Carb 1# is too rich dispite resetting the fuel/air screws

[blittlej]

I have a 1979 GS1000EN. The previous owner had the carbs dipped and can cleaned. 2-4 have perfect color on the spark plugs. Always a lite tan. The 1# carb is alway too rich. So, I have started with seating the air and fuel screws. The fuel screw is out 3/4 of a turn and the air screw was out 1 and 1/4 turns. But, I have the air screw out 1 and 3/4 turns. Stock exhaust and air cleaner. It is still too rich. So, it maybe that the richness in not in the pilot circuit? Where would I go next? Or just continue with adjusting the pilot circuit?

[Gorminrider]

I'm no expert on carb tuning but just to do some of the typing that experts can copy and paste ,

The first question must be: does turning the idle mixture in and out have any effect? You need know that the idle mixture is mostly about idling-that is, lower rpm. It looses its relevance as a proportion at higher rpm. This is harder to discover on a 4 cylinder bike than a two cylinder but I'd start with a vacuum gauge or maybe a special spark plug that shows the flame ("colortune")

given you can say or even guess that it is NOT "working", you have to revisit cleaning the passages,the idle jet and the screw itself.

Given you can say it is working, AND/OR you need then to move on to non-functioning "choke" (enricher), carb sync, main jets(clean? worn? sticky slide in CV types?) fuel level (float needle and seat) and whatever else you can add to these.

simplest of all, maybe just swap your spark plugs around. Try Easy first. ..and, I forgot to ask "How bad is it?" wet? or just a bit of soot, which is my assumption-oilygummy is somewhere else altogether and that'd ask for different tests.

[Big T]

Things to check:
Is the pilot fuel hole larger on #1?
Someone may have tightened it down hard and expanded the hole
Are the pilot jets all #15?
Are the main jets all #95?
Are the needles all in the same position, or is one shorter?

The biggest problem with older bikes is previous owners

[blittlej]

Thank you both for your input!!! I will tear into carb 1# this weekend. I was hoping that I left something out. But, I need to check the size of the jets and etc.... Did not want to take off the carbs again. But, no way around it.

[Big T]

Well, now that you're an experienced carb guy, you can do a tear down in no time

Just write down your findings

[Gumpster]

I would tighten the fuel screw ( bottom one ) 1/4 turn and adjust air screw to highest idle. As mentioned, that carb might have an enlarged pilot fuel hole. VM carbs use the pilot circuit through the entire range and from my experience, are more critical than on CV carbs. Best of luck!!

[Spyder]

Have they been vacuum synced, and are the floats set to the same height? It is not uncommon for the mixture adjustment to very from carb to carb. If you are convinced it is the idle/pilot circuit, I would just lean that carb out in small increments until you are burning clean...

[chuck hahn]

Negative on Gumpsters post. VM carbs use the pilots very little in throttle positions ( grip twist ) past 1/8 turn. And secondly the pilot screws ( bottom ) are set and forget. 3/4 to 1 out MAX!!!.

VM carbs are richened or leaned via the AIR screws on the sides at about a 45 degree angle. Turning them IN will richen that cylinder and OUT will lean that cylinder. Turning them in decreases the air in the mixture and turning them out increases the air in the mixture.

NOW..IF you cant lean the cylinder out no matter how much you turn the air screw out then youve got to do the wet fuel level exam on that carb. So many folks think that if the service manual says the float height is XX.X mm then there isnt any way the fuel can rise too far in a bowl. This is simply NOT true. Float hts get you "within range" but the wet fuel level will tell the truth.

And finally you gotta be certain the float needles dont leak and be sure the float seat seal ( fiber or metal crush washer on most carbs and orings on CV carbs ) also dont leak past the float seat.

[Gumpster]

image.jpgThe attached illustrates how the pilot circuit affects total fuel through the entire range.

[chuck hahn]

its of no consequence past 1/4 throttle...this aint my first day. It helps transition but its NOT the cause of the posters isuues.

[chuck hahn]

http://zeus.mtsac.edu/~cliff/storage/gs/GS1000.pdf

start on page 8-3.

[Gumpster]

Second day?

[blittlej]

It was the pilot fuel screw. I went down to 1/4 turn from 3/4. So, I think it is an oversized pilot fuel hole in carb 1#. Now it is too lean (looking at the spark plug). But, that is fixable. The spark plug looks good and does not have any popping. But, the bike something stalls after coming off the highway and waiting for a stop light. I have seen several posts that say to adjust the Air screw to max idle. But, on a 4 carb setup, adjusting one air screw on one carb does not make a difference that I can see. I am adjusting by small adjustments in the air screw. Cleaning the spark plug and then driving it for thirty miles and looking at the spark plug again. Another guy at a motorcycle show said he does it with a temp gun at the tail pipe. Getting both exhaust pipes to be the same temp.

[Gumpster]

Good to hear the progress. Little tweaking and you're there.

[KiwiAlfa156]

VMs fitted with air and fuel screws are fussy to tune. But having spent many hours adjusting them, here's what I've learnt. The pilot system governs idle and off idle performance up to around 1/8 throttle. But off idle performance is a transition to the needle jet, so this too affects off idle fuelling. I spent fruitless hours trying to tune the pilot circuit to tune out an off-idle (just cracked throttle) richness until a specialist mechanic told me to look at the needle jets as the needle wears them oval. Sure enough that was the problem. Moral of the story is that carbs don't read tuning manuals and all the circuits do effect each other to a more or lesser degree.

Back to fuel screw/air screw. The air screw and pilot jet determine the pilot circuit fuel mixture. That's all it does. The jet meters the fuel, the air screw adjusts the amount of air and thus mixture strength in the pilot circuit (some carbs use an air jet rather than a screw presetting this mixture.



Important to note that once mixed the fuel is often described an 'emulsion'. This emulsion helps the fuel atomise when it enters the inlet.

Where it enters is through one, two or more 'by-pass' orifices - which 'by-pass' the main circuit. One of these by-pass orifices is the pilot orifice and is metered by the fuel screw. It meters some of pre-set fuel air mixture entering from the pilot circuit. Most of the mix with the engine on a closed throttle and progressively less (as a proportion of the mixture entering the engine) as the slide rises -the other by-pass and needle jet (really all in combination) taking over. The by-pass is there because the air flow under the just open slide isn't sufficient to create enough vacuum to pull fuel up through the main and needle jet, let alone atomise it. More by-passes are required to meet increased demand as engine speed rises, finally being supplimented by the needle jet, which then takes over the lion's share of supply from that point until the main jet is the flow limiting factor. The fuel circuits aren't discrete, they play as teammates.

So the best way to think of the screws when tuning, is air screw is overall mixture for everything that comprises the pilot circuit-too rich may affect fuel atomisation, and the fuel screw adjusts how much is going when the throttle is closed or barely cracked open.

Some folk think screw settings is a numbers game and all the settings should be unform across the carbs. The whole point of the screws is to tailor the idle mixture to the variations of each cylinder.

With the above in mind, the method I use is setting air screw mixture at high idle using a color-tune when the fuel screws impact is less. Then tuning the fuel screws at normal idle when nearly all the mixture is passing though it.