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VM30 carbs

Started by RDSingh, May 26, 2020, 06:22:21 AM

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RDSingh

Hello, I want to change carbs on my 78 rd400. Its a ported setup. Stage 1 with Yz85 reeds and rz350 cross-over. Bike has spec 2  exhausts. Could anybody tell me the jetting numbers for vm30. Read many positive reviews regarding tm34 also. So just confused with what to use vm 30 or tm34. Not sure if tm34 are good option for stage 1.  :umm:

Seizer

Here's a good baseline for VM30's:


3.0 cut slide
6DH3 needle
159-P2 needle jets
2.0 air jet
45 pilot
280 main
40:1 premix


motodreams

I was also after this info.  There was quite a bit of discussion on the old 2-stroke site and broadly speaking they approved of the VM30s as best for the 400.


Yamaha 179

That's goofy.  For street riding, even hot street riding, with up to Stage 2 porting work Mikuni VM 28s are fine.   Tractable around town and runs well on the big end.  Raced with them for years with Stage 3 top ends and chambers; never a problem.  You can use a Y boot filter easily and there is a lot of jetting info available. 
Lyn Garland

motodreams

I honestly think you were involved in the discussion regarding VM30 on the old forum Lyn and may have even provided the specs.  This is likely the Spec2 'hot' porting which I have no clue on the specific stage name.  Bike has pipes ignition etc.  Daytona 400.  So you think sticking with VM28's is a better decision that running VM30s?

m in sc

here's my much less experienced take on it (compared to  lyn):

i've seen a 70 hp rd with 28mm carbs. it absolutely can be done.  -however-, when there's no race class restrictions, it opens up other options. the larger carbs will, in my experience, let the motor get into the powerband quicker with less efficient porting work. its kind of a cheating mod, the bigger carbs, in some ways. also, bigger carbs just look meaner , which has no performance value. also, you really really know how to tune for big hp with smaller carbs as well. they are more sensitive to changes IMHO.

a 30mm carb isn't  a 'big' carb at all, so id def consider it.  IF i were to go with smaller carbs on a build, id go with pwk 28s or 30s. the easiest carb to tune IMHO and very, very effective.

Or, stick with the 28's, its simple for sure and can work extremely well. Boils down to preference.


SUPERTUNE

Quote from: Seizer on May 26, 2020, 10:24:29 AM
Here's a good baseline for VM30's:


3.0 cut slide
6DH3 needle
159-P2 needle jets
2.0 air jet
45 pilot
280 main
40:1 premix

This is a great baseline I use for 30mm carbs.
Chuck
RD machine work, boring, porting, cranks and engine building.


Chuck 'SUPERTUNE' Quenzler III
Team Scream Racing LLC
1920 Sherwood St. STE A
Clearwater, FL. 33765
cqsupertune@tampabay.rr.com

Yamaha 179

There's a lot that goes into changing carbs and I think that you should look at the whole picture.  I don't think anyone can fault the VM 28's for all round high performance street riding.  The single action choke for both carbs, the ease of fitting good air filters, use of oil pump, availability of parts, and information are all big pluses.  The problem may be available RD carb bodies.  I haven't tried to buy any RD carbs but they may be scarce.  I know SUDCO still lists VM 28s in their current catalog but they are not RD types.  Problems going to VM 30s include replacing the reed manifolds, throttle and throttle cables, maybe air filters, and oil delivery systems.  Only the pilot and main jets will cross over from the VM 28s to the VM 30s.

The limiting factor in the RD engines is the transfer port system.  The transfers just aren't very good.  They can be made better but it takes a lot of work to improve them very much.  Usually a Stage 1 or a Stage 2 porting job does not involve a lot of transfer work.  Raising the ports, blending them and matching the casting to the liner sure; but changing and redirecting by changing the wall and roof angles is unlikely.  That is serious work and takes time and effort; more like Stage 3 or the ultimate job.  A light job on the transfers will not require 30mm carbs. 

So, is it worth the extra money to put VM 30s on in place of 28s?  I think not.  I'm old school but if I were thinking of changing carbs on a mildly tuned RD 350/400 I would put a flat slide 28.  Probably be a much better use of your money. 

Disclaimer:  Couple - three years ago our rider won the WERA vintage Formula 250 championship with his RD 250.  He had VM 35.5s on it for one race and VM 38s for three races.  Significant transfer porting work was done and the carbs had dividers on both the intake and output sides to improve airflow.
Lyn Garland





m in sc

to Lyns point & play devils advocate:

remember, rz350s stock come with 26mm carbs. Just food for thought.  :twocents: :vroom:

Jspooner

FWIW - the 350 I just sold had stage 2 porting by Spec II and I had VM30s on that. I have no leg to stand on as far as a comparison between those and the stock VM28s but I will say it sure ran good with the 30s on it. That thing did not stop pulling until you let off the throttle and they did not make it hard to ride at lower speeds and RPMs. I would not hesitate to put 30s on one again. With that being said, If I had a good set of RD carbs I'm not sure I'd replace them just for the hell of it but if I had to buy carbs anyway I'd probably go with 30s in either variety (TM or VM).
"Just quit brain fucking it and get it done"

Jspooner

In Lyn's defense, there other things to take into consideration when switching out carbs like, cables, autolube ports, carb spigot size, physical carb body size, choke (two levers VS one) so its not just swapping out carbs and back in business in a couple of hours, you need to be ready and able to make other changes.
"Just quit brain fucking it and get it done"

rodneya

Quote from: m in sc on May 26, 2020, 11:28:02 PM
here's my much less experienced take on it (compared to  lyn):

i've seen a 70 hp rd with 28mm carbs. it absolutely can be done.  -however-, when there's no race class restrictions, it opens up other options. the larger carbs will, in my experience, let the motor get into the powerband quicker with less efficient porting work. its kind of a cheating mod, the bigger carbs, in some ways. also, bigger carbs just look meaner , which has no performance value. also, you really really know how to tune for big hp with smaller carbs as well. they are more sensitive to changes IMHO.

a 30mm carb isn't  a 'big' carb at all, so id def consider it.  IF i were to go with smaller carbs on a build, id go with pwk 28s or 30s. the easiest carb to tune IMHO and very, very effective.

Or, stick with the 28's, its simple for sure and can work extremely well. Boils down to preference.

I have to agree that bigger carbs look soo much better, even more so if they are flatslides or Lectrons.

RDSingh

Thanks for great info regarding vm 30. What is the base line jetting for tm34
.

Hawaii-Mike

To save money, try UFO's on the VM28's.  I used them on my H2 (which happens to have VM30's) and the UFO's smoothed out the midrange power significantly.

rodneya

Quote from: RDSingh on June 01, 2020, 04:00:46 PM
Thanks for great info regarding vm 30. What is the base line jetting for tm34
.

Not sure for a 400, but I recently bought a set of TM34's to use on a banshee 421 motor with pods and Shearer pipes and came up with the following online.

change 389Q2 needle jet to Q0
300 to 310 main
40 to 50 pilot