1978 CB750A Fuel Problem?

[Jabulani]

Hello fellow CB750 enthusiast,

  I recently pulled my dads 1978 CB750A out of the garage after being let sit for 10 years.  I was lucky that he drained the gas tank and carbs before he stored it.  I did end up pulling the carbs off and soaking/cleaning them anyways just to make sure.  Everything went back on the bike great (new spark plugs, oil change/filter, and air filter.)  Had the carbs synced and runs fairly well in first gear except when its cold.  Second gear is pretty good between 35-50 and then at 55-65 the engine begins to sputter and run like junk.  If I throttle it continues to sputter until about 68-70 and then it catches again and tries to almost lift the front tire off the ground.  It continues to have good power all the way until about 80 (scared to do much above that at this point in time).

  What would make it only sputter intermittently between 55-65 mph?  I am currently getting 28 mpg and I think it has to do some with my hwy driving is about 60 mph and it doesn't run very well at that speed.  Does anyone else have problems like this?  Anyone know of any causes for this / possible solutions?  I have gotten rid of my car insurance and have the bike for the summer.  It runs reliably at this point, but kind of hoping on taking it on a few longer rides and want to get better gas mileage than 28.  My honda civic gets 39-42 hwy but I can't afford the insurance on it right now so thus the motorcycle.

Thanks in advance,
Stephen

[Re-run]

Well I do not have the A but I do have a 78. The carbs are hard to clean. You need to pull them off the engine and tear them apart. Pull all the jets out, yes idle jets too. They are press in so you will need to wrap them with some cloth and grip with pliers and twist a little while yanking. They should pop out. You will probably need a thin copper wire, very thin, and clean all the emulsion holes, then blast with carb cleaner. As long as you do not remove the slides, you should have to resync. Then set your fuel screw, should be right in front of the bowl on the engine side. Set about 1.5 turns out from lightly seated.
Once you put the carbs back on and turn on the gas, twist the gas a couple times to make sure the accel pump shoots gas from each of the brass nozzles at the carb openings. You could also to this before taking them off, you just need to airbox off so you can watch them. Engine does NOT need to be on for this.
Try this and see if it helps.

[dave Esch]

I just bought my first 750four, a 76 model. This was also my first bike. After leaving the previous owner's with my new wheels I filled the fuel tank with regular gas and everything was fine. The next day after about 25 minutes of riding, and at about 50 mph, the engine began to sputter and miss and I had no power. When I came to a stop I could barely keep it running long enough to get off the road. After ten minutes it was fine, but would act up every couple miles, dieing out completely. After calling the previous owner he informed me that the bike would not run on regular octane. I drained it, filled it with 94 octane and have had no issues since. Could this be your issue as well?

[Re-run]

That should not be an issue unless engine mods were done.
That 76 either had engine work to raise compression, the timing is off, or there is some other issue with ignition. NONE of the sohc bikes required premium fuel from the factory.

[dave Esch]

Carbs jetted, after market cam. Anything else I do not know.

[Re-run]

Well a cam should not usually require higher octane as it has nothing to do with compression. I would have to guess your timing is advanced and that would require higher octane depending on how far it was advanced.