1978yamaha400

Each of times we started it, it took about thirty - forty seconds of run time from idle till we killed the switch around 5000RPM. We did this 5 or 6 times. So we really only had 3 or 4 minutes of total run time. After each run, I pulled a plug to see it was soaked, it never was. Will try the starting fluid next time. I did notice the fuel vapor return line on the right intake throat was off and put it back on, I also backed out the nylon? half inch turn screw at the rear of the carb (not the two slotted mixture screws towards front of the carb) one full turn. After that we could not start it again. Day had grown short, and we were hoping to find some pointers like you all have given before starting again. Have not gotten back to it yet, but pretty sure the fuel and carb cleaner should have burned off with the runs. Will take the carb off again, and this time inspect the vacuum chamber springs and diaphrams to see if one or both is stuck open. Thanks

PS, this is a 4stroke and we did throw some raw gas in the cylinders before the choke cleaner. Thanks again.

Thanks for the info. The throttle cable and assembly seem to travel normally. But while I checked the floats in both chambers, I did not check the upper assemblies with the diaphrams and springs. Another bit I read online said something about control needles breaking on Yamaha dual carbs, effectively making the throttle action useless, is this so?

In the process of fixing 400, mounted rebuilt carb with "factory" settings. Right ignition point, was not opening, set with difficulty as outlined in Hayes. After lots of kicks and priming the cylinder with choke cleaner, engine started, but immediately ran up to red-line. After 5 or 6 starts it keeps doing the same thing.... does not seem to be flooding, any thoughts?