Hi everybody, I was recently cleaning and installing a kit into my carbs for my 1980 GS250 ( I'm fairly sure I'm going to have to reinstall all the stock parts) and I noticed that the slide needle does not sit into the slide as it should. It sits at a slight angle. The plastic retainers in the slide both have a little bump on them where they contact the needle. You can see this in the shadow caused by the flash in the attached picture. Also, the bike only runs with the choke on. I've reinstalled the original piolet screws and set them to where they were at before. The bike still runs very poorly, do I have to reinstall the original piolet jet?
The spring loading of the needle at an angle is a design feature and is normal. The spring and bump in the plastic cap preloads the needle at an angle and stops it from rattling around in the needle jet. Remember the flow through the carb is stop-start with only 1 stroke out of 4 being intake, so there will be a flow reversals at low RPM, and air flow though the carb even at WFO is 'lumpy'.
The needles and jets do wear over (a long) time. If you think about it, the needle is under sideways pressure from the air flow through the carburetor and would be pushed against the aperture of the jet anyway. I can't remember which direction they angle to, but its uniform across all the slides and probably against the direction of air flow into the engine to counteract this loading to minimize wear. Worn needle jets, BTW have an oval/egg shaped hole where the needle sits, and cause just off idle (1/16th - 1/8th throttle) richness that you can't dial out with the pilot screw, and make the bike a bitch to ride around town. I've had this problem before.
If the bike is only idling on the choke, there's a couple of things to look at (assuming Mikuni CVs)
Idle speed knob. The choke is actually a self-contained mini-carburetor that is cast into the main carb body, and provides a rich start mixture when opened. As such has its own air intake under the diaphragm through from the carb bell mouth and fuel directly from the float bowl, so it operates independently of the main throttle butterflies, pilot and main circuits.
The warm idle speed is set by the Idle Speed knob between the carbs. It cracks open the butterflies to bleed enough air though the carb to keep the engine ticking over via the pilot circuit, when the choke is off. So if the idle knob isn't open enough the engine will run on choke but die when its shut. Try progressively screwing in the idle speed knob to see if this allows the engine to hold a normal idle. If it can only hold a high idle like 2000-3000 rpm, the problem isn't the knob position.
Pilot Circuit. This is the pilot jet, the pilot air jet, the pilot mixture screw and pilot orifice into the venturi, the adjacent bypass orifices under the closed butterfly plate and all the inter connecting passage ways that join it all together.
All of these parts/passages have to be surgically clean. Any sediment, grit, gum, dirt, etc. will stop the pilot circuit from operating correctly and you spend a lot of time on a motorcycle on public roads is running on the pilot circuit. Make sure the pilot jet you replaced has exactly the same Mikuni part number and square in a square symbol stamped on the side as the one you swapped out.. I'm assuming you haven't disturbed the pilot air jet. Make sure you can pass aerosol carb cleaner through the pilot circuit. Squirt it in though the pilot air jet in the bell mouth. If should come out past the pilot screw, and out of the nearby small bypass holes as we as out of the pilot jet. Spray it up the pilot jet, and it should come out the air jet and the pilot and bypass orifices. If carb cleaner can't get through, air and fuel can't either. Make sure that you haven't inadvertently broken the pilot crew needle tips off by over-tight seating of the screw. Easy to do.
If the pilot circuit is confirmed as being unclogged, then the next thing is to recheck the pilot screw settings. Lightly seat the screws and then go out 1 ? to 1 ? as a baseline.
Once you get it running and fully warmed up you can fine tune the pilot mixture screws for best idle mixture.
