1978 400C losing power at 80mph

[ItsBork]

Got just about everything dialed in on this bike after a long road of work. I've been taking it out on longer rides and giving it more beans on the back roads and so far everything seems mostly good. The one thing I've noticed is the bike seems to get super gutless at about 80mph, almost like it's being choked out by a limiter. I can be in high 4th or low 5th gear and once the speedo hits about 85mph the bike just seems to give up (for clarity i do not have one of the 85mph speedos). Revs stop climbing and if I give it more gas it just seems to bog. Bike doesn't struggle to accelerate or idle and when I checked the plugs they were a nice shade of tan. Bike is running full stock except for a Dyna S ignition and VM30 carbs. Stock airbox and exhaust. carbs are jetted for stock parts by Speedmotoco, don't have the numbers on hand.

Now I know this is only a 400cc and even the spec sheet lists the top speed at 160kph/100mph. So is this just "how it is" after 50 years of age? I'm certainly not going to throw the bike away because it won't do 100mph I just want to make sure there's not something I'm missing.

Might not be relevant but I've also noticed a lot more deceleration pop and general gurgle since the new carbs were fitted. It's not what I'd call excessive but the exhaust tones are definitely different than they were with the stock carbs, a little "buzzier" at high revs and a little more rumble at low rpms. Sounds good to me so I haven't been worried but it might matter. It also might be entirely my imagination.

Thanks for any feedback. I'm so close to being "done" with this bike to the point where I have the confidence to actually ride to to destinations and I just want everything to be as good as it can be.

 

[Nessism]

Sounds like a carb problem. A GS400 in good working order will top out at about 100mph. Of course, that assumes everything is honky dory with the engine, such as proper valve adjustment, good compression, and properly setup carbs.

 

[GregT]

Sounds like it's a little lean at full throttle. You need to do a proper plug chop. Wide open for at least a half mile, then switch off and pull the clutch. Read the plugs before firing up and riding off.

 

[Rob S.]

But how fast did it go with the old carbs? If it went 95 or 100, there's your answer. And does this "Supermotoco" have a dynomometer, and did they use it to jet your bike? If not...:grumpy:

 

[zed1015]

Are those plugs tan after a high speed plug chop or just when you have checked them when you have stopped after a ride.
Unless it was a plug chop the colour of the plugs only indicates the running conditions at idle.
There's a chance that the mixture is too rich at the top end which will cause it to bog down.
This can usually be clarified by backing off the throttle slightly which then improves the running.
There is also the possibilty that the carbs are too big and there's not enough velocity in the venturi at WOT causing fuel drop out..