Topic: CB750 tank mileage

[mycb750k6]

How far can you go on your stock CB750 gas tank before you have to switch to reserve? I'm not getting very far and don't want to find out how far I can go once on reserve.

[bender01]

136 til reserve. Did it twice  this past sunday. It died like clockwork the second time also. I was paying attention. I get poor mileage (38) so you may do better. 74 750k stock.

[andy750]

Last Saturday I took a spirited ride out to Western Mass. Some high speed runs and lots of twisties. Was getting 46 mpg (or 160miles/tank). Bike was running great!

Stock CB750 K4 (stock exhaust, airbox, K&N filter and needle in 2nd position, air screws 1 turn out).

In the past 150 miles has been more normal for my 3.5 gallon tank. Should get at least 20 miles out of reserve.

Andy

[Jerry Rxman Griffin aka MuthaF'er]

Myles, you should expect low to mid 40's mpg even with the kit. I was getting +/- 45mpg with my kitted engine as long as I was riding fairly normal.

[bender01]

Im going to check my needles. Is that 2nd position from the top or the bottom?

[andy750]

Im going to check my needles. Is that 2nd position from the top or the bottom?

From the top. Stock position is 3rd from top but runs a tad rich.

[mycb750k6]

Hmmmm. I'm running 120s w/clip in center and stock pipes. I went to reserve at 90 miles and put 2.8 gallons in the tank. Are petcocks different for these bikes or can they vary as to when reserve comes on?

[andy750]

On my K2 (810cc) with stock HM300s, K& N air filter, stock airbox, clip in 3rd position from top, 120 mains I turn on reserve at 130 miles.

For the K4 see above (mains are 110).

Andy

[Jerry Rxman Griffin aka MuthaF'er]

Plug chop Myles. You may be running rich?? You should have at LEAST 3 gallons on main tank (not sure exact). Just figure your mileage and find the tank capacity.

[trueblue]

If you really want to know how far you can go on a tank, carry a small fuel tin with you and when you run out that will give you the exact milage on your tank, saves a lot of speculation.

[WhyNot2]

Just filled up.

Had 110 on the speedo since last fill-up and had not turned on reserve, took 2.99 gal

Not the best milage..........stock air box w/K&N filter, str8 4-2 pipes, no baffles.

I ride to work every day it ain't rainig, 16 miles one way.

Sometimes I go out for a small ride on the weekends to the flea market, or around checking out yard sales, looking for parts.

[run-tmc]

Since this is a similar topic to what has been posted, I thought I'd just keep the original thread going.

My CB's mileage is atrocious. It runs okay, but I am barely managing 25 mpg around town when others on the forum complain about 36 mpg being bad. I'm trying to ride above 5,000 rpm for the most part as it seems to like higher revs and there is a lot of support on the forum for keeping the engine running fast.

It's a '76 CB750 K6. Here is what I've done to it with lots of help from members here (thanks!):
-cleaned, rebuilt carbs: 2nd clip from bottom on needle, 105 mains, 40 slow
-new OEM air filter in OEM airbox
-new OEM carb boots, intake and airbox side
-new Pamco ignition
-new HO Coils
-new NGK plug wires
-new NGK plug caps
-new NGK DR8ES-L plugs
-MAC 4-1 exhaust/can
-new battery
-new sprockets front and rear, OEM tooth #s front and rear that escape me at this moment
-new 530 o-ring chain
-Blade Style Fuseblock
-new tires
-new wheel bearings
-rims trued
-I weigh 165 lbs
-not riding with a load, just me
-mostly in the city, but have done some highway riding as well
-elevation is 3,400 ft above sea level
-riding style would be best described as conservative, not leaning it over to far and occasionally ripping it up at 11,000 rpm when merging onto a freeway, but otherwise attempting to abide by speed limits

Last night the bike died at approximately 70 miles on the odo. Had to flip to reserve, where it fired right back up. In a similar experience, after 116 miles, it is completely empty on reserve. It seems to me that I should be able to get more than 100 miles before switching to reserve and perhaps closer to 140 miles on reserve. Does this sound like the right ballpark?

I was experiencing a significant bog between 4-5000 rpm, that new airbox side boots largely solved. Now it is only a slight bog at about 4,200 rpm. Perhaps X-24 plugs would solve the slight bog?

