Author Topic: 1976 Cb750 supersport ignition and running issues (Read 4159 times)

Jon.Sorber · « **on:** August 06, 2016, 08:22:19 PM »

Hello Everyone,

This is my first post on this forum and my first post on any forum. I'm having issues with my 1976 CB750 supersport after it starts. When i get it started it will idle fine, riding it however is a different story, when i let go of the clutch and open it up it will die and sputter out. I will then have to kick start it to get it to come back to life. I've checked my battery and i read 11.95 VDC. All my electrical works. I put in new coils and cleaned the carbs in the past week. it has a non point system. It's some type of electronic ignition system that was in the motor when i purchased it. I can take a picture and post it up here if some one can identify it and point me in the right direction of the literature for it. I've had the bike for about 2 and a half years now and have riddin it all over the country, haven't ran into this before. Any advice would rule. I live in arizona now and the bike hasn't seen heat like this, wondering if this could be part of my issues, maybe the air screws on the carbs. Anyways any info/ suggestions would be awesome. Thanks guys

scottly · « **Reply #1 on:** August 06, 2016, 09:46:26 PM »

Welcome Jon! What is the battery voltage with the motor running at 3000 RPM? (Put a fan in front of the motor.) There is one particular electronic system that is known to fail under higher temps, usually causing the bike to only run on two cylinders. A picture of the ignition would help.

scottly · « **Reply #2 on:** August 06, 2016, 10:13:03 PM »

Some new members (but not all) have trouble posting pics until they have 5 posts. If you are having trouble, just make 4 more quick reply's to this thread.

Jon.Sorber · « **Reply #3 on:** August 06, 2016, 10:47:38 PM »

Scottly,

unfortunately i do not have a tachometer so i do not know my RPM's. I'll get some pictures of the bike up here soon. Thanks for the response and Pics quotes.

Jon

Jon.Sorber · « **Reply #4 on:** August 06, 2016, 10:54:26 PM »

Also my issue was happening before i cleaned my carbs. the initial issue happened about 2 months back. the bike was running fine about 60MPH for 15-20 minutes, 2 back fires in a row. Made it home bike wouldn't start the next morning. check coils and spark, no spark, tested coils. bad. replaced coils. did not check plugs. got bike running with new coils. the starting of current issue: at high RPM's and clutch let go bike will sputter and die.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #5 on:** August 07, 2016, 08:38:13 AM »

Jon, I'd start with the battery. 11.95V isn't a good sign. Charge it and take it for a LOAD TEST out of the bike. VOLTAGE doesn't tell the full story even if it shows 12.6V after charging. I use Advantage Auto Parts. They also have a good AGM battery. Get a battery tender style charger too.

Jon.Sorber · « **Reply #6 on:** August 07, 2016, 08:17:43 PM »

I've got the battery on the trickle charger, any other advice to check when i put it back in the bike.

scottly · « **Reply #7 on:** August 07, 2016, 08:34:34 PM »

Check the plugs, and monitor the voltage of the battery while on the "trickle charger".
One more post and you should be able to post pics.

Quote from: Jon.Sorber on August 06, 2016, 10:54:26 PM

did not check plugs.

flybox1 · « **Reply #8 on:** August 08, 2016, 11:09:26 AM »

2 hours after the battery comes off the charger, it should read 12.5v or higher at the terminals.
My 2 year old Scorpion AGM stil hovers around 12.8v

points cleaned? timing and advance done?

Jon.Sorber · « **Reply #9 on:** August 09, 2016, 07:36:53 PM »

all right guys, here's some updates. battery holds a good charge at 12.9. hooked it back up check my plugs. cleaned. black soot, not wet though. put them back in, turned my air screws 1 whole turn out. bike fired right up, let it idle for a bit, opened up throttle in neutral bike died. started it right back up. let idle for 5 or so minutes. hit kill switch to turn bike off. bike off. would not start back up. could not get bike started after that. check battery again 12.9 again. now we're here. i have a couple of pictures of the electronic ignition component that i would love to have identified. i have a tack on the way should be here friday. any thoughts? thanks guys

scottly · « **Reply #10 on:** August 09, 2016, 08:04:08 PM »

