Author Topic: CB750 (K4) Ignition Coil Assembly... What are my options? (with Pics) (Read 23614 times)

sbparks · « **on:** May 11, 2010, 06:20:46 AM »

Hi Folks,

I am working to reassemble the ignition coil assembly for my bike.

A few comments... Its tough to fit the two coils in the frame where they belong when the wiring assembly AND the carburetor cables have to share the same space. I did some test fitting and it will be tight. Stock handlebars and stock cables.

A few questions: How should the carb cables be routed past the coils? I have pictures from my disassembly, but that wasn't a stock setup, so its not helping me much. Should the cables be routed past the right headlight ear, to the right side of the steering stem (by the top of the frame) past the gas tank mounting rubber and then under the coils? There almost seems to be a space under the coil for the cables to fit. The wiring harness will go underneath the coil as well, right?

Also, What options do I have for replacing my spark plug wires (that come off the coil)? Mine appear to be attached (and the parts fiche shows them as a single unit). My plug wires seem pretty hard... Would be nice to replace. Am I talking about replacing the entire unit? Are there aftermarket coil assemblys that would work for my K4? A search of Dennis Kirk didn't show anything that jumped out at me as being right.

Hoping for some guidance and for some options if necessary.

Thanks guys!

sbparks · « **Reply #1 on:** May 11, 2010, 06:37:15 PM »

Bump... No other coil options?

Is there a way that I could keep the original coils and get a fresh set of plug wires?

Thanks again!

scottly · « **Reply #2 on:** May 11, 2010, 06:51:28 PM »

Those coils fit into that space when the bike came from Honda, so they should fit now. Yes, the wires tend to get stiff, especially when they are cold, but unless the insulation is cracked, or damaged, they will work fine. Look closely where the wires come out of the coils. If no cracks, you should be OK. People have posted ways to replace wire with a special splice if there is enough good wire, and others have posted methods for replacing the whole wire inside the coil. After-market coils are available, too.

Johnie · « **Reply #3 on:** May 11, 2010, 08:01:42 PM »

Here are some pics of my stock K4 coils and wiring. The throttle cables go to the left side of the stem and under the coils. The wiring harness goes on the left side of the frame and mine went over the rubber as you can see by the pic. Honda made room for the harness to fit next to the frame back bone under the tank space. As far as the plug wires, I would just keep them unless you are having an issue. Mine are all a bit stiff but I attribute that to the age and motor heat. I did replace the plug ends on all 3 of my 750's and the coils and plug wires are fine. Click the pics to enlarge...

sbparks · « **Reply #4 on:** May 12, 2010, 06:04:33 AM »

Thanks Guys for the suggestions and Thanks Johnnie for the pictures - That says it all!

Johnnie - You replaced the plug caps? These are the 10K resistor caps from Honda (NKG brand)?

I guess to replace the wires (and keep my coils), I could splice new wires onto the place where the original wire exits the coil. I'll have to decide if I really need to do that or not. I did notice a small crack in the plug wire's insulation where the wire comes out of the coil. Maybe just some sealant over the crack. I was mostly concerned that they were hard and a bit unsightly.

Thanks again.

andy750 · « **Reply #5 on:** May 12, 2010, 06:19:58 AM »

You can also use CB900 coils. This is what I am using for my CB750K4 - have been for years. You can then replace the spark plug wires with whatever you choose. I use copper wires but have used graphite in the past.

Photo of my setup...

Good luck
Andy

sbparks · « **Reply #6 on:** May 12, 2010, 08:24:34 AM »

Andy,

Another option... Thanks. The ideas are what I'm looking for.

Did you need to do anything special to attach the signal wires (not the plug wires) to that CB900 coil? It looks as if the connections are made at the rear (the side closest to the seat) of the coil. Same Blue, Yellow and Black w/White Stripe wires on the colis?

Thanks again!

tlbranth · « **Reply #7 on:** May 12, 2010, 08:25:43 AM »

Andy, are those cb900 coils the same resistance as the stock ones (5 ohm I think)?

andy750 · « **Reply #8 on:** May 12, 2010, 08:52:54 AM »

Quote from: sbparks on May 12, 2010, 08:24:34 AM

Did you need to do anything special to attach the signal wires (not the plug wires) to that CB900 coil? It looks as if the connections are made at the rear (the side closest to the seat) of the coil. Same Blue, Yellow and Black w/White Stripe wires on the colis?

Thanks again!

