Finessing Carb Tune

**Nessism** · 10-03-2023, 07:58 PM

Have you checked fuel level using the clear tube method? If not, that's where I'd start.

Where are the pilot fuel and air screws set? The fuels screws should be set about 3/4 of one turn open, and the air screws twice that.

Vacuum synced yet?

BTW, the main jets are ~~95 stock. If your bike has 75's, that's a problem.~~ Edit: I didn't realize you had a 550. Never mind with this last comment.

**KiwiAlfa156** · 10-03-2023, 09:39 PM

Stock air box or pods? At what speeds, gears, load and throttle position does it seem to be carbureting badly? Just at idle, or at WFO as well?

Partzilla lists the std main as 80 size for the VM22s on the 78 GS550 B, C, N and EC/EN with optional main jet sizing from 75 to 85
https://www.partzilla.com/catalog/su...or-gs550b-c-ec

75 is the leanest in the range of stock options, and might be a choice for high altitude (2 sizes leaner than stock), at full throttle and high rpm. But only a AF ratio sensor and gauge or WFO plug chops will confirm or disprove this.

In the pilot (slow running) circuit, the air screw sets the AF ratio for the entire pilot (slow running) circuit in tandem with the pilot jet. So from just off idle to 1/8th or slightly more throttle the bulk of the fuel is coming through the pilot outlets, rather than the mains/needle jet. The fuel screw (on the bottom/engine side of the carb) only trims the pilot outlet which -in the VM22- has a diameter of 0.6 mm when not obstructed by the tip of the fuel screw. The other outlet on the pilot circuit, the bypass, which is un-trimmed has a diameter of 1.0 mm. See https://gsarchive.bwringer.com/bikec...at_height.html

In the case of the VM22 the pilot outlet has an area of 0.28 mm? fully open. The bypass has an area of 0.79 mm?, so the fuel flow from the bypass is significantly greater than that of the fuel screw controlled pilot outlet. With the throttle closed and the engine idling, the fuel screw is in full effect, as soon as the throttle is cracked the by-pass will flow the lion's share and the position of the fuel screw has limited effects on pilot mixture.

The fuel screw trims idle mixture only. The air screw on the side (airbox side) effects up to around 1/8th throttle. My method is to do a plug chop (hit the kill switch and clutch simultaneously and cruise to a halt, the pull the plugs on the side of the road) cruising at around 1/8th throttle for 1/2 mile or so and adjust the air screws to correct the color of the plugs. It may take a few runs. This is easy because you can do it at legal speeds.

Once you are happy with the color when cruising around on the pilots, then you can get back to the garage to set the fuel screws for idle. Either using a color-tune plug (my preference) or adjusting each fuel screw to attain the highest smooth idle (my least preferable). The engine should be at operating temperature and being cooled by a large fan to do this.

**swishalump** · 10-04-2023, 05:42 PM

Thanks for all the info! Stock airbox on there and carbs have been vacuum synced. Fuel screws are one turn out and air screws are bottomed out across the board. I understand the standard is two turns out, but I have been having a tough time getting a good idle and off choke once I start messing with them. Nessism, are you checking float bowl fuel height with the clear tube method or gas tank? The tank was topped off during all of this. Idle sounds ok, it's an aggressive throttle snap from idle to 1/4 or 1/2 on that almost kills the engine. When I'm smooth on the throttle after 1/8 throttle to WOT that produces intermittent and stuttering power, but doesn't get anywhere near killing the engine. 1st feels the worst. 2nd the best, 3rd and 4th are still meh. When I'm real smooth and gradual, no problems overall.

**Nessism** · 10-04-2023, 07:07 PM

You are off the playbook when it comes to your pilot screw (fuel and air) settings. Given that, I'm at a loss as to what to suggest.

BTW, the clear tube test is detailed in the service manual.

**Who Dat?** · 10-04-2023, 08:30 PM

One thing that I see missing from your original post (and your profile) is ... WHAT BIKE???

There have been some guesses and/or assumptions, but it really is nice to know.

**swishalump** · 10-04-2023, 09:02 PM

How do I add a signature with my bike? It's a 78 GS550

**KiwiAlfa156** · 10-04-2023, 10:12 PM

I'd start with Nessism's fuel and air screw settings as a baseline. Be VERY careful when seating the fuel screw the pilot outlet is just 0.6mm and the and the brass fuel screw tip is very fragile and prone to snapping off.

