Topic: New to the 750, need some help from the guru (PIC HEAVY)

[vtwin51]

Hello all,

I'm pretty new the old bike scene. I picked up a 74' CB 750 last OCT2012. Its a complete custom build done by a guy up in PA. I'm located in Northern VA.

So there is a lot to this bike that I cant find info about in manuals. So I've been having an issue with how this bike runs after riding it for more than a few miles. The same symptoms have shown up on a few different occasions. So while I was riding the bike stalled out going about 70MPH. It didn't lock up or anything but just made a sound like it lost complete power. I quickly dropped it a gear and dumped the clutch and it started up again. But then died another few hundred feet. So, I pulled over and tried to kick it over(its a kick only system). It refused to start. It had good compression but wouldn't start. So I waited about 20 minutes, intermittently trying to kick it, and it finally started. I got off the exit and turned around. after about another 2 miles it died again while riding, I dropped gear and dumped clutch and got it started again. And drove it almost all the way home when it died again with same symptoms. After letting the bike sit for some time it will start back up.

So a little on the design of this bike. It is a batteryLESS system using a large capacitor. Again, I didn't build this bike. So i know Ill catch some flack for that. I have done some research into this and see that it is a usable system if I'm riding with minimalist functions (Headlight, brake light).I replaced the previous 5600UF CAP with a 10,000UF CAP hoping that was the issue. But no change. 

It doesn't have a starter and all wiring is stored under the starter cover. I believe a picture says a thousand words. so here goes some pictures.

I can take pictures of anything else you all might think could help. This bike really is an interesting beast. I'd really appreciate any input you all might have.


Thanks a ton.

[vtwin51]

More pictures to show the internals and parts.

[tomkimberly]

Did you hear a hissing sound? You could have a blocked fuel tank vent.

Tom

[HondaMan]

Those electrics sound a little spooky! This bike's alternator requires a current thru the field coil to generate any power at all, so a capacitor-type system is probably a pretty shaky foundation. Can you at least install a small 12 volt battery of some sort? A gel-cell or maintenance-free type would be fine.

[78whiteorbs]

put fresh sparkplugs in ...see what happens.

[vtwin51]

Tom, My first thought was fuel. But I immediately checked my two separate fuel filters and both bulbs were full.

HondaMan, I've gone back and forth at rewiring the whole bike. I just haven't yet cause i didn't want to miss the good weather. I also made a quick connect for the battery so I can put on a larger 12V. I just don't have a place to mount it yet as there's no battery tray. So I'm in the process of fabbing one to it.

Before the bike died last time I did some circuit tests with multimeter. That CAP was charging at 14.5V. Also the key switch is a 2 stage. Turn it once to right bike has power. Turn it again it has lights.

Is there any tests I can do in its current configuration. I understand most basic mechanical tasks and have a small cache of tools.

Thanks

[Evening Echo]

I had a similar issue on an old bike. The coil was failing and would break down when it got hot. As it cooled it would be fine again. It had me scratching my head for a while. I'm not familiar with the dyna coils but might be worth looking into.

[vtwin51]

Echo, That's something I've been reading into. Its just hard for me to believe that these coils could be bad seeing how the whole bike was built less than 2 years ago using mostly new parts to include the coils. But seeing how it always dies once I've been riding awhile and how only time helps before it will start, that might be something to look into.

[Evening Echo]

It wouldnt surprise me. It was happening on my '62 bmw after I had fully restored it, with new loom, reg, condenser, coils etc. I reinstalled the original bosch coil and the bike is still using it! Would you have another bike or know someone to swap them out with. Might save a lot of hassle. Intermittent faults really are a pain to get to the bottom of!

[Stev-o]

Tom's 750 w/Dyna coils failed when hot last weekend. We put stock coils in and it ran great.

[chewbacca5000]

Hondaman sells a resistor pack that I use when I run those dyna 3 ohm coils it is inexpensive here is the link

http://sohc4shop.com/catalog/product_info.php?manufacturers_id=12&products_id=44&osCsid=29aeaf77e0fdc7898f55fea0ce45eff7

Either that or go back to 5 ohm coils.

[vtwin51]

Thanks a ton for the info and the link. Is there a way I can test the coil to be sure they are the problem? I love an easy fix but hate chasing a problem by purchasing parts.

thanks

[lucky]

Did it slow down before it died?

It could be seizing up from a lean mixture at highway speeds because it has velocity stacks and if it was never rejetted it could seize completely and throw you over the handlebars or just lock up the rear wheel.

If you give it quick sudden throttle while sitting, still does it bog down or quit running?
That is a sign if a lean mixture.
Any backfiring?

[chewbacca5000]

Thanks a ton for the info and the link. Is there a way I can test the coil to be sure they are the problem? I love an easy fix but hate chasing a problem by purchasing parts.

thanks

Not sure how you would unit test the coils.  I just know the 3 Ohm really use alot of juice and the 3 ohm resistors will help if you want to stick with that coil.  By default the coils are 5 ohm.  Lower ohms use more juice.

  Make sure you answer Lucky and find out what the jetting is.  If it is seizing do to heat from a lean mixture it could be dangerous.

[Stev-o]

When the bike dies, pull a plug and check for spark.  That will give you a clue on if your coils are working.

