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Ignition issues

Captain Dave

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Hi all I am a new member and this is my first post. I purchased a non-running 1980 CB750C. Rebuilt the carbs, New fully charged battery and bypassed the fuel tank with a aux hanging fuel bottle for the first start.

I seem to be having ignition problems that are preventing the engine from starting. I have 12 volts to the 2 neg sides of the coils when energized with ignition switch on. I tried to check the static timing.

I disconnected the yellow positive side of one of the coils, hoked up a test light with a 3 way pigtail to that coil connector, reconnected the yellow lead to the pigtail, the 3rd line to my test light and grounded the test light. When I rotate the crankshaft the test light remains on through 360 degrees. Same thing with the blue side of the other coil.

I did a visual inspection of the coils and one of the coils has 2 large cracks. I removed the coils and tested them. They both have a resistance of 480. Manual states 530 +-50 so that seems to be at the limit but passed. I have ordered 2 new coils due to the cracks.

I then removed each ignition wire and checked the resistance through them. #1 5.06, #2 5.43, #3 zero and #4 5.14 all on 200K scale. Obviously # 3 is shot for some reason. I dissembled the plug caps from the wires. The # 1 & 4 have resistors in their perspective caps. The # 2 & 3 do not appear to have resistors? This seems odd but they look original. 2 & 3 are Nichiwa SDO5F 1 & 4 are Nichiwi XDO5F.

I say the # 2 & 3 caps don't have resistors because there isn't a slot for a straight blade screw driver to remove the resistors like there is on the # 1 & 4. This is puzzling any help is much appreciated.
 
Somebody has simply replaced the caps that do not unscrew, later ones are molded in resistor. Test them for resistance, if no resistor you will get zero, resistor around 5K.

You have tested the coils completely backwards, the yellow and blue are the NEGATIVE leads. Black/white wires are the 12V positives.

Do a compression check to weed out lack of compression as making you think the carbs need rebuilding, which they likely need anyway. Everybody rebuilds the carbs and then bike runs like utter crap due to (1), the carbs still not being right, they often have to be cleaned 2-3 times before even 'experts' get them right; and (2), the bikes are 40 years old and nobody ever set the valves on them to run compression way down and why most of them are sitting instead of being ridden. Run the valves and high likelihood of several almost closed up, what most of them do. A reading of .002" is the same as a zero and will burn valves. Why we here tend to set them at .005" regardless of what the service manual says. You are looking for 170 psi of compression and likely will not have it in at least one place.
 
Thanks acm49.

The ignition wires appear to be original. They are each labeled on the coil end coil 1 through 4 and on the plug end engine 1 through 4 accordingly. The wires are very stiff and in-flexable.

Can I go to NAPA buy off the shelf ignition wire and cut them to fit?
Is it 7mm wire?
I assume copper core?

I thought it was odd the ignition switch hot wire was connected to the negative side of the coil. I'm going to connect the coils to a charged battery correctly and see if I can get it to fire a plug. Obviously a previous mechanic had switched the low voltage coil leads.

Compression test is a good idea. I'll give that a test while I'm at it. Once I get it to run I'll take a look at the valve lash. Any advice is much appreciated.
 
I have dome a little more investigating. The plug caps appear to be original see attachment. The 90 degree ones both have a resistance of 94200. Both of the straight ones seem to be have infinite resistance. The meter remains on 1.0.

I connected the coil that didn't have a crack to my truck battery. Positive to + low voltage side of the coil and negative low voltage side to ground. I then put a coil wire into one of the high voltage outputs along with one of the 90 degree plug caps and a spark plug. I grounded the spark plug to the negative side of the battery and the coil bracket.

I was thinking I should see a spark when I open and close the low voltage ground. No spark. It could be the coil wires are bad or I am not doing this right?

I'm looking for some help. Is the coil test I'm doing a valid test or have I done something wrong again?

Can I get straight plug caps with the proper resistance at the automotive store?

Thanks in advance for any pointers.
 

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Get rid of the resistor type plug caps entirely, you don't need them and they just cause trouble. I have used automotive ones before and wires too but be aware that most car ignition wires are carbon dust coated string, they do not have metal wire conductors like motorcycle stuff does. You CAN get true wire plug wires at parts store but you will have to specifically demand them.

The coil is a waste spark type, you cannot just put one plug wire in, you have to have BOTH wires and plugs in place and grounded. IIRC you don't even have to ground the plug ends as such, all they have to do is to positively touch each other on the bottom firing end. The secondary voltage there runs down one plug and up the other to complete ground. Why you need both wires.
 
I went to the local motorcycle repair shop and got some 7mm cooper core wire $2.00 per foot. They also had new NGK caps $6.00 a piece I got 4. I'm waiting for the new coils to arrive and will test the system then.

