• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

btom750

CB750 New member
Messages
2
Reaction score
0
Points
1
Location
Berlin Maryland
So I just bought a cb750. 1st bike, though experience with cars. Its a bobber, custom frame, tank, seat, forks, handlebars, oil tank, ect. Anyway, the guy I bought it from said the engine and carbs were rebuilt. I got it cheaper because it wasnt running, which he said was due to electrical issues. The factory wiring harness is pretty much non existent from the work done to it and pretty much all electrical accessories shaved, so I went and redid all the wiring and now the bike starts on one pump.

Now I went to put gas in the tank and it came out the release valves in the bottom of the carbs. (the guy had drained the tank and carbs of gas). I tightened them up a turn and the gas pretty much stopped except for a few drips, so I connected them in pairs with hose so that they would not leak. When I start the bike its obvious that there are issues with the carbs, I called the guy back (without telling him there were issues) and he assured me that he had rebuilt and rejetted the carbs himself and that they didnt have any issues and that they were good to go.

There are several things: the bike only runs 'strong' with full choke. As soon as I start releasing the choke, the third carb begins making a somewhat loud sucking/whistling noise. Which gets louder and louder as I release the choke until full release the engine's RPMs fall and it shuts off. So one questions is what is this noise?

Also, after running the bike on full choke for a couple minutes the exhaust pipes on cylinders 1,2,3 are pretty hot and would definitely burn you, however the pipe on cylinder 4 is scalding hot, like really really hot (these minutes consist of mostly idling and some revving, also revving seems somewhat hesitant but not that hesitant, I don't really know what to expect since this is my first bike). So there are no air filters on the carbs right now and I tested putting a rag on the mouths for a second while the bike was running. What I noticed is that they all spit out gas on the rag, however when covering carbs 2 and 3 the engine speed did not seem to change, though when I covered the 1st carb the bikes RPMs jumped up really high and when I covered 4th it nearly made the engine stop.

I don't know what to do at this point and I don't want to waste time or money on anything that I dont need to do. Especially since this guy assured me twice that the carbs were good to go. Does anyone know why the bike is having any of these symptoms and what I should do? All help is appreciated.
 
Also, I put Stabil in the fuel that I am running. It says put an ounce in 2.5 gallons, I put about .75 in 1 gallon. I ran the bike yesterday and the exhaust pipes on 2 and 3 were cold, with 1 warm-hot and 4 hot hot. I let the bike sit overnight with the fuel mix in the carb bowls, and when I ran it today, as I said, pipes on 1,2,3 are hot and 4 is really hot.
 
The carbs must be "synchronized" after disassembly, which is done with air filter/s in place. (Requires Specialized Tool)

That's step one.
 
If you are getting fuel dripping out of the bottom of the bowls then your carb float levels are set too high or they are sticking and allowing too much fuel into the bowls. The carb overflow is out the little nipple on the bottom of the bowl, they each need there own tube running out away from the carbs to allow excess fuel to run out of the carb. If you connect them or plug them then you will start to have too much fuel in the bowl and run very rich or flood all together.

You should check the sync. You also need to start checking for air leaks then you need to get back into the carbs. It sounds to me like the carbs are definately not good to go no matter what he said. Having to run with the choke on indicates a very lean condition. Need to start checking for blocked jets and passages as well as float levels and air leaks in the boots. Check what the main and pilot jet sizes are as well as what level the needle is set to. Sounds to me like you are going to be getting back into the carbs
 
Any non-runner is immediately suspect. In my experience, phrases such as "needs minor work" are generally code for, "I'm either scamming you or I'm a complete moron. Or both."

"Ran when parked," makes me laugh out loud.

Not to be alarmist but the symptoms you describe are textbook low compression. The trouble could indeed be 100% inside the carbs but I won't even begin to consider taking a project without getting out the gauges. If it makes the seller edgy, all the more reason to be wary.

Hopefully you'll report that it's perfect but I'd rather look like chicken little than see you chasing ghosts. If it's good, the rest is just details. If not, you've got some work ahead.

These are amazingly fun bikes, but think of them as very high-maintenance girlfriends. The more trouble you can anticipate the better your overall experience will be.

If you don't have one yet, get the Honda Shop Manual and use it for bathroom reading.

The rest is getting familiar with your new toy.

OSC
http://www.oldschoolcarbs.com
"Borrowing from one person is plagiarism. Borrowing from many is research!" Alfred E Neuman
 
Back
Top