Choke vacuum when cold GS750 VM26.

[Guest]

So went to sync the carbs after recent top end rebuild - upgraded to 850 pistons and cylinders with new rings and hone - I've done about 150kms and noticed a bit of black smoke when warm at idle which wasn't there before.

Plugged in the carbtune to check sync - a little uneven but not too bad and then it got dark so left it til the next day to adjust.

So then I started the bike from cold with choke and noticed carbs 1 & 3 were pulling vacuum whilst 2 & 4 had none. More choke and higher rpm made 1 & 3 go higher whilst the other two still showed nothing. With choke in and throttle open all carbs were then showing vacuum which became higher as bike warmed up.

So I synced the carbs when warm. But this black smoke showed up again on this mornings ride when hot and rpm's starting to dip too at idle with a blip of the throttle getting it right again.

Stripped carbs and replaced all o-rings a year ago.

Any thoughts appreciated before I pull carbs again. Thanks.

[Steve]

Only thought I have on this is ... don't measure your vacuum when on "choke".

Not sure why they weren't all the same, but the "choke" on these bikes (regardless of carb type) opens a separate passage for an enriched mixture of air and fuel that bypasses the normal throttle mechanism. Because of that, any attempt at getting meaningful vacuum readings while on "choke" is futile, at best.

.

[Guest]

Only thought I have on this is ... don't measure your vacuum when on "choke".

Not sure why they weren't all the same, but the "choke" on these bikes (regardless of carb type) opens a separate passage for an enriched mixture of air and fuel that bypasses the normal throttle mechanism. Because of that, any attempt at getting meaningful vacuum readings while on "choke" is futile, at best.

.

Thanks Steve, I wasn't interested so much in vacuum levels when cold, but was wondering whether this may indicate the choke circuits are blocked. I'm thinking perhaps the ones with no vacuum are clear since they are overiding the slide height and the ones with increased vacuum may be blocked?

I guess the richness when warm may be unrelated but it wasn't there before rebuild.

[tkent02]

Thanks Steve, I wasn't interested so much in vacuum levels when cold, but was wondering whether this may indicate the choke circuits are blocked. I'm thinking perhaps the ones with no vacuum are clear since they are overiding the slide height and the ones with increased vacuum may be blocked?

I guess the richness when warm may be unrelated but it wasn't there before rebuild.

It seems like something clogged in there might do this...
Dunno, but it's pretty easy to find out.

[Guest]

It seems like something clogged in there might do this...
Dunno, but it's pretty easy to find out.

Right you are. I'll get the colortune out tonight and see whats going on at idle and maybe pull the carbs anyway to check out the choke passageways. Cheers

[Guest]

Update

Ok so wasn't able to get back to the bike til the weekend.

As a reminder I'm chasing down black smoke when the bikes hot. All the plugs are sooty black.

Checked with a colortune and found i was getting good colour on the pilot settings - bunsen blue - and when I accerate gently up to 3000 rpm or so colour stays the same, and more heavy handed throttle goes orange then blue as it's supposed to. Cylinder 4 was flashing orange at 3000 rpm so I decided to check the wet fuel level:

1 - 0.5mm
2 - 1mm
3 - 2.5mm
4 - 4mm

So the only cylinder with the correct fuel level was the one which showed it may have been too rich a float height???

I've now adjusted all the floats to a wet fuel level of 3mm.

She driving great warming up well enough to drive after a minute or two and pulling to redline really nicely. However theres still black smoke again when hot - especially when pulling up after a bit of 5 - 6k rpm accel and decel. And a blip of the throttle makes the idele drop if not stall.

GS850 cylinders and pistons with new rings and hone.
Foam Uni filter.
4 in 1 pipes and muffler (Formula 1 - aussie brand)
Pilot fuel screws are at 1 1/6 turns for driveability - below 1 turn won't idle.
Air screws 1 1/4 turns - highest idle and colortune. Any further out and idle drops and it pops on decel.
Main jet 102.5
Pilot 15
Needle in middle position whereas stock for this year is 2nd from top.

I tried the needle height to stock last year (2nd from top) and I had trouble synching the carbs and the idle raced as if very lean. But this was before the rebuild. And I didn't have the black smoke then either.

So I'm thinking of trying the needle height at stock again.

But I've also read that the foam elements can break down over time restricting airflow??? I don't know how old mine is.

Any other ideas appreciated.