Jetting for Mikuni vm34

[CJF]

Could use some help getting in the ball park for the right jetting. I have a set of vm34 on my T500 race bike. The stock settings from sudco are: 260 main, 35 pilot, Q2 needle, 6DH4 throttle needle, 3.3 cut away. I had it on the 3rd notch from top neddle setting. The bike has '72 cylinders (stock ports) and Jemco race chambers. I am running open carbs with just mesh covers on the intake (no airbox). The bike idles perfectly and starts right up. The problem is that it is dragging from 1/4 throttle on up with no real acceleration and just kind of bogs throught the entire throttle positions once past 1/8th to 1/4 throttle.

[ja-moo]

Q-2's have shown to be pretty big for a 250cc cylinder. A P-4 to a P-6 is probably what you will end up with.

[CJF]

Thanks...I was also thinking this is where i needed to go. I've tried 2 larger sized jet needles and it has gotten a little better. Next, I'm going to try the small needle jets hoping that gets me the rest of the way there.

[diamondj]

Chris,

Sent you a PM on this but I don't know if it actually sent or not - sitting in my outbox now.

Anyway, Ralph Spencer (Desmocat) passed along his jetting on his T500 racer to me:

Pilot - 35
Cutaway - 2.5
Main Jet - 260
Needle Jet - 159Q0
Needle - 6DP17 in second groove from top

He also told me later that the 36mm Mikunis from Sudco worked even better straight out of the box.

Hope this helps,

Jim

[CJF]

Thanks for your help Jim...got your PM. I ordered the Q0 needle jet and will try the jet needle you ahve listed if the needles i have don't work. Appreciate the help.

[tgrogan]

Sorry to be so late. First, if you have racing chambers and stock porting, you are asking for all sorts of mixture problems. Second, 260 main jets are for heavily ported engines - I used GT750 race porting and seldom went over 230s and always ran fairly rich. Third, you should not worry about low engine speed performance when racing. Needles, needle jets and slide cutaway are just non-issues. Just be able to keep it running while waiting for the start - then it's WFO only. It's enough of a problem to get high speed jetting dialed in.

[CJF]

This is where I have landed with jetting to date:
260 main
Q2 needle jet
6D17 jet needle

It runs nice through the entire throttle range with the exception of a slight flat spot at around 3/4 throttle, then seems to clear at full throttle. I'm hoping the jet needle position will clear this or a combo of the jet needle position and a smaller main jet. To your point, I may need to drop the main down some. I have mostly gone from feel of the bike while riding to get to this point and have only done a few plug chops. the chops I did looked pretty good, but may have been a little on the rich side. The plug color looked like a dark brown as opposed to a milk chocolate brown.

[tz375]

With a plug chop at WFO throttle I'm looking for an almost white tip to the insulator, a 1-2mm wide mixture ring deep down inside, no sings of cement boil, no sign of overheating on either electrode, a clean discolored band on the side electrode.

3/4 throttle richness is probably the needle if the mains are close. I find my lap times are much better when I get transitional jetting right. It makes it so much easier coming out of corners hard when I can roll it on smoothly. Faster riders may not share that need, but for me, the easier the bike is to ride, the smoother and faster I go because it inspires confidence.

[CJF]

Yep, I definitely need to lean out the main if that is the coloring I should be looking for. I agree with you on the needle. My only problem now is that it is 30 degrees out and i don't want to freeze my arse doing plug chops!!

[tz375]

30 degrees - we'd like that. It's 10 here.

[diamondj]

Won't your jetting requirements change when it gets warmer? Unless you are going to race in 30 degree weather?

[tz375]

Yes Jim, they will change quite a bit when the air is warm and less dense. If you have a weather station, that's OK, just dial in the numbers and let it work out the rioght jets for warmer air. If not, it's probably better to wait or run it on a dyno in a warm shop.

One thing I forgot to mention earlier, and I'm picking up on what Tgrogan said, and that's get the main jet right first and then work on needle and N/J - assuming it idles OK.

The main jet has an effect all the way down the rev range at full throttle and it also impacts part throttle down to about 50% or less. So that's the jet to focus on and then fine tune part throttle

[CJF]

Hmmm, now you tell me . I have been working on the jetting from the opposite direction trying to get the 1/4-3/4 right and then once in a while checking wfo throttle to make sure it is staying consistent in terms of throttle and engine response. I'll work on the main jet plug chops next to try and get close to the color you mentioned and then work my way back down to see what might need to change. I'm beginning to not like winter anymore!!

[tz375]

Chris, and of course you must work down from too rich towards the right jet size not up from too lean.

I should have picked up on that earlier. The sequence I use is:
Idle and just off idle
Main jet
.
.
.
.
.
The rest
Main jet again to be 100% sure I didn't screw something up