Topic: Mileage musings...

[evanphi]

So I get really bad mileage from my 1975 CB750. Like 21MPG. Boo.

If my bike sits for a few days with the petcock closed... when I turn it on I notice a lot of fuel comes down the tube before it gets all the air out. The fuel is going somewhere out of the carbs when the bike is sitting.

I took a bowl off last week after the bike sat for about 4 days, and it was about half empty. Normally much more fuel when I pull it just after turning off the fuel. It would make sense to me that the overflow tubes have some sort of micro crack in them or something, and are leaking fuel very slowly that is evaporating before it can make a puddle under the bike. Or would this just be normal evaporation?

Thought: Would riding the bike, thus air flowing past the overflow tubes, cause some sort of venturi-like effect on the tubes, causing more fuel to drain out? My mind says no...

I have the day off Thursday, so I intend to shrink-tube the overflow tubes to see if that helps any with my mileage. I will run a whole tank through on a nice long ride, too. If not, replacing float valves will be next. They are all working correctly, though and tuning is good all through the rev range.

[flybox1]

evap.
your carbs are vented to help with fuel equalization while riding.
too much evap (bike sitting with fuel in bowls) and you get varnish.
21mpg is low.  what do your plugs look like?

[evanphi]

evap.
your carbs are vented to help with fuel equalization while riding.
too much evap (bike sitting with fuel in bowls) and you get varnish.
21mpg is low.  what do your plugs look like?

I never get varnish in the bowls, though. I've seen that before, but I even mistakenly left fuel in my bowls over the winter (2-3 months), and had absolutely no varnish. Pristine bowls in the spring.

lunch bag brown on the plugs, through the range.

[dhall57]

When I first got my 750 K6 I check the mileage and I was expecting at least high 30s or low 40s mpg but was shocked when I was not getting much better than you evanphi. Found out my front brake was dragging pretty bad. Rebuilt front caliper fixed that problem and mpg out on the open road is between 45 & 50 mpg.

[evanphi]

When I first got my 750 K6 I check the mileage and I was expecting at least high 30s or low 40s mpg but was shocked when I was not getting much better than you evanphi. Found out my front brake was dragging pretty bad. Rebuilt front caliper fixed that problem and mpg out on the open road is between 45 & 50 mpg.

I have a fully rebuilt caliper and MC, no issues with drag. This is one of the first things I did.

On that note... all my bearings are good, both wheels spin freely and completely.

[dhall57]

So how long as the bike been getting this sorry mileage. Does it run ok and has no fuel leaks.

[70CB750]

When I first got my 750 K6 I check the mileage and I was expecting at least high 30s or low 40s mpg but was shocked when I was not getting much better than you evanphi. Found out my front brake was dragging pretty bad. Rebuilt front caliper fixed that problem and mpg out on the open road is between 45 & 50 mpg.

Ha, that was my first thought.  Experienced dragging front brake mileage first hand after installing second disc.

To OP - do you close the petcock everytime you park it?  It could be as simple as petcock leaking fuel overnight.

[TwoTired]

Any MPG quest should begin with a full 3K tune up checklist completion, including renewed air filter.

A changed-from-stock air filter and exhaust will definitely alter mileage.  An impeded air filter will increase fuel consumption rate.

Changed sprocket ratios, tire heights, prolonged idle periods at stop lights, a stationary "warm up", and operating primarily in lower gears will also impact MPG.

Do you know what the jetting/settings are for your carbs?  Float height setting?   Care to share that data?

How long are your carb drain hoses and vent hoses?  Evaporation occurs faster with short hoses, as the air can exchange more quickly.

You can do an experiment.  Put some gas in a shallow jar and set it where it can evaporate.  Gas IS volatile and it's very root of usefulness is its ability to vaporize, atomize, evaporate into an atmosphere.

With your open container experiment, you can then judge it it is "a lot".

[MCRider]

Just throwing this out there. CB750s have been known to "leak" gas into the sump. So, do you turn the gas off EVERY time you shut it off? Even before you shut it off? Do you park it on the side stand or center stand?

How it happens is: fuel tap is left on, and esp its on the sidestand. Fuel or vapors leak past the float valves into the cylinders. It condenses and drips into the crankcase. It evaporates from there into the ether. Explains the por mileage and the different float levels. May not explain everything.