Quote from: Jon.Sorber on August 09, 2016, 07:36:53 PM

check my plugs. cleaned. black soot, not wet though. put them back in, turned my air screws 1 whole turn out. bike fired right up,

That may be a Prestolite ignition? Anyway, that's not your problem, nor your charging system.
Black sooty plugs can only be caused by too much fuel or lack of air. Pull the plugs again and see if they are sooty again. BTW, how are you cleaning the plugs? The best way is to burn the black off with a propane torch.

goldarrow · « **Reply #11 on:** August 09, 2016, 08:05:57 PM »

Prestolite ignition

Jon.Sorber · « **Reply #12 on:** August 09, 2016, 08:12:30 PM »

will do. i'll check the plugs again and burn the black off.

scottly · « **Reply #13 on:** August 09, 2016, 08:14:11 PM »

What was the last thing to change before this trouble started?

scottly · « **Reply #14 on:** August 09, 2016, 08:17:26 PM »

Take a picture of all four plugs before you barbecue them.

Jon.Sorber · « **Reply #15 on:** August 09, 2016, 08:19:14 PM »

i used a small silver wire brush and some carb cleaner to clean the plugs. any other advice on my air issue. i cant think of anything i changed before the issue started. it was kind of out of the blue. will do. i probably wont get to that untill thrursday or this weekend. thanks again sottly

Jon.Sorber · « **Reply #16 on:** August 10, 2016, 08:05:28 PM »

here are some pictures of my plugs, also wondering about the coil wiring from left to right. coil wires 1 and 4/ 2 and 3. just looking for confirmation on which cylinder they go to. left to right. the plugs were a little wet with gas when removed. i tried starting it one more time before removing them. it seems like an air issue, maybe more is need to allow spark. is this thinking correct?

Thanks for the help guys

scottly · « **Reply #17 on:** August 10, 2016, 08:19:45 PM »

Did the bike sit for an extended period? When did it last run right, and when did the problem start?

scottly · « **Reply #18 on:** August 10, 2016, 08:24:56 PM »

Were those same spark-plugs in the motor when the problem first started?

Jon.Sorber · « **Reply #19 on:** August 10, 2016, 09:11:01 PM »

the last time the bike ran well was about 8 weeks ago. the bike sat for maybe 6 weeks and yes the same plugs but in my attempts to fix it over the past couple weeks i've cleaned them once or twice. just started noting everything i do on the bike a habit i should have gotten into when i got the bike.

Jon.Sorber · « **Reply #20 on:** August 10, 2016, 09:15:43 PM »

heres a pic of my work area. A 6x10 enclosed trailer

scottly · « **Reply #21 on:** August 10, 2016, 09:18:17 PM »

I don't like to jump to conclusions when trouble-shooting, but this time I'll make an exception. The gas left in the bike 8 weeks ago may have gone bad over the last 6 weeks in the Phoenix heat, fouling the plugs. Don't even bother burning off the old plugs; install brand new plugs and report back.

madmtnmotors · « **Reply #22 on:** August 11, 2016, 04:24:21 AM »

Quote from: Jon.Sorber on August 09, 2016, 07:36:53 PM

hit kill switch to turn bike off. bike off. would not start back up. could not get bike started after that.

A couple things to check:

>Did you turn the kill switch back to the "run" position? (Hey, I've overlooked it at least once)
>Have you checked to see that the kill switch is working properly?
>Have you checked to see if you are getting spark?

Jon.Sorber · « **Reply #23 on:** August 11, 2016, 06:26:26 AM »

Scottly

I have brand new plugs on the way along with a tach. to help me diagnose how the engine is running. thanks for the tips. will let you know the out come in a couple of days. Thanks again.

Jon.Sorber · « **Reply #24 on:** August 11, 2016, 08:29:32 PM »

i just had my my rear tires tube replace. wondering about a good grease for the bearings and for the rear axel? any suggestions or tips? thanks

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Author Topic: 1976 Cb750 supersport ignition and running issues (Read 4159 times)