I think I have a general understanding of the ignition system with one exception. What is the Spark unit and what does it do?
 
Power flows into the coil + and out the - like on oldschool points type system, the power then goes to the spark unit, which is a transistor switch that simply replaces the breaker points. The transistor switching to off is the same thing as the breaker points opening up to kill coil current, the plug fires at that point. Actually, a PAIR of plugs in the case of waste spark system. You may be interested to know that in that type one of the pair of plugs fires BACKWARDS, or from the outer wire electrode to the inside center core rather than the normally understood other direction. Why the two plugs in a pair must tie together at the outer shell to properly 'ground'. Power runs down one plug like normal then jumps to the other to run back up it backwards to complete the secondary circuit.
 
amc49 thanks for the lesson on the ignition Sys. Is the Spark unit the 2 aluminum components OKI MPS-200 on top of the battery housing?

Compression test isn't good. 90 90 60 90. Motor hasn't run in several years. Any possibility the numbers might improve if I get it started? Bike shows 30000 miles
 
Yes to question.

See what I meant by nobody ever setting the valves?, the compression drops like a rock when the exhausts burn. How most of them die way ahead of normal time.

You can try setting the valves and then trying to run it, a few come back from the dead. If those compression numbers are real the engine will run like pure crap.
 
Yours and do as you will.............the engines are odd in that they close up valve clearances to zero to then lose compression or burn the valves. It's because the valves are crap steel with barely a heat treat on top, then they were created for a time of low lead fuel. No lead at all has them wear through the heat treat to the soft steel underneath then they recede like lightning on the exhaust side to burn quickly. Owner had to keep valves set on a regular basis to keep any of that from happening, THEN they can go long distances with no problems. The rings by contrast are durable and hardly ever give trouble. Not running valves plus the carb issues the complicated carbs have are why you always find them 'running' but not really. The bikes run hard enough to scare the crap out of you if you keep the valves set though.

Anything under a true 100 psi of compression will have trouble keeping spark plugs hot enough to keep them from fouling out.

The alternator rotor is another common issue, if the bike is running then it may well have a charging problem, the rotors tended to fail left and right.

Luck.
 
Hello, I've been out of the country for 2 weeks got back yesterday and the new coils had arrived. Installed the coils spark looks good. Cranked it over with starter fluid and got it to fire up for 3 or 4 seconds. tried again and no luck.

Prior to installing the coils I ran a pint if Mystery oil through the top end. That brought the compression to 1/90 2/95 3/85 4/105

Removed the plugs they looked good. Cleaned them anyway tried again and can't get it to fire.

Pulled the valve cover to see if things were spinning, they are. Out of time for today i'm going to check valve clearance next. I'm off to Israel tomorrow 4 day trip. I'll try again when I return.

Any suggestions appreciated.
 
Well 2 weeks turned into 15 months. Long story short I got the bike running and it is running better than I could have imagined. Adjusted valves, Rebuilt carbs and tuned. Charging system seems to be OK. I've put around a thousand mils on it in the last few weeks and still cranks over with vigor.

Next step is to repaint tank, side covers and replace bent handle bars.
 
Hi all I am a new member and this is my first post. I purchased a non-running 1980 CB750C. Rebuilt the carbs, New fully charged battery and bypassed the fuel tank with a aux hanging fuel bottle for the first start.

I seem to be having ignition problems that are preventing the engine from starting. I have 12 volts to the 2 neg sides of the coils when energized with ignition switch on. I tried to check the static timing.

I disconnected the yellow positive side of one of the coils, hoked up a test light with a 3 way pigtail to that coil connector, reconnected the yellow lead to the pigtail, the 3rd line to my test light and grounded the test light. When I rotate the crankshaft the test light remains on through 360 degrees. Same thing with the blue side of the other coil.

I did a visual inspection of the coils and one of the coils has 2 large cracks. I removed the coils and tested them. They both have a resistance of 480. Manual states 530 +-50 so that seems to be at the limit but passed. I have ordered 2 new coils due to the cracks.

I then removed each ignition wire and checked the resistance through them. #1 5.06, #2 5.43, #3 zero and #4 5.14 all on 200K scale. Obviously # 3 is shot for some reason. I dissembled the plug caps from the wires. The # 1 & 4 have resistors in their perspective caps. The # 2 & 3 do not appear to have resistors? This seems odd but they look original. 2 & 3 are Nichiwa SDO5F 1 & 4 are Nichiwi XDO5F.

I say the # 2 & 3 caps don't have resistors because there isn't a slot for a straight blade screw driver to remove the resistors like there is on the # 1 & 4. This is puzzling any help is much appreciated.
Throw them all away and buy new ones. I had my carbs apart 3 times and finally figured out that the wires were probably original (1979) Quite the improvement ! I just used automotive ones that would fit.
 
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