advice please on fettling new 389

[mikeb]

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

[Triton Thrasher]

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?

[mikeb]

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

[Scott and Jay]

HI Mike, I don't think you said what speed you did the plug chop at. To test the main jet you have to be doing about 70 mph, in top. I mark the throttle to look at it and see it's in the top 1/4 of the range. Doing this is top is hairy, of course. See if you can do it in third going up a slight gradient (like you said) for a km. I have a 320 main jet, with a KNN air filter (also a 389). You're suppose to do it with new plugs. With mine, I started at 280 and went up (you can do it in increments of 20 - to make much difference, with a Monobloc). It missed and lost power, going up hills - until I got to 310. Then it was fine. The other thing about starting lean (cautiously), on the main - is you can use the choke as a tuning guide. If it gains power when you put the choke on (in the main jet range) - you can go to a bigger, richer jet size..Regards

[mikeb]

cheers Scott. my understanding of a plug chop on a monobloc is throttle between 7/8 and fully open, preferably up a slight incline. yes you can run out of road quickly. i was fully open up some gradients last weekend on the Auckland BSA club run to Port Albert but i was having too much fun to stop and take the plugs out! it was a great ride and the 25th anniversary of the club's 'Rods of Bullen' trophy, tho we were infiltrated by a couple of vincents (RR!) and a tuono.

back to the carb - i've now got the 106 needle on 4 (richest) and a 420 main still, tho a 440 on the shelf should i need it. and dropped the plugs to a BP8ES from a BP7ES (alloy head), so it should be cooler. I really must have a proper look. I've dropped the idle jet to 20 tho its still a bit rich (less so, but still a bit lumpy and won't falter with extra air bleed) so i wonder if there some other problem with the new carb.

[RDfella]

A difficulty with plug chops is that you're supposed to run flat out and then immediately cut the motor - without letting it 'wind down' ie it shouldn't fire again. So it's clutch in and cut ignition - bit hairy if the cut out is on the mag..... That ssid, 2nd plugs do look a trifle lean.

[clive1]

Hi Mikeb,
Have owned My RGS since new (1962)
assuming you have cut the engine at full throttle,
having been pulling hard uphill for at least some distance
(for the plug reading to be accurate)
then the colour of your centre electrode is spot on,
recommended to be from grey to light brown

The plugs should be B7HS