1982 yamaha seca 400 bog and sputter

[yozzie]

I just bought a project seca 400. Been through the carbs twice, finally got it starting good, idles nice, takes off great but once I get to higher rpms and crack open the throttles under load it starts to bog and sputter. With no load it revs great. Changed spark plugs, checked plug wires, changed coils with a spare set I have. Just wondering if it has something to do with the diaphram slides. I tried putting weaker, shorter diaphram springs in and it seems to have helped a bit. running it with the air filter removed seems to help a bit as well. The main jets are stock, maybe the slide needles are too long and they don't open the main jets enough? Not sure what the previous owner may have changed..
thanks for any suggestions.

[xs650mike]

First thing you want to check is the gas cap vent. Run it till it bogs then stop and open the cap. If there is vacuum in the tank the vent is blocked. It's only a little pinhole, should be visible in the center of the underside of the cap. Not sure where it is on the Seca 400 but I'm sure you'll find it. Run something like a guitar e-string or a number 80 drill through it. Sure sounds to me like that's the problem.

[yozzie]

I had tried the gas cap already. Fuel flows fine from the tank, let it run into a gas can. The previous owner had rebuilt the top end, starting to wonder if he got the timing out a tooth. I've had the carbs apart a few times, everything is clean and all jets are stock. One thing it does is kind of "gallop" when I am spinning the engine over with the electric start. Sounds strange. I think I am going to pull the valve cover off and see if it's in time.

[FJ111200]

I had a Seca 750 here in UK once, and it was doing exactly what you describe. I tried allsorts, until i blocked off all those breather pipes that come off the carbs. Then it was sorted and ran like a good 'un.