How to diagnose and repair an old motorcycle PT1. In this video we take a close look at a 1972 Honda CB750 for a customer who has had some running issues with it in the past. His reports say that the bike seemed to be losing ignition power and would feel like it was down on cylinders. With that information I go through and test the ignition components with a multimeter as well as make some basic adjustments to some of the components. I also remove the carburetor float bowls to get an idea of their condition to best build a quote up for all of the needed repairs.
The tests, component checks, and eventual repairs are all things you can use on your old bike to get it running or help keep it running at its full potential. I always highly recommend a service manual for your bike as well as a digital multimeter as those two tools greatly help you in any repair work you must do. Instead of guessing what's wrong and asking random people on the internet you can do the tests yourself and get a clear understanding of how to repair as needed.
The figures below apply to this specific motorcycle. Your motorcycle may have different specifications to be aiming for so be sure to do your research.
Test results on the coils:
Primary Resistance (coil harness leads to each other) - Goal 5k OHMS +/_ 10%
Coil 1-4 = 4.9k PASS
Coil 2-3 = 5.0K PASS
Secondary Resistance (spark plug wire leads to each other) - Goal 15K OHMS +/_ 10%
Coil 1-4 = 18.9K = FAIL too high
Coil 2-3 = OL Open Line FAIL
Spark Plug Resistor Cap Resistance - Goal 5K OHMS
1 = 8.4K FAIL
2 = 10.16K FAIL
3 = OL Open Line FAIL
4 = OL Open Line FAIL
Further notes - The plug wires were very deteriorated with poor connections. The points plate was out of time which you see me adjust. The points themselves needed to be filed. The carburetors had varnished fuel requiring a rebuild. The fuel tap was seeping and I chose to go for a rebuild there as well.
There will be much more in episode 2 so don't miss that!
Clymer/Haynes/Factory Service manuals are a must with any motorcycle you own. They are an incredibly valuable tool and only cost about $35 which you will end up saving 10 fold with time and saved money by not buying parts you don't need. The manual linked is a Clymer which is an Amazon affiliate link where I get a small commission if you make a purchase.
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A digital multimeter will cost between $25-35 for a basic one like mine. I have had mine for about 15 years now and it is one of my most used tools in the shop. I have linked it below which is an Amazon affiliate link where I get a small commission if you make a purchase.
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Part 2 where we do the repairs and fix the bike:
• How To Diagnose And Re...
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