1980 CB750 engine dies after riding 10-15 minutes

[Astronomichael]

Hello everyone. I've found this forum to be invaluable so far and would first like to thank everyone for their contributions.

Sorry for the long post to follow, but I want to be as thorough as possible.

I recently bought my first motorcycle. A 1980 CB750k. The first night I got i home I rode it around my neighborhood to get a feel for it. Had fun, all was good. The second day I took it out again and it died while turning around at a dead end holding in the clutch. I tried to start it up again and it refused to start. Let it sit for a couple minutes and tried again. The cranking from the battery started getting weak and I knew that the battery was dying. Fairly new battery. I pushed it home and came on here looking into what it could be and why it wasn't charging. After reading quite a bit about it I decided to test my rotor. It gave me a reading of 2.7 ohms. Being out of spec I decided to order a new Rick's Powersport rotor.

The new rotor tested 3.8 - 3.9 ohms so I installed it. Jumped the bike with a non-running car battery with the cables going from pos to pos and neg to bike frame. It started right up. Well, started right up like it did before. The guy I bought it from said it was "cold-blooded" and needed to be given some gas to warm it up a little before it would idle on its own. After about a minute or so of giving it a little throttle it idles. At idle I tested about 14.4v across the battery. Good, right? I decided to let it idle for about 5-10 minutes to charge the battery up a bit before riding. Noticed a little bit of white smoke or vapor coming from the exhaust while idling that seemed to clear up quickly. I rode it in my neighborhood again for about 10-15 minutes and it died in the same way without restarting. I didn't expect the battery to be fully charged after only a few minutes of riding, but also didn't expect the engine to die.

Now the guy told me that the carbs had been recently professionally rebuilt. I don't know if he was completely truthful, just going on his word. Seemed like a stand-up guy. Just adding this tidbit for thoroughness.

While riding the bike seemed to have good power. Quick responsiveness to throttle twisting. No bogging down or anything like that.

I've come across a few things like maybe the coils going bad, needing carb attention (cleaned or rebuilt), ignition attention, overheating, etc. Can anyone please help me diagnose what this might be?

Thanks,
Bryan

[TOOLS1]

Carry a spare spark plug, and when it dies pull a plug wire and put it on the spare plug to check for spark.
TOOLS

[Astronomichael]

Thank you. I'll probably try that tomorrow. Does it matter which plug wire I use?

For now I removed a couple of the spark plugs and the ends have a dry black deposit on them. I read a couple of charts and what I got from them was that it's either too rich or there is an ignition problem. Another thing I noted is that it doesn't seem to make any difference in the way the bike starts or runs with the choke lever either all the way in or all the way out.

[shinyribs]

The spark plug says the engine is rich.

True,it could be a sign of a weak ignition not burning all of the fuel mixture,but since you mention the choke has no affect on the engine I'd put my money on rich.

I'd be tempted to remove a carburetor float bowl and see where your jetting is for starters.

[Astronomichael]

So I just started it up and warmed it up to a hands-free idle. Touched all 4 of the exhaust pipes coming out of the engine and they were all equally hot (ouch!). I heard to do that to check and see if all the cylinders were active. Idled about 1 or 2 minutes. Revved the rpms up to between 3000-4000 and let it return to idle. Seemed ok. Revved it up a little past 5000 and it started to skip out a little while returning to idle. Gave it some throttle to keep it running and it was rough. Let it go back down and it died. Took off the far left spark plug wire and put it on a brand new NGK D8EA plug and tried to restart. First time no spark. Tried again and there was a slight dim spark. The garage was fairly dark and it should have been a pretty bright spark by my imagination, but it looked weak.

[shinyribs]

OK, so you know all four plugs are firing and your battery has full charge/voltage before you start the bike. And you charging system is good. All those things should lead to a healthy spark.

A fouled plug will create a weak spark no matter what the ignition/battery/charging system is doing.

Is your engine grounded back the battery well? That can hinder the plug grounding and create a weak spark.

You might also want to check out your jet sizes and see if they are out of whack.

[Astronomichael]

Thank you so much for your help here. I really need my CB750 for transportation, so getting it running properly is vital.

