Survivor Chop: Tune-up

 When this bike arrived, it needed a little bit of everything, including tuning.  It arrived with 150 mains, which are huge for a stock engine.  After some trial and error, seat-of-the-pants tuning, and a little cheating with an AFR meter (that never read correctly because of air leaks in the exhaust), I arrived at 130 mains, #3 clip position on the needles, 40 pilots, and 1.75 turns on the air screw.  Floats were set and all carbs were synchronized.  The ignition was set statically and the points were gapped according to the shop manual. This made the bike run relatively smoothly, with good throttle response through all positions except a stumble around 3/4 throttle.  It did not like any quick crack of the throttle. The bike would never start cold on the kick starter and required several rotations of the engine to begin roughly idling for a moment.  Idle at full operating temperature was also a bit rough for my liking.  For the time, it was sufficient to move on to other work.

After the trip to Gettysburg, I decided I wanted to see what I could improve in the tuning.  While it started and ran ok, it was certainly not a cold kickstart bike, the idle seemed choppy, and its mannerisms did not exude confidence or reliability. Honestly though, most CB750 chops I see running around seem to be about the same, but I know they could be better.  I guess it just depends on the performance you're willing to accept.

Ignition Precision (actually, accuracy, but that didn't sound as good)

In the past, I've always set my points gap with a feeler gauge and set the timing statically.  I rarely yanked out the timing light to check operational timing and full advance.  This was good enough, but clearly there are more accurate ways of achieving the proper settings.  I was curious how much there was to gain from setting dwell time instead of gap and setting timing dynamically.  Spoiler:  setting dwell time instead of the gap on the points made the biggest difference, but using the timing light to ensure your mechanical advance is operating properly is a good idea.

Setting the dwell time (or dwell angle) on the points is much easier (IMO) and more accurate than using a feeler gauge to set the gap on used points.  Used points have pits all over the surface and this affects the true break point of the electrical connection.  You may set your gap properly, but the points actually electrically short at a different time than is expected based on the gap you set.  Setting the dwell angle directly sets the time that the points are in electrical contact and the time that the coils will charge.

To set the dwell angle on the CB750, which has a set of points for each pair of cylinders (1/4, 2/3), you must set your dwell meter to 8 cylinder.  It is a wasted spark system with 2 sets of points, so you cant use the 4 cylinder setting. The 4 cylinder setting on dwell meters is based on a single points set and no wasted spark.  Connect your leads to each side of the points so that it measures across the points.  Start the engine and set idle around 850-900 rpm. When I checked the dwell on the Waffle, it measured 19 degrees with the points freshly set with my feeler gauges.  The proper dwell angle should be 24 degrees.  This means that my coils were charging for 20% less time than they should.  I adjusted the points until I achieved a dwell angle of 24 degrees and then set the timing back to factory specs.

The timing was set using a timing light, then checked statically to see how much of a difference there was between the methods.  The difference between static timing and using the timing light was negligible, but I preferred using the timing light.  While I had the light hooked up, I spun the engine up in rpm and verified a smooth advance that ended between the two hash marks on the advancer.  Base timing, advance, and dwell were all now within spec.

With the ignition system set properly, the bike idled a lot smoother, both at operating temp and cold start.  While riding, the throttle response seemed more crisp as well.  Cold start was still about the same.  Meaning that the bike did not want to kickstart cold, but would fire up on the starter after several rotations. Overall, it seemed to start easier, but it still wasn't a kicker.  On top of that, the stumble at 3/4 throttle was still present

It looks like I'll be doing a few things differently from now on. This includes how I set float height, which I discuss below.  Improvements were made by setting the ignition system via dwell angle and a timing light, but I still suspected fueling issues.  

Float Height Quandary (Using float height for fine tuning)

I felt that my main jet was sized properly and any change in the needle clip position from #3 resulted in a worsening of the throttle characteristics in other areas where it was previously good.  At #3 on the needle clip, the bike still exhibited what seemed to be fuel starving around 3/4 throttle.  

While doing some research on float heights for these carbs (Keihin 657A/B), I noticed a discrepancy in where the height was measured from.  The shop manual isn't explicit on where to measure the height from.  The most agreed upon method is to measure 26 mm from the lower level of the flange on the carb body.  I also found a wet fuel height, which at some point must have been determined by some racers back in the day because Honda never specified a wet fuel height. 

Lower flange on carb body (picture rights: Hondachopper.com).

The wet fuel height is measured using what some call "the clear tube method." This method simply uses a clear tube connected to the drain of the float bowl and routed up against the body so that the opening of the tube is above the float bowl gasket surface.  This allows you to see the height of the fuel in the bowl.  The ideal height is claimed to be 3-4 mm from the gasket surface.

Measuring wet fuel height with clear tube method.