Nothing special although my blue goes to yellow and yellow to blue...not sure why that is but it works (and dosent work the other way)

Tlbranth - sorry no idea if resistance is same as stock. I have a Dyna S to go with the coils and no problems in 40,000 miles. Touch wood

. Plugs - I use Denso Iridiums although found no difference with regular NGKs D8EAs but they may run cleaner

cheers
Andy

sbparks · « **Reply #9 on:** May 21, 2010, 09:58:52 AM »

To continue this thought...

I purchased all of the equipment I thought would be necessary to make this change... A CB900 coil setup (eBay), a length of solid core 7mm wire, 4 new plugs (NGK D8EA), and 4 new plug caps (Dennis Kirk). So, all set to go...

I test fitted the coil just yesterday.

There's really no way to use the original CB750 coil mounting brackets as the CB750 coils need the bracket to go under the coil whereas the CB900 coil bracket HAS to go on top of the coil due to the coil input wires being situated in the rear (facing the seat) of the coil.

OK, no biggie, so I use the mounting bracket that was intended for the CB900 coils.

The fit is different due to the fact that the 900 coils mount wider than the 750 coils. But they still mount (nicely) in the frame.

Problem is when I try to position the gas tank on its mounting rubbers. The coil (both sides actually) comes into contact with the inside recess of the tank.

Andy750... Did you have the same issues when you tried to fit your tank? What brackets did you use to mount your coils?

I could try to dimple the area where my tank touches the coil. I think if I get it past the contact point, there's more room behind that as I slide the tank into its final resting point.

How risky an operation would it be for me to try to 'take some room out of the tank' (put a hammer to it)?

Suggestions, ideas?

Thanks, Scott

PS. Pictures to follow when I get another moment...

andy750 · « **Reply #10 on:** May 21, 2010, 10:11:07 AM »

Quote from: sbparks on May 21, 2010, 09:58:52 AM

Andy750... Did you have the same issues when you tried to fit your tank? What brackets did you use to mount your coils?

I could try to dimple the area where my tank touches the coil. I think if I get it past the contact point, there's more room behind that as I slide the tank into its final resting point.

How risky an operation would it be for me to try to 'take some room out of the tank' (put a hammer to it)?

Suggestions, ideas?

Thanks, Scott

PS. Pictures to follow when I get another moment...

Pictures will def help as there may be differences in coils across the years. Mine were also bought off Ebay many moons ago and I dont recall the year they came from. In any case Ill take more photos of mine if required. I used the stock CB750 mounting brackets and have no clearance issues with the tank - are you sure your coils are on the left side? I mounted mine on the right side by mistake and then you have clearance issues. But other than this they work fine and tank (stock 74 750 tank) fits on the original mounting rubbers with no mods required. Dont bash your tank just yet! Photos will help here...

More pics of my setup:

Andy

sbparks · « **Reply #11 on:** May 21, 2010, 11:06:42 AM »

Andy,

Thanks for the reply.

A few comments... It appears that you mounted your coil 'underneath' the mounting bracket. You have to mount the coil this way due to the electrical 'tabs' on the coil are towards the lower half of the coil. This is how I positioned mine, except I used the CB900 coil brackets.

The 900 coil brackets are stamped steel vs. the 750 coil brackets appear to be cast. Both brackets use integral threads (no need to use a nut to accept the bolt that goes through the coil's mounting tabs).

Mounting the coil assembly left side vs. right side... You are referring to what side of the frame you are mounting the side of the bracket to, Correct? I have mine over on the right side of the frame. Why? Because my coil bracket is offset (left to right) and the way I have it best centers the coil assembly within the frame tunnel. I could attempt to mount the opposite way (the left side of the frame), but I would have thought that would make for a poorer fit. Besides, my tank seems to contact both sides of the tank anyhow.

I like your suggestions, but I'm not convinced mine is gonna fit (without some banging involved).

Pictures forthcoming...

Scott

sbparks · « **Reply #12 on:** May 21, 2010, 04:55:33 PM »

Here are the pictures of my 'new' setup. Judging from some of Andy's pictures, I'd say my setup (especially on the left side of the coils) appears more centered than your setup. Check out the detail of my coil brackets.

I still have the tank interferrence.

