Atlas monoblok 689/15

Condo · Feb 15, 2025

Gday gents, wondering you could help with carby settings on my 1965 Atlas 750. The bike was fitted with 2 689 Monoblock non matching carbys when I acquired it. I have gone with one 689/15 carby on a dual manifold. Starts and ideals ok but it’s very rich . The jetting is as follows.
MJ-340, slide -689/3, throttle needle -D(top clip)
Jet holder-106. Needle jet 25.
The previous owner chromed the slides , so if I need a bigger cutaway I may have to machine it myself .
Would anyone have any ideas of jetting please .
Condo
I will post pictures on my
Condo’s Old Timer Model Page , if that helps.

Triton Thrasher · Feb 16, 2025

At what throttle opening is it too rich?

kernel65 · Feb 16, 2025

Jetting seems okay, appears you have gone after the needle first to lean it. Guessing rich at cruising speed and only in the right cylinder? I would start at the float level/needle valve/seat. Easy first check.

Condo · Feb 16, 2025

Triton Thrasher said:
At what throttle opening is it too rich?

All

Condo · Feb 16, 2025

kernel65 said:
Jetting seems okay, appears you have gone after the needle first to lean it. Guessing rich at cruising speed and only in the right cylinder? I would start at the float level/needle valve/seat. Easy first check.

No, very rich at 1000 to 2000 rpms, keeps wanting to stop when going slow and I pull clutch , have to keep nipping it to keep it alive , a real pain in traffic

kernel65 · Feb 16, 2025

Got you. I’d still start in the float bowl, as mentioned before. Check to make sure your float still floats too.

Condo · Feb 17, 2025

Ok. , going to add air down low by increasing slide cut away to app 3.5. Then ride around at 800/1000 rpms and check plug , if still sorry going to a 20 needle jet and plug check again.
Wish I knew how to post pictures as clicking camera not working,,,,

Triton Thrasher · Feb 17, 2025

Condo said:
going to a 20 needle jet

Do you mean pilot jet?

The needle jet is the one that’s likely to be 106.

baz · Feb 17, 2025

Condo said:
No, very rich at 1000 to 2000 rpms, keeps wanting to stop when going slow and I pull clutch , have to keep nipping it to keep it alive , a real pain in traffic

Forget totally about what revs it's playing up (unless it's an electrical issue)
It's solely on carburettor position
It's worth marking the throttle to see exactly where you need to jet
Also fit a new needle jet before you start playing and always get the engine up to temperature

Condo · Feb 17, 2025

Triton Thrasher said:
Do you mean pilot jet?

The needle jet is the one that’s likely to be 106.

yes sorry pilot jet from 25 down to 20.in my diagram they sometimes describe needle jet as jet holder ..[106 now]going to 105 .i pulled plugs today after a 10 minute ride around the streets at less than 1/4 throttle ,the plugs were sooty and blacker than the ace of spades. since then i have increased the cutaway on slide but haven't ridden it yet.[still original jetting ,to get base before changes].

Condo · Feb 17, 2025

baz said:
Forget totally about what revs it's playing up (unless it's an electrical issue)
It's solely on carburettor position
It's worth marking the throttle to see exactly where you need to jet
Also fit a new needle jet before you start playing and always get the engine up to temperature

but do i fit a 106 or go with a 105 and new needle.

baz · Feb 17, 2025

Condo said:
but do i fit a 106 or go with a 105 and new needle.

I'm not sure on a monobloc
But go with whatever the spec is for a single carb on an atlas
Because if it's worn you'll be pissing in the wind playing with other settings
If it still isn't right, then consider changing the needle jet size and the pilot jet size

norton bob · Feb 22, 2025

Probably seize up with a 105

marshg246 · Feb 22, 2025

Like @baz said, you must state/know the throttle opening before working on it.

0-1/8 Throttle = Pilot Circuit. 25 pilot jet is correct.
1/8-1/4 Throttle = Cutaway
1/4-3/4 Throttle = Needle position. Needle jet involved but if you have 106 that is not worn then nothing to change
3/4-Full Throttle = Main Jet.

Fuel height can affect all. If rich across the board, then probably way too high (flooding)

Since you state 1000-2000 and since normal idle is 1100, I'm guessing you're talking about 0-1/8 throttle so forget everything but the pilot circuit.

BTW, generally, worn slide = leaner, worn needle or worn needle get - richer

Stalling at lights is usually too lean, not to rich unless VERY rich caused by flooding.

Are you sure the black on the plugs isn't oil?

Condo · Feb 22, 2025

Gday thanks, since I posted this I have ridden the bike at 1/8 throttle for 2 miles , the plug looks like a sorry chimney. , and no it is not black and sticky so I going to say it’s not oil related. I have ordered a #20 pilot jet, new #D needle and needle jet, haven’t tried them yet.

Condo · Feb 22, 2025

Plug Looks like sooty chimney

marshg246 · Feb 22, 2025

Condo said:
Gday thanks, since I posted this I have ridden the bike at 1/8 throttle for 2 miles , the plug looks like a sorry chimney. , and no it is not black and sticky so I going to say it’s not oil related. I have ordered a #20 pilot jet, new #D needle and needle jet, haven’t tried them yet.

marshg246 · Feb 22, 2025

At 1/8 throttle, not pilot jet, not needle, not needle jet! Most likely flooding (float sticking, badly adjusted).

Atlas monoblok 689/15

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