Is it normal for 4th gear to feel absolutely gutless below 6500 rpm? Is it normal for 5th gear to lack any power whatsoever? My reference for a streetbike is my brother's 1997 CBR900RR and that thing seemed to pull in any gear no matter the rpm. I recognize that my experience on the CB won't be the same, but only 3 gears out of 5 having power is a surprise to me...thoughts?

Thanks in advance for your help and please let me know if you need more information!
Tom

[Schnell]

I get about 120 miles on a full tank. K5
Stock airbox, aftermarket 4 into 2
Have not been into the carbs so I don't know the jet size or the needle clip position.

[andy750]

120 miles is low Schnell. My K4 gets 140+ in any weather. See above.

[Jerry Rxman Griffin aka MuthaF'er]

Drop your needle! Try the middle clip position as it comes from Honda. You are running too rich and your rpm range is right at the heart of the needle's range. No wonder it bogs at that rpm range and your mileage is so #$%*ty. Why do you have it set that way?

[70CB750]

I run needles in same position as you (#4 counted from the top) but I have variation on breadbox as air filter and pretty much wide open exhaust.

I get 110 miles on tank before switching to reserve, but my reserve does not work - I refill every 100 miles.

[lrutt]

pretty easy 150 on my K6.

[run-tmc]

Thanks for all the input guys.

If I'm adjusting my needle position, I'm going to have to re-sync, won't I?

Jerry, the reason I ran the needle in that position was that it came that way and I didn't think I had any reason to make changes before I got it running. Once I tore the carbs down I decided to keep the needle position the same way rather than muck with it before I could test it (winter carb clean/rebuild).

[PeWe]

Ignition OK?
I have fouled some sets of plugs, running rich due to too advanced ignition timing. The play in the advancer unit delay the mark ~3 degrees. I have to remember that when checking the timing with lamp. F just right of the case index mark

[Phoenix]

I ran out at 161 two weeks ago.  Stock k1 with 341's and no diffusers, 120 jets, regular gas
, slight carbon on mid-range plugs.  As an aside, HM is suggesting I add diffusers and got to smaller jets and, maybe change pin position.  I'll do that as soon as I understand his instructions.

ps. When I ran out above, it started sputtering so I went o reserve.  I don't think I got more than 5 miles before she ran dry. 

[HondaMan]

Hmmmm. I'm running 120s w/clip in center and stock pipes.

Ah, there it is...should be more like: 110 mainjet, set to #4 position of the needle (4th from top). Air screws then at 7/8 to 1 turn. If bowls are also deeper (like I make them, around 25mm), the 105 main might be easier on the tankful, too. Honda's "trick" of the 'legendary' K4 of 1974-75 was to have 26mm bowls with 105 mains and #4 slots on the needles, with 3 degrees less total spark advance and 2 degrees more static, so it would idle. These carbs are numbered "086a", and matched up with petcocks that have just one outlet spigot, like the F0/F1 bikes.

I went to reserve at 90 miles and put 2.8 gallons in the tank. Are petcocks different for these bikes or can they vary as to when reserve comes on?

If your petcock has 2 outlets, the full-to-brim to tip-of-reserve tube distance is 3.0 gallons on the K4/5 tank, and slightly less on the K6 tank. The single-outlet K4-5-6 petcocks have a similar height, but the mesh filter over them stops the fuel from spilling in very well as it approaches the 3-gallon mark, so you sputter.

The reserve runs out at about 0.8 gallon, making you til the bike over toward the petcock to get the rest of it. 

[Jerry Rxman Griffin aka MuthaF'er]

Naw, needles have nothing to do with sync. You should be fine provided they do have a good sync.

[HondaMan]

I ran out at 161 two weeks ago.  Stock k1 with 341's and no diffusers, 120 jets, regular gas
, slight carbon on mid-range plugs.  As an aside, HM is suggesting I add diffusers and got to smaller jets and, maybe change pin position.  I'll do that as soon as I understand his instructions.

ps. When I ran out above, it started sputtering so I went o reserve.  I don't think I got more than 5 miles before she ran dry. 

Umm...maybe time to remove the petcock and have a look-see? Two things can cause this:
1. If the rubber 4-hole thingie that fits under the lever is leaking between the ports, it is basically always running on Reserve, or:
2. If the base of the brass Main tube is loose or got a crack in it down near the petcock, it's leaking the reserve fuel in before you turn the lever.

I won't go into detail how I learned about #2 above...it's embarrassing...

[lucky]

You need to tell us what year the CB750 tank is.
 The later models have 5 gallon tanks.

[Hon3ybadger]

Like my F1. I think its 4.7 gal?