So how much dismantling of the carbs do I have to do to check the jet sizes? Could it be that someone installed jets that are too big? I was looking at this: http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revD.pdf and I have to admit that getting into the carbs looks quite intimidating to me.

The way I understand it is the black carbon on the plugs is simply an indication of improperly burned fuel whether it be from either weak spark or a too rich mixture, or both. Is that right? Could it be anything else?

I'll check to make sure the engine is grounded and clean up the contacts to properly ground it if needed. What wire and connection location should I be looking for? Is there a quick and easy continuity test I can run with my multimeter to rule out an improper ground?

Like you said with the condition of the choke operation and this type of plug fouling coexisting in my CB750, I'm leaning towards thinking I'm running too rich. Could running rich be what causes that sputtering down after revving to 5000 rpm and then being unable to restart?

I really think that once I get this problem nailed down and fixed I'll have a good bike.

[Astronomichael]

A little update. I called the guy who sold the bike to me and he didn't sound too certain about whether or not the carbs had been rebuilt. I drilled down into specifics a little bit.

Would there be a choke problem on a newly rebuilt carburetor bank? I wouldn't think so. The choke cable engages when the lever is operated. Would a newly rebuilt carburetor be running rich?

Can anyone say with confidence that a complete and competent rebuild would fix my problem? I am seriously considering sending my carb bank to MacGregor Carb Services in Washington. They advertise a complete rebuild as shown in the guide for $320 including shipping. I estimate it would come close to costing me the same amount dollar-wise to do it myself needing to buy tools, equipment, chemicals, and the carb kits. Also I really don't think I have what it takes to do that kind of deep-down meticulous work as indicated in that guide.

[shinyribs]

Astronomichael wrote

So how much dismantling of the carbs do I have to do to check the jet sizes?

 

You only have to remove the float bowl to see what jets are inside. You can do this with the carbs still on the bike. The #4 (far right when seated) will be the easiest to get to since the starter cover kills some clearance on the left side of the engine. There are 3 phillips head screws that hold the bowl on. You might have to shop around for a screwdriver that will fit in there,but if you find one you can get it off easy. I just recently did it myself on an '80 F. I used a 90 degree screwdriver.

Like this:



^^^That only worked on the two rearmost screws. Due to clearance on the forward screw I had to use a phillips bit and turn it with a wrench.

A bit like this:



You can probably find both of these at Lowe's,Home Depot or any hardware or auto parts store for less than $10 for everything.

Astronomichael wrote

The way I understand it is the black carbon on the plugs is simply an indication of improperly burned fuel whether it be from either weak spark or a too rich mixture, or both. Is that right? Could it be anything else?

In my opinion,correct. But if it is a too rich mixture finding the cause is the solution. It may be jetted too large or it may be something else. <- I had an idea about this earlier today.See below... *

Astronomichael wrote

 Is there a quick and easy continuity test I can run with my multimeter to rule out an improper ground?
 

Check resistance(ohms) between the negative terminal of the battery to the spark plug while it is installed in the engine. Don't check to the metal tip that the plug wire attaches to,check the metal portion that the wrench goes on.

Astronomichael wrote

I really think that once I get this problem nailed down and fixed I'll have a good bike.

I sure hope so!

* My idea from earlier today:  I remembered you saying that the choke seemed to have no affect on the bike whether on or off. Are you sure that the choke cable is actually operating the choke mechanism?

An easy test of this would be to remove the cover to the air filter. Once you remove the filter you should be able to look up inside the air box and see the throat of the carbs. Or at least 1 or 3 of them. You'll see the round plate,or butterfly,that is your choke plate. See if it opens when you turn the choke off. It would be awesome to find out that your issue is a improperly adjusted choke cable or maybe a broken choke cable.

I hope this helps somehow. If I can come up with anything I'll holler at you. I know how much it stinks to have an issue like this.

My apologies if I have seemed to have insulted your intelligence on anything I wrote here. It wasn't my intention,just trying to be as thorough and clear as possible. Good luck and keep us updated!

[Astronomichael]

Ok! I really appreciate the thoroughness and clarity in your explanation. Not insulted at all, in fact grateful.

