Help tuning VM26 carbs with Gunson Colortune

[Chuck78]

Trying to learn how to tune the pilot fuel and air screws on our GS's VM26's has been quite a learning experiment to say the least. I have an 844cc GS750 with a very very free flowing MAC 4-1 with a custom baffle I made, and K&N dual oval open element filters (like pods but way more filter surface area and airflow, 2 large oval shared filters with two carb inlets each).

I was told to start at one turn on the fuel screws, and 1-3/4 or 2 turns on the air screws. Others said 7/8 or 5/8 even on the fuel screws, but if more than 3 or 4 turns out on the pilot fuel screws are needed after pods and pipes, go to the next pilot jet size up. I messed around in this area but couldn't get a good idle or running condition. 1 turn had way too much hesitation.

I just read a post earlier, said to start at 1-3/4 on the air screw, and adjust the fuel screw until the flame (as viewd through the Colortune clear spark plug) was a perfect blue and then back it out just a hair more until you see a slight flicker of yellowish flame. Then rev it to 2000 rpm and observe the color and tune air screw for the best darkest blue before going to yellow.

I also will tell you that the colortune will allow you to see flame color changes as you turn the pilot screws as well.

You will turn the pilots till the flame is the described "bunson burner blue" at normal idle. Then I turn it out and look for just the SLIGHTEST filcker of orange/yellow. At this point, the pilot is ever ever so slightly rich.

Then you will fine tune with the side mixture screws at the higher RPMS where they are transitioning to the needles...usually around the 2,000 RPM range. I run the RPMS up to the 4000 to 4500 range and observe the flame after a second or two to allow for the cylinder to "settle down" a bit. Do each carbs screws and move to the next.

Well, with my nonstock setup, I figured 2 turns out on the air screw and 1 turn out on the fuel screw would be a good start. 1 turn out again proved too lean. Well, I backed the fuel screw out to 3-1/4 turns before I started seeing somewhat frequent yellow flashing but still mostly blue. I thought that maybe I wasn't seeing that it was really more white-blue at 2 turns so I kept going. I had an intermittent misfire (haven't checked this again since new plugs today), but it was mostly a nice blue flame with a yellow flash now and again, and I never got a solid yellow flame but closer to it by 4 turns out. I have stock #15 pilots still, and some say upgrade to 17.5. This could explain the excessive amount of turns on the fuel screw.

Do I need to do this in the dark to see blue vs blue-white flames? It seemed to make very little difference from 1 turn out until 3-1/4 turns out with the color of the flame. Carbs were dipped twice in 3 days, then in the ultrasonic for 20 minutes in simple green, all times blown out with 100psi of air after. New o-rings and gaskets.

No carb synch done yet as today is the first day of jetting and pilot settings that it actually ran respectably.

I ended up at about 2 turns out on both the fuel and air screws to get it running the best. Old plugs looked darker tan/brown, but after another 10 miles on the brand new plugs, they looked pretty nice and tan to brown on the ground strap and the porcelain insulator was very lightly tanned.

Still not idling perfectly smooth, but MUCH improved. These settings shoot down the baseline of roughly double the turns out on the air screw vs fuel screw. Am I doing something wrong, or is my stock pilot jet with extremely high flowing exhaust and intake filter legitimately giving me these far from the norm settings and flame color readings?

[chuck hahn]

I can see the way out numbers seeing you have all the mods and a 4 into 1.

The 3/4 to 7/8 on the pilots and 2 out on the mixture screws is for a stock bike setup.

No you don't need to be in the dark..its plenty just to be in the shade of a tree or at the front of the garage. Look for the bunson burner blue as you tune them. Remember you tune the pilot first till its just right at a high idle..like 1500 or so.

Then you check the higher RPM ranges and adjust the side mixtures to get both the pilot and the main circuits tuned good. Goona take some fiddling on the first carb, but once you get that first one down youll be close to do a baseline setup on the other 3..then it goes easy.

[azr]

When I use my colourtune I always do it in as dark as lighting as I can get, usually in the shed with the door mostly shut and the fan full blast. I find it really helps to see the colour difference. Why are haven't you done the vacuum synch yet? Morgan actually talks about doing both at the same time but I find that to be trouble some sort of. I always do my synch, then fine tune the carbs.

[Steve]

No carb synch done yet as today is the first day of jetting and pilot settings that it actually ran respectably.

Why not a carb sync? You could be trying to tune the mixture on a carb that is not open far enough to flow any fuel.

Why are haven't you done the vacuum synch yet? Morgan actually talks about doing both at the same time but I find that to be trouble some sort of. I always do my synch, then fine tune the carbs.

