Topic: 750k2-running on 3 cylinders- ideas?

[birdy]

Finally got it running with a new battery...
Running rough though- running on 3 - what should I check first?

[ekpent]

Check to be sure that cylinder is getting spark and gas first.

[scrapvalue]

Once you have figured out which cylinder it is, check the spark plug.
Go ahead and switch the plug with a cylinder you know is working.
If it then misses on that cylinder, you know it is the spark plug.
If the problem persist, it is most likely the carb.

[tlbranth]

While you're testing all these theories, put some 2-stroke oil in the tank. If the problem isn't fuel, you'll be running an unlubed piston up and down an unlubed cylinder and doing some damage.

[birdy]

Eh? I thought the engine oil would lube the cylinders regardless of if the cylinder is not firing?
If I put 2 stroke oil in the tank won't this mess up the engine running?

[Terry in Australia]

The 2 stroke oil (or Auto Trans Fluid, my preference) will help lube the upper cylinder that's not firing, particularly if your idle jet is blocked and there's no fuel/air mix getting in. If it's not a bad plug/lead, I'm guessing that it'll be a blocked idle jet, the most common cause of poor running on recently revitalized CB750's, and lastly, you may need to check all the valve clearances, just in case you've got a tight clearance and one valve isn't completely sealing. Cheers, Terry.  

[Spanner 1]

Lads, have yis lost your minds...!! Do not put oil in the gas , unless you want to run on 0 cylinders with 4 fouled plugs !
Our bikes can run all day on 3 cyls. with no damage as the oil pump has complete responsibility for lubing the cyl ( oil holes in the ring grooves). If anything, better lubed without firing and burning-off any oil blowby on the cyl. walls.    How many guys have ridden home, some long distances, on less than 4 cylinders with no damage ?........IMHO.

[Terry in Australia]

No mate, a little two stroke oil or ATF (I'm talking 70:1 fuel/oil ratio) won't foul your plugs, unless you've got the weakest ignition system in motorcycling history. Upper cylinder lubricants have been used since the invention of the internal combustion engine, and the benefits include an anti knock cure via reduction in carbon deposits in the combustion chambers and on piston crowns, carbon buildup in ring grooves (great for unsticking stuck rings) reduction of carbon deposits on inlet an exhaust valves allowing better valve sealing and gas flow, and lubricating the cylinder above the rings.

Here in Oz a lot of guys who ride older "pre-unleaded" bikes without hardened valve seats run an upper cylinder lubricant to reduce the risk of burning valve seats, and you can buy an upper cylinder lube kit for older cars at most auto parts stores. Another benefit for our bikes is that the oil mist created by the combustion of an upper cylinder lubricant reduces corrosion in our exhaust systems (ever seen a rusted out 2 stroke pipe?) and is particularly beneficial for anyone installing expensive NOS OEM pipes to CB750's, "back in the day" some OEM pipes didn't last a year or two before they rusted out, due in part to the chemical residue left behind after the chroming process. Cheers, Terry.