Confusing VM carb issue

[dbarnes42]

Ok, so today I spent the better part of the day working on this thing. Here is a rundown of what I did:

Pulled the carbs off and checked the float level dry and it was withing specs.

Changed the clip on the needle back to the #2 position.

Set the fuel screws at 5/8 turn out and the air screws at 1. 5 turns out.

Bench synched the carbs. This became somewhat difficult because no matter what I did with the screw on the slide for #1, the slide would not go up or down. Not really sure what I did to solve this, but eventually it started moving.

Cleaned out the airbox. It seemed to have water in it, so I resealed it. I am not sure if it was water, but it didn't have any odor. Plus, I had riden in the rain a couple of days ago.

Performed a wet fuel level check and it was within spec also.

Got everything back together and started it. Started fine, idled good, but the problem was still there. I then vacuum synched the carbs, but the problem remained. I was pretty much ready to put a for sale sign on it at this point. But I am persistant. So, I unhooked th throttle cable and started over. But this time I set the fuel screws at 1/2 out and the air screws at 1.25 out. I vacuum synched the carbs again. At this point, the bike was idling very smooth, and without the throttle cable hooked up, I moved the slides by hand. The problem WAS NOT there.

Out of curiosity, I put caps over the vacuum port on the carbs and the petcock and left the petcock in prime. I was trying to rule out some type of vacuum leak in the line from the carbs to the petcock. It did not make a difference, so I put a new piece of line on just in case. It seemed to have no affect on anything.

So, I rerouted the throttle cable, made sure that is was pulled tight with no slack and hooked it back up. Once it was hooked up, I adjusted it so it had a little play in it.

As of 9 pm eastern, the problem was not there. But I have not had a chance to ride it any distance because of rain. However, in the garage things seem ok.

I know that the fuel screws are not close to the stock setting, but this is what the engine seems to like. Once I can get the bike out, I will do some pkugs chops and see where I am at.


Thanks to everyone who helped me on this.

 

[dbarnes42]

I finally have good news! I was able to ride the bike to work today. It started up fine and after about 30 seconds, I started closing the choke. Within a minute or two, the choke was fully closed and the idle was good and smooth.

Jumped on and off I went. Before after about 10 miles or so the rpm's would hang when I stopped. NOT TODAY!!!

After my 42 mile ride to work, when I pulled in the parking lot, the bike still idled fine. Performance on the interstate was great. There was no surging/wandering at highway speed. The acceleration was even better!

I am not really sure what cured my problem, but for the time being it is running great. I just hope I can keep my hands off of it for a while and not find something else to mess with!

Here are the carb settings I ended up with:
Pilot fuel screws 5/8 out
Pilot air screws 1.25 out
Clip on needle in second notch from top
Bench and vacuum synched carbs

Thank you very much to everyone who gave me advice and pointed me in the right direction.

 

[almarconi]

Glad you finally got it sorted out :clap:

 

[Wallowgreen]

Glad she's running well for you now, enjoy the ride

 

[KEITH KRAUSE]

Interesting experience you had. Sorry I arrived late to add my thoughts.
Two best things you did was put the jet needle position back to where it should be, and pulling the slides out (which was necessary anyway).
On these older carbs, the slide(s) will expand a little after full warm up and cause a hanging idle. "Blipping" the throttle or letting the throttle "snap back" on its own from 1/2 open or more can sometimes help if the slide isn't too bad or there's no serious build up of fuel varnishing or mechanical issues. Snapping the throttle isn't good for the carb synch though.
The slides benefit from a good polishing of the slide bores. A little Semi-chrome or Mothers will polish up the bores nicely and clean them well too. Your bike was running rich as far as the jet needle was concerned (approx' 1/5 to 3/4 throttle position). With poor fuel/air ratios and added mechanical isues, all kinds of bad stuff happens.
The side air screws should be adjusted using the highest rpm method, not just set at a ballpark point.
The little dimple in the brass plate goes down. The dimple actually fits in a little indentation in the plastic ring for the jet needle. Hard to line them up though during re-installation. A dab of grease helps. Some 750 carbs don't use the plastic ring ('76/'77?) above the e-clip but the brass plate still has the dimple.
Glad you got it running right.

 

[cyclefvr2]

maybe a air leak? head gasket/ base gasket
or the cable is sticky