OzVMX Forum
Clubroom => Tech Talk => Topic started by: tony27 on October 02, 2016, 10:15:22 am
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What is a baseline setting for the float height on a new VM Mikuni?
I just bought a new 36mm to complete converting my CZ to reed valve & suspect the float height is a bit high, the overflow tube has a bit of fuel sitting in it & exiting corners the motor isn't running clean
I tried lowering the needle to the top clip which didn't really help & because I was running the motor in I couldn't bring myself to spend the time at higher revs to get around it
Really need to sort before I go back to work tomorrow night as the next meeting is on Saturday
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Float height won't be affecting that, your looking at slide cutaway and needle jet , if the needle is running at the 1 or 5 positions you need the next richer or leaner tube
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Tony check with hatrix Auz there base settings are very close
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From memory it is 17 to 19mm if it has the bent float arm. Definitely worth checking before diving into other changes, I've seen a few near new carbs lately with the float level to high.
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Float height certainly does have an effect Tony. Get that sorted before anything else. You can then start the tuning process proper.
As Mike says TD Hatrick will be able to help.
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Float height is easiest to check at the moment. Just found the float height mentioned in the specs on Sudco's site as 17-19mm so I'll see where it is in comparison
I was supplied a jet kit for running the reed which from memory has a 6DP1 needle where the carb came with a 6FJ6 needle which looking at the chart are quite different, the 6DP1 is richer if I understand right
Both the needle jet supplied & fitted to the carb are the same 159-Q5 & I'm pretty sure I won't have any others in the 159 length or time to get 1 before the weekend
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The 6DP1 is richer at the top end but slightly leaner at the bottom end. That's the one I would be using if it's blubbery off the slide.
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6DP1 is what is fitted at the moment, was supplied with the clip in #2 if #1 is the top notch, I tried the top notch & it was slightly better but still quite rich
Needles I could get delivered in the next couple of days are 6DH3, 6DH8, 6DP4, 6F5, 6F15, 6H2, 6DP10, 6N7. Quite cheap so I don't mind getting more than 1
Would you suggest any of these as a possible improvement?
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Hi Tony,
It's a bit hard without listening to the bike but I would probably pick the 6F5, followed by the 6DH3 and then the 6F16, the 6DF15 and 6DH8. I did a quick spreadsheet to compare them but I am O/S and don't have details on the 6DP4 or the 6H2, so that's about the best I could do. Like you say, they aren't very expensive and can go into the jets box for the future.
Pete.
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Tony
Had good results with 6F5 double taper 38mm Mikuni on the big bore ZEDS , no reed valve , running a 340 main Q6 needle jet
With a Reed have to come down on the main ???
Cheers
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Tony
Had good results with 6F5 double taper 38mm Mikuni on the big bore ZEDS , no reed valve , running a 340 main Q6 needle jet
With a Reed have to come down on the main ???
Cheers
Should be close then Tony as your Q5 needle jet is one step leaner than the Q6.
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Tony,
I bought a 36VM from MikuniOz. When i fitted it to my 125, it was flooding so bad that overnight, it filled the cylinder. I rang them up and he said the float was "factory set" and should be OK and not to touch it. I did some research and it was set wrong. The floats and everything are different than the older ones, so i don't think you can use your old measurements from the original workshop manuals.
I just checked my notes, and i had to set the float to 22mm, and i believe i checked it using this method on YouTube.
https://www.youtube.com/watch?v=PHuB1lyagKM
Cheers,
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Have ordered the 6F5 needle, hopefully it should be here Thursday as I will have ordered too late in the day
I'll check the float height tomorrow arvo before work to 18mm if it is less than 17mm, may just swap the arm out of the old carb that was used with the original cylinder & see what it measures.
I can't go off the Manual Alex as none off them give a float height for Mikunis as they weren't factory fitment even though they were commonly used
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Tony,
If you can't get a definitive measurement on the float height just lower it until fuel stops dribbling out the overflow. If the float level is too low it will give you a hesitation when you snap the throttle and it may well starve towards the end of conrod straight, but as you're probably not going to race it at Bathurst that won't be a problem. ;D
Pete.
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Been thinking about float height versus fuel level and it strikes me that they might not be the same thing.
That video above is measuring fuel level, but it might not correlate to a defined 'float height' measurement from a manual, depending on float design, float arm design and carby design.
Or have I completely missed that point?
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Been thinking about float height versus fuel level and it strikes me that they might not be the same thing.
That video above is measuring fuel level, but it might not correlate to a defined 'float height' measurement from a manual, depending on float design, float arm design and carby design.
Or have I completely missed that point?
Fred, as you know, a liquid always finds it's own level. So when you put a clear tube in the bottom of the float bowl and run it up the side, the level will be the same in the bowl and in the clear tube. When you compare them side by side, your tube should have fuel up near the top of the bowl as a rule of thumb. If the fuel in the tube was only half way up the bowl, or right up over the bowl, then you know your float needs adjusting.
I did mine on the bench, using some fuel going into the normal fuel hose to fill the carb, and using a clear hose to check how much was in the bowl (as my new VM36 was severely flooding). I just adjusted the floats until i got the correct height of fuel. Then i measured my float height and it was 22mm. The factory setting is 18mm https://jetsrus.com/carburetor/carb_mikuni_VM_series.html , which was causing the fuel to overflow the bowl and fill the cylinder. I am not sure why the bowl overfills at 18mm factory setting but it sounds like Tony has the same carb problem. I don't think a defined measurement from a book is that important. The factory probably did the same check as in the video, recorded the float height from that, and took that as the official float height and put it in their owners manuals.
Cheers,
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That is exactly what I was thinking. Fuel level should probably be around the top of the float bowl, but float height measurements would be dependent on the design of the carb, and the buoyancy of the floats etc. For example, if brass floats have been repaired with swags of solder, then the stock measurement will not work.
Bloke in the video was saying he was setting the float height using the fuel level, which are different terms.
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Same Same, but different...
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Decided to check on a hunch while changing the filter, I had cut the long bellmouth that the new carbs come with down to 15mm (roughly half original length) but shining a torch down the hole it looks like it may be making contact with the aluminum airbox
Would that cause the problem? Airbox is rubber mounted
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Finally got to try out the changes at a vintage atv event today, started out fine then rained while we were waiting on the line. Called it a day after the fourth race when it started raining again, felt I'd learned enough & have next round of the eNZed series next week
From memory I put the clip in the middle groove of the 6F5 needle as well as lowering the float height to 19 or 20mm. The blubbering has gone which I'm happy with but I'll put the float height back closer to 17-18mm as every now & then it felt a bit flat, normally after winding up through the gears uphill & through a flat up corner with a decent dip after it or down a straight that had a few dips in it
I had also turned another 3mm of the airbox side of the carb so there was no clearance issues
(http://i788.photobucket.com/albums/yy166/tony27_photo/CZ%20981%20random%20stuff/DH%20reed%20valve/4%20petal%20unit/Muddy_zpscah8pipi.jpg) (http://s788.photobucket.com/user/tony27_photo/media/CZ%20981%20random%20stuff/DH%20reed%20valve/4%20petal%20unit/Muddy_zpscah8pipi.jpg.html)