View Full Version : 85 Honda 250SX Carb Help/replacement

09-23-2020, 03:10 PM
Recently grabbed up my dad's old 1985 Honda 250SX that had been sitting in the elements for 15+ years. Engine was seized (as was the carb). I have since rebuilt it over the summer of 2020, carb was cleaned thoroughly and new carb kit installed. The machine started right up for me after the rebuild, but during my first ride I noticed some rev hanging, seemed worse after it warmed up (almost sent me into a tree during my ride and makes it hard to shift properly because I have to let the rev hang calm down before upshifting). Anyway, to me it sounds like it is either an air leak or the throttle needle plunger is getting hung up as the throttle closes. When the machine is cold I have taken the gas tank and top of the carb off, the needle valve/plunger seems to be operating fine. I have adjusting the air/fuel mixture screw as well and turning it in seemed to make it worse, but nothing with the air/fuel screw resolved or remedied the issue. I tried spraying carb cleaning spray all around the boot and even along the top end since I had done a rebuild, no discernible air leak noted. I have just installed a new choke cable (uses the choke plunger/cable system) and that seems to be working (I was hoping that the old cable was keeping the choke open a little and that was the problem, but the new cable didn't resolve the issue). I am posing 3 questions;

- should I just go ahead and purchase a new boot/carb insulator in case there are some very small cracks (which wouldn't surprise me since EVERYTHING else on the trike needed a lot of TLC or replacement) - despite no change in RPM when spraying thoroughly with carb cleaner
- should I take the carb off (again) and try to clean up the needle valve/plunger again and maybe even sand it down a little with some 600 grit and gas? Or grease it a little?
- should I just replace the carb? If so, has anyone had any luck with a particular new carb on their 85 SX? (I would appreciate links if you have them). I don't see many carbs with the plunger system on it for the choke anymore, and I would be ok with that (too much that can go wrong - although that means I've wasted money on a new choke cable lol)

I just don't want to waste much more of my money or time on this carb if you guys think it might just need a new carb. To me, the symptoms scream air leak or needle valve hanging up on the downstroke, but I'd appreciate other input if you guys/girls got it. As I said, the throttle cable is free and working just fine, no hang ups at all.

10-03-2020, 01:07 PM
Good news everyone! It ended up being extremely easy to fix. The choke plunger assembly was hanging up and not entering its non-choked position. Part of the brass choke needle valve broke where the cable bullet connects so it went a little lopsided on me. I can't find a replacement kit that has the right choke plunger (all of them have o-rings, which is the '86 and '87 set up - which also have shorter needles). I used a second set of hands to help reinstall the cable/plunger and she runs beautifully now! Sure is excellent to hear her purr like a jungle cat for the first time in over 15 years. Happy rippin' everyone!