Topic: 1975 CB750 Condenser Question

[John G]

I have the part number; which is 30250-300-154.
Is there a cross reference number for any usable condensers?
Also I did as they say in the Honda manual I have to hook a battery to the condenser and then touch the end to the body of the condenser to check for a spark. I don't get a spark so I'm assuming I need new ones.

[newday777]

Beware of aftermarket condensers. They die very fast.
Use only Honda condensers.

#10
https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB750KA_(76)_750_FOUR%2c_JPN%2c_VIN%23_CB750-2540001_TO_CB750-2575894/POINTS/b3d05c73-22e7-4620-b018-132301d96b6e/80fb05af-446d-41cb-a88f-21b6993df839/y

[69cb750]

Original condensers are good, aftermarket are bad
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2323012.m570.l1313&_nkw=30250-300-154&_sacat=0
If bike runs good condensers are good.

[bryanj]

If they are relly selling the complete assembly for$89 thats a steal!!!

[HondaMan]

I have the part number; which is 30250-300-154.
Is there a cross reference number for any usable condensers?
Also I did as they say in the Honda manual I have to hook a battery to the condenser and then touch the end to the body of the condenser to check for a spark. I don't get a spark so I'm assuming I need new ones.

This was a silly (bordering on stupid) test that someone at Honda put into (many of) the manuals. In order to see the spark from a 12 volt charge you would have to be in almost total darkness when you touched the lead-to-case together to see the spark from a 0.24uF capacitor at 12 volts: you can barely see a spark from a 1,000uF capacitor at hundreds of volts!

A much better test is to run the bike in a darkened garage and watch the points for arcing. If it is a tiny spark bouncing about the contacts, the condensor is good. If there are sparks flying off like a 4th of July sparkler, the condensor is open (i.e., no capacitance) - a test you can also mimic by just disconnecting the condensor from the points and running the engine that way. If it is shorted, or 'leaky' as a partial short, the bike will struggle to run after warmup. The latter is the symptom and signature of many Chinese-made crap condensors.

[John G]

Thanks HondaMan,
The bike didn't like to idol after a 2 hour highway run. I checked the valve adjustment and the thought I would check the electrical components. Now that I've watched the university of youtube I can use my meter to check the condensers. Your explanation is also helpful and easy to perform. Again thanks. I'll put a message out when I get it figured out what's the issue with the idol.   

[Deltarider]

This video may help you diagnose a faulty condenser:

At the breakerpoints a bit of sparking is normal, excessive sparking or arcing is not good and can indicate a bad condenser. In this vid the left condensor (1+4) is bad, right condensor (2+3) is good.
Video: courtesy Ulf Penner

[bryanj]

I do not remember seeing that test in a proper Honda manual, all the one i had said to use the Honda "suitcase" tester with capacitance meter

[MauiK3]

Great little video, really shows what we need to know.

[Medyo Bastos]

If you listen closely you can hear that erratic spark from a short distance


Sent from my iPhone using Tapatalk

[HondaMan]

Excellent video, Delta! That's the best way (outside of Honda's old tester) I know of to diagnose a bad one.

[jwurbel]

Southbound has been out of that part for quite some time.  Just letting you know as I tried recently.

[trigger]

I had a condenser go last year on a CB750. No unusual sparks at the points, the bike would rev fine standing still but, would misfire under load out on the road. Can not complain as, the bike is 48 years old and it was still on the original condensers .

[HondaMan]

I especially miss the old Hitachi (and the American-made copies from Blue Streak, sold during the 1980s) condensors. They were somewhat self-healing after a long sojourn without power applied: their capacitance would fade a little to about 0.16-0.20uF, but using them for a couple of weeks would make them come back to 0.24uF or more. They were of a stiff waxed-paper core. The insulation went to polymer insulation around the late 1990s, so they either work and stay working, or they die almost as soon as they are used. The testing causes a large failure rate, from what I've heard: those that are not tested get to die in your own bike. 

[HondaMan]

I do not remember seeing that test in a proper Honda manual, all the one i had said to use the Honda "suitcase" tester with capacitance meter

The one we had never worked for condensors, from the first day we got it. I repaired it at one point (broken wiring, I think it was) but then the vibrator tube that was supposed to simulate points to run coils for their testing also quit, and we couldn't get one. It was cool when it worked, showing clearly good vs. weak coils. After that it just became a bench volt-ohm-amps meter in a suitcase.

[bryanj]

One for sale on ebay US at the moment for $200+ but missing the strobe

[Deltarider]

Excellent video, Delta! That's the best way (outside of Honda's old tester) I know of to diagnose a bad one.

Actually, it's a miracle, at least imo, the bike runs the way it does run. I mean, seen the arcing, I would have expected the engine to run way worse than it does.
Now that electronic thingies are becoming cheaper and cheaper, would any of you guys know of a thingy to test coils. What I have in mind is a device that simulates running, preferably with a switch so you can bring 'the rpm' up to 10.000. So by have it going for 15 min. you'd detect intermittent firing or even quitting and you would know your coil has aged and should be replaced.

[bryanj]

Its not the time you test ist the max gap

[Deltarider]

Its not the time you test ist the max gap

I was told that often you can start your car or bike and everything seems OK and then 10-15 minutes later a coil starts malfunctioning.

[bryanj]

Yes but having used the suitcase tester mentioned above that shows up as spark failure at a smaller gap, on the tester you increase the gap until spark failure, from memory 7mm is ok

[Deltarider]

Ok, so you referred to that suitcase (see pic), me, I referred to a big workbench I've seen in an oldfashioned workshop specialised in electric equipment, like starters, alternators, distributors, coils etc. I'm afraid these specialised workshops have dissapeared. At least the one in my neighbourhood did.

[bryanj]

I think that is just a different version of the "suitcase" tester mentioned above, either or were a requirement in Honda dealer workshops same as the special tools board and vac gauges

[Oddjob]

The part labelled service tester was for the coils IIRC. The little perspex window covered the movable poles with the dial at the bottom increasing the gap between the poles. There was a capacity tester for condensers on there as well, we used to charge them up and thrown them at other mechanics shouting catch as we did so, if they caught them they got a bit of a belt from earthing them out through their body, not a PC thing to do now but we had to make our own amusement back then  .

TBH the suitcase hardly got used, it was too much of a faff to get all the stuff out, we also had the roll cabinet one with the oscilloscope screen in it, only ever saw that used once.

[bryanj]

Ken, there were two different versions of the "suitcase" tester.
One had one analogue dial to do everything with the selector switch
The other had 2 analoge dials where the rh one did dwell, points resistance and capacitor testing, the lh one did resistance,volts,amps.
Both had the adjustable spark gap tester, not sure if both had the strobe unit.
At Meads it was screwed down on a movable cabinet at about4ft high with a car battery in the bottom to power it. I used it most weeks for something, usualy points condition, frqentlycoil test

[scottly]

What I have in mind is a device that simulates running, preferably with a switch so you can bring 'the rpm' up to 6000. So by have it going for 15 min. you'd detect intermittent firing or even quitting and you would know your coil has aged and should be replaced.

I built an ignition tester that can spin the points cam up to 10,000 RPM.