I just took my air filter out and felt in there and from what I could tell the choke plates are open. The edges of the plates are facing straight rearward with open space above and beneath. I pushed the edge of one of them up and encountered some spring-loaded type of resistance. It did move, but didn't seem to want to let me push it all the way closed. When I relaxed my pressure it pressed back against my finger all the way back to its open horizontal position. When I did that I noticed that the visible part of the choke cable on top of the carbs moved a bit. I pulled out the choke lever and the cable then pulled straight up out! The catch piece on the end of the cable is now in clear sight. Moving the choke lever now moves the cable and the catch piece freely in the space above the carbs. I think it was only slightly connected to the choke mechanism before, and certainly isn't connected at all now.

Incidentally I removed the air filter and felt in there a day or two ago and the plates were open then too. I didn't know what I was feeling at the time though. I think the choke has just been stuck open the whole time. Maybe that explains why I have to give it throttle upon starting it cold. But in my mind doesn't explain the richness.

Now it seems like I really need to check those jets. I'll get to that tomorrow for sure.

Also I tested resistance across the negative battery post and the bolt part of one of the connected spark plugs. It fluctuated a bit before settling on 0.1, or was it 0.01. I had my multimeter set to 200 ohms. Either way it didn't read infinite.

[Astronomichael]

By the way here is a video of my CB750K idling, just to show you who we're talking about. Warm started.
http://vimeo.com/72022118

[Astronomichael]

Tried to get the bowls off on carbs 1 and 4, but those screws won't budge! I did notice that the boots between the carbs and the engine (front side of carbs) were totally loose. All 4 of them. The clamps just spun freely and even had back and forth play. I tightened the clamp screws. Seems odd, if the choke is stuck open and the boots are loose I'd think that would make me run lean not rich. Anyway, I'll try again to get those screws off the carb bowl covers and get to those jets.

[shinyribs]

I agree with that would cause it to be lean. But engines can do some awfully strange things sometimes.  You could always get those boots sealed up tight and reinvestigate. Seems like it would make it even richer, but you just never know.


And those screws are a pain! I wish they had hex or Allen heads on them. I intend to do that...one day...

[Astronomichael]

Well seems like I can set the idea of richness aside for the moment and possibly turn my attention to the ignition system. I am going to try and test the coils under the tank soon.

[Astronomichael]

I think after all it was just that the battery wasn't ever fully charged after replacing the rotor. It did need the new rotor for sure, but after running the battery down on the bad one it never got fully charged again. I'd only jumped it from a car battery to get it started. I invested $30 into this little beauty: http://www.amazon.com/gp/product/B000GZL29M and let it charge for a full day and night. Now my CB750 seems to be doing great. Still running rich, and the choke cable needs to be reattached, but she runs strong non-stop now.

[mike69]

i have the exact same problem also hard to start. also looking for answers. took out the carb man said they where rebuilt but two fuel rail donut gaskets leaked, and float took in gas had a hole after straighting this out still had the 15 min. ride

[Pcontrer714]

Hello everyone, I have a 1981 CB750 K and I have been experiencing the same issue however, this issue seems to come and go. It seems to happen once the bike is fully warmed. I been cruising the neighborhood at about 30 MPH and the it starts acting funny sputtering then dies. The next day starts and idles well then I take it out and it happens again and I have only ridden about a mile. The thing that baffles the mind is I rode a good 6 miles last week with no complication. I hope this adds to the discussion and resolve. Thank you.

[TOOLS1]

You guys have vacuum leaks. As the engine heats up the rubber boots between the carbs and head soften/expand allowing any cracks to open up, creating a vacuum leak.
TOOLS

[motogrady]

I know low voltage messes with mine.
The charging systems on theses are marginal at best.
I think they only start charging after 3k rpm.
You lug that thing around, you ain't charging battery.
And with the headlight always on, well that don't help.

Monitor your battery voltage.

I fixed mine when I found the molex electrical plug from the stator was half burnt near the fuse box.
Put a switch inline to the headlight and don't have it on during the day.
Seems to help it charge itself.

The battery gets weak, they run sluggish, the battery dies, so dies the bike.

Get a cheap battery charger, it runs good after a few hours on the charger, then begins to fade, you got electric/battery issues.