Thanks, Scott

Terry in Australia · « **Reply #13 on:** May 21, 2010, 05:31:22 PM »

Hey Scott, did you pick up on Andy's comment that he's running a Dyna S ignition? The CB900's ran electronic ignitions, not points like the SOHC 750's, and from memory the 900 coils were only 2 or 3 ohm, not 5 ohm like a CB750's, so while the 900 coils will work well with Andy's Dyna S, I'm not sure that they will if you're still running points? Cheers, Terry.

andy750 · « **Reply #14 on:** May 21, 2010, 05:50:43 PM »

I see your problem!! Your coils are on the wrong side -see pic DSC_0596.JPG - I made the same mistake and this is what I mentioned above. Look at my last photo - the front tab is on the left side of the frame. Look at the front bolt that slides through the frame...the coil should be on the other side of the frame when the bolt goes through. Try that and I bet it fits ok with the tank. If not Ill take a top photo and right side pic of my setup to illustrate.

Good point by Terry but try it and see...

good luck
Andy

sbparks · « **Reply #15 on:** May 23, 2010, 04:03:32 AM »

Folks,

With a little extra work, I am now able to fit the tank over the coils... Thanks Andy.

And no, I didn't exatcly focus on the statement that Andy made on the fact that you were running an electronic ignition (until Terry pointed it out to me). What is the 'Dyna S' and where do I get one? What's involved with the setup?

I was aware that the CB900 coils were lower in resistance, but I thought I read some posts that indicated that it would still work.

Again... suggestions please... Should I be looking at a new (electronic) ignition? Will my old one (the points) work at all?

What should I do here guys?

To refresh your memory, the whole reason I started this switch over was to get away from the nasty plug wires that are integrated into the stock coils.

Thanks for the ideas.

Scott (in Binghamton where the weather is FINALLY decent enough to ride!)

sbparks · « **Reply #16 on:** May 23, 2010, 06:06:03 PM »

Guys,

One more question to add...

I mounted my battery into the bike today for and performed a few tests.

Most of the systems were working, a few, not so.

One thing I noticed that was related to my coil installtion... The right coil... had the Blue wire attached to it, actually became warm as I had the power on at the key (engine not started, never tried). The longer I had the power (by way of turning the power on by using the key), the warmer the coil became. I believe the inner mounting rod (that goes through the coil) was what was getting warm, then the mounting brackets.

What's going on here? Bad coil? Mistake while reinstalling?

Thanks for the help!

Scott

scottly · « **Reply #17 on:** May 23, 2010, 06:13:51 PM »

Current flow through the coils will cause heating. Not a good idea to leave the ign on with the motor not running. The effect of the lower resistance coils is you will get a hotter spark, at the expense of shortened point life.

tlbranth · « **Reply #18 on:** May 23, 2010, 06:37:47 PM »

the other coil's points were open so did not warm up.

sbparks · « **Reply #19 on:** May 23, 2010, 07:12:38 PM »

So, you're saying that it is working as expected?

I just happened to have the engine sitting with one (the blue one) point closed and that caused the heat to build up?

Thanks!

scottly · « **Reply #20 on:** May 23, 2010, 07:16:09 PM »

Start her up!!!

tlbranth · « **Reply #21 on:** May 23, 2010, 08:40:03 PM »

I don't know how much heat should be expected, sorry. I just wanted you to know why one heated and the other didn't. Scottly seems to think it's OK.

andy750 · « **Reply #22 on:** May 24, 2010, 04:35:04 AM »

I tempted fate....when removing my tank at the weekend I caught the screw on plastic connector at the plug/coil interface and cracked it $:-\$ . Spark plug wire still attaches but no longer held in by the plastic piece. Still its last for the last 9 years so cant really complain....

Glad to hear yours is working. Just a note that may/may not apply to your set up. I need to reverse the blue and yellow wires to the hook up the coils to work so that blue goes to yellow and yellow goes to blue. Not sure if thats due to my Dyna S but thats what works for me. It wont fire/start in the correct order of the colours match.

Never noticed any heat build up (that was detrimental).

Good luck
Andy

Skunk Stripe · « **Reply #23 on:** February 07, 2011, 09:34:49 PM »

Sorry to dredge this up! But the cb900 coils are 2.5 ohm correct? My 78 750 charge system seems fine with no issues and I never need a charger to keep the battery up. These style of coils will be ok with the dyna S? Seems so from Andy's experience. Anyone else use these and the S?

Terry in Australia · « **Reply #24 on:** February 07, 2011, 10:45:40 PM »

Probably not mate, I just installed a new Dyna S ignition and in the instructions it says to use coils that are at least 3 Ohms. Kong was selling a set of Dyna 3 Ohm coils and leads for 90 bucks this morning, if they're not sold yet. Cheers, Terry.

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Author Topic: CB750 (K4) Ignition Coil Assembly... What are my options? (with Pics) (Read 23614 times)