Rough running at low revs

[Peterwh]

I have a 1978 CB 750 in mint condition that I bought about a month ago.  Everything is fine except for two things.

First, it runs very rough at low revs, say below 2000.  Above 2000 revs it sings.  Timing is perfect.  changed the plugs.  Fresh gas.  Cleaned and gapped the points.  The guy I bought it from was sick and I think it had been standing in his shed for maybe 18 months or two years.  I haven't done anything with the carbs though I'm going to check the balance tomorrow.

Second, all the lights work fine.  The rear flashers work as turn signals but they don't come on solid otherwise, and I think they should.  I think they did when I first got the bike but then I noticed they stopped.  It seems strange that they flash but don't light up under normal conditions.  I'm wondering if its a weak earth connection but I'm not sure where to start.  I need to get the annual PA inspection done and I'm guessing it will fail with this problem.

Any ideas with either or both of these anyone?  

[TOOLS1]

The rear turn signals only use a signal filament bulb, and do not work as running lights like the front ones. You would have to change the sockets, and bulbs. Plus add wiring to make them work as running lights. I think it does not have running lights on the rear, so that when you use the turn signals. They will catch the eye better coming on from nothing. Rather then just getting brighter.
If the bike has been sitting, for a year, and a half. Your carbs are probably gummed up. I will also guarantee your accelerator pump is not working. The two passages with check balls become clogged very easily. and are a bear to clean out. If you can stand a little rough idle, go on, and ride it. then rebuild the carbs next winter.
TOOLS

[Peterwh]

Thanks. I had a good idea I need to rebuild the carbs, I'm just a bit wary of being able to do it successfully, like why fix it if it isn't (too) broke.

The lights are just a bit confusing. I grew up in the UK where there was no requirement for running lights and I'm not sure exactly what the law is here.

[TOOLS1]

Your good on the law. don't worry
TOOLS

[sgtslag]

Buy a can of SeaFoam (~$9, available nearly everywhere), add 1 oz. per gallon of gasoline, in the tank.  Run it for 5 minutes, shut it off, let it sit overnight.  Then ride it a good long while to really flush the system out.

SeaFoam will dissolve varnish deposits in the fuel system.  It is inexpensive, and it works -- it isn't perfect, but it is good.  It will not harm your engine.  If it works, you will have saved yourself a lot of hassle; if not, you will have managed some cleaning of your fuel system, and your gas tank, so nothing lost.  Cheers!

[Peterwh]

Thanks for allbthe advise.  It was the carb balance. They were way out.  Once I got them balanced it runs great right through the range.  I could hear it improving as I brought each one into sync.

Also cleaned the air filter while I was at it.

[TOOLS1]

That is the worst I have ever heard of carbs being out of sync. had someone messed with it?
TOOLS

[Peterwh]

They were certainly all over the place and the screws and lock nuts looked a little chewed up but I don't really know much about the history.  I just bought it on eBay from a guy in central PA.  If I knew how, I'd upload some photos.  It's in pretty good condition for a 33 year old bike.

I bought the Vacuum gauges from Vintage CB750.  If anyone else is thinking about it, they are pretty good but they could do with some damping.

Just one other question, could someone please send me a close up photo of their centre stand stop.  I noticed today mine currently uses the drive chain.

[TOOLS1]

That's the pretty common center stand stop. I just have a piece of 1/4 X 3/4 strap steel bolted to the passenger foot peg, for mine. Vintage sells one all ready made up, and it has a rubber bumper.
To up load pictures just use the insert image link in the tool bar above the box you write in.
TOOLS

[Nina]

Thanks so mush for the info about no running lights on the rear, just signals.

My husband and son just got in an argument about them.  My husband was sure the Cb750
had them.  I printed your article and explained it to them both.

You saved a war!

Thanks!!!!

[TOOLS1]

Wow I usually start them
Thanks TOOLS