low mpg, engine seems too hot

Hello everyone,

I have a 1982 kz 750 M1 that I LOVE! I got it for a gift a few weeks ago, and I notice that my gas mileage is lower than it ought to be. I'm only getting about 20-25 mpg. When my dad purchased the bike, he put the stock size jets back in it (the previous owner had increased the size). I think i'm still running rich...I'm also getting a lot of backfiring out of the 2-to-1 exhaust when I let off the accelerator. Can anyone tell me how to decrease the amount of gas in the fuel mixture and/or if this is the right thing to do?

Thanks!

Welcome and a tip... read the coil repowering mod in the site's FILEBASE. You may not have jetting issues. You may also want to adjust your valves, if that hasn't already happened. Sooty/fouled spark plugs are from incomplete combustion caused by poor combustion but poor combustion can be poor spark or poor compression and not just a rich fuel/air mixture.

Welcome to the forum.

What was the reason that your dad put the stock main jets back in?
What type of airbox or air filters are you running?
Have you pulled a spark plug to check its color and condition?
Backfiring could be from several reasons. If the motor is really running hot, it could be that it's actually running lean.

I'm sure you'll get some great help here.
Answering the above questions will probably help others give you some better ideas/suggestions than I have.

the reason that he replaced the jets was because it was running too rich before...the plugs were coming out black, and misfiring after a very short period of time...

I couldn't tell you about the air filters yet, as I don't know off the top of my head, and i'm not near the bike right now....if memory serves me correctly it appears stock though.

Thanks for any help you folks can offer.

Chris

Also check that your mechanical advance mechanism (behind the RH ignition cover) is moving freely. The engine will run very hot (and otherwise run poorly and backfire) if it is stuck up.

If the stock jets are in, and you've installed a 2-1, you most certainly are not running rich. Follow WG's advice, low-power spark will make the plugs black and sooty (which indicates either a rich mixture OR a LOW-POWER SPARK). Your manual may or may not tell you that, and if your used to dirtbikes/newer bikes low power spark is probably new to you (newer ignitions are much more powerful)

NBK

Wow! That's half the gas mileage you should be getting!
The 82 KZ750M1 I had averaged 50-55 mpg, and that was with it usually cruising ~80 mph, my 79 an 80 750 twins did the same, with sane riding it would probably get 60 mpg.
Forget points, that bike has electronic ignition, and instead of a choke lever it has a choke knob on the left side that you pull to engage choke, so make sure it's pushed in. And the petcock has three positions, On, Reserve, and Prime, do not leave it in Prime or you will probably be filling your crankcase with gas and/or dumping gas on the ground.
At 25 mpg you are not riding very far between fillups, IIRC that has a 3 gallon tank.