How To Diagnose And Repair An Old Motorcycle PT1 - Honda CB750

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.

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.

Part 2 where we do the repairs and fix the bike:

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[michaelmaletich]

I purchased this exact same bike, orange 1972 CB750 K2 brand new in 1972 (just graduated from high school) for $1820 out the door. It was one of the best bikes I’ve ever owned. Open the throttle and it screamed. But just cruise around and it purred. Can’t ever remember having a problem with it. Wonderful bike.

[stephenschneider]

I'm an old school motorcycle dude started riding in the 70s. As much as I love my new bikes I really dislike and struggle with all the technology crammed up my ass. I don't want traction control, abs, riding modes that each have their own riding modes! Wtf? I just want to ride. I miss my old bikes with carbs and "manual" controls. I love your videos. To see the younger generation embrace the early machines and keep em going, in original condition gives hope for our planet.

[jackfarr]

These old Hondas are lovely machines, very well presented, keep the content coming!

[kihestad]

Very informative, learned a lot! 👍 My first bike was a 74 CB550 super sport, this was like a walk down the memory lane 😊

[Martin_IPL]

I really enjoyed that, it's good to go through the basic stuff in the detail you did. looking forward to the next one.

[blazesboylan]

Excellent video, especially for those of us still riding carbureted bikes.

[kevinharmon]

My 1st bike was a 1976 750. With my father-in-law and his 73 750 and his son on a 500 we took our first cross country from Topeka to Roesburg Oregon. I still see the 1st time seeing the Rockies later Yellowstone then the Pacific. What a great trip for this flatlander and what a great bike!

[TheDafxf]

Better bikes than todays complicated machines 👍🏻🏴󠁧󠁢󠁥󠁮󠁧󠁿

[kevinbrislawn]

My younger brother sadly passed away in 2006 owned this same color CB 750 He really took extreme good care of it. I miss him a lot.

[mr.mikeyg.]

Beautiful bike. I had the year/color back in the 70’s. Sure miss that one.

[sqdl]

I just got a 1978 CB 750 and I pick it up today. When I was young, that was the bike I loved.

[iantobanter]

I ran a cb400f1 for 18000 miles bought from new in 1976-issues were sticking front disc calliper hinge and water ingress to coils- location was prone to attracting road spray. The calliper hinge was alloy and the pin it pivoted on was steel hence prone to seizing-addressed through stripping and copper grease. Bear in mind it was my daily ride in darkest wettest South Wales. One strange issue as failure of the frame tube due to corrosion at the prop stand bracket. It was followed by a cb550f2. I covered 62000 miles on that orange beauty over 4 years. I fitted electronic ignition and a Peter Furlong full enclosure chain case-ugly but effective in extending the life of oil sintered Izumi chains. A Cibie bright headlight conversion also improved matters. The nylon swing arm bushes were replaced with bronze items. A pig of a job-I think the 400 had grease nipples as does my current ‘03 Thunderbird. I always fancied a Difazio box section swing arm but they were super expensive. Keep an eye on the mudguards. My rears rotted through in no time-that old Welsh weather again. Rear brake cam was also prone to seizing-due again to steel to alloy interface-there’s a felt washer which you should keep oiled to keep the h2o out

[jamesthurber]

Diagnostic genius. Methodical and knowledgeable will win. Points timing as I practiced a few hundred times.

[jamiequinte]

I just bought a 1975 CB750 and really appreciate your knowledge being passed on.

[BoBjjjjs]

I just love the look of this machine, the right colour and just gorgeous. The model I always wanted but never had the money. Love the look of it. 😎

[robwinter]

i was 17 when that bike came out, i can remember the color being popular, i have never owned or ridden such a big bike, rob

[moochamber]

Don't worry; I will stop sending as we know that you are a busy man. I get chatty sometimes. have a good one. Enjoying the show!!

[danthemowermedic]

Excuse me while I binge-watch your channel! Love the video-style editing. Simple and fun are good! Works and flows well. Getting nervous sweats because I want to ride my 79' so bad. Getting paint-matched bags back from the painter here soon and a top-end rebuild over the winter! I can finally tune the carbs in because the gaskets are leaking so bad lol.

[ADVRider]

Nice and informative! I am in the process of convincing my wife we need a CB750. Found one for just over $1000. Your vids got me wanting one!

[sugarskulllyfe]

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