Topic: CB750 MPG seems bad. Running rich or gas leak?

[boots]

My cb750 k7 got 28 MPG on its last tank. That seems darn wrong.

I can't recall what mileage I was getting last season.  I've noticed a small drip from the output of a new fuel petcock when in the stop position, could this be the culprit? But I don't ever see that fuel has run out of the carbs when it's parked, which is where it would go if the drip is pushing fuel through the whole system.

I'm running a 4-into-1 pipe with a baffle (which is new to the bike, the old pipes were 4-into-2 no baffle), a stock air box and stock jets.  Mixture screws are turn 1 1/2 out from completely closed, idle speed is proper. Could my new pipes be affecting the mixture?

Any thoughts on diagnosing this? Or better yet, what mileage do you think I should be getting?

[Really?]

I wonder if your mileage is fine but you are not using the fuel as you think?  Leaking petcock, fuel coming out of overflows when parked overnight or down the carbs and into the cyl's and into the oil sump? 

If you pulled the fuel line, with the petcock off, at night.  See if anything came out in the morning?


Dunno, just thinking.

[lrutt]

man, that is bad. my k6 will get 50 or a little better all day long. my k8 with sidecar 45.

[curemode2002]

I had a drip from the petcock on my cb650 that was dropping my MPG about 11 MPG new petcock got that all resolved now I am back to 42-45 MPG. I even shut mine off every night before and after but that pesky leak killed me even though it was just a slow drip.

[boots]

Yeah, you guys are probably right. It's way too bad not to be a leak. I'll be pulling the petcock and seeing what's what with that drip.

[steam-powered man]

maybe check the oil level / sniff the oil?

[dhall57]

Take a look at your front brake caliper also and make sure brakes aren't dragging. My 750K6 was doing that and I was getting 25 mpg on hwy. Fixed that and now gets 50 mpg.

[BobbyR]

You may have a leak and not know it. I have noticed that this ethanol fuel dries very fast compared to real gasoline.

[hman0217]

Take a look at your front brake caliper also and make sure brakes aren't dragging. My 750K6 was doing that and I was getting 25 mpg on hwy. Fixed that and now gets 50 mpg.

would one feel the drag of the caliper in this case? I certainly don't feel a loss of power but i just filled up my tank and calculated 19 mpg on the last tank. sheesh...

[ekpent]

Take a look at your front brake caliper also and make sure brakes aren't dragging. My 750K6 was doing that and I was getting 25 mpg on hwy. Fixed that and now gets 50 mpg.

would one feel the drag of the caliper in this case? I certainly don't feel a loss of power but i just filled up my tank and calculated 19 mpg on the last tank. sheesh...

Put it up on the center stand if you have one or a jack and see if it has any drag. Also if you have a drag the disc and caliper will be hotter than normal when being used. As for mileage 45-50 mpg is normal for me on the stock 750's.

[flybox1]

When was the last time you did a complete tune-up?  (Pay special attention to timing/advance and a proper vacuum sync of your carbs)
How do your plugs look after riding?
What do they look like after an idle plug chop?

[ofreen]

Looks like the OP is long gone.

[flybox1]

Take a look at your front brake caliper also and make sure brakes aren't dragging. My 750K6 was doing that and I was getting 25 mpg on hwy. Fixed that and now gets 50 mpg.

would one feel the drag of the caliper in this case? I certainly don't feel a loss of power but i just filled up my tank and calculated 19 mpg on the last tank. sheesh...

Hahahaha....Thanks for diggin' up an old one, hman0217 

[hman0217]

Hahahaha....Thanks for diggin' up an old one, hman0217 

Well I didn't want to get yelled at for asking a question without trying the search tool first!

Put it up on the center stand if you have one or a jack and see if it has any drag. Also if you have a drag the disc and caliper will be hotter than normal when being used. As for mileage 45-50 mpg is normal for me on the stock 750's.

Well tried that and that wasn't it. Thanks though

[ekpent]

 Old crusty dirty aged stock paper filter won't help the mileage either if you have the stock air box.

[754]

I had a similar problem  with my K4..usually got about 20 mpg.., had them apart many times..had one plug that would foul once in a while and need clean/replace.
 Sometimes when iI took a left turn lane in warm weather and I was idling, I could see folks in cars fanning their face..the fumes were that bad.
 Then one day i threw another set of roundtops on , sealed a few leaks/weeping..and away it went 40-55 mpg..
 I can only surmise it had bad brass parts..

[hman0217]

I had a similar problem  with my K4..usually got about 20 mpg.., had them apart many times..had one plug that would foul once in a while and need clean/replace.
 Sometimes when iI took a left turn lane in warm weather and I was idling, I could see folks in cars fanning their face..the fumes were that bad.
 Then one day i threw another set of roundtops on , sealed a few leaks/weeping..and away it went 40-55 mpg..
 I can only surmise it had bad brass parts..

pardon my ignorance...what are you calling a roundtop?

[754]

What year is it? 750 i presume...69- 76 has roundtop carbs...look in for sale for pic
77-78 has PD carbs or coffin tops they are sometimes called..

[hman0217]

yea it's a '75 K5, so  guess I have roundtops. Just rebuilt them too. One thing...the boots from my carbs to the engine are the originals and I have reason to suspect they may be leaky because of two symptoms:

1. sometimes after the bike warms up the idle stars climbing dramatically. I've mitigated this by setting the idle low and holding the throttle when I cold start so the high idle is actually a good idle.

2. Occasionally,the engine loses a bit of power suddenly and the revs seem to drop (from the sound of them)

So I think I'll try that, since you pointed to carbs/leaks as a reason for getting crap efficiency.

Thanks