Just took first place in the lowest HP contest at the local dyno shootout!
Looks like I have some more carb tuning to do!
The tech said the RPM reading was about 1000 less than actual??
I'm guessing it is 1/2 actual??
Dyno Results
I told the tech my clutch was slipping and to take it easy, I really just wanted the A/F ratios more than anything else, but I didn't detect any slippage during the runs.
My guess is that the selector setting wasn't right on the dyno tach. The motor is supposed to put out 23 HP at the CRANK at 7,500 RPM per original Honda ads. 15.4 probably is not that far off at the rear wheel. 66 dream, 78 cb750k, 02fz1, 09 wing
Hi,
Very good that you putted your bike to the test on a dyno, if you can "read" the dyno results it will tell you much about the state where it is in, and if the engine is happy and will stay that way. Are there more companies in the neighborhood with a dyno ? Reading the results of a dyno is difficult and easy misinterpreted, especially with young people who used to new machines. If I see this run I would not be sure of the capability of the tech who made it. These young guys do know a lot about modern 4 cylinder engines, but if there 's a cb72 on it they mostly do know as much as yourself. Did he rev it to the red line ? If he did he should be aware of the wrong tacho results and first solving that before going further. But maybe I'm mistaken. Is it a CB72 or a CB77, a c72 or a C77 ?. when I know what it is I can compare it with the results of mine and give some advise / explaining. I put my CB72 and others on regular base (around 10.000 km) on the dyno. First thing I see is that the rpm range is not good, I think it's indeed the halve of the real rpm or so, and indeed it depends on your freshness of the tacho how accurate is is. I've seen also errors of 1000 rpm, don't worry about it. It's good that you measured the gas (lowest curve) it say everything about the carbs indeed. Next time when you put your motorcycle on a dyno, measure the gas from both exhausts apart from each other and compare them. If you have a CB, it says everything about the leg you're measuring. Does the guy who made this run know that these are no vacuum carbs ? these slide carbs should be opened gently, and not in once as modern carbs. The engine is running rich and poor in one run, so carb work is necessary. It's important to fix the clutch (if it's slipping), before running again on the dyno, also make sure your chain is lubed and the rear tyre is in good condition. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen,
The bike is a 1966 305 Dream (CA77) with a 360 degree crank (pistons rise together), single carb and single point set (even though the points cover says Type 1). The tech is an experienced dyno operator, but admitted he had never tested one of these before. They have been in business as a Honda dealer for many years, and there are no other Dyno facilities nearby. He did accelerate slowly and I did not detect any clutch slippage by sound during the runs. When it does slip, I usually hear a rapid increase in RPM under load which stops as soon as I let off the throttle. When I questioned him about the RPM after the test, he said the reading was about 1000 RPM low, and said I was running rich at low and high speed and lean in the middle. I'm no expert on dynos, but I believe the Dynojet measures HP and calculates the resulting torque from the RPM reading...which leads me to believe the torque numbers cannot be correct. Also, the HP and Torque curves do not cross at 5252 RPM if you double the RPM scale. I don't think he hit the red line, since even if you double the RPM readings, the curvre would stop at 6800. Hopefully, the A/F readings are correct so I can work on the carb? Any coments welcomed!! 66 dream, 78 cb750k, 02fz1, 09 wing
Hi,
A CA77, is comparable with a C77, so I will compare with a C77. Since a CA77 has no tacho, I don't understand his conclusion about 1000 rpm higher / lower, please explain. before you do another dyno run : Make sure the air filter is clean (the influence of the air filter is huge with a C77, and thus also with a CA77). Make sure that the RPM scale is ok, indeed the dyno uses the rpm to calculate things, without a good calibration of the RPM it's difficult to say something about the figures. Maybe configure it as a single cylinder machine (on the dyno), but that's depending on the software version they use. Measure the compression of the engine on both cylinders. If the compression is not within limits, it's useless to run on a dyno. Make sure the engine is warmed up good, a dyno run is an heavy load for the engine, it should be up to it's task. Make sure the ignition works flawless, the dyno measures the rpm on the high tension cord, so if the ignition doesn't work correct, it's impossible;e to get a good run. The dyno curve fits the curve of my C77 if the rpm is multiplied three times (?), but that makes no sense either. It's dangerous (for jetting the carb) to conclude anything from this run if you're not sure of the rpm range. But if 1000 rpm on the scale is 2000 at the crank, I would advise a smaller main yet, run again to see how the needle will fit in, but start with the main jet, aim at 13 for the ratio (air/fuel), that's fine, not leaner. Does this dynorun shows the hp of the rear wheel or is it recalculated for the crank including losses ? jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen,
Air filter is new, carb is clean, valves adjusted, timing set with light for max advance match, (dances slightly at idle), new points, condenser, and plugs, compression 150 in each cylinder. Altitude 2500 FT, Temp 60 degrees F Prior to the dyno i ran the bike for about 5 miles, mostly wide open throttle and pulled the plugs and they were chocolate brown color I recalculated and plotted the torque curve assuming RPM shown is 1/2 actual, and assuming the HP curve is correct with the following result: The HP and TQ curves now cross at 5252 RPM so maybe this is correct? I don't have any figures on the stock torque values, but my guess is that with this engine torque should not be twice the HP?
66 dream, 78 cb750k, 02fz1, 09 wing
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