QuestionI inherited a 1975 honda cb 750. It had been sitting for about 20 years. It runs (pretty smoothly) My problem is I get gas draining through tubes at the bottom of the carbuerators. It drains with the bike running and continues when its off. It seems to be only from two of the carbuerators. I also get a periodic backfire when I accelerate. Any direction you can give me will be helpful. thanks
AnswerGeorge, it sounds like you just poured gas in it and a new battery and off you went! Wonder that it runs at all, if those are the circumstances!
Pull the carb rack off and clean the bowls out, check all the jets and float valves. The carbs are draining fuel, probably due to worn/varnished up float valves. There are O-rings and gaskets that need inspection and replacement, along with the float valves. The whole fuel system needs checking and cleaning, from the gas tank (including vent) to the fuel tank, petcock, fuel lines and carburetors. The leaking float valves are flooding the cylinders, fouling the plugs and causing backfiring.
You really need to go through it thoroughly... check compression, adjust valves and camchain, replace the plugs, check plug caps for resistance issues (5k ohm is stock), check spark advancer behind the point plate, then clean, lubricate and adjust points and timing. The air filter may need replacing, as well, as they can plug up with evaporated fuel vapors, as well as dirt and moisture.
Bike should have a full oil/filter change, inspection of drive chain, fork seals, electrical connections, all cables, brake fluid system, etc. Tires are probably hard as a rock and the tubes should be changed, as well. Check it over very carefully, before you go roaring off into the sunset....
Bill Silver
www.vintagehonda.com