When I originally set the float height on the Waffle's (the blue survivor chop) carbs, I set it based on the lower flange on the side of the float bowl gasket surface per the Honda spec. This is about 2 mm further down than if set from the gasket surface. Measuring the wet fuel height using "the clear tube method" after setting the floats by the Honda spec, the fuel height was around 6 to 7 mm from the gasket surface.  There is a discrepancy of 2-3 mm between the two methods.  

After setting all other variables (synchronization, main jet, needle height, pilot, air screws), changing the fuel height in the bowl can offer some fine tuning... or, another way to put it, compensation for not quite getting the other variables correct.  

In an attempt to remedy the lean condition off idle and at 3/4 throttle, I moved the floats up toward the carb body 2 mm to wet the emulsion tube more.  This richens the mixture slightly throughout the throttle range, which is similar to a slight increase in jet size. This mostly eliminated both issues. Off idle manners were perfect. Although it still seemed very slightly lean at 3/4 throttle, I didn't want to bugger with it because 95 percent of my riding is going to be low in the throttle range.  Fixing the slightest bit of perceived power loss at 3/4 throttle might sacrifice the off idle performance.  

The final float height was 24 mm from the lower part of the flange.  The bike kickstarts well, has a good cold start idle, good off-idle throttle response/cruising behavior, and great response with quick throttle changes.  When the engine is cold, the off idle response is a bit sluggish, but this isn't unexpected because the intake tract is still cold and fuel is dropping out of suspension. Even though there might be a barely noticeable loss in power at 3/4 throttle, the hard stumble is gone.  Overall, response and power throughout the throttle range is very good.  Using the float height to fine tune the fuel mixture worked well in this instance.

Final Carburetor specs for the Waffle:

Stock engine with breadbox air filter and 4-4 drag pipes

Altitude: Sea Level

Main: 130

Pilot: 40

Needle Clip Position: #3

Air Screw: 1.5

Float height: 24 mm from the gasket surface (not the lower flange on the side)

Other useful notes:

- When setting your float height, ensure that the float tang is just barely touching the float needle.  I set my float height with the carbs turned sideways.  This way, I can set my calipers to the desired height and push the floats in so the tang contacts the float needle.  I can vary the pressure to verify that the contact is present, but not compressing the spring.

- The float needle spring should compress completely under the weight of the floats when the carbs are held upside down.  If this is not the case (which can occur if aftermarket float needles are used), you may need to set your float height a few millimeters higher to achieve the wet fuel height of 3-4 mm.

- In the event that you need to swap points or an entire points plate roadside, some spare wire and a lightbulb can be useful for setting your timing.  You can carry wire and rob a lightbulb from your bike temporarily or carry a couple LED's with integrated resistors  spec-ed for 12 volts in your tool bag.  I purchased 20- 3mm LED's with integrated resistors for this purpose.  Simply disconnect the blue or yellow ground wire from the points and the capacitor, clamp the short lead of the LED to the point under the nut and run a wire from the battery positive to the long lead on the LED.  Now you can set your points based on when the light turns off.  Below is a picture to help clarify this procedure.

Survivor Chop: Repairs and upgrades everywhere

 

After a weekend trip to Gettysburg with a couple friends, I found a few things on this bike that could use some love.  On top of that, some of the equipment is just old and could use replacement so I don't have to deal with issues roadside.  This includes some electrical repairs, exhaust repairs, new oil and vent lines, new wheel bearings, new front rotor, and a tighter rear suspension. 

The last one in the list above was a bit of a surprise to me after I loaded the bike up with my gear for the weekend.  I went over a bridge and heard a screeching sound like the wheel locking up.  It turns out that the rear wheel was rubbing the fender... a lot.  To remedy this, I asked John to whip up some spacers out of scrap to preload the rear springs a bit more. I measured the spring rate to be about 185 lbs/in, so we went with 1/4" spacers to add about 90 lbs more preload (46.25 lbs on each side).  This helped quite a bit, but it still rubs slightly over heavy bumps.

I slapped some black paint on the underside of the fender to cover the raw metal from the tire rubbing over the years.  This also gives the tire a fresh canvas to scuff up later.  Overall, the bike rides about the same, but the tire scuffing is only slight.  John made up a second set of spacers to add which are 1/8".  I may add those and see if I can avoid tire contact with the fender altogether.

I hope to use this bike as my touring ride for a long time to come.  With that in mind, I want it to be as reliable as possible.  And while the original regulator and rectifier on the CB750's is quite reliable, it is already 49 years old.  So out with the old and in with the new.  A new regulator rectifier combo from Rick's Motorsports (model 27-10100).  This reg/rect combo is plug and play with the factory wiring harness, which surprisingly, this chop has.  Removing the old parts and installing the new combo unit was a huge pain only because the main wiring fixture from the original CB750 was mounted under the oil tank, making it impossible to remove the original regulator and rectifier without removing the rear fender.  This was one big reason why I did this upgrade. I would not have been able to replace the original parts roadside.

