"New" bike owner looking for guidance on my CB750

[ClusterFix]

Hey guys!

I recently picked up a non-running 1982 CB750c that was someone's cafe project. The wiring job on the bike was garbage so I completely stripped it and got everything reran using the typical minimal wiring diagrams. I replaced the plugs, and last night the bike would fire up at full choke, but the throttle was very sluggish. This morning it wouldn't fire up at all. The last owner says the carbs were bench synced but needs vacuum synced. What would the salty members of this prestigious forum recommend? Pull the carbs? Sea foam? Any help or guidance would be greatly appreciated.

 

[Flogger]

Welcome... Sea Foam is just a preventative in my opinion...

 

[Dave]

Me, I would pull the carbs and inspect and readjust everything.

 

[Jdlong27]

Also check the charge on your battery. If you fired it up but didn't ride it and depending on how long it "ran"; I've heard these bikes don't charge until you're over a certain rpm. Also, if you rewired make sure it's correctly wired to not just start but, to also charge. If not that, turn the Petcock on a bit and off, then drain a carb float bowl to make sure it's getting gas. It's still a good idea to pull the carbs and go through them thoroughly, but I'd check the easy fixes first.

 

[amc49]

Seafoam and the like is the mark of the unlearned, I should know as I sold thousands of gallons of it.

Compression test to make sure all of your time is not wasted These are famous for valves not sealing because nobody ever sets them, it then makes people go after the carbs since they think they know what they are doing. The carbs ARE well known for clogging up very badly in the idle and primary circuits as they feed off each other and one stopped up makes the other run super rich. The aircuts give trouble as well.

Bench synced is pretty much vacuum synced unless again the compression is down to pull one cylinder way off in draw. Of course the bench sync has to be done competently and I personally know maybe 5 people that can do it right.

Battery must be hot, the electronic ignition is pretty bitchy about it. The bikes shell out alternator rotors like other bikes do spark plugs. They don't pass break even on charging until around 2000 rpm or so, even when right.

 

[ClusterFix]

Battery is charging, double checked my wires and cables, I pulled the carbs and went through them, new jets and seals, and I vacuum synced the carbs, so they're all set. Using the guitar string method for bench syncing the carbs initially actually got me pretty damn close. So it's up and running, but i'm having a little throttle drift at idle. I checked for vacuum leaks, and everything seems to be good. From what I read it could be running a bit lean. Thoughts? Thanks again for all the info.

 

[amc49]

You're assuming a non-running 34 year old engine is in perfect shape and almost always a mistake. Run a compression check looking for around 170 psi. Compression off due to leaking valves (common on these) will make the idle hunt. If you checked long enough you would see it in the vacuum sync, they tend to change there too. A rock solid motor does not hunt at idle. Have a look at idle mixture screws too, they can affect that somewhat. Do all have an effect when closed off one at a time? If not then carbs not clean despite what you did, common on these.

 

[ClusterFix]

Haha, I'm definitely not assuming there's not some work involved. I bought the bike as a project so I've been enjoying going through the steps. I don't have anything to test compression yet, but that's on my list. I just used the default idle mixture screw setting, but I've read a few ways to get that set right, so I'll give that a go first. But now that you've got me thinking out loud, valve clearance would/could be made worse by coming up to temp, which is when the idle starts fluctuating. Any idea what a good idle speed should be for these bikes? The manual says 1000, but that seems a bit high. Thanks again for the info AMC.

 

[amc49]

I use 1000-1100, who cares what the manual says? You want the engine to roll over comfortably.

If the idle limiter caps are still on remove them by heating the screw heads with like a soldering iron to melt the loctitie they are fastened on with, then they pull right off. Careful, the screw head is extremely small right under it and breaks in a heartbeat with the wrong force used there. Intentional to keep owners from messing with the EPA mandated mixture settings.

You can take the fuel bowls and grind the tab off the limiter strikes to stop too, another way to get around it. What I did, mine began to idle funny about 4 hours after bike bought new and the available adjustment wouldn't fix it. 30 minutes and and bowls all ground to allow freedom of adjustment and the idle then was perfect. You can often get a slightly smoother idle by richening it up a bit. Say up to 2 1/2 turns out on the screws.

 

[Flogger]

AMC, what is the normal compression range?

 

[amc49]

Book figure is 170 psi, all plugs out, engine hot and spun until the gauge quits rising. Need wide open throttle as well or false low numbers. They will run fine at 140-150, once you get down below 120 things start getting worse quickly. 100 is the number we used at the garage for ANY engine type as being unrunnable until rebored or other if it was a 4 stroke unless a crap low dog engine like lawn equipment. Around 100 or so the compression induced heat drops off enough so that the spark plugs quit running clean and then they begin to fuel foul rapidly to give big trouble.