Topic: '75 CB750 - Trying to Diagnose Problem

[CB750_KBC]

Recently bought a 1975 CB750K. It ran well when I bought it and for about a month.

First trouble came on a short ride. The bike just started stalling. I'd pull over, crank it a few times and eventually it would start back up. But it sounded different, maybe like it wasn't firing on all cylinders. I'm new to this kind of bikes, so any help would be appreciated.

So when I got back home I also noticed a new, small oil leak. Very minor, but alarming for sure. I checked the plugs, they were charred black. Too rich? So I replaced them with new D8EA plugs. Right plugs? After the new plugs were in, the bike struggled to start but I did get it going. It would barely keep idle so I revved it a while and upped the idle screw a little. It ran for a bit, I brought the idle down a little, just under 2000. That's when I noticed the bike was running super hot, the engine and oil reservoir seems on fire they were so hot, even the key was hot when I turned it off. Typical?

So even with the new plugs, I can barely get it running and when I do I have to open the throttle and rev it just to keep from dying. And it gets really hot really fast. And the idle won't keep.

Oil delivery problem? Any help/guidance appreciated.

KBC

[Johnie]

Did you get a history with the bike? Could be a fuel delivery problem, carbs dirty, gas tank dirty, old points and condensor, etc. Screen in petcock plugged, etc. I would drop a carb bowl and see if you have some dirt in there for starters. If you do then tackle that problem and move on from there. As far as overheating, if you run it at idle for any period of time it is a good idea to put a fan on it. Last, be sure to check the oil only when the motor is warm like after a ride. Oil in these bikes tend to drain into the oil pan when they sit overnight. Then the new owner checks it cold, thinks it is low and adds to it. Hence, way over full. Not a good thing. Let us know how you come out...and welcome to the board.

[GammaFlat]

I vote for the sock/filter in the tank (happened to me).  The blackened plugs can be from a number of things and may be a red herring.  To check the sock, remove the petcock (remember that fluid goes the way of Newton's apple) and inspect the sock on the top of the petcock.  Mine was plugged with silt. 

[GammaFlat]

If you're having problems when hot I'd be thinking about bad capacitors...  Which could impact your spark.

[KB02]

Makes me think some dirt got in the fuel delivery system and clogged up the slow jets in the carbs. Pull the carbs and give them a good, thorough cleaning. Then recheck.

[CB750_KBC]

Thanks again all.

I've heard sync'ing the carbs can be a challenge. Does removing and cleaning them require you to sync them upon re-install? Or

KBC

[Bob Wessner]

Does removing and cleaning them require you to sync them upon re-install?

KBC

Sorry, but yes.

[CB750_KBC]

I've heard that syncing them requires a special tool...or is there a trick of the trade to sync them relatively easy?

[ev0lve]

I've heard that syncing them requires a special tool...or is there a trick of the trade to sync them relatively easy?

As trick as it gets
http://www.carbtune.com/


You would bench sync them prior to installing them - then fine tune that with a carb sync.

[MCRider]

I've heard that syncing them requires a special tool...or is there a trick of the trade to sync them relatively easy?

To your first post, if you're running it doing all your diagnosis while its stationary its going to get really hot. Shouldn't run for more than 2 minutes sitting still in a garge. Put a floor fan on it per Johnie.

As to synchronizing, you can "bench sync " them without a tool. It may still run poorly but if you've fixed the problem a bad sync job isn't anything as bad as you're describing. I'd say the carbs have to come off. Or heavy doses of SeaFoam and carb cleaner thru the fuel and crossed fingers.

Oh yeah, gotta check the petcock as suggested, if for no other reason than it needs it.

[GammaFlat]

Thanks again all.

I've heard sync'ing the carbs can be a challenge. Does removing and cleaning them require you to sync them upon re-install? Or

KBC

It depends on how far you broke them down.  If you took off or loosened the adjuster nut on the top, then yes... otherwise you don't need to (at least because of your cleaning).  The adjustment that the carb syncing addresses is the below adjuster nut.  If you haven't loosened these and you haven't lost track of which one came from which carb, your carbs will be synced about as well as they were before you took the carbs apart.