Author Topic: 1975 cb750k no spark  (Read 565 times)

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Offline cado007

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1975 cb750k no spark
« on: October 02, 2025, 02:33:20 PM »
Well if you have been following my thread we successfully opened the bottom end and replaced a broken shift drum. I rode it the past Sunday 120 miles ran great no leaks however around 70 mph and in hi end it was a little boggy. This bike has a PAmco ignition and always started - idled - ran like a sewing machine and now shifts great but the top end speed barely get to 80. I pulled the carbs which were dirty looking but again - started idled great. Also I put new plugs and ngk caps and wires on it. Anyway I pulled  the 086a carbs and they had some generic mains that said 223 AB-( the previous owner had been running pods) I also has a set of 0647b with  the proper 105 kehin mains and 40 slow. These were clean and rebuilt but were given to me and have been sitting in a bag for a decade = I pulled bowls sprayed cleaner but everything looked new. I did not check needle level hoped it was set to 4 - Anyway put new spark plugs - put vacuum gauges on - had fan running - I was having an issue with them not idling all the way down like the idle screw was not touching and it was idling at 1500 rpms - again they have been on shelf for decade - anyway I got them where I thought was good went to go for a ride and I had forgotten to put the #4 plug wire on. I had been dicking around for at least 5 minutes with it unhooked - yay. So I plugged it in and went to resynch and it had a weak spark  initially - now it seems to have no spark on all 4 and won't fire. Did leaving it unhooked damage the pamco or the coils. I checked the primary resistance on the pamco coils and it was 2.8 - 2.6 taking in the resistance of meter on both coils. I confirmed that I have power to the coils - any thoughts - can u still buy replacement pamco coils or use a different coil with the pamco system although I thought that the hi input coils are around this spec - so any advice on what I have done so I do not have spark and also  why I was able to idle without idle screw touching at 1500 rpms would be great. I have both push and pull cables installed. Thanks Damon.       after more research - I think I may have fried the pamco - man just trying to fix one thing now back to square one for a dumb mistake - anyone happen to have a pamco plate for sale - does Pete even still make them for 750?   I could be wrong but that what I am afraid might have happened - I guess if I can't find. one its back to points or buy a dyna.
« Last Edit: October 02, 2025, 03:04:17 PM by cado007 »

Offline HondaMan

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Re: 1975 cb750k no spark
« Reply #1 on: October 02, 2025, 07:53:54 PM »
Well if you have been following my thread we successfully opened the bottom end and replaced a broken shift drum. I rode it the past Sunday 120 miles ran great no leaks however around 70 mph and in hi end it was a little boggy. This bike has a PAmco ignition and always started - idled - ran like a sewing machine and now shifts great but the top end speed barely get to 80. I pulled the carbs which were dirty looking but again - started idled great. Also I put new plugs and ngk caps and wires on it. Anyway I pulled  the 086a carbs and they had some generic mains that said 223 AB-( the previous owner had been running pods) I also has a set of 0647b with  the proper 105 kehin mains and 40 slow. These were clean and rebuilt but were given to me and have been sitting in a bag for a decade = I pulled bowls sprayed cleaner but everything looked new. I did not check needle level hoped it was set to 4 - Anyway put new spark plugs - put vacuum gauges on - had fan running - I was having an issue with them not idling all the way down like the idle screw was not touching and it was idling at 1500 rpms - again they have been on shelf for decade - anyway I got them where I thought was good went to go for a ride and I had forgotten to put the #4 plug wire on. I had been dicking around for at least 5 minutes with it unhooked - yay. So I plugged it in and went to resynch and it had a weak spark  initially - now it seems to have no spark on all 4 and won't fire. Did leaving it unhooked damage the pamco or the coils. I checked the primary resistance on the pamco coils and it was 2.8 - 2.6 taking in the resistance of meter on both coils. I confirmed that I have power to the coils - any thoughts - can u still buy replacement pamco coils or use a different coil with the pamco system although I thought that the hi input coils are around this spec - so any advice on what I have done so I do not have spark and also  why I was able to idle without idle screw touching at 1500 rpms would be great. I have both push and pull cables installed. Thanks Damon.       after more research - I think I may have fried the pamco - man just trying to fix one thing now back to square one for a dumb mistake - anyone happen to have a pamco plate for sale - does Pete even still make them for 750?   I could be wrong but that what I am afraid might have happened - I guess if I can't find. one its back to points or buy a dyna.

If you go to points you can also use my Transistorized Ignition, which stops them from wearing and improves the spark in the process.
See SOHC4shop.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book
Link to My CB500/CB550 Book: https://www.lulu.com/search?sortBy=RELEVANCE&page=1&q=my+cb550+book&pageSize=10&adult_audience_rating=00
Link to website: https://sohc4shop.com/  (Note: no longer at www.SOHC4shop.com, moved off WWW. in 2024).

Offline cado007

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Re: 1975 cb750k no spark
« Reply #2 on: October 02, 2025, 07:59:18 PM »
Thanks Mark,
My friend Flip has your box  on his cb750 and loves it.
question will points work with these Pamco coils that are metering about 2.6 or would i need coils too?
 

Online scottly

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Re: 1975 cb750k no spark
« Reply #3 on: October 02, 2025, 08:03:07 PM »
You should use 5 ohm coils regardless of points or Dyna ignition.
BTW, there are some inexpensive electronic ignitions around that some members have reported positive results with. Do some research. ;D
« Last Edit: October 02, 2025, 08:14:13 PM by scottly »
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Offline cado007

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Re: 1975 cb750k no spark
« Reply #4 on: October 03, 2025, 08:08:21 AM »
Thanks Scottly
I found one of your old threads that was great.
I ordered the ebay dyna copy coils and the dyna copy ignition plate
They ship from us and say they are returnable - i will give it a shot'Thanks for the advice
« Last Edit: October 03, 2025, 08:11:24 AM by cado007 »