1981 cb750c - Wont Turn Over / Charge / and Stalls Out

[bcombs99]

I recently bought a rebuilt 1981 cb750c - completely bobbed out

2 MAJOR ISSUES:

Its a gorgeous bike, but I can't keep it running.

(Problem 1)
When I picked it up it was running fine.  Then 4 hours later when I got home I turned the key and it would rev twice then die.

The guy that did the rebuild put in a new battery, stator, rotor - etc...  And I've tested  them all with a multimeter and they are all reading good.

Therefore, the only way I get it running is when I jump it from my truck.

(Problem 2)
When it IS running after I jump it...  It is running really rich...  The Choke cable has been jammed so we disconnected it and installed a spring to have the choke run half open.
-Carbs have been completely cleaned
-Rejetted
-Float Needle Valves have been replaced on all 4 carbs
After this was done it was running okay... It would start up fine (after giving it a lot of gas) and it would go into 1st gear okay...

However after about 10 miles or so - the tank got low and I stopped for gas... Since then It hasn't been the same...

I jumped it to get it started and tried to go into first several times and it would stall every time... Even when I would try with the throttle wide open...


Does anyone have ANY ideas???

Below is the bike:

[TOOLS1]

COOL BIKE!!!!! First you need to repair the choke, and jet the carbs accordingly. Second, have you checked to see if it is charging 14 volts at 3000 RPM?
TOOLS

[bcombs99]

Thanks for the reply!

I had a buddy break the carbs down / clean them / jet them / replace the float valve needles...  And this got it to fire up...

But it was running really high RPMs and Back Firing...


I have yet to jump it since last night when the shit hit the fan... but I've been reading about testing the battery to see if it running 14 volts at 3000 rpms...  But the Tac has been removed so I don't know for sure when I'm at 3000 rpms..
   - but just to confirm what this will look for is the following?
        - to see if the battery is being charged while its running?  
        - if the battery is not running 14 volts at 3000 rpms that means its not getting charged
   - Is that correct?

Also,
What are you referring too when you say "jet the carbs accordingly"?  My mechanic said he jetted the carbs, shouldn't this have taken care of it?  Or, can you jet the carbs - but then ALSO have to sync them up or something?

He mentioned that I would need to adjust the "Valve" at some point too...  I'm not sure which he was talking about...


Another theory I had is when I was riding low on gas last night - the bike cut off and I coasted into the gas station...  I was thinking since the tank is super old and there is probably rust in it...  Some of the junk could have been sucked in somewhere and clogged something....
   - How likely is that?
   - How would I be able to troubleshoot/fix?

Thanks again for a GREAT website and all of the insight!



 

[TOOLS1]

To test for charging, all you need to do is rev the engine. You do not necessarily have to have it at 3000 RPM. For jetting. You can not just "jet" a carb. If your exhaust, or airbox is changed, you need to do plugchops, and read the plugs. This will tell you what jets to use at idle. half, and full throttle. However when you said it idles high, and pops, then add in that it needs choke to run, that is telling me you have a vacuum leak. Also the carbs must be synced for the engine to run correctly. BTW have you downloaded the factory service manual from the home page here?
TOOLS

[bcombs99]

Thanks for the insight...

Yes, I downloaded the repair guide... great tool!

Today I jumped the bike and got it started - as I did that...  A bunch of oil started leaking out of the bottom of the Generator cover - where the Stator and Rotor is lodated...
(Seen in the picture below)




After the oil stopped leaking - I measured the Volts on the Battery to see if it was being charged as the bike was running...
       -Turns out the battery is not being charged, as it read 11.59 volts
I read somewhere that this means that this could be the 'Regulator/Rectifier'? (does that sound correct?)



So I rode it around for about 10 minutes to see if it would charge and it was doing okay... but then about 10 minutes/15 minutes into riding it around...  As I would try and give it throttle the engine started getting weaker and weaker and would miss...  Until it got to the point where the engine just cut off.   After it cut off... I got nothing from trying to turn it on...

Also, I think the "airbox' has been removed from this bike







Today I checked the Voltage after I jumped it and had it running...  The battery was still at 12v so from what I've read it sounds like the "Regulator/Rectifier"?

Also,

[icerigger]

Yeah, could be. If the PO replaced the rotor cause it was bad there's a good chance he needed to replace the regulator as well. Sometimes the rotor will take the regulator with it when it dies.

[Murrayf]

you need to follow the tests in the fsm and test each component one by ine to find the culprit

and start by charging the battery and making sure it is good

[TOOLS1]

The manual has detailed instructions on testing each component of the charging system, or just do a search on the forum here. Other guys have documented testing their charging systems. Also your stock airbox, and exhaust are long gone on that bike in the picture. Those pod air filters make tuning those CV style carbs a bitch! You might want to check into a Dynojet jet kit.
TOOLS

[bcombs99]

Right...  I'm sorry I should have mentioned that...

I found that thread of troubleshooting the charging system and from following those instructions it has made me believe it is the regulatory /

I found a thread of someone testing the all charging components and it was as follows:

1. check the battery make sure it has 12 v  -  confirmed

2. check the rotor verify at least 4 ohms - confirmed

3. check brushes to confirm new / check to see conductivity - confirmed both

4. cut the engine on and test the battery to see if it is receiving more than 12 v (to verify being charging)
            -if not receiving more than 12 v, battery not getting charged while engine running
            -this means  rectifier/regulator is faulty


Just from inspection - I can tell the rectifier/regulator looks to be original

So I hope this should fix the charging issue
=======================================================

The stalling out and etc. seems to be the major issue now...  

I appreciate everyones insight...

[motogrady]

Yes, nice bike.

My 2 cents, big and foremost problem is a no charge to battery.
They get low on voltage, they run like crap.
You gotta figure out the electric thing first.

The jetting thing, when you go to different exhaust pipes, and drop the airbox
for pods, much more air moves thru the motor.  
They breath better, so you must increase the amount of fuel to match.
Bigger jets do that.

If it runs good on a fully charged battery, forget about all the rest for now.
Get that sucker charging.

[bcombs99]

So after looking at some more threads - I think I might have found some problems/fixes with the bike cutting off....

It looks like the PODS on a DOHC seem to be an issue if not correctly JETTED as mentioned by TOOLS1

Here is a THREAD I found discussing it:

http://honda-cb750-s.456789.n3.nabble.com/Pods-on-a-DOHC-options-td3867628i20.html


NOW, I just have to figure out how to properly JET the Carbs, as I have no idea what I'm doing... but this site is FULL of great mechanically oriented minds that have fantastic enough  to share their knowledge!

Thanks again...

[easyrider12]

Hey bcombs,  welcome to the forum.  Nice bike you have there, I have the 81c as well but I've been lucky to stay away from those issues.  Sorry I can't offer tech advice I'm just the class clown I guess lol.  see you around bud.