main and raised the needle to 3 (=richer), that's the second plug chop pic. better, but to my untrained eye it still looks too lean.
So comments please on the bottom pic (420 main) -  should i go to a 440 main (with air filter)? and/or '4' on the needle? that would surprise me. I know the needle is independent from the main but covers much of the range. no obvious running difference between 2 and 3 on the needle.

BTW the '3' slide seems OK and the pilot is too rich (with filter and without) in that it has a 'lumpy' idle and the air bleed screw doesn't slow it down when turned out past 1-1/2 turns. The pilot was loose in its thread but no difference when tightened. I think it needs a 20 pilot.

also the back corner of my mind says some people report float height issues with the new 389s (or was it just the new banjo arrangements?) - i don't know how to tell.

thanks
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Online Triton Thrasher

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Re: advice please on fettling new 389
« Reply #1 on: 27.10. 2019 10:30 »
Don’t be afraid to try richer on needle position.  It doesn’t cost anything.

When weakening the mixture screw (out), are you trying slowing it down with the throttle stop?


Online mikeb

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Re: advice please on fettling new 389
« Reply #2 on: 28.10. 2019 01:29 »
yes TT, winding the pilot air screw out it doesn't make it falter on either fast or slow idle.
I will try the needle on 4
any thoughts on those plug colours?
its a bit surprising to have to richen it up after having added an air filter
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS