Motorcycle Repair: bucking at just on gas,cruising at low rpm., honda vf750c, honda magna

Question
i have a 82 v45 honda magna. i changed the oil and it never worked right again. at cruising speeds the bike bucks. i then run wide open to clear problem, usually 10,000 rpm problem clears. this happens everyday. could the oil level be to high.

Answer
Dana, changing the oil should never be a factor in the bike's performance, unless you didn't put any back in and it is trying to seize.  Too much oil can lead to problems as the oil can get air bubbles trapped where it is being whipped up by the crankshaft.

There are various ways to help determine the cause of the mid-range misfiring and surging. Running it at 10K rpms is not a good long-term solution.

Eary V45s had issues with camshaft/rocker arm wear, camchain tensioner that failed, spark units that die after awhile, carburetor.

You have to isolate areas to inspect and evaluate, before you go to the next ones.

Start with valve adjustment and compression test. If all 4 cylinders are pretty even in compression readings, then you can remove the spark plugs and take a look at how they are burning. Black, fuel-fouled plugs are getting too much fuel or not enough air. After almost 30 years, the carbs should be removed and cleaned with new gaskets/o-rings changed out. A restricted fuel filter or weak fuel pump can aggravate the fuel flow issues, too.

Check all the spark plugs to see that they are all firing the same. If the tachometer becomes intermittent, then the probable cause is one of the spark units that triggers the tach unit.

All the spark plug caps/wiring should be checked for around 5k ohm resistance values.

Air leaks at the intake manifolds and vacuum lines can all affect this problem.

The oil level is checked through a sight -glass on the lower portion of the clutch cover, when the bike is up on the centerstand.

http://www.cmsnl.com/honda-vf750c-v45-magna-1982-us_model761/partslist/ for parts references.

These bikes are quite complicated and have expensive parts to try to source. Try to find a good independent repair shop to help you sort out the actual source of the problem.

Bill Silver