Vanguard 23 Carburators

[slonomo]

I thought I'd do a little aside to explain what I've learned about the Mikuni VM carbs. This is new information to me, as I am learning on the fly. Please do not judge my knowledge (or lack of) and if you know something I don't, please contribute.

Mikuni VM34 34mm

There are different versions, and from what I can find there are 3 different versions. Each differing model was designed with OE MFG specs in mind.

The difference, in general, has to do with:
1. Where the fuel mixture screw is located
2. Where and what type of choke is used
3. If there is a main jet port on the bowl or not

Tuning:
Each type is tuned the same, in theory, but there are two different mechanical differences, speaking of the fuel mixture screw specifically.

On most carbs, the fuel mixture screw should be set around 1.5 turns out at the beginning of the process. This is the starting point at which you need to keep track of how many turns you are at. 1/4 to 1/2 turns are used to adjust this circuit, at operating temp.

After an adjustment, you have to wait 10-20 seconds to allow the engine/carb to catch up with each other. Note whether the idle sound got better/smoother or worse/stumbles.

Keep track of how many turns you are at.

Fuel Mixture Screw types:
1. Air - Mixture Screw - located toward the "air intake side" of the carb
2. Fuel - Mixture Screw - located toward the "engine side" of the carb

The Air-Mixture Screw
This screw limits the amount of "air" allowed to mix with the pilot jet fuel supply. The amount of fuel passing through the circuit is fixed. Adjusting the screw "out" (counter clockwise) means you are allowing more air into the mix, which makes the mixture leaner. Adjusting the screw "in" (clockwise) means you are allowing less air into the mix, which makes the mixture richer.

Screw out:
If you let the screw "out" (ccw) more than 3 turns to run well, the jet is too big, and you are running rich. You need to move down 1-3 pilot sizes. Basically you need more air to get the mix right, more air than the screw will adjust to. So to get the mix right you need less fuel.

Screw in:
If you adjust the screw "in" (cw) less than 1 turn out to run well, the jet is too small and you are running lean. You need to move up 1-3 pilot sizes. In this case, you need less air to get the mix right (richer). So going up in pilot size will add more fuel to the mix to allow more fuel.

Fuel Mixture Screw
This screw limits the amount of "fuel" allowed to mix with the pilot jet fuel supply. The amount of air passing through this circuit is fixed. Adjusting the screw "out" (counter clockwise) means you are allowing more fuel into the mix, which makes the mixture richer. Adjusting the screw "in" (clockwise) means you are allowing less fuel into the mix, which makes the mixture leaner. It's the opposite as the "Air Mixture Screw".

Screw out:
If you let the screw "out" (ccw) more than 3 turns to run well, the jet is too small, and you are running lean. You need to move up 1-3 pilot sizes. Basically you need more fuel to get the mix right, more fuel than the screw will adjust to. So to get the mix right you need more fuel.

Screw in:
If you adjust the screw "in" (cw) less than 1 turn out to run well, the jet is too big and you are running rich. You need to move down 1-3 pilot sizes. In this case, you need less fuel to get the mix right (leaner). So going down in pilot size will add less fuel to the mix.

[slonomo]

Here is the Mikuni Tuning Manual for VM carbs.

[slonomo]

Update:

I installed the #35 main jet, same issue. Only idles when the air screw is adjusted all the way in. Ordered some larger jets, should be in Wed/Thurs.

[slonomo]

Update:

Finally got to work on the cart tonight. Man it's been a long month with the Covid19 stuff and the Black Lives Matter protests and riots.

Anyway, I ordered new pilot jets that came in a couple weeks ago.

For the record these jets are VM22 style Mikuni pilot jets with the 8 holes.

Sizes tried: 37.5, 40, 42.5, 45, 47.5, and 50

Ended up trying them all just because I could. 37.5, 40, 42.5, and 45 wouldn't idle unless the air screw was all the way in (bottomed out).

