Just bought a 1999 honda magna with 4000 miles on it. previous owner said he had carbs cleaned. bike starts fine and idles fine. it surges at all speeds up to about 55 mph. after that it runs great. does it need the carbs to be cleaned again or shim the needles or both. or is it something else? love the bike but need to get it running like it should.
The carbs may need to be cleaned. Try running seafoam in the tank first and see if that clears some of it up. The slow jets may be plugged up inside the carbs. If the snorkel had been removed from the airbox, it could cause siminlar problems as well. have you changed the spark plugs yet? be sure to check out the Honda Magna forums for more info:
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Sounds like you need to have the carb jets cleaned. You can try seafoam that you add to fuel tank and ride it to see if that will clear it up, but I have had the same problem with my VF750C Honda Magna with only 8000miles and mine turns out to be fine tuning the injectors. This is only after the carbs have been reworked and cleaned.
Before you go changing jets make sure your factory air screw setting was not changed and has the correct amount of turns out from seated position. Second check for intake leaks at the carburetor joints/manifolds ( with the bike running spray WD40 around the carb joints and if the engine changes rpm's there is an intake leak replace carb joint,gaskets or o-rings depending on your model.
Sounds like it sat allot.. That causes problems with the carbs.. I would say it runs pretty good for sitting so much.. I would suspect the carbs need cleaning.. or maybe rebuilt,, jetted and synced.. Try some carb cleaner and work you way up from there..
Drain the carburetors. There should be a screw on the lower side of each carb float bowl. Remove the screw then replace it after the fuel drains. Remove the water trap bowl at the bottom of the petcock, (gas valve). Is there any water or trash in the bowl? Drain a cup of gas from the tank. Is there any water or trash in the cup? Dump it, clean it and re-mount it, (not all bikes have a water trap bowl). Turn the gas back on and wait a minute for the carbs to fill with gas. If the bike doesn't start and run properly then shut off the gas and remove the carburetors from the engine.
FOR EACH carb > Remove the float bowl and clean the entire carb with a spray carb cleaner from the auto parts store. Wear protective goggles to avoid getting spray in your eyes. Spray into all the little airways and fittings in the carb. Remove the idle screw and the air screw on the outside throat of the carb and spray into the screw holes as well. < < READ CLOSELY > > Be sure to put these two screws back in the same hole they came out of. IMPORTANT > do not tighten these two adjusters down. Only screw these in until they LIGHTLY seat. Now turn each adjuster one and one half turns outward. Put the rest of the carb back together, clean the air filter and install the carb. Before putting the slides back in the throat of the carbs, Install an in-line fuel filter. Also, sync the carbs such that the slides on both carbs move at the same exact instant when the gas is slowly increased from idle speed. Let the float bowl fill then start the engine. This process should get you back on the road. Please rate this solution. Thanks!