It might be a good idea to pull the carbs off (I know it's a pain with the stock airbox) and just check that the pilot circuit is clear in all the carbs. If you remove the fuel screws (keep then in order so the go back into the same carb) you should be able to see right thru the pilot outlet (or shine a torch thru it). When you put the screws back in you can seat the screw and visually see the tip of the screw close off the pilot outlet. This will give you the feel of what the screw bottoming is like without the risk of buggering it. Back it out 3/4 of a turn and your baseline is set.

If you squirt aerosol carb cleaner with a straw on the button in to the pilot air bleed in the bellmouth (with the bowl off and slide raised) you should get streams of cleaner squirting out the pilot jet, the pilot outlet and the bypass. This is positive proof that the passages of the pilot circuits aren't blocked.

If the air screws are bottomed then the pilot mix is gonna be very rich. Air screw in reduces the volume of air mixed with the fuel metered by the fixed pilot jet and feeding the pilot circuit outlets. Turning an air screw out, adds more air to this fuel and leans this mix out. At some setting the air screw will give the correct amount of air for the fuel passing thru the pilot jet. This fuel mix (+ air passing under the slide) is what you are running on up to about 1/8 throttle after which the needle and needle jet start adding fuel air mix from the main circuit

Remember it is this pilot mixture, set by the air screw, that the fuel screw trims. Which is why it makes sense to get the air screws correct before trimming the fuel screws.

The reason it probably runs on the 'choke' (which is more accurately a 'cold start enrichment' circuit and is in functional terms a seperate 'mini-carburetor' that sits beside the main venturi), is that the 'choke' even though it's adding more fuel it's also adding a lot more air. So the mixture overall, though rich, is leaner than what you are feeding it thru the pilot circuit with the choke closed.

If you get the pilot mixture sorted, then you can get into the upper rev range to check things there. When this is sorted, you might (might not) have to re-address the screw settings

I would also return everything to the factory settings jet and needle clip wise as the bike is pretty much standard. 80 main, middle clip, etc, as this is likely to be really close to what you need. Stock pipes or after market?

**swishalump** · 10-04-2023, 10:24 PM

Thanks Kiwi! Just did a full clean, Berrymans soak, ultrasonic cleaner, air compressor to clear out passages. I'll start with the fuel screw adjustments and see what happens!

**Nessism** · 10-04-2023, 10:40 PM

Originally posted by swishalump

...air screws are bottomed out across the board.

If the bike isn't running well for some reason with the screws open, then there is something wrong.

**swishalump** · 10-06-2023, 02:23 PM

Got the stock exhaust on there, needles are where I found them, middle settings. This bike was running and ridden frequently 2 years ago, so I assumed the carbs after a clean and fresh o-rings would be pretty straightforward, but here we are!

**KiwiAlfa156** · 10-06-2023, 04:40 PM

Stock pipes and air box. So stock jetting/screw settings should be correct. Also have you checked that the choke plungers are clean and closing completely? If you can pass aerosol carb cleaner thru all the carb plumbing (this gives you a visual confirmation that compressed air doesn't). I'd reassembly them with the fuel screws at 3/4 and the air screws at 1 and a half out.

Just a note with your comment "

...it's an aggressive throttle snap from idle to 1/4 or 1/2 on that almost kills the engine.

The VMs are slide carbs and the throttle slide is directly controlled by the twist grip as opposed to CV that use the vacuum of the air passing thru the carb to lift the slide. With the CV the twist grip controls the air flow via the carb butterflies. The reason for this indirect connection to the slide is to stop the slide being lifted too quickly as this will destroy the vacuum in the carb and stop the fuel flow and the mix goes lean and the engine dies... momentarily.

Long story short, aggressively whacking the throttle open from idle on slide carbs and having the engine momentarily stall is 'normal'. Accelerator pumps are fitted to some carbs in order to alleviate this momentary leanness. If the idle mix isn't right (or timing is out) it can exacerbate the stalling/dying behaviour.

The ignition timing and ATU operation (spark advance) is another thing you might want to look at after you get her idling.

**jive060** · 10-08-2023, 11:41 AM

http://www.factorypro.com/tech/tech_...le%20/%202k-3k)

Try to follow this

Stay stock before your try new directions bt changing things

Finessing Carb Tune

Finessing Carb Tune

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