Fix: replace the flaot valves, ALWAYS turn off tap. I turn it off before shutting down the engine so the bowl levels are a little lower than normal while parked. If parked for more than a few hours, use the centerstand. Floats can stick open when leaning on sidestand.

[evanphi]

So how long as the bike been getting this sorry mileage. Does it run ok and has no fuel leaks.

Mileage has been like this since I bought the bike. This is my second season with it. Runs perfect, no leaks.

Any MPG quest should begin with a full 3K tune up checklist completion, including renewed air filter.

A changed-from-stock air filter and exhaust will definitely alter mileage.  An impeded air filter will increase fuel consumption rate.

Changed sprocket ratios, tire heights, prolonged idle periods at stop lights, a stationary "warm up", and operating primarily in lower gears will also impact MPG.

Do you know what the jetting/settings are for your carbs?  Float height setting?   Care to share that data?

How long are your carb drain hoses and vent hoses?  Evaporation occurs faster with short hoses, as the air can exchange more quickly.

You can do an experiment.  Put some gas in a shallow jar and set it where it can evaporate.  Gas IS volatile and it's very root of usefulness is its ability to vaporize, atomize, evaporate into an atmosphere.

With your open container experiment, you can then judge it it is "a lot".

3K tuneup was just done this spring. New paper filter. Sprockets are stock. Tires have 32PSI front and rear. Bridgestone Spitfire (100/90-19 front, 130/90-17 rear).

Minimal warmup each go... kick it, choke for less than a minute, and ride off, choke off down the road. Mileage is the same whether all city or all highway. I start stumbling at ~160KM on a fresh tank.

Jetting is stock for the carb stamp, 40/120 middle clip. 26mm float height. All jets are clean, all bowls are clean, etc... just put the 120s in two months ago, and gave everything a good once over.

Drain hoses go to the swingarm and are all held in the little holder back there.

Bike is always parked on centre stand at home, with petcock OFF.

[evanphi]

More thoughts:

Something I haven't done is a compression check.

Oil smells like oil. No gas.

Just upgraded my reg/rec, and spark is great on all 4 cylinders.

2/3 was not reaching full advance, and at idle they are jittery. I think it may be the shaft causing the jitter. I'm going to set 2/3 to full advance for the time being since I spend minimal time at idle. Almost never in stop/go traffic. Upgrading to HondaMan ignition next year.

I've been avoiding replacing the float valves because they all work just fine. They all shut off just fine. No leakage.

[TwoTired]

Have you investigated the emulsion tubes?  And the air jet passageway that feeds it?

Do all four spark plugs show the exact same deposit pattern?

[evanphi]

Have you investigated the emulsion tubes?  And the air jet passageway that feeds it?

Do all four spark plugs show the exact same deposit pattern?

Yes, yes, and yes at idle. I'm doing a mid-range plug check on Thursday.

[evanphi]

So I bit the bullet and replaced my float valves tonight... All floats set at 26mm.

Old float needle, you can see how worn out it is:



KeiHin (left) vs Keyster replacements (right)



Hopefully the .5mm smaller inlet won't affect performance... The spring in the new needles is much stiffer than the old one. It is 10:30PM now, so I can't start her up to see how things are. But no leaks or anything with carbs back on! I'm getting really good at removing my carbs and getting them back on.

[TwoTired]

So I bit the bullet and replaced my float valves tonight... All floats set at 26mm.
The spring in the new needles is much stiffer than the old one. It is 10:30PM now, so I can't start her up to see how things are. But no leaks or anything with carbs back on! I'm getting really good at removing my carbs and getting them back on.

If the spring is stiffer than stock, expect the fuel level to be different in the bowls, even if you set them mechanically as the ones with the softer needles.  A clear tube up the side will confirm or deny that.  In theory, the level will be lower, which should lean out the whole range operation.

Might help with mileage.  Might also starve for fuel in some throttle position. 