I fully agree that a vacuum sync needs to be done FIRST, then the mixture tuning.

.

[Chuck78]

I did bench synchronize the carbs when I rebuild them, but I thought that I needed to get the mixture in the ballpark before trying to sync them.that will be my next step then. I suppose that I should set up all of the pilot air screws at the exact same setting for synchronizing?

[rustybronco]

I suppose that I should set up all of the pilot air screws at the exact same setting for synchronizing?

And the fuel mixture screws as well before you synchronize the carbs.


Two turns out on both the fuel and pilot air mixture screws? In my opinion there is Sum-ting-wong...

[Chuck78]

yes I was kind of wondering why both screws were turned out the same as opposed to the air screw typically being twice that of the fuel screw. That was one of my concerns. Maybe I am just not reading the flame color correctly? I did not try backing the air screw out double as far because that would have been 4 turns, and the mixture color seemed pretty good with the screws turned equal amounts out

[rustybronco]

It 'might (?)' be caused by having the throttle plates open a bit more than normal. What RPM is it idling at?

[Chuck78]

1100rpm. when its colder, it idles slightly lower, not nearly as drastically as it did when I had the fuel screws 1 or 1-1/8 turns out. I had a slight hunch that it might be due to having the throttle plate open more than needed. I will try that next time I get a free minute to work on it

[MisterCinders]

With those mods, you very likely need to go up to 17.5 pilots.

Have you rejetted the needle and mains? No point synching and fine tuning the carbs if you will tear them down again to get to the needles.

Definitely synch the carbs before colortuning. You should be able to get synched and started with fuel screws out 1 turn and air screws out 2. Good rule of thumb is air screws = fuel screws x 2

[Chuck78]

122.5>120>117.5 & still a tad rich. Needles raised 1 notch but lower rpm's at higher loads (highway speeds or going up hills in 3rd gear or higher) have no power past 1/4 throttle until I get above 5500 rpm.. No surging, but flat acceleration or maybe bogging. Need to play with needles, maybe try swapping to a thinner needle if raising it to position 5 helps.

[MisterCinders]

122.5>120>117.5 & still a tad rich. Needles raised 1 notch but lower rpm's at higher loads (highway speeds or going up hills in 3rd gear or higher) have no power past 1/4 throttle until I get above 5500 rpm.. No surging, but flat acceleration or maybe bogging. Need to play with needles, maybe try swapping to a thinner needle if raising it to position 5 helps.

Forget RPMs, VMs are strictly a throttle position creature.

It does sound like you are a bit lean (flat) in the mid-throttle circuit. You might do well to shim the needle instead of jumping a full slot. A plug-chop at mid-throttle would tell for sure.

[Chuck78]

I am still struggling with this tuning, & I suspect that I need 17.5 pilots. With the air screws at 1.5-2.5, turning the fuel screws out quite a lot never seems to get me a yellow flame, but at 1.25-1.75 out on the fuel screws while turning in the air screws will definitely give me a yellow flame.

The problem is that I am getting an inconsistent flame. If I turn the air screw in, I have a wide range of maybe 1/4+ turn where it will be fairly blue but still lighter, but will flicker yellow every 5-10 seconds, AND I feel that there is never a good bunson blue flame like on the Colortune box's photo, to me it seems to go lean to rich from white-blue to darker white blue with occasional yellow flashes, to solid yellow.

The yellow flashes make me wonder it I am needing to do a 3-angle vslve job, as maybe the valve seats aren't sealing all that well 100% of the time, maybe they rotate a little & sometimes seal, sometimes not a perfect seal...overthinking it?

Any thoughts?

[Chuck78]

I replaced one leaking seat and all 4 needle valves, and lowered the float height twice (lower fuel level, greater distance float bottom to gasket surface). Maybe I should do all 4 seats with new? Swapped one from a junk gs1000 carb to fix a fuel overflow tube seeping issue..

I also wondered if maybe two dips in berrymans for days followed by an hour in sime green/water in the ultrasonic may have left some oxidation in the pilot passages? The ultrasonic did leave some of that on the outsides, but I sprayed tons of carb cleaner and 100psi of air through them afferwards.

[Steve]

I replaced one leaking seat and all 4 needle valves, and lowered the float height twice (lower fuel level, greater distance float bottom to gasket surface).

Just to make sure you know, but changing the fuel level by adjusting the float height will affect ALL of the circuits in the carbs. If you lower the level to lean out the pilot mixture, it will also lean out the needle and the main, but probably in varying amounts.

It is far better to start with something at the proper setting, then do all your jetting changes based on the proper fuel level.

.