Weird issue here with some VM24 carbs. I have a 79' 650 with pods (foam style, no inner lip) and a 4-1 header. I bought some VM24 carbs on ebay for it that were said to have sat on a shelf after the owner upgraded, and i believe it, they were pretty clean besides some corosion. I did a quick vinegar ultrasonic clean on them and then washed them out with water then gasoline... point is they are pretty immaculate. They have also been synced.
Problem is that they are running way richer at/below 1/4 throttle than they should be when they all have 15 pilots at 2 turns out (which should defintely be a bit lean for my setup). On one carb my AEM AFR gauge is pegging 10 (lowest it reads) with any throttle input over 0 up to 1/4 (above that is fine because thats into the needle/main jet). At idle it's at 12ish AFR. On another carb it only goes to 10.3 or so afr with throttle and 13 at idle. So basically i'm getting the exact inverse of the typical 1/4 throttle lean spot that we see with pods.
Seems to me that my carb bodies are damaged? The only thing i can think of is the vinegar ultrasonic clean, but they were only in there for 10 min. Also, the chokes are all the way down and the rubber seal at the bottom of the plunger looks fine, just a little imprint from the seat.
-I set the fuel levels in the bowls on all the carbs as well. 3mm +/- whatever from the bottom of the lip carb body.
-It has 1.75 CA throttle slides.
It sounds like you know what you are doing, so I am not questioning you. Things I might suggest to take a look at: fuel level in bowls, turn out the air screws further that 2 turns, install smaller pilot jets. what is the cutaway on the slide as it plays a part in the throttle range you are having an issue with.
Ok the fuel level sounds good. To the best of my knowledge all the vm24 carbs that came stock on (K)Z650 that had 1.75 cutaway slide came with accelerator pumps. Do these carbs have one? If so then make sure it is not pumping fuel before it is intended to.
I have not researched the size of pilot air jets used on the different carb sets as they are pressed fit and not threaded for removal or replacement like some of the larger carb have. Maybe smaller ones were used in the carbs with 1.75 cutaways
Ah dang it, forgot that too. Pumps were deleted by previous owner. I finished the delete by putting on some VM22 carb bowls and tapping the pumper pathway (that goes into the airstream) to install some little grub screw plugs.
Only other odd thing was a bunch of air sneaking past the slides during the sync. Normally the AFR doesnt change much during a sync compared to with the carb tops on. With these, the AFRs were all over the place when doing the sync. Had to tweak the sync screws then put the carb tops on and then check the sync Hope the throttle linkage seals stay good
I hear you about the air around the slides, but you are rich not lean. Where did you plug the accelerator pipe at. In the carb throat on the bowl side? Make sure there is no fuel getting by at that location.
Keep in mind that you have not only altered the air intake and exhaust, but also the carbs. Do these carbs have adjustable needles or do they only have a single slot on them? Some not all of the accelerator carbs came with needles that were not adjustable.
At this point I would try turning the air screws out farther. In general a rule of thumb is if the air screw is less than one turn out you need a larger pilot jet. If it is more than 3 turns out you need a smaller pilot jet. Please understand that is a general rule of thumb, not some absolute.
If that does not solve the issue I would then raise the needle by going down one slot. If the needle only has one slot then the use a a washer the thickness of the slot needs to be added below the clip.
I should of asked before what sort of AFR meter are you using. Also there should be a stamp just below the choke lever to indicate the carb set.
Looks like a ton of fuel is coming out of the pilot hole on the little throttle blips. I don't know what it normally should look like though.
I didn't say it before, thanks for stepping through this with me!
Ive watched my carb video a few more times. Looks like the fuel is coming from further in past the slide, which would have to be the choke? The pilot circuit comes out just past the main jet, but still "within" the slide, right?
As your raise the needle it makes for leaner mix. Are you using the needles that were in the vm24s or the vm22s? It is my understanding that the needle used in the 1208 set (5CL30) were non adjustable while the needles used on the vm22s (5CL10-4) were adjustable.
First, i used another carb bowl and blocked the choke port in the bottom of the bowl with a a bit of hot glue. AFRs on that cylinder did not change. So it's defintely not some weird choke issue.
second, i took an old main jet and soldered the end of it, essentially capping off the main jet. That cylinder idled at 16+AFR and, of course, immediately pegged full lean with any bit of throttle. I'm surprised the needle/needle jet have this much affect on the idle. Ill the diagrams ive seen say its 1/4 throttle or higher.... Anyways, good to know.
I pulled a needle out and took some measurements. No markings, but right off the bat its .5mm longer than a 5DL31 and it has 6 slots. These measurements dont match up to any 5 series mikuni needles. They are more aggressive than a 5DL31
b 23mm ish
When i got the carbs they were clipped to #4 and i put them to #5. I have moved it back up to #4 and will try that out. Putting the unknown needle at #4 side by side with a 5dl31 at #3 shows it should be pretty close except for the more aggressive taper on the unknown needle.
Also, i took inventory of my spare parts. I have:
4x y63 (same as 5DL31)
It probably is a dynojet needle then. I cant find a spec sheet with all of their needles and specs... not terribly important i guess.
So i got the needles back in and went for a test ride. It definitely needs a re-sync but off idle AFRs are much better.
Idle is still around 13.8.
1/8 is around 12.5 (used to be ~10.1)
1/4 is around 13.5
around 1/2 is a bit lean though. I saw some 14-15-16 AFRs.
Full throttle is ok around 13.8/14
What are your guy's thoughts? If i bump the needle up another clip, the 1/8 will be better, maybe even too lean, and the 1/2 throttle will definitely get too lean. Im thinking a half clip higher on the jet needle and go up to 115 from 112 on the main jet?
What outside temperatures are you dealing with? And are you testing in high humidity or very dry (desert) type environment.
AFR's change a bit with temp and humidity.
I try to do my tests around 70 to 75 deg on an average humidity day. That is about the midpoint of the weather I typically ride in.
Are you holding the throttle at those positions and letting the bike get up to speed?
At any rate, the your 1/8 is a bit rich. It will run fine but gas mileage will be a little low.
Your 1/4 would be ok if it was getting richer at larger openings, but I'd rather see 1/4 at about 13.
Your 1/2 is definitely too lean. 13.8/14 is more a target for 1/8 doing light steady cruising (60 to 70 mph).
If you twist and hold to 1/2 throttle, it should start out in the 11's or even 10's, and as the RPMs increase, the AFR's should sweep through the 12's and hopefully end up right around 13 at redline.
Be aware, most throttle stops are set so the WOT goes slightly past fully open. That is, the slide goes beyond the maximum opening of the carb throat. That is a little safety device to enrichen the mix at WOT in case someone is just holding it to go top speed. That's fine, but it gives a skewed representation of your jetting. Because just slightly off WOT, when the slides are exactly at fully open, it will be leaner than the throttle-stop position. You can adjust the stop for testing purposes, or just remember to not go to WOT and stop your hand just short of WOT for testing.
If your results were on my bike, I would lower the needle yet again, probably half step using a shim, then put in a larger main. If you see any throttle higher than 3/8 going beyond 14, let off the throttle and get a larger main.
Don't mix Dynajet main jets with Mikuni. Dyna jets are to be used with their needles and the numbers on them do not match Mikuni's numbers. It will drive you crazy trying to tune the carbs if you start mixing them,