New Jet Blocks for the VM29 Carbies

I've been spending a lot of time on these carbs recently. This is mainly because they were beat to death before they ended up in my hands. I got the rack for 75 bucks, which turned out to be about all they were worth. Damage inside and out.

Luckily over the years, these carbs have become more and more desirable and the once obsolete parts are now coming back. Gaskets, jets, screws, etc. Have been available but hard to find on occassion. But nothing was worse than the jet block.

The jet block is a cast piece that rests in the throat of the carb and makes the transition in the slide region smooth. It is obviously critical to the function and performance of this carb, but is also the weakest link. The area of failure is where the needle jet threads into the bottom of the jet block. The cast aluminum is soft and porous, making for fragile threads. Many people who have worked with these carbs have dealt with stripped jet block threads by boring the boss open, pressing a brass slug in, and retapping. This makes a better block than the original but it must be perfectly straight.  Others have just used epoxy to glue in the needle jet. This can also be tedious and it's not permanent. I ran a set with an epoxy-ed needle jet for two years and it was ok. Eventually it began to disintegrate even though I used PC-9, which is approved for fuel.  All these bandaids aside, there is a much easier way.

Z1 Enterprises has been making new billet jet blocks for a few years and they are quite nice. For about $40, you get a block that is nearly drop-in with stronger threads. Don't  forget to replace the small paper gasket underneath and clean the area well. The jet block must be centered well in the slide bore to avoid rubbing.

That being said, when I installed a new Z1 block, I used the small screw and the jet needle to get the new block seated as well as I could. With a few iterations it only has a minor bit of contact on the inside of the slide. It slides smoothly but witb a bit more friction than the others old blocks. I would expect that the interface will clearance itself a bit after some short use. I may mark the contact areas and polish the block where needed to prevent scoring on the slide.

Tuning Results: Testing a few adjustments on Rock Flute

I've spent some time verifying the settings on nearly everything for the engine. I also adjusted the compression on the rocker bolts for the front end.

First off, VM29 carbs don't have vacuum ports for synchronization so I added some. After the vacuum sync, the off-idle stumble is gone, idle is much smoother, and the throttle response is more crisp. A well synced set of carbs is paramount.

The plug gap was increased from .026 to .036. This made a huge difference in the heat of the plugs. They're running much cleaner and the exhaust note sounds a lot sharper. I'm going to bump up to .040-.045 soon.  There is plenty of spark energy to cross a bigger gap. The plugs should take the heat too. The resistor plugs have helped with noise in the Tachometer signal and should also help develop more spark duration. I am planning on testing out some 10 kohm Resistor caps to increase the spark duration. The duration on a dyna spark is about 0.9 ms. The stock points system had a duration of 1.5 ms and the cb operates best with a minimum duration of 1.0 ms according to the wisdom of Hondaman.  With the addition of a 10 kohm cap, I can run non resistor plugs and increase the spark duration by 15-20%.

I also verified the ignition timing and found that 1/4 was set to 39* and 2/3 was at 40*. Not a huge difference but certainly not good. I also noticed that the screws for the 1/4 pickup weren't very tight. After some thought, I decided to reduce full advance back to around stock so it's now set at 35*. I changed the timing curve to setting 5 on the Dyna 2000 which moved full advance up to 6500 rpm from 3500 rpm. When riding the bike after these changes, I noticed that the bike would load up easier at lower rpms. I think this might be due to the lack of timing advance below 6500 rpm so I switched the the timing curve back to curve 4. This curve has full advance coming in at 3500 rpm. More like stock, which works well for many differently built engines.

All in all, cleaning up the timing, opening up the spark gap, and synchronizing the carbs has made a significant difference in the behavior of the engine. It has a much smoother idle, easier start, smoother transition off-idle, and a crisp throttle throughout. The suspension adjustments have allowed the front girder to move easier and I'm running a lighter pressure in the air shocks. I will post an update after I do some testing to optimize the spark gap.

I also lubed up the cables and affixed the speedo pickup to the rear stay to reference the rotation of the rear sprocket. Road tests were done with a temporary fuel tank and no rear fender since the originals are out for paint. A bit awkward...and a bit illegal.

Installing Vacuum Ports for Synchronizing

Here's a quick and reliable way to install vacuum ports on your bike if your carbs don't have them. My vm29s don't and I want to ensure good balance. Ensuring good balance between carbs has made a big difference in ride quality and performance on many of my other bikes and it's bothered me that I've never had a way to check these.

I bought 5 mm threaded vacuum ports to install in the rubber boot. Other places to add vacuum ports are on the carb body, behind the slide of course, or on the cylinder head by tapping into the intake port. There is no room on the vm29 carb and the head of a cb750 requires removal of cooling fins on the out cylinders to make a good spot. I don't want to do anymore work on the cylinder head so to the boots it goes.

This is easy enough. Just drill a hole in the boot the same size, or close, to the minor diameter of the threads on the vacuum port. Clean off the debris nd test fit the port. Add some gasket maker to the threads and screw them into the boot. Let them dry and cap them with the supplied acuum plug.  Done and done.

Now you can go back to the precious article and vacuum sync your carbs. Woohoo!!!!!

Vacuum port with gasket and sealant.

Installed.  Maybe a little tacky looking, but with the rubber caps and the carbs back on I dont think they'll be very noticeable.

