Topic: '75 CB750F-Tuning Issue or just bad gas mileage?

[weasel897]

I just bought my 75 CB750F in march.  The PO did a full tuneup.
This guy does full restorations so I figure he knows what he's doing.
This bike was in great working order when he got it so he just did a once over on the bike.
He has listed everything that he did(I can share this info if needed) and I can't see why I'm only getting around 30-33 miles to the gallon. 
My plugs are covered with black soot and so is the opening around the muffler.  As far as I know, everything is stock and mostly original(other than new tires, battery, plugs, etc.)
I'm not hard on the throttle(most of the time).
Mostly in-town driving, but I do occasionally take trips to near by towns for a few hours.  The highway mileage doesn't seem to be much better.
Bike doesn't seem to have any problems.  It runs great.  No sputtering, no sluggishness at anytime during a ride.
What are your thoughts?
Thanks,
Weasel

[kvallelunga]

That seems low. I get on average 40 mpg. It sounds like it's running rich. Is the choke opening fully? Do you have a shop local that can put it on a dyno to get good fuel mixture readings?

[Gordon]

If your plugs are sooty black then you're running rich.

A list of what was done to the bike would definitely help.

[TwoTired]

You have the stock muffler?
 How old is the air filter?
Age matters, as over time, the filter fibers shift and make the pores smaller.  And some of the plugged pores can't be cleaned.

750's seem to wear the slide needle and needle jet orifice with vibration. Makes them run richer.
Aftermarket replacement kits often have a different needle profile/size than Honda original parts.  Many report richer settings.

Check the air bleed screw setting.  Closing off the air too much improves throttle twist response, but makes the bike run richer.

Count the sprocket teeth front & rear and compare to stock specs.

Tire size stock?

Other things:
Float height too high.
Emulsion tube air bleed holes plugged.

[Don R]

Thanks from me too, after cleaning carbs multiple times replacing needles, jets, etc. and going 1 notch down on the needles my F0 finally is rideable but mileage is still not good. It has a glasspack muffler but is otherwise stock. Now I have things to re-check.

[weasel897]

ok, here goes.......(relevant stuff anyway)

1.carb rebuilt, and sync job - float needles, seats, and jets.
2.full tune up kit - points, plugs, condensers.
3.adjusted valves, cam chain, ignition timing.
4.changed oil and filter
5.new 17 tooth front sprocket.......I'll have to check the back to see if it's stock.
6.new 102 link 530 roller drive chain
7.New avon road runner tires (again I'll have to check the sizes)

He never list any part spec.  Just the fact that it had been done. 

Muffler is stock, although I'm looking for a replacement stock one (small hole on the underside that was temp fixed)

I'll check the filter and the air bleed screw setting first thing in the morning.

I wish there was a shop around here that could look at it.  One of the BENEFITS of living in a small town

[dhall57]

Make sure the frt caliper is releasing all the way, if not frt wheel of course will drag and not turn freely causing terrible gas mileage.

[BeSeeingYou]

With my all original stock 76F I get around 40 also.  The stock jets would be 105 and 40 for the idle and spark plugs should be NKG D8EA  or ND X24ES. 

[Jerry Rxman Griffin aka MuthaF'er]

I would expect 40-45mpg. What's the condition and brand of air filter? Drop 1 float bowl (after shutting off the petcock, draining the bowl) and check the jet sizes to make sure they are what SRust has indicated.

[weasel897]

Ok, so I cracked open a carb this weekend.  It's got a 110 main jet and a 40 pilot jet installed. 

Would the difference between a 110 and a 105 main jet cause that much difference in mpg?

[weasel897]

Forgot to post about the air filter....

There was not a lot of trash.  It looked pretty new from what I could tell.  If I had to guess, I'd say it was less than a year old.

[TwoTired]

You can not tell by looking at the filter if it is clogged up or the pores have closed up due to sagging fibers.  It has a 1 year replacement cycle.  And if you don't KNOW its age and use, it is time for a replacement, ESPECIALLY if your engine is running rich or getting bad gas mileage.
Besides, if you are going to the trouble of tuning the carbs, you will want to tune them to a brand new filter, not some unknown.
If a new filter makes no change, then you have a spare for next time by putting the old one in.  If a new filter DOES make a change, then you now the old one overdue for change.

From the 1/4 to 3/4 throttle positions the slide needle dominates the fuel mixture ratio.  Change the main if you are running rich while the throttle is between 3/4 and WOT.

Cheers,