VM28 carbs

[gargantua350]

Hello All,

How are the new VM28 carbs?  I have seen them for sale in various places.   Mine are worn out and has bowl leakage that couldn't be solved with new gaskets. I think they are warped.  The slides seem completely worn out. 

My question:

with new carbs (vm28), Is the procedure for the DF mods the same as in the original carbs?  would I effectively get the same results?  Would it be better to just go with flat slides?  The bike btw runs great.  It's just these minor things I'd like to fix.  any opinions are appreciated.

set-up:

stock RD350 motor with Vitos crank
ProX pistons on std bore
Cheap cylinders from ebay
spec 2 classics
UNI air filters
HPI ignition at 1.8 mm BTDC
Main jet - 190
Needle #2 position from top
Pilot - #30
2.5 slide
1 turn out on air screws

[m in sc]

yes they will work but get genuine ones, stay away from ebay or amazon specials.

fwiw, summitracing has them for about 106.00 https://www.summitracing.com/parts/mku-vm28-49 but then youd have to set up for oil injection, jet, etc...

or, you can get the whole shooting match ready to go, jetted, fresh cables, etc, no guess work from john at economy.

https://www.economycycle.com/shop/yamaha-rz-350/fuel-and-airintake-yamaha-rz-350/carburetor-rebuild-kits-parts-related-fuel-and-airintake-yamaha-rz-350/carburetors-yamaha-rd250350400r5ds6ds7-parts-carburetor-rebuild-kits-parts-related-fuel-and-airintake-yamaha-rz-350/replacement-rd250-350-400-r5-ds7-carburetors-mikuni-vm-round-slide-series-28mm-carburetor-pair-includes-jetting-and-throttle-cable/

[gargantua350]

thanks Marc. Had no idea Summit sold bike parts.  I usually get my nova parts from them.  good to know

[m in sc]

neither did i . i was looking for some jets for a 4160 a few years ago and a mikuni jet popped up in the list. i was like 'wtf'?

[Perattzi]

I'm in the same boat, my carbs leak no matter what I try. New gaskets, orings, seats, viton tipped valves. Floats checked numerous times. 🤷
I'm ready to just order new ones for that money. I have the short top carbs with a 1 into 2 cable(premix)from economy or HVCcycle can't recall. Will the cable and all my Dave mods swap over just the same is my question? Thanks!

[m in sc]

yes

[Perattzi]

Thanks for confirming Mark. I ordered a set for $212 shipped from Dennis Kirk.
The right carb fuel inlet had become loose as well. 50 years old I'd say the carbs have fulfilled their duty. 🫡

[gargantua350]

Hello,

If 2 of the vm28s have left side chokes is there an interference issue?  I am assuming because of the absence of the cross over tube both carbs will need choke levers correct?

[m in sc]

there wont be an interference issue, but tripping the RH one will be tricky.

[Joe6v6]

I just got these carburetors fitted to my bike, brand new genuine Mikuni VM28's. Im haveing an issue hopefully I can get some direction here. When I open the petcock the gas flows & fills the float bowls but the fuel line itself does not fill back to the petcock. When it is running as gas is consumed I can see more gas going thru the line. This seems ok at low rpm, cruising at 4k it runs fine. Above 5k I can feel it running out of gas, sputtering. .  Is there a reason why gas would only flow until the bowl is full & closes the needle?
Opening the gas cap does not help, it is not a vacuum above the carbs
There are no filters, the tank is pristine, gas flows freely from the petcock with  the lines removed.
I know it is running out of gas at higher rpms because, I removed the fuel lines at the petcock & prefilled them and did not have an issue at all until after 10 min or so when the line level dropped back down to the bowl.   
 

[scully]

check float heights.... otherwise fuel will only flow to carb when the needle is open.

[m in sc]

yeah dont sweat air pockets int he lines, thats normal. as scully said, and make sure petcock is flowing well.

[Joe6v6]

yeah dont sweat air pockets int he lines, thats normal. as scully said, and make sure petcock is flowing well.

I understand air pockets are normal, but the whole line is air right up to the carburetor, its running out of gas at high rpm. My old carbs, original vm28's didnt do this. They filled up almost to the petcock. The petcock flow just fine without the line attached. .

[m in sc]

I mean, ive had new carbs w sticky floats as well. you didnt put the floats  in upside down by accident did you? my tms had issues for the 1st few days.

[Joe6v6]

I mean, ive had new carbs w sticky floats as well. you didnt put the floats  in upside down by accident did you? my tms had issues for the 1st few days.

I never opened them up before I put them on, im gonna have to take a look.