Carburetor Bench Synchronizing: Mikuni VM29 Smoothbore

This goes for most mechanical carbs. The hardware is just different.  First take the top caps off, pry back the locking tab on the lock screw, loosen the lock screw and slide adjustment set screw.

Carbs off the bike.

The lock screw is toward the back and the set screw is the one toward the air filter (both are brass).  Ensure that your cap gaskets are in good shape.

Slide adjustment set screw and lock nut fully loosened.  Lock tabs are bent back and the lock screw is also loosened.

Next, unscrew the idle knob down to a low setting. A couple millimeters from the threaded steel mount is good.  Screw the slide height adjustment down on each carb until the slide is all the way down. I do this by watching the idle adjustment cam and the tops of the other slides to see when they begin to rise. Once they begin to rise, you know the slide you're adjusting is seated. Just turn the set screw back a bit to just before the others rise. Do this for the remaining three carbs.

This is the idle screw (knob) on VM29's unscrewed to a low setting.  Note the small gap between the throttle cam and the threaded steel mount.  You can close this gap completely for setting the bottom travel of the idle adjustment, I just prefer this way. 

Now your slides are pretty close to even but you want to ensure that you have the same slack in each linkage system. To do this, crank the idle knob in to lift the slides up. Snag a drill bit of any size. 1/8" is good. Tighten  the slide adjustment nut on the set screw and the lock screw on one carb, say the #4 carb. This will be your reference carb.

Drill bit under the carb slide.  

Place the back of the drill bit under the reference carb slide and turn the idle knob until the slide makes contact with it. You want to make sure that the slide isn't resting on it and become familiar with how it feels to slide the bit in and out. You will now set the height of the other slides to match the feel of the reference carb.

Move to one of the other three carbs, say carb #3. Place the drill bit underneath of the slide and lower the slide by adjusting the set screw down. Match the feel from the reference carb and then lock the set screw and lock screw. tightening the lock screw can make a slight change in the slide height so double check the feel on that carb after locking it all down. Repeat this for the final two carbs and you've successfully bench synchronized your rack-o-carbs.  If you dont want to, or cant vacuum synchronize the carbs, bend the locking tabs back over the lock screw and replace the top caps. You're done.

Locked and ready to roll.... to vacuum sync, if you're man enough.

The best method for synchronizing carbs is with vacuum gauges with the engine running. A meticulous bench sync can get you close, but a vacuum sync can get you dead on and compensate for any cylinder variation.

To vacuum sync, place the carbs back on your engine and warm the engine to its operating temperature. Next remove the top caps and pry back the lock tab.

Install your vacuum gauges or gauge and restart the engine. Set the idle a bit higher than normal and check the vacuum on each cylinder.

If they are different, you'll need to adjust them. I like to choose a reference carb that is in the middle of the readings. Set all of the other carbs to the same vacuum reading as the reference carb by adjusting the set screws. Once complete, lock it all down, replace the top caps, and drop the idle back down to normal. You're done... Again.

Check your vacuum every couple of months and adjust as needed. A well balanced set of carbs will provide optimum performance. Do yourself a favor and make it part of your regular maintenance program.

Winter work

I finally got the courage to start the mods on Rock Flute I want to complete before the 2014 Whcm. Part of which is eliminating the off idle stumble while cruising. I hope that after having to bash a groove back into a mutilated air jet that increasing them to 1.1 from 0.9 will lean out the emulsion. Ohh how I miss riding this bike.

Next bike build on the list is the W650 digger, the attic cycle.

Rock Flute: Tuning Session 1

I've done a little work on the VM29 smoothbores since the engine rebuild. I bench synced the carbs to a random .116" because I don't have vacuum ports to sync them on the bike. First few runs its been running rich on 120 mains and 20 pilots. Tuning from the bottom up, I swapped jets to 115 mains and 17.5 pilots, which is the spec for a stock cb with vm29s. The bottom end was still incredibly rich but a lot better with the air screws out over 2.5 turns.

I dropped the pilots to 15s and kept the mains at 115. Idle looked good with 1.5 turns on the air screw but off idle was rich enough to cause a stumble. Plugs showed a lot of soot. Not wanting to mess with the pilots anymore, my suspect now is the needle and main. WOT pulls well without hesitation which indicates the main is close. To alleviate the stumble in the transition from idle to 1/8 throttle I dropped the needle down. The stumble was a lot less noticeable but still their.

I went for a ride to pick up a starter switch for johns bike and was going to drop the needle to its lowest position when I got back. Unfortunately I didn't make it home under my own power due to this little gem:

Jeff Yarrington and the Scooby Van to the rescue!  My million year old, burnt, dry clutch cable finally gave up the ghost. I couldn't find a picture of Jeff's sweet van but I did find a picture of him sleeping in it. Thanks Jeff!  He got me back to John's place where I dropped the tuning for a while and resumed wiring John's black chopper.  So many things to do.

 

My fruitless quest for a starter switch was kindly remedied by Jeff. When he picked up my bike he brought two momentary switches. What a friend. Thanks AGAIN!

Ok sorry.  Back to the tuning.  With a broken cable, I was down for the night.  My next step will be to drop the needles all the way down and then refine the a/f ratio based on some plug chops. Hopefully I'm good with the 115 mains because I feel like 112.5 or 110s on a hopped up engine is a bit absurd.  Just one more week before we leave for the Roost.  TIME IS RUNNING OUT!!!