Rebuilt Mikuni VM26's

[Mdelosrey]

Recently acquired a 78 GS750, Ran but needed a high idle (choke?) to stay running. Owner gave me an extra set of VM26's. Found a rebuild kit, cleaned em up, and installed all the new brass. I wanted to install these until I could rebuild the VM29's.

Installed them onto the bike but now it does not want to start at all. Checked spark and it has spark. I checked for fuel at the bowl, I cracked open the drain bolt and fuel came out. As far as setting wise, I set the slide at .030" gap. Air screw was seated and backed out 1.25 turns, Pilot jet is seated all the way in (unsure if this is correct), and needle is at lowest c-clip.

I noticed these VM26's have an extra brass port opposite of the choke plunger on carb 2,3, and 4. The VM29's only have these ports on carb 2 and 4 and the lines just led to ground (guess they're drains/overflow?). I'm not sure what I should do to the VM26 ports?

I'd appreciate it if anyone can help point me in the right direction...

VM26's rebuilt with Keyster KS-0231:

VM29's:

 

[Agemax]

the port on #3 carb is the vacuum port for the fuel tap. if you are not using a vacuum tap then that port needs to be blocked off.
1 1/2 turns on air screw, but what have you got the fuel screw set to?

 

[tkent02]

And don't break off the tips finding out!!!

That may be your problem already, it's easy to do.

 

[Mdelosrey]

the port on #3 carb is the vacuum port for the fuel tap. if you are not using a vacuum tap then that port needs to be blocked off.
1 1/2 turns on air screw, but what have you got the fuel screw set to?

Fuel screw same as Pilot (under carb)?

(VM26) So run #3 port to petcock? What about the ports on 2 and 4?

Thanks for your help!

 

[Mdelosrey]

And don't break off the tips finding out!!!

That may be your problem already, it's easy to do.

Wait what tips?!

 

[Agemax]

fuel screw under carb should be approx 1 turn out to start with and the air screw on side approx 2 turns out. use that as a base setting before tuning.
the ports on 2 & 4 are vents. leave as is or run the tubes same as the other carbs.
have you set the float heights? try adjusting the idle screw as well, may idle too high to start but you can lower it once the rest of the carbs are tuned in.
tkent was referring to the tips on the fuel screws. they can break off if you screw them in too tightly, before backing them out

 

[Mdelosrey]

fuel screw under carb should be approx 1 turn out to start with and the air screw on side approx 2 turns out. use that as a base setting before tuning.
the ports on 2 & 4 are vents. leave as is or run the tubes same as the other carbs.
have you set the float heights? try adjusting the idle screw as well, may idle too high to start but you can lower it once the rest of the carbs are tuned in.

Ok I will back out Fuel screw 1 turn and set Air screw 2 turns out.

I will lead port 2 & 4 to ground.

Float heights were set to 22MM.

Idle adjustment is handled through the knob next to where the throttle cable goes correct?

 

[Agemax]

correct on the idle screw.
float height should be 23-25mm.
also the clip on the needles should be in the middle, as a base setting.
what air filter set up are you running?

 

[Guest]

Carburetor rebuild kits are notoriously bad and might well be your problem. 24hr. soak in carburetor dip, replace O-rings (cycleorings.com) approx. $20.00, use all Suzuki parts if any replacements are needed. except gaskets and O-rings.

 

[Mdelosrey]

correct on the idle screw.
float height should be 23-25mm.
also the clip on the needles should be in the middle, as a base setting.
what air filter set up are you running?

Ok Ill pull the carbs back off and reset the float height to 24mm.

I can also set the clip on needle in the center.

Currently no filter, it has velocity stacks. Will order filtered stacks once I get it running.

Will update this thread after I make these changes. Thanks again!

Carburetor rebuild kits are notoriously bad and might well be your problem. 24hr. soak in carburetor dip, replace O-rings (cycleorings.com) approx. $20.00, use all Suzuki parts if any replacements are needed. except gaskets and O-rings.

Crap, I thought that these Keyster kits were decent quality since they were "made in Japan"

Hopefully the kit is not the problem...

 

[Guest]

You'll need jetting for a stage three jet kit for velocity stacks unless those carburetors have been re-jetted for that purpose already.

 

[Guest]

Leave 2 and 4 vent tube off if you are using velocity stacks or pods. Unrestricted air intake changes the entire equation.

 

[chuck hahn]

Actually Oldvet. Vm 26s are way more tolerant than Cv carbs side thay are mechanically operated and not vacuum operated. and he may NOT need rejetting..depends on what the bike runs like once he has it back together properly.Needs to get it running, set the valves, do the timing, get the carbs vacuum synced and then start fiddling with stuff.

 

[Mdelosrey]

You'll need jetting for a stage three jet kit for velocity stacks unless those carburetors have been re-jetted for that purpose already.

I would definitely rejet them if need be, but my plan was just to get the bike rideable with the VM26's and once the VM29 rebuild/cleanup is done, I will put those back on.

Leave 2 and 4 vent tube off if you are using velocity stacks or pods. Unrestricted air intake changes the entire equation.

Will do!

Actually Oldvet. Vm 26s are way more tolerant than Cv carbs side thay are mechanically operated and not vacuum operated. and he may NOT need rejetting..depends on what the bike runs like once he has it back together properly.Needs to get it running, set the valves, do the timing, get the carbs vacuum synced and then start fiddling with stuff.

Good to know.

Yeah at this point I just want it to start with these VM26's.

 

[Mdelosrey]

I went back to the bike a few minutes ago to give it another try with just carb cleaner sprayed into vstacks, I heard a little sizzling sound and found this!

Luckily I've been in contact with C5 Performance ignitions. They're working on making a new optical ignition kit for my bike. I guess I'll just have to work on the VM29's while I wait for the new ignition kit. I will still attempt to get the bike to run on the VM26's after ignition is fixed.