[tlbranth]

When I first put my bike together after a 25 year sit, it ran, but not all that well. It burbled when I opened the throttle. The plugs were black. I was getting terrible mileage - about what you're getting. First thing I did was toss the new air filter I bought from PartsNmore because I was pretty sure it was restricting air flow. I'm pretty sure it was an Emgo - a brand I steer entirely clear of now for other reasons. I fixed two leaking floats and I lowered the fuel level a tad. Mileage improved, burbling lessened. The big change though was lowering the needles a notch (raise the clip). Now I can actually mess with the idle mixture without ruining low speed running. It was a long process and it's continuing. There's probably no magic bullet for you here. Like TT says, tune it. The spark stuff requires no intuition - it's a matter of accurately setting stuff. Do that and adjust your valves. Make sure the advancer is working correctly, check compression. Then start messing with the carbs. That's where intuition, magic and luck come in. But you'll know it's not ignition. My 750 now gets an average close to 50mpg but the bastard won't idle too well. These puppies take a lot of tinkering.

[Don R]

 I had to adjust my floats with the replacement float valves. I adjusted them and rechecked with the clear tube method. i've experimented with float level. It makes more difference than I ever dreamed it would.

[HondaMan]

3K tuneup was just done this spring. New paper filter. Sprockets are stock. Tires have 32PSI front and rear. Bridgestone Spitfire (100/90-19 front, 130/90-17 rear).

Jetting is stock for the carb stamp, 40/120 middle clip. 26mm float height. All jets are clean, all bowls are clean, etc... just put the 120s in two months ago, and gave everything a good once over.

There it is.

The post-1974 bikes have the following jetting, which matches their cams and spark advancers:

40 idle jet, air screw at 1.0 turns out.
105 mainjet, needle in 4th clip from the top.

The 120 mainjet with needle in center clip was dropped when the cam duration went below inlet +3 BTC opening. The very latest 750 I ever saw with a #120 mainjet from Honda was a K1 from the Old Factory, circa 1971. They went to #115 in 1971, then #110 in 1972 when the HM341 pipes and smaller-slit airboxes appeared, and to #105 partway through 1973 (K3 model). I have not ever seen a virgin 750K4-K6 with more than #105 mainjet and #4 clip position, but have seen a few with #3 clip position (still had #105 mainjet).

The tires: metrics should be run a bit higher than the 32 PSI mentioned, usually: 34-36 nominal is normal for a 150-lb (dressed) rider. I have a Vetter fairing aboard, and need 38 PSI front to support it: the rear is never less than 36 PSI with metric sizes, too. Most of the time I run 38 rear PSI unless carrying another person: then it goes to 40.

In 1973 (or it may have been early 1974) Motor Cycle Magazine had an "ex spurt" write an article about "Tuning the CB750 carbs". In this article, he espoused how "most of the lost horsepower in the 750 has come from Honda leaning out the mainjets to make them appear to be the 'good guys' by improving MPG on this bike."
 That guy was so full of BS I could smell it in the ink on those pages... But, this started a lot of people down the trail of installing bigger mainjets, because it is easy to do and makes the bike feel stronger (until the plugs foul from too rich mixtures). It also washes the lube off the valve stems and shortens the valve guide life. For the Saturday Night rider, it improved the stoplight-to-stoplight times, which could have been better done with a smaller countersprocket and the normal jetting...

You may wish to try the #105 or maybe #108 mainjets. Also make sure the sparkplug caps have the correct resistance (and are within 500 ohms of each other on any one coil). This should at least get your MPG into the high 30s for in-town riding.

[evanphi]

The reason I went to 120 mains is from this site: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

I have 7A stamped carbs, which indicate 120/40/middle. I'm also not on stock exhaust... 4-1 with baffled reverse cone muffler. The bike came with 110/40/middle and still ran the same mileage. No difference whatsoever.

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When I put the needles in and was setting float height, most didn't need to be altered much, maybe half a mm on one side for maybe 3 floats (considering two per carb). I'm going to start it up and take a short ride to work this morning and see how things feel. Also going to make up a clear tube insert for Thursday.

[Dunk]

My K1 is getting 40-45 MPG depending on how spirited my riding is. I've only been tracking for a few tanks so I'll know better once it gets up over a dozen and variations average out. Only changes are ported head, 4 into 1 pipe, and Dyna S with Dyna coils and iridium plugs. I seem to recall my K2 (memory may be fuzzy, this was years ago) got low to mid 30s. My last K1, which has a more aggressive cam seemed to get in the 36-40 range the few times I checked.

