79 cb750 cuts out at 6500 rpm

[maxx]

Hello I just bought my bike a couple days ago and it runs great besides it wont rev over 6500 rpm. Also the headlight dims while idling but if i rev up the motor everything works fine. does anyone have any suggestions for me? thanks maxx

[sgtslag]

Welcome, Maxx.  The '79 models are the first DOHC's, and they are very good bikes.  The alternator will not generate enough amperage to charge the battery, or run the lights fully, below around 2,000 RPM -- that is normal.  Idling any bike, below 2,000 RPM, will slowly drain the battery, so don't let them idle for very long -- ride it, or shut it off.

With regards to the engine cutting off at 6,500 RPM, there are a few possibilities:  clogged main jets within the carburetors (add 1 oz. of SeaFoam, per gallon of gas, to the tank, then ride it for 150 miles, before re-fueling, to allow it to dissolve varnish deposits within the fuel system -- may cure the problem, may not, but it is a very good idea, all around, as it does a great job cleaning the fuel system, including the gas tank's innards); cracked, leaky spark plug wires, or end caps; timing/advance not set properly; bad coils; bad plugs; or?...

Since you just got this bike, I would strongly recommend a thorough safety check:  chain condition, and tension check; brake check; fluid checks (forks, engine, brakes); and anything else your (trusted) mechanic thinks is a good idea.

When I buy a used vehicle, I budget a minimum of $1,000 for maintenance fixes needed, and general fluid changes all around.  If you want to tackle it yourself, start by downloading the Factory Service Manual (FSM), from the Home Page of this site, and get familiar with it, and your bike.

For starters, all fluids, and filters, should be changed as soon as possible:  oil and filter (expensive motorcycle oils are unnecessary, and will burn a hole in your wallet... any SM-rated automobile oil will work, but those without the energy star/friction modifiers are better, like Shell Rotella T/T6 Diesel Oil, rated SN for cars, no friction modifiers, inexpensive, high quality oil); fork oil (use inexpensive Dexron III/IV/V ATF fluid, from Wal-Mart, around $5/quart); clean/replace the fuel filter (if it is missing, add a paper fuel filter, between the petcock, and the gas tank).  The Brake Fluid is DOT3, and the lines need to be properly bled (see the FSM, and do some searches on this forum for Tools' idea on reverse bleeding with an old-fashioned, pump oil can).

These bikes have a semi-automatic Cam Chain adjustment.  This is a critical maintenance issue which needs to be done on a regular basis.  Check the FSM for the procedure, and frequency.  The carburetors may need to be disassembled, and cleaned, then they will need the floats adjusted, and they will need to be balanced.  This can be done by you, or a mechanic, but it is critical to get the best performance out of the bike.  Check the FSM for procedures, and tools necessary.

Whatever questions you have, share them here, and the members will try to help out.  Cheers!