I also replaced the glass tube fuse holder with a more modern blade type fuse.  While this bike retains the factory harness, someone consolidated the fuses to just a single 20 amp fuse.  I would prefer separate fuses, but I've owned and built many bikes with just one main fuse without any trouble.

The exhaust repairs included replacing the no. 10 bolts on the exhaust sleeves at the tube joint near the footpegs with 1/4-20 bolts.  The right side exhaust came loose while up in Gettysburg and I had to use bailing wire to strap it to the frame.  I don't like the fact that the sleeves are a tad small and simply black pipe, but they will work for now. I also sealed some leaks at the spigots with aluminum borrowed from an Arizona Green Tea can.  Nothing but the finest material.

The new oil and vent lines were brought to you by Home Depot.  Reinforced clear vinyl tube for the oil lines and clear non-reinforced vinyl tube for the vent lines.  I like that the oil flow is visible.  This material gets soft as the are heated, so be sure to use reinforced tube for the oil lines.  

I have run many bikes on the original wheel bearings for years without noticing any issues, until I actually replaced a full set on a bike.  The ride was significantly smoother and handling in corners was notably better.  The bearings on this bike had their time in the spotlight and exhibited some rough spots. And while I would have just run these in the past (and did for a while this past year), bearings are cheap, replacing them is good insurance, and the difference is quite noticeable.  

The bearings for the hallcraft front hub were conversion bearings to adapt the wheel to the 5/8" axle.  The part number from NSK is 6203-625VVC3.  Simply drive the old ones out carefully with a punch, clean all the surfaces, inspect the inner sleeve for damage, and install the new bearings.  Be sure not to compress the bearings in so tight that they bind from compressing the inner sleeve.  Press the bearings in until the inner sleeve is in contact with the inner race of each bearing, but both bearings spin easily.  This goes for the rear hub too.

While doing the bearings, I cleaned and polished everything along the way.  It's easy to get to those small spots now, and not so much later.  

The rear wheel has three bearings, two in the hub, and one in the sprocket carrier.  All Balls sells a great kit that I have used many times.  The part number is 25-1362 for the CB750 rear wheel.  It comes with all three bearings and a new shaft seal for the sprocket carrier.  This shaft seal was important for the factory bearings, which were not sealed.  However, the All Balls kit comes with sealed bearings.  

The sprocket carrier has a bearing retainer on it.  When I cant find my retainer tool, I just use a set of hex keys or punches that are close to the diameter of the holes and rotate them with a pry bar.  To prevent the aluminum threads from galling, clean the retainer and spray the threads with penetrant.  Leave the sprocket carrier in the wheel while working on the retainer as it will help hold the piece steady.

Once the retainer is removed, drive the bearing out from the other side and remove the shaft seal.  Clean all the surfaces and install the new bearing and shaft seal.  When installing the new bearing, you can use the old bearing as an intermediate surface to bash on.  It's the same size as the new bearing and will distribute the force evenly. One other tip: If the shaft seal is being a pain to remove, heat the retainer with a torch and melt/soften the rubber shaft seal.  It will pull out easily.

The rear hub also has a bearing retainer and I use a similar method as above.  Instead of hex keys though, I use flathead screwdrivers to engage more of the surface of the retainer. Use penetrant to minimize/prevent thread galling.

Drive the bearings out from each side, clean, and replace.  Once again, the old bearings can be used to assist with seating the new bearings.  And once again, make sure to inspect the bearing sleeve that rests between the hub bearings and ensure that it is in contact with the inner races of the bearings and the bearings spin freely.  Reinstall the bearing retainer and you're all done.  This is also a good time to inspect your kush drive rubbers, your rear brake shoes, and your brake liner.  Ohh and clean/polish everything!  One more pro tip:  Matching the bearing seal color to the bike adds 20 horsepower at the wheel. Luckily, the All Balls kit is blue!

Last on the list for this post is the front rotor. The original one was heavily pitted.  They were made by Hurst-Airheart, now known as Airheart Brakes.  You can purchase a rotor that's similar in dimension, but will need some modification to mount to the Hallcraft hub.  The part number for this replacement is 3301-1003 (7.75" diameter, 0.119" thickness, 3.50" bolt circle diameter, 0.194" bolt hole diameter).  The bolt holes need to be counter-sinked and opened up to accept the 1/4-28 bolts that the Hallcraft hub uses.

I didn't purchase a new one and go down the route of modifying it to work because I had a few of these rotors sitting around.  I checked the potential replacement for warping, pitting, and thickness.  Then cleaned, sanded, and painted it to make it look purdy.  I installed with the original bolts because they were in good shape and used blue loctite.

So that wraps up a good bit of work to get this bike, that my friends have named "Blue Waffle", in top shape.  Better electrics, newer bearings, fresher brakes, new lines, tighter suspension.  After this, I'm going to spend some time tweaking the tuning a bit.  I feel that there's plenty of room to improve this bike's performance and reliability.  So next we'll take a look back at the carburetors and the ignition.