If the changes on a K5 requiring in leaner jetting were more restrictive airbox baffles, more restrictive exhaust, and less timing advance... Wouldn't he be able to run the jetting he has (or closer to it) if say the bike has a 4 into 1 like most do, and if he drilled holes in the larger airbox baffles or trimmed them down, and set the total timing advance to be the same full advance at the same 2500 RPM or less? Initial timing shouldn't be a huge deal wherever it falls on these since they advance quick and most riding is around 3000 RPM or greater > full advance. I suppose there could be so little mechanical advance in the later advance mechanisms that it may become hard to start, but I doubt that. At that point it would be the cam holding him back, not sure how drastic the differences are or how that alone would affect jetting requirements.

[HondaMan]

The reason I went to 120 mains is from this site: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

I have 7A stamped carbs, which indicate 120/40/middle. I'm also not on stock exhaust... 4-1 with baffled reverse cone muffler. The bike came with 110/40/middle and still ran the same mileage. No difference whatsoever.

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When I put the needles in and was setting float height, most didn't need to be altered much, maybe half a mm on one side for maybe 3 floats (considering two per carb). I'm going to start it up and take a short ride to work this morning and see how things feel. Also going to make up a clear tube insert for Thursday.

Ah...someone has put K1 carbs on that bike. The old factory K1 came with the 7A carbs, and those had the 120 mainjets. Mine was one of those: it was a plug-fouling, rat-racing, tire-smokin' K1 that got 32 MPG. I didn't have it long because the frame was make crooked: Honda swapped me for the K2 I have now.

The 7A carbs bodies are the same air passages as teh 657A/B series carbs, though. And, they develop the same HP. But, the 7A carbs usually have the hole-in-the-tip idle air screws: make sure yours are clean.

Next: drill the emulsifier tubes out to at least .037" hole size in the 8 larger holes. consider also opening up the lower holes to be .025". Typical max for those upper holes is .039", but that won't make much difference over the .037" size. Yours may be as small as .033" now, if stock.

[evanphi]

So I went for a nice long ride today... made much better with mileage. I had done 50KM on the previous needles, and then swapped to the new ones. My total ride distance before switching to reserve was 185KM. Big improvement. Now to see how it fairs with a full tank on the fresh needles.

But, the 7A carbs usually have the hole-in-the-tip idle air screws: make sure yours are clean.

Going to put new ones in now, too.

[evanphi]

Will putting the air screw with the holes in them change tuning? The ones I just took out were solid, and the replacements have holes.

[HondaMan]

Will putting the air screw with the holes in them change tuning? The ones I just took out were solid, and the replacements have holes.

If yours were solid, and you think they are original that way, then we're into the "weeds" for verification. The little brass hole in the bell mouth of the carb is smaller if the idle air screws have a hole in their tip: it is bigger if Keihin installed solid-tipped air screws. The difference is .006" hole diameter. I don't know if I have a set of 7A carbs around, I'll take a look to give you a "good number" to check.

Most of these carbs have the solid-tip air screws. All the "new factory" carbs came that way, while the "old factory" bikes were the source of the ones with holes in them. I've found (by accident first, experimentation later) that the ones with holes in their tips will adjust from 1/2 turn out to a little over 1-1/2 turns out, while the solid ones only adjust between 3/4 and 1-1/4 turn. Beyond those limits, there is not further "authority" and the mixture does not change further. If you go more closed, it just stops the fuel altogether: if you go more open, it can't lift any more fuel than it does at the max rating, so it doesn't get richer (it does, however, get irregular and "clumpy" when open too far, making black sparkplugs quick!).

All this is a busy way to say, "If you change from solid to hollow tips: it won't tune the same". You'll find it has to be tinkered before it will settle in. I've usually found the Keyster hollow-tipped air screws to be very rich, making the carb idle settings run nearly 3/4 turn out (instead of nearer 1 turn) in order to keep from wetting the plugs too much at speeds below 3000 RPM.

The Keyster needles for the slides are also much leaner than the Keihin needles (number #27201 should be the correct ones in yours), and are sometimes also longer. This tends to make for flat performance in the 2500-5000 RPM range, which is right where I usually ride.

There are also 3 distinctly different needles in the Keyster kits. I have samples of all 3 around here somewhere. That's just bizarre!

[evanphi]

Ok. Thanks! The needles are still the KieHin. They are still good. Just the float valves and air screws are Keyster.

I had the solid needles at 1-1/8 turns out, so I will put the holey ones at 7/8 and see how that goes.