When I tried the 47.5 it would idle pretty good at 1500 rpm with no popping, with the idle screw set at 3/4 turns out.

When I tried the 50 it started to idle good, but stalled and flooded and wouldn't restart. Took the carb off, changed back to the 47.5 and vroom!

Just to double check myself I tried all the other jets again, just to see if it would duplicate my findings....and they did.

[slonomo]

So, as it sits right now: July 4, 2020

VM34 Mikuni Carb
180 main jet
47.5 pilot jet
needle clipped on middle notch
air screw set at 3/4 turns out
980 ft elevation
80-82 degrees F
50% humidity

Performance at this point is good. Sitting in neutral the rev is quick and smooth. It settles down slowly though, I would like to see it settle much faster.

There is a bit of a hesitation just off idle, so I'm betting I need to adjust the needle.

Acceleration is fantastic. Not as strong as with the 42mm CV carb, but still a world above the factory 2 barrel carb. The engine finds 5000 rpm very quickly and wants to keep on going, but I stopped at 5000 rpm since I don't yet have the rev kit installed. At 5000 rpm the cart was clocked at 33 mph according to 3 different GPS apps.

Throttle is touchy, probably due to the short throw of the throttle linkage I have hooked up. I need to adjust that to even out the pedal with the linkage, or rework the way it's hooked up.

Still some fine tuning to do, but overall very satisfied that this carb is the right choice.

[slonomo]

The throttle action is not that great with this setup, since the carb has much more travel than the golf cart pedal does. A bellcrank device is the answer.

I made a rough diagram (below) and a cardboard model (video coming) of what I'm going to make out of aluminum stock to make the bellcrank throttle pull device. I needed it to be at least a 3:1 pull ratio, with multiple adjustment points to fine tune the pull of the cable to the accelerator pedal and the cable to the carb. This will also work for the choke system.

video of the throttle bellcrank device
https://www.youtube.com/watch?v=AhLAou4KFQA

Here are some additional photos of bell cranks that I used for inspiration.

Then I made a rough one out of some misc materials I had in the shop, and it seems to work, so now I'll remake it so it's nicer and prettier lol.

Here's a photo of the type of cable ends I intend to use. I didn't really want to have to solder anything, although I can if I need to. By using the screw type cable stops and ends it makes installation much easier.

[raydhd]

Is it possible that the speedway motors bell crank I used for my 440 cc sled build would be easier and safer to use ? It has a ball bearing pivot and can have hole drilled for whatever ratio you need. Also the arms can be be bolted on for whatever angle and it costs about $20. Pics of it are on pages 5,6 of my 440 sled build.

[slonomo]

Here's the finished product. It works great. Its much smoother and feels more controlled than using the Briggs linkage with the springs and such. It took a few tries to get the ratio right, but it only took a few minutes.

https://www.youtube.com/watch?v=lZyeDKMY40Y

[slonomo]

Originally Posted by raydhd

Is it possible that the speedway motors bell crank I used for my 440 cc sled build would be easier and safer to use ? It has a ball bearing pivot and can have hole drilled for whatever ratio you need. Also the arms can be be bolted on for whatever angle and it costs about $20. Pics of it are on pages 5,6 of my 440 sled build.

Thanks Ray, I dont think I've read that thread. I'll check it out!

[slonomo]

Update:

I spent a few hours in the shop over the weekend....with the A/C cranked up of course!

It appears that my intake gasket was not sealing properly. I removed the intake and carb and installed a new gasket.

I also put the VM34 carb back to stock. So back to 180 main, 30 pilot, and the pilot air screw set at 1/2 turn out.

Now she idles at 1100 rpm and revs nicely. I think I have a winner!!

I also changed the orientation of the carb, so now it's facing the inside of the cart instead of the outside. The carb sits right above the engine and allows the heat from the engine to warm up the carb. The warmer carb allows the fuel to atomize much better when warmed up.

Next up is to change the throttle cable bellcrank and make one for the choke as well. Both needed return springs.

Pics and video coming.