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.
https://amzn.to/3IpjBnB
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.
https://amzn.to/3prAipN
Part 2 where we do the repairs and fix the bike:
https://youtu.be/GQWFTC_9kXo
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I have to rebuild my carbs every season, I have considered pulling them and taking them into my house for winter months.
My bike I would have just replaced the coils and wires and rolled it out and ran with it.
I have a 1979 cb750 four K and it's locked up tighter than a drum, but was free when I got it. Only thing I've done is rebuild the carbs and try to start it.
I think the hardest part is getting all four carburators working in harmony.
Great video man!!
Nice old bike thanks for the info .
Looks to be way more than 38k..just curious
Very educational dude. Thanks. regards from Sydney
That was super informative! I just got a 73 and this is so so useful! Thank you!
I took your advice and am watching these videos. Im really getting a lot of great advice on what to look for. Quick question I noticed I have oil in one of my reflectors on my forks. I’m figuring it’s a busted seal, but it still has me freaked out a little. Dude are you In Missouri or Vermont? I’m in Oklahoma bro. Anyways you rock!! See ya.
How to repair cb750 step 1 they don't breakdown.
I just picked up one of these a few days ago. Same bike. 500 bucks, barn find. I will restore it completely, down to the frame.
Is it best to drain the float bowls if the bike is going to sit for awhile?
I just got a 1978 CB 750 and I pick it up today. When I was young, that was the bike I loved.
Great video and tutorial. I miss my old 83 Night Hawk 750. Looking at getting another. Your videos definitely inspire me.
Just got this exact bike yesterday. My seat is locked up (aftermarket lock). Once I get past that I can get going.
its amazing how much of a headache these bikes must have been for mechanics…. im amazed at all cb videos ive seen someone has always tried to repair different or find some janky repair etc
I had my carbs sonically cleaned, did a basic balance myself and fitted electronic ignition. Made a heck of a difference. Only bad bit it got rid of the popping in one of the exhausts when throttling off. They never did that back in the day but it was sort of quaint for a 50 year old bike.
I didn't read through all the comments, but that green liquid, in carb 3/4, looked like fuel stabilizer.
Another great video mate! Enjoying your presentations down here in Australia
can you give me any idea about my 1978 yamaha 100 enduro kickstart only halfway thats why it wont start?its suppose to kick start all the way down,but mine seems like only kick start halfway?thank you very much
Hi BJ enjoy the vids, bring back memories. I was a child of the 70's and worked on many old bikes.
I know this is a year old vid now, but just wanted to say I would not have set the timing the way you did.
The back plate was way over to the right as you said and this shows that someone had wrongly set the timing up. What I would have done is set the back plate to the centre of the 3 holes.
Then get cylinder 1 to 4 to the firing mark, set up the bulb as you did and then move the back plate for points 1 to 4 only – back (retard) until the light just goes out. Thats 1 to 4 set, then turn engine – 2 to 3 cylinder firing position, and adjust that back plate until the bulb goes out. Both sets are now correct, but check with a strobe light. The bigger back plate should be always in the centre of the three screws, and the two sets of points – back plates – only are moved.
Hope this helps.
Essa parece que esta melhor, mais bem cuidada do que aquela 750 blue.