1979 CB750 Not running

[vintagehondaguy]

Hello fellow CB750 enthusiates. First let me start out by saying that I am new to this site and am not very familiar with bikes in general. I am pretty handy working on cars and I understand that these older Hondas are not very complicated to work on so i plan to do most if not all of the work on my bike in the future. Anyway I purchased my fist bike last October, a 1979 CB750 Limited Edition with just under 12,000 miles on it. I rode the bike a few times after buying it before putting it away for the winter and it seemed to run very well. I recently took it out and discovered that during my first ride the bike ran a little sluggish but nothing really too noticeable. A week or so later I decided to take it out again and ir ran fine until I got about two or three miles down the highway and the bike began to lose power almost as though it were running out of fuel. I pulled over and checked the tank and it was fine. I tried to start it and it would crank over but would not start. I tried cranking it until i finally killed the battery. Well needless to say I had to have the bike towed home. When I got home I decided to connect the battery to my charger but I noticed that the positive battery post had melted and the cable bolt was loose. I summized that the cable bolt must have been loose to begin with and so it was likely that it was arcing out and overheating the post thus melting it. Well I replaced the battery with a new sealed fully charged battery and started the bike. The bike ran rough at first but then smoothed out. I decided to take the bike out again and the same thing happened minus the battery post melting. The battery is not completely dead as I do have lights but not enough juice to turn the engine over. Can someone out there tell me if this sounds like an electrical problem or a mechanical issue or fuel problem?. The fuel in the tank is fine but i'm now wondering if maybe i have a fuel delivery problem or something electrical that may be the cause. I would appreciate any input or suggestions as to how to proceed with diagnosing the problem. Thanks  

[Kha]

Have you check your fuel filter? Dirty filters will do that, happened to me.

[vintagehondaguy]

I have not but I will do that today. Thank you!

Sent from my iPhone

On May 21, 2014, at 2:35 PM, "Kha [via Honda CB750'S]" <[hidden email]> wrote:

Have you check your fuel filter? Dirty filters will do that, happened to me.

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1979-CB750-Not-running-tp4051229p4051230.html

To unsubscribe from 1979 CB750 Not running, click here.
NAML

[TOOLS1]

It sounds like your charging system is bad, which is common on the DOHC bikes. You can download the Factory Service Manual for free from the manuals thread at the top of the home page here. It has very detailed instructions on testing the charging system. Also do a search for "DOHC charging" in the search feature. You will get hundreds of results.
TOOLS

[vintagehondaguy]

Thank you I'll check it out and post my findings.

Sent from my iPhone

On May 21, 2014, at 2:49 PM, "TOOLS1 [via Honda CB750'S]" <[hidden email]> wrote:

It sounds like your charging system is bad, which is common on the DOHC bikes. You can download the Factory Service Manual for free from the manuals thread at the top of the home page here. It has very detailed instructions on testing the charging system. Also do a search for "DOHC charging" in the search feature. You will get hundreds of results.
TOOLS

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1979-CB750-Not-running-tp4051229p4051232.html

To unsubscribe from 1979 CB750 Not running, click here.
NAML

[sgtslag]

Did you prepare your bike for the storage period?  The gas needs to be stabilized prior to storing the bike for several months; the battery should be connected to a smart trickle charger (Battery Tender Jr. is a great unit for this:  microprocessor-controlled, four different charging modes, self-monitors, changes modes as needed; sells for $20-$35, depending on sales).

Have you run a cleaner through the fuel system yet?  I use SeaFoam for both storage preparation (gas stabilizer for up to two years), and as a fuel-system cleaner (absorbs water, dissolves varnish) -- I add it to the gas tank on all of my vehicles, around once per month, as a PM.  You might have some partial blockage of the jets, due to varnish.  SeaFoam is fast acting, and inexpensive at $6-$9 per 16 oz. can.  If it works, great!  If not, you are only out a few dollars, and a few minutes of your time.

Back in '79, the API oil standard was SE...  Today's API standard is SN, and there is no comparison between the quality of modern oils versus the SE standard of yesteryear.  I owned a '79 K model, purchased it in 2006, with 9,600 miles on it.  I added SeaFoam to the crankcase, ran it idling on the centerstand for five minutes, shifting through all gears, to circulate the SeaFoam throughout the engine, and the transmission:  it turned my caramel-colored oil coal-black, due to dissolved carbon deposits, and other crud.  I changed the oil and filter to remove it all from the bike.  After another 5,000 miles of hard riding, I repeated the process:  the oil never darkened, even after 300 additional riding miles -- I changed out the oil and filter just to be safe.

The '79 models run at 250 F, which was the thermal limit of the SE oils.  This explains why I got so much crud out of my engine with the first treatment.  The modern oils, even the conventional oils, do not break down at 250 F -- their thermal limits are higher (how high, I don't know).  I would recommend treating your engine/transmission with SeaFoam once, then use modern API-rated oils (use only the non-energy conserving oils, though, to avoid clutch slippage caused by these additives!), or any API-SL or higher rated oils.  Shell Rotella T conventional/T6 Synthetic diesel oils (rated SM for cars) are excellent, have no friction modifiers/energy conserving additives, and they're inexpensive ($12/gallon, and $22/gallon, respectively, at Wal-Mart).

I would also recommend changing out the fork fluid.  Mine appeared to be original...  I used Dexron III ATF fluid in three different bikes.  It is what Honda used/recommended in the '79 model.  It is equal to around 7 weight fork oil, but it costs less than half of what fork oil will:  <$5 as opposed to $15, for a quart.  It improves the front suspension, handling, etc.

Best of luck with your bike!  Cheers!

[vintagehondaguy]

Great advice thank you!! I never realized how much the oil type and quality mattered. I'm gonna go ahead give this a shot. I did add ethanol shield with stabilizer 1oz. /5gal as recommended on the bottle and ran it through the carbs. I also used the battery tender as well (picked one up on E-Bay) for $25 before I put the bike away. I will try the Sea Foam. Thanks again.

Sent from my iPhone

> On May 21, 2014, at 4:54 PM, "sgtslag [via Honda CB750'S]" <[hidden email]> wrote:
>
> Cheers!

[vintagehondaguy]

Thanks I'll check the charging system as well.

Sent from my iPhone

On May 21, 2014, at 2:49 PM, "TOOLS1 [via Honda CB750'S]" <[hidden email]> wrote:

It sounds like your charging system is bad, which is common on the DOHC bikes. You can download the Factory Service Manual for free from the manuals thread at the top of the home page here. It has very detailed instructions on testing the charging system. Also do a search for "DOHC charging" in the search feature. You will get hundreds of results.
TOOLS

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1979-CB750-Not-running-tp4051229p4051232.html

To unsubscribe from 1979 CB750 Not running, click here.
NAML

[icerigger]

+1 on the charging system.  Try this link for a very good troubleshooting method.

http://www.motorcycleproject.com/motorcycle/text/DOHC_charge.swf