What octane should I be using ?

Hi Guys

1980 KZ750 LTD
I'm getting some pinging around 3000 - 4000 rpm, more when the bike is warmed up.
I've been using premium gas, 91 octane, should I be using a lower grade?
Should I spray some water into the intakes to clean out the combustion chambers?
Should I switch to higher heat spark plugs?
...Or....should I do all of the above?
This pinging is annoying, it's only under load but I'm scared I'm gonna hole a piston. Any advice would be appreciated.

Gregory.

First I would check the ignition timing across the RPM range up to 4k. Yours may be set too far advanced. If your bike is stock, it should not ping on 91 and should probably run OK on 89.

To clean out carbon, do the seafoam thing where you draw it in through a vacuum line until it kills the engine. let it sit about ten minutes, then run it out. You will look like a crop duster for a while when you are riding from all the smoke coming out, usually lasts maybe 15 - 20 minutes.

I was hoping I wouldn't have to buy a timing light. Oh well, I'll see If I can borrow one.

Thanks

Where are you at? I would come by and help you if you are near me!

Requiescat wrote:

I was hoping I wouldn't have to buy a timing light. Oh well, I'll see If I can borrow one.

Thanks

Make sure your mechanical timing advancer is working right. Those suckers gum up and get sticky over time. They can screw up your timing.

Ping is detonation = bad. Higher octane fuel reduces detonation by allowing the fuel to burn rather than detonate. I've always used 93 octane, as suggested when I bought the bike new, with no ping. I suggest you fill the tank with 93 and see if the ping goes away before doing too much other digging into the engine. Ed

no reason why a stock 8.5:1 motor should need 93 octane gas.give her a good tune up and check timing,if the bike gets mostly city driving take it out on the highway and wind her through the gears mostly in lower gears to keep rpms up to blow some carbon out of it.

I use the 93 or better octane it eliminates(ping) and just runs better.also gets sligthly better milage.

Plummen wrote: "no reason why a stock 8.5:1 motor should need 93 octane gas."

I didn't know KZ750s had such low compression. The stock compression on the 1977 KZ650C1 is 9.5:1. Ed

Check ALL tune up spec's, timing, plug heat range, etc. These motors are designed to run on 87 octane, any higher is a waste of money as the higher octane fuel actually burns slower. You can actually LOSE power using it in a low compression motor. You mentioned it pings when lugging the motor........ DON'T DO THAT! As plummen said, run these things. They really enjoy spirited acceleration and will run better! Let us know...


Plummen wrote: "no reason why a stock 8.5:1 motor should need 93 octane gas."

I didn't know KZ750s had such low compression. The stock compression on the 1977 KZ650C1 is 9.5:1. Ed

That is correct (9.5:1) compression for our 650's and should be for it's big brother 750 also.

650ed wrote:

Plummen wrote: "no reason why a stock 8.5:1 motor should need 93 octane gas."

I didn't know KZ750s had such low compression. The stock compression on the 1977 KZ650C1 is 9.5:1. Ed

i guess im used to the john deere tractor compression of a stock 900-1000 :laugh:900-1000 is only 8.7:1

Requiescat wrote:

Hi Guys

1980 KZ750 LTD
I'm getting some pinging around 3000 - 4000 rpm, more when the bike is warmed up.
I've been using premium gas, 91 octane, should I be using a lower grade?
Should I spray some water into the intakes to clean out the combustion chambers?
Should I switch to higher heat spark plugs?
...Or....should I do all of the above?
This pinging is annoying, it's only under load but I'm scared I'm gonna hole a piston. Any advice would be appreciated.

Gregory.

If resorting to a different spark plug heat range, would go only one step to a colder plug, and not a hotter plug.

For example, if NGKB8ES is standard, then NGKB9ES is colder, and NGKB7ES is hotter.

The hotter burning plug helps keep it clean under easy riding conditions, such as a riding style of continuous plonking around at lower rpm's and without a regular blast through the gears. Imo, it would tend to increase pinging.

The colder burning plug helps keep temps down under racing conditions or high speed riding for extended periods. It would tend to foul more under "normal" riding conditions.

Btw, does slight application of choke lessen the pinging?

Good Luck!