Tune up ??'s... mpg dropping and backfiring...

Looking forwards to what to work on first on my 76 kz750b1 this winter and right now she starts up and runs fairly reliably. But the gas mileage has been steadily dropping each time I fill up by about a mile or so and while driving the engine backfires a lot when I let off the gas while the clutch is engaged. If I grab the clutch and it goes to idle it seems to be ok.

When I bought the old thing in July I dusted the cobwebs off and drove it 100 miles and got about 45 mpg the first couple weeks. Was about half and half city/highway miles. Last few weeks its been mostly city miles and at last fillup I was getting 35 mpg.

Is city mileage really that bad?

What should I be looking to work on first this winter?

re set your point gap and ignition timing.

.014" gap (.012~.016 is the tolerance range)

use a test light and the light goes on at the "F" mark.

the points are wearable and when the backfiring begins and the mileage goes away, time to .....

Ashbane, check your plugs. If they are black and sooty consider the WG Coil Repowering mod. See WG`s web site for details.

If exhaust smells excessively gassy, consider checking/adjusting the service level of the fuel in the carb float bowls. See your Service Manual for the proper level. Also check/replace clogged air intake filter.

If one or more exhaust pipes are running cool/cold, cylinder may not be firing. Consider adjusting the air/fuel mixture settings as per your S.M. Also consider testing/replacing the associated ignition components (as per trippivot above).

Best of success.

PS
Nice looking bike.
Bill

Ashbane wrote:

Looking forwards to what to work on first on my 76 kz750b1 this winter and right now she starts up and runs fairly reliably. But the gas mileage has been steadily dropping each time I fill up by about a mile or so and while driving the engine backfires a lot when I let off the gas while the clutch is engaged. If I grab the clutch and it goes to idle it seems to be ok.

When I bought the old thing in July I dusted the cobwebs off and drove it 100 miles and got about 45 mpg the first couple weeks. Was about half and half city/highway miles. Last few weeks its been mostly city miles and at last fillup I was getting 35 mpg.

Is city mileage really that bad?

My '79 750T gets about 38 mpg in city only mileage.

Are these the original ignition coils,sparkplug wires and sparkplug caps? any/all of them may be the reason for poor milage.

The coils and wires break down from heat and vibration. And the caps from age. Look at replacing all that is mentioned.The "Wired George" coil mod using a relay really works as many guys here have done it. The wiring in these older bikes has corrosion and this reduces the current to the coils and that means weak spark.

His website is www.wgcarbs.com it lists all that is needed and a wiring diagram to follow.

Thanks for the replies, sorry for the delay.

It's the original coil and points, but I did the WG wire mod within a couple weeks of buying the bike and finding this website. One of the first things I did was going over the wires and fixing all the bad connections I had. Several lights didn't work when I got it and the original fuse box had partially melted away so a lot of it got replaced and I added the wire mod at the same time.

The backfiring is the most prevelant thing. When I let off the gas and it winds down it's like a popcorn popper in my engine.

Hopefully there's a guide on the site here for checking out the points or replacing them. I've never looked at them before but I been wanting to learn.

I've put my hands behind the mufflers while running and both sides seem to be running ok. The pressure from one seems a little stronger than the other, but I also have a big rusty hole in one pipe that I screw clamped a sliced soup can around as a patch, so that might be where I lose some exhaust that doesn't go out the pipe.