Thursday, July 2, 2015

Bore Scoping a Couple of CB750s

I borrowed my friends bore scope just to poke around a bit.  Did I find anything interesting?  Ehh, I'll leave that up to you.  It's not the best scope but it was good for some more than basic diagnostics.  It was fun to poke around.

Here are a few shots from my 836 engine.  Ported head, oversized intake valves, and about 10k on it. The #4 cylinder burns a tiny bit of oil noticeable on startup.
#4 cylinder

 #4 cylinder.  Hone-y isnt it.

#4 cylinder
#1 cylinder.  More carbon.

#1 cylinder.

#1 cylinder
#2 Intake Valve

#3 intake Valve

The next engine is a 1978 F3 but the top end was rebuilt with K model parts.  No clue if it's any good. I found some debris, rust, and what looks like bad honing patterns. The #4 intake spigot is also broken but that's another story:
#1 cylinder

#2 cylinder.  Maybe has a little rust from sitting on the right side of the picture

#3 cylinder.

#4 cylinder.  It has some debris in it.
Ohh and I found a little kernel in the #4 intake.

Friday, June 12, 2015

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.

Tuesday, May 19, 2015

Carls Konged '78 CB750

Spent some time over at Carl's today to help get his bike back on the road. Needed lots of love after being his sole mode of transportation for a long time and then shoved in the corner for a few years like an old broom.

Just a few things off the checklist (stock k8 engine):

Set points:
Gap 0.012" to 0.016"
Static set 1-4 timing on F-mark with ohmeter
Static set 2-3 timing
Lock plate and roll.

Valve Lash:
E.O.I.C method
Intake 0.002"
Exhaust 0.003"

Set cam chain tensioner:
Push kicker to get tension on the front of the cam chain and slack on the rear. Loosen lock nut on tensioner and back out the set screw. Listen for movement in the tensioner and lock down the set screw and lock nut.

After setting the valve lash, I successfully snapped a Tappet cover off. Good word of advice: Just snug the Tappet covers down lightly. This one was fatigued or already broken because I didn't even get the gasket seated to the valve cover before it popped.

Meanwhile Carl replaced the fuel pump in his car. Lovely day.

Monday, May 18, 2015

Rock Flute Specifications:

For my own notes and for those interested, here are the specs on Rock Flute.  I'll update this as things change... because they always do. For instance, the green paint in the picture is actually a brighter green than the color code listed below.  I will update the picture as soon as the bike is repainted.

10.9 to 1 CR
1978 CB750 F bottom end
1976 K model cylinders, Decked
1976 K model Head, Stage II porting, decked by Mike Reick
                33.5 mm OS intake valves, stock exhaust valves
                Megacycle 125-75 cam, 0.005” Int/Exh tappet clearance, 105 int 104.5 exh lobe centers, no adv.
                                (More cam specs in chart below)
                Adjustable cam sprocket
                HD Valve springs and titanium keepers
                Late model cam towers with unbolted early model rocker shafts
Wiseco 836 forged piston kit
CycleX Super Rods
New primary chains installed
Reinforced valve cover with cam tower stabilizers

Transmission/Final Drive:
F model transmission, backcut with close ratio 4th and 5th gear
             Primary: 1.708:1
            1st:           2.500:1
            2nd:          1.708:1
3rd:           1.333:1
            4th:           1.133:1
5th:           0.969:1
Barnett Clutch plates and springs
Drilled clutch basket with oil grooves for added oiling
Final Drive: 17/48 2.82:1

Dyna 2000 ignition with 3 ohm Dyna minicoils
Total Timing Advance: 35*
Timing curve: 4 (changed 6/12/15 from 5 which may be causing low/mid stumble)
Rev limiter: 10,500 RPM
NGK DR8EA 5 kohm Resistor type spark plugs @ 0.036” gap *test gap up to 0.050" for better burn, idle, and mpg (resistor plugs used to eliminate noise in digital tach and increase spark duration)

Ramflo Filters
Sudco VM29/CB750 Adapter boots with vacuum ports
Mikuni VM29 Smoothbore Carburetors
               Mains: 115
               Pilots: 15
               Air Jets: 1.1
Air Screws: 1.5 Turns
Needles: #5DL31 with clip @ #1
Needle Jets: 0-6 (verify)
Throttle Valves: 1.5
Float Height: 23 mm

4-2, 2” long baffle with 1.25” inner diameter

Cycle One Manufacturing rigid
2" forward
4" up
36* rake, Harley 1" neck
5" ride height
Paint: Sikkens Rally Black
Fuel Tank: 1969 Triumph Tiger Daytona 500
Rear Fender: 5" Ducktail ribbed fender, 7 Metal West
Paint: Gloss Black/MD State Police Green - BMB1B (ICI Autocolor paint code)
Seat: Satin black powder-coated Accufast seat pan with single leaf spring suspension
Suspension/Front End:
Vincent Style girder with Cannondale MTB air shocks @ 60 psi each
Front Wheel:  Satin black powder-coated SOHC CB750 rim laced to a CL450 dual leading shoe brake hub
Front Tire:  Shinko 705 110/80-19
Rear Wheel:  Satin black powder-coated DOHC CB750 rim laced to a late model CB750 drum hub
Rear Tire:  Shinko 705 130/90-19
Gauges/Info Center:
Vapor Computer System
     Speedometer, Tachometer, Trip/Odometer, Top Speed, Time, Head Temperature, Shift Lights/Redline Warning
Analog Oil Pressure Gauge
Headlight:  Cycle Standard 4.5 inch early style headlight with high beam indicator
Taillight:  Bullet style dual filament

Capacity: 5 Quarts
Mobil 1 20w-50 Full Synthetic

Capacity: 3.7 Gallons
Premium 91+ Octane

Curb Weight:
450 Lbs


Cam Specs
Valve Lift
Int. Open
Int. Close
Exh. Open
Exh. Close
Lobe Center
tappet Clearance
Sealed Duration
Megacycle 125-75
262 @.04
257 @.04


Saturday, May 9, 2015

Rewelding and cleaning up some of the parts on Rock Flute

So I found out my kickstand can't take the entire weight of the bike dropping down on it. 450 lbs bent the tab enough for me to have to reweld it. While I had the welder out, I also added a bit of metal to the headlight mount because it had a tendency to vibrate at high speed. Both problems fixed and covered with a half assed attempt to blend satin black paint.

Wrap up with some ginger beer

Wednesday, May 6, 2015

Carburetor Return Springs

Brought to you by you local hardware store.

Monday, April 27, 2015

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.