1976 dt400c runs ok on stop but take it up the road and it's awful, here's what's been done, strip and rebuild, new piston etc, carb rebuild, bike runs ok on stop under load and it stutters and surges, checked the breather, same. new ignition coil,same. disconnected the ignition switch,same. disconnected the kill switch,same. have taken the carb off for a mate to look at as I am not convinced all is OK with it, before I took it of I could see maybe an inch or more of fuel in the clear pipe, when the pipe came off the fuel tap the level dropped straight into the carb leading me to think it's not breathing properly, if nothing shows up with the carb he reckons maybe the stator needs rebuilding.

That sounds like a symptom of a plugged gas cap vent. Can you safely run it with cap loose or off to see if it helps?

Yeah it's clear, tried it after clearing it and fuel runs freely.

what kind of air filter are you using and if you use a stock airbox how much oil is on it
Doug

It's a uni filter, stock airbox and as for oil I sprayed aerosol oil all over until it was covered but not dripping

First & only thought at this stage without dissecting things too much is Exhaust & the spark arrestor deep in the tail muffler & build up where the tail muffler fits into the main pipe. I'll go read the story again.
Yep i'll still stand by my guesswork. Also, fit a black fuel hose so you can't see what's going on, seriously.

Ok will remove the muffler tomorrow when I get a chance, I removed the arrestor, blasted clean and repacked as part of the restoration.

why the black pipe?

black pipe so you can't see & won't worry as they do bubble away & look half full & all that. I'm sure its why they don't run clear hoses originally. That goes for oil pipes as well. What you can;t see you won't worry about--especially if its running ok, but i realize yours is not. If they had a clear float bowl there'd be a party going on in there with vibration but mostly on top of the fuel not at the jet where its submersed. Also, i think you said you repacked the spark arrestor, but 'That' is deep down past anything re-packable in the end. Look deep down in & there's a twirly jet engine thingy & you can't see right through it as is angled & do get lumps of carbon trapped there. Anyway--just my thoughts & not easy without being there or hearing an audio. Bad exhaust restriction will kill any performance in stages of severity depending how restricted.

Wayne

Humour me here(!), but is there any difference if you a) ride around without the airbox lid on and b) without the air filter in place? Obviously don't ride too far without the air filter!

I can't remember if you bought UK bike or import, but pretty sure that UK bikes don't have the spark arrestor.

yeah it's an import, will try without the air filter when I get the carb back.