[scottly]

In the pic of the spark-plug, it appears to be carbon fouled; too rich, not lean.

[vtwin51]

Lucky, When the bike dies it doesn't lock up. It makes a sound like all the cylinders are moving without the spark. Almost like a fast humming sound if that makes sense.

The picture of the plug was the #4 plug. After I saw that I pulled #1 and it was much cleaner.

When its running I can give it quick throttle and it responds very quickly. I do believe the builder did rebuild the carbs with proper jets.

And no backfiring except when engine braking going down a hill.

Also I forgot to note. I thought it a small issue but after I have the motor running for a while, if I leave the petcock on "on" it will leak fuel out the bottom of the carbs. My solution to this is to put petcock on "off" after riding. Im not sure if that could effect it any way.

Thanks for all the info. I'll be purchasing the resistor pack tonight. Figure they cant hurt.

[HondaMan]

Lucky, When the bike dies it doesn't lock up. It makes a sound like all the cylinders are moving without the spark. Almost like a fast humming sound if that makes sense.

The picture of the plug was the #4 plug. After I saw that I pulled #1 and it was much cleaner.

When its running I can give it quick throttle and it responds very quickly. I do believe the builder did rebuild the carbs with proper jets.

And no backfiring except when engine braking going down a hill.

Also I forgot to note. I thought it a small issue but after I have the motor running for a while, if I leave the petcock on "on" it will leak fuel out the bottom of the carbs. My solution to this is to put petcock on "off" after riding. Im not sure if that could effect it any way.

Thanks for all the info. I'll be purchasing the resistor pack tonight. Figure they cant hurt.

The Resistor Pack will help reduce the current to those overkill coils, leaving a little more power available for other things. That might help? You can use the standard 1 ohm coil, but in a weak charging system like this one, I might recommend the 2 ohm version (like for the smaller Fours that got stuck with these 3 ohm coils somehow). While this reduces the spark voltage somewhat, it is still more than the stock coils (although the duration is shorter).

One thing that Dyna coils are famous for: if the sparkplug caps are 0 ohms (or just metal caps) or if the short wires from the coils to the plugs are the silly silicone resistance wire Dyna sells, the coils tend to overheat after a while. So, while this next suggestion may seem complex, it is a semi-analysis of the setup you seem to have:

1. If you use the 1-ohm Resistor Pack, use the 5k ohm (5000 ohm) sparkplug caps at the minimum. If you can find the 10k (10,000 ohm) version, so much the better. Use sparkplug gaps of about .040" to .045".
2. If you use the 2-ohm Resistor Pack, use the 10k ohm plug caps. This will extend the spark duration, leaving the peak voltage at about 2kV more than stock. Use stock plug gaps with this arrangement (.028" nominal).

Either of these arrangements will cool off the coils, which will reduce their current when hot. Maybe this will help?

[scottly]

Also I forgot to note. I thought it a small issue but after I have the motor running for a while, if I leave the petcock on "on" it will leak fuel out the bottom of the carbs. My solution to this is to put petcock on "off" after riding. Im not sure if that could effect it any way.

Yes, the fuel leaking out the bottom of the carbs may be part of the problem. What do the number 2 and 3 spark plugs look like?

[754]

If it starts again after cooling a few hours, it really sounds like coils..but then only 2 cylinders should quit. ..maybe ignition ?

[vtwin51]

So I'm definitely agreeing with the coils being the culprit. I went out to the bike and it kicked over first try. I ran it for a little over 10 min and it idled fine without a hickup.

Sooo some things to note. I got down and looked over everything I could see and noticed it seems like there seems to be excess fuel going in carb #4. There was some fuel near the screen inside the velocity stack. Not a whole lot but it was moist. I pulled the float chamber off and there was fuel in there as I believe there's supposed to be. the floats looks fine and moved freely.

Also I wanted to get an eye inside the AC generator case so I pulled it off and look what I found. This doesn't look factory at all.

[HondaMan]

No wonder the starter was removed! 

So...next question: is there a voltage regulator? I wonder if that cryptic "16A" on the windings indicates "16 amps"? If so, you're 3 amps ahead of a stock alternator.

I don't see any brushes in the picture, so am supposing it to be a field-regulated alternator. So, there should be a regulator and rectifier somewhere.

[vtwin51]

HondaMan, there is a regulator/rectifier. I pulled it off to find make model info and all there was is a sticker on back. I'll include a photo.

Also a little embarrassing to show but I guess I didn't tighter the gasket down tight enough cause my wife sent this photo to me at work today.
So I got to spend my evening spreading kitty litter everywhere and trying to scoop up about 4QTS of oil. I was due for an oil change anyway.

[HondaMan]

I'm not gonna laugh at you (well, maybe not), because I've NEVER done that myself...(Yikes, was that lightning???!). 

Have you written me at my HMAN750 e-mail, from the SOHC4shop website? I think I'm conversing with you there, too?

[vtwin51]

yes, I sent you two emails asking about the resistor packs and what you thought best and how to purchase.

From what I got from the part number on Reg/Rec is it is a part for a harley. and I got that stator/rotor is a WASSELL. And from what I read they typically go on brit bikes. Any thought? This kind of thing is out of my knowledge base. I know how to find compatible OEM parts but these parts are all foreign to me.