Hesitation with my VM32's at certain rpm

[AzHonda]

As the title states. On my 74 Yamaha TX500. I have two vm32's. The mains are 185 and pilot jet is bumped up to 140. When I shift into each gear, things are fine until the rpms hit right where you need to shift (no tach installed). So I would say around the 5000-6000 range. What happens is the bike starts sputtering. Once I shift into the next gear it goes away until I hit that rpm range again. Once in 5th gear (no 6th) I can only get up to around 58-60 mph and the bike starts to sputter.
My thinking is maybe more taper on the jet needle? Or a larger needle jet? Those two items are still stock.

Ideas?

 

[teazer]

Pilot jets don't go to #140. That must be a typo.

At flat out wide open throttle the jet that controls most of it is the main jet. Try a larger main and see if it gets better or worse and then try a smaller main. Those carbs were never stock on that bikes, so it's a guess where to start. Please list all the jets

Main Jet
Pilot Jet #
Air jet #
Needle jet # and size
Needle #
Float needle and seat #
Slide #

How do the plugs look - white, black, wet, dry?

 

[AzHonda]

Pilot jets don't go to #140. That must be a typo. Yup. Typo. It is a 40

Main Jet # 185
Pilot Jet # 40
Air jet # BS30/97 2.0
Needle jet # and size 159 Q-6
Needle # 6DP17
Float needle and seat # VM34/39-3.3
Slide # VM34/110 3.0

How do the plugs look - white, black, wet, dry? Dry and brown with a little white

 

[teazer]

Try a much smaller main jet with that needle and needle jet to see if it cleans it up a bit - say 150 - 160 with a #30 or 35 pilot jet.

 

[cxman]

needle jet is a fire hose and dumping fuel

i would go done to a q-4 or q-2

that is probably most of the problem

 

[AzHonda]

Thanks guys!
Teazer-I have tried that option. I also have a couple other jet needles I have tried-6DH8 and 6N1

I'll go ahead and try the smaller needle jets. School me here. Why wouldn't I want to use larger needle jets? It seems to me that the engine is starving for fuel at those RPM's mentioned. Wouldn't a larger size let more in or would that throw everything off?

Thanks for the input guys. I know I will get this sooner or later and will have learned a lot more on carbs.

 

[teazer]

We believe that the jets are too large. The question is where is it too rich, 1/2-3/4 throttle or flat out or both. If it was OK wide open and rich in the middle I would agree try a smaller needle jet, but it sounds like it is rich all teh way to the top and that says the mains are too large.

If you already tried a 150 ish main jet and it made no difference and the midrange was so fat that you never got to WOT, then I'd try smaller needle jets. The thing about a needle jet change is that it has most effect (proportionately) at low to mid throttle.

Your two needle alternatives have different taper and taper start points and they are both leaner at small openings and richer from mid to 3/4 and up than the 6DP17, so that will confuse the heck out of things.

Rule of thumb: Get idel and off idle right first, then wide open (main jet) and after that work on the needle and needle jet.

I have seen CB450s with Q-2s and small main jets and others with 150+ mains and much smaller needle jets. The other thing is that you really would be better off with Bleed type needle jets. In this case #176 series in a slightly smaller size. In theory most of the fuel at 1/4 throttle up is coming through the main jet and when it's mixed with air the needle and jet restrict it at anything less than WOT.

That's why I like to get an hour on a dyno to get main jets spot on and then fiddle with the needle and jet afterwards.

Grab a copy of the Mikuni carb manual and any other books on bike carbs to start putting the pieces of the jigsaw puzzle together.

 

[cxman]

here is a working setup

main jet=200 (7mm hex style)
pilot jet=35
needle jet (primary type with shroud)= 159p6
jet needle=6dp17 with e clip in center spot
air jet= 2.5
K&N pod filters
Stock engine
1976 xs 500c

i have heard that dcc has a decent kit for your bike call them and ask for the base jetting

 

[AzHonda]

Awesome guys. Thanks for all that knowledge and time spent explaining things.

I do have the Mikuni carb tuning manual. It's been a great help. Cant wait to get this issue fixed. Been living with it for over a year now. Just can't stand it anymore.