Hard start/sluggish throttle response

[olegja]

Hello everyone!

I'm currently restoring a b4 (GK75a) that I bought in boxes. Previous history of the bike is unknown, all I know is that it was crashed some 20 years ago, and left to rot...

I have assembled the bike, and started it. Seems to be quite hard to start, initially it will fire up instantly, then die. Then it will only start with the choke on, and only after some cranking. Any throttle past idle will instantly kill the engine.

If left to run for a while, I can ease it of the choke, and give it some throttle. Throttle response is very sluggish, and I guess that it is pretty much unridable now (still missing some vital parts).

I did clean the carbs, and replaced the float and diaphragm-lid o-rings. I suspect that there is a giant air-leak somewhere. Problem is, I can't find where. Any suggestions?


Thank you.

[Squishy]

Sounds like a carb problem.

First of all, it's supposed to only start with choke on.
Secondly, did you set the carbs to stock settings from the manual? (mixture screws, float height). Assuming the bike is stock and the airbox is fitted.
How are the spark plugs?

Maybe carbs need syncing.
How is the air filter?

[ventYl]

If it sat down in boxes for 20 years I expected some huge carb cleaning will be needed. Also dry O-rings in float bowl comparment will be probably not sealing properly so you should replace them all (good news: they are of fairly standard metric dimensions so you can buy them in store selling bearings, sealings and O-rings). In US/CA there's good option in buying a set of Bandit 400 economy carb rebuild kit which contains all the O-rings, float needle and float bowl rubber sealing.

There's one "secret O-ring" under slider holder. You can get there after you unscrew main jet, remove diaphragm lid and slider. then you can press slider holder upwards by your fingers. Most often this O-ring and O-rings on float assembly (one around float needle seat into carb body and one at choke fuel line) are responsible for fuel problems in these bikes.

[Gouraami]

Is the air box on? Sounds very similar to what I was experiencing without the air box on

[olegja]

Thanks for the replies everyone.
Airbox is on. I did clean the carbs, but did not replace all the o-rings (only float and diaphragm-lid/vacum take-of). Neither did I check floats or mixture (assumed that no-one had ever touched them).

Going to try that, thanks.