Float height and richness

[Coady]

I've been chasing a richness issue on my 1979 GS750 with VM26 carbs.

She's driving great, warming up well enough to ride after a minute or two and pulling to redline really nicely. However there's been black smoke when hot - especially when pulling up after a bit of 5 - 6k rpm accel and decel. One rev at idle clears the smoke and it will not blow any more smoke when revving at idle.

I tried leaning out the needle 1 position but it ran really lean and the smoke still appeared when hot.

So I figured its the pilot circuit and have finally eliminated the smoke by putting the airscrews at 2 1/2 and then fuel screws at 1 1/2 to maintain idle, but plugs are all a similiar darkish colour.

So I'm wondering whether these pilot settings are compensating for the float height?

I've adjusted all the floats to a wet fuel level of 3mm. Specs say 3mm - 5mm for my year, but 3mm for earlier years. Does the float height have such a large effect? Should I try dropping it to 5mm or will this effect other throttle positions adversely?

Thanks



GS850 cylinders and pistons with new rings and hone (done about 500kms).
New dyna s with good spark and 12 volts at coils.
Recently cleaned and lightly oiled Foam Uni filter.
4 in 1 pipes and muffler (Formula 1 - aussie brand)
Main jet 102.5
Pilot 15
Needle in middle position whereas stock for this year is 2nd from top.

[MisterCinders]

If you are getting rich results at higher RPMs, wouldn't that be from upper end throttle? What size are your main jets?

Edit: I see you have 102.5. So nvm.

[tkent02]

However there's been black smoke when hot - especially when pulling up after a bit of 5 - 6k rpm accel and decel.

Can you explain this a little better?

[Coady]

However there's been black smoke when hot - especially when pulling up after a bit of 5 - 6k rpm accel and decel.

Can you explain this a little better?

OK. When fully warmed up, if I cruise along at say 4k rpm, only moderate throttle opening/acceleration, pull up at the lights and blip the throttle then no smoke.

If I fang it between lights (this is on my commute) with throttle 1/2 to 3/4 or even full, and then pull up quickish with some engine braking, if I then blip the throttle when stopped I get a single puff of black smoke...

Quick revs in neutral after this - no smoke.

Cheers

[OldVet66]

Try it without downshifting when you are decelerating. My old motor had marginal valve seals, and would smoke if I used the motor for deceleration.

[Coady]

Try it without downshifting when you are decelerating. My old motor had marginal valve seals, and would smoke if I used the motor for deceleration.

Thanks for the tip, but it's definitely black fuel smoke and not oil smoke (I became an expert on oil smoke before I replaced the rings) and I replaced the valve seals a year ago.

What's more, I have gotten rid of the black smoke by opening the air screws up to 2 1/2 turns - essentially leaning out the pilot jet. I'm just wondering whether this is compensating for richness somewhere else in the carb, or whether I should stop worrying and just run it like this?

[OldVet66]

My #3 carb will get stuck and squirt gas out of the overflow tube in a rhythmic pattern when at idle. If I doink the bowl with a screwdriver handle, it will start working right. It has only happened twice. It's possible one of your floats are sticking, but I think you would have seen some overflow by now if it is. If the bike has good pull off of idle and accelerates smoothly through Red Line, I wouldn't worry about it unless it gets progressively worse. Too rich on the mains will get you a mid range stutter and an unwillingness to smoothly accelerate to Red Line. Try riding without flogging it so hard and see if it continues.

[Coady]

Maybe I am being a bit hard on the old girl and whacking the throttle open pretty quick.

I've been thinking about your earlier comment about the valve seals though OLDVET66, and thinking maybe with a lot of vacuum on decel, fuel could get sucked through the choke plungers if the pads are marginal.

Think I'll put the air screws back to 1 1/2 and fuel screws to just under 1 turn like I had it before, and see if I still get still smoke with no engine decel.

Mind you I'm no sure what to do if this is the case as those choke rubbers are unavaible?

Thanks

[Coady]

update

Just a follow up for anyone that may be searching this stuff in the future.

Basically my pilot fuel screws were out too far making the black smoke. Set them to 3/4 and air down to 1 1/4 and goes great (still need to fine tune these settings).

Kinda obvious really but I'd been stuck on old settings that worked before I overhauled the top end and replaced the dubious points ignition and reset the float levels.

Previously I'd always needed air screws out atleast 1 3/4 to eliminate rich bogging off idle which I now believe to be caused from too low a float height - only checking the wet fuel level showed how far out they really were. So I'd been compensating for the wrong fuel level with the air screws and then needed to open the fuel screws to maintain an idle...

Guess I'll do some plug chops and see if I really have sorted the richness issue.