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
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 046A), I noticed a discrepancy in where the height was measured from. The shop manual isn't explicit on where to measure the height from. Some guru's online say to measure 26 mm from the lower level of the flange on the carb body while others say to measure from the gasket surface of the 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.
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. This is about 2 mm further down than the gasket surface. Measuring the wet fuel height using "the clear tube method," the fuel height was around 6 to 7 mm from the gasket surface. This is when I began to really question the reference point for float height. If the ideal fuel level is 3-4 mm from the gasket surface, it seems that the 26 mm measurement should be made from the gasket surface.
So that's what I did. I reset the float heights based on the boss that's centered on the gasket surface to a measurement of 25.7 mm. This increased my fuel height by a few millimeters, thereby wetting more of the emulsion tube. Below is a picture of the wet fuel height after setting the floats at 25.7 mm from the gasket surface instead of the lower flange area. The wet fuel height is about 3 mm from the gasket surface.
Before I made this change, I verified that the carburetors were synchronized and the ignition system was set as described above. The carburetor's vacuum was even across all 4 and I made no other adjustments other than increasing the float height to achieve a wet fuel height of 3-4 mm from the gasket surface.
The results were surprising. The bike now kickstarts on the first kick, even when the engine is cold. Drop the choke immediately and it idles well (with the engine cold, idle rpms were low, but it doesn't stall). Throttle response seems a little sluggish just off idle when the engine is cold and this is likely because of the fuel dropping out of suspension while the engine and intake tract are not up to operating temperature. This is very minor though. The rest of the throttle range improved and the stumble at 3/4 throttle is gone. The entire throttle range is excellent and feels fully tuned. It has great manners at cruising position and it accelerates smoothly when the throttle is rolled open quickly.
What did I learn? I will be setting my float height at ~26 mm from the gasket surface from now on and verifying that the wet fuel height is 3-4 mm from the gasket surface. I will also be setting the dwell angle and using a timing light to set timing at base time and full advance. John is currently tuning up his Amen Savior chopper and will be doing the same. Hopefully I get a good report from him because his bike had similar cold start manners as the Waffle.
Final Carburetor specs for the Waffle:
Stock engine with breadbox air filter and 4-4 drag pipes
Altitude: Sea Level
Needle Clip Position: #3
Air Screw: 1.5
Float height: 25.7 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.