Topic: Runs out of gas at 70 miles but 1/4+ tank of gas left???

[tensi0n]

Ok I have a 71' CB500 and for some reason the bike starts to run out of gas after about 70 miles.  When this happens I switch to the reserve and fill up at the nearest gas station.  However every time I look into the tank there is a 1/4+ of gas left.  When the tank is filled up the bike runs fine.  Any ideas what's going on? 

[martino1972]

-is your tank rusty on the inside?
-on reserve,can you keep going till tank is empty?
-might wanne take the petcock off and clean tank,petcock,screens etc.
-check if the vent hole in gas cap is open(unlikely though,hearing ur problem)

[bryanj]

Nothing is going on, if there was no gas left in the tank there would be no reserve! tank doesnt hold that much either

[martino1972]

unless it consumes way to much gas,how big is that tank?? 70 miles on a tank of gas seems a little thirsty..

[bryanj]

Its not 70 miles on a tank its 70 miles between full and reserve and thats about 2 gall so 35 mpg which is normal for town work

[martino1972]

does a cb500 have such a small tank then??? wow,didnt know that..

[tensi0n]

There is not rust at all inside the tank.  I haven't taken the petcock off yet, I wanted to post the problem here first.  I'll take it apart and give it a good clean and see if that fixes the problem.  The gas tank holds a little less than 4 gallon and the reserve holds 1.5 I think. 

Nothing is going on, if there was no gas left in the tank there would be no reserve! tank doesnt hold that much either

But when I look into the tank there looks to be enough gas for at least 40-50 more miles. :\

[martino1972]

so,lets say you drove 75 miles(70+5 on reserve).....how much does it take to fill her up all the way??
that way we can figure out the mpg..

[tensi0n]

so,lets say you drove 75 miles(70+5 on reserve).....how much does it take to fill her up all the way??
that way we can figure out the mpg..

Well I remember the last time I filled the tank when it was completely empty I put about 3.6 gallons in it.  I didn't fill it to the brim but it was as full as the pump would let me get it.  Ever since I've had to refill once I drove like 70-72 miles.  Then I would switch to reserve and usually only have to ride about 1 mile to the nearest gas station.  I only really drive around the city hardly any highway.

According to the shop manual the gas tank holds 3.7 gallons and the reserve holds 1.6.  Based on that I figured I should get more millage between full-ups. 

[martino1972]

so,its safe to say you use around 3.5 gallon for every 70 miles,right???
thats 20 mpg(us gallon),thats way too low...

is the airfilter clean??
when was the last tune up??
what do the sparkplugs look like??

[tensi0n]

so,its safe to say you use around 3.5 gallon for every 70 miles,right???
thats 20 mpg(us gallon),thats way too low...

is the airfilter clean??
when was the last tune up??
what do the sparkplugs look like??

I changed the air filter last month.  It was tuned up about a year ago (I don't drive it every day or go very far).  I took a look at the spark plugs and they look normal, they're black but I don't notice any oil or anything else on them. 

[martino1972]

look at this website,it has all sorts of sparkplug colors on there,what # would be close to yours you think??

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

p.s. the correct way to read the sparkplugs,is to give the engine hell for lets say halve a mile,then kill the engine,and take plugs out..
but be safe doing that!!

[TwoTired]

From the 1972 CB500 owner's manual.
Fuel tank capacity 3.7 US Gal. (14.0 Liter)
Fuel tank reserve - 1 US Gal.  (4.0 liter)

70 miles on 2.7 gal. is 25 MPG.  This sucks.  Either you don't use any gears above 3rd, you have a fuel leak, a fuel thief, your stand pipe is taller than stock, you have carb adjustment issues, or a clogged air filter.

BTW, the reason it looks like there is plenty gas in the tank is because all of it is not usable.  The gas tank sort of drapes over the frame spine and their is no cross feed between sides.  The only way to get fuel from the right side of the tank to the left is by sloshing it from right to left. An extra heavy application of brakes does this quite well as the fuel's momentum forward equalizes the level on each side.

Cheers,

[tensi0n]

From the 1972 CB500 owner's manual.
Fuel tank capacity 3.7 US Gal. (14.0 Liter)
Fuel tank reserve - 1 US Gal.  (4.0 liter)

70 miles on 2.7 gal. is 25 MPG.  This sucks.  Either you don't use any gears above 3rd, you have a fuel leak, a fuel thief, your stand pipe is taller than stock, you have carb adjustment issues, or a clogged air filter.

BTW, the reason it looks like there is plenty gas in the tank is because all of it is not usable.  The gas tank sort of drapes over the frame spine and their is no cross feed between sides.  The only way to get fuel from the right side of the tank to the left is by sloshing it from right to left. An extra heavy application of brakes does this quite well as the fuel's momentum forward equalizes the level on each side.

Cheers,

Yes I use gears above third. I don't have a fuel leak. I haven't check the stand pipe yet.  Sounds like its a carb issue.  The air filter isn't clogged. 

If it's a carb issue what do you recommend?  I synced the carbs as good as I could last month and they almost spot on.  What else could it be?

[martino1972]

how long did you have this bike??
did it do it right from the start??

[tensi0n]

how long did you have this bike??
did it do it right from the start??

I've had it for about 2 years now.  It was sitting in my garage for about a year then took it to the shop after about 10 months of it sitting.  I started riding it to save gas and now noticed how horrible the gas millage is on it.  So to answer you questions, I'm not sure if it did it from the start.

[martino1972]

well,i would start by reading the sparkplugs after a small trip of giving her hell,and see what color the plugs are,they can shed some light on the issue

[tensi0n]

well,i would start by reading the sparkplugs after a small trip of giving her hell,and see what color the plugs are,they can shed some light on the issue

Yeah, I have some time tomorrow to work on it.  I'll post back here the end results. 

[martino1972]

sounds good,sparkplugs can tell you alot of things..
also,while your at it,check if your choke works correctly,a incorrect working choke could very well cause your problem,check if they open all the way,by looking into the carb opening on the airfilter side..

[TwoTired]

Yes I use gears above third. I don't have a fuel leak. I haven't check the stand pipe yet.  Sounds like its a carb issue.  The air filter isn't clogged. 

If it's a carb issue what do you recommend?  I synced the carbs as good as I could last month and they almost spot on.  What else could it be?

Possible carb issues:
Float height too high
Float valves sticking open
Air screws turned in too far
main emulsion tube cross bleed holes plugged.
Choke butterflies not fully opening.
Slide needles clipped in a too rich position.
Main jet o-ring leaking
Main jet too large.
Main and/or slow jet orifice size altered.

Do your brakes drag?  Wheels spin freely?
Are your tire pressure correct?

Do you only drive uphill? 

Has your air filter gotten wet or oily?

Does your mileage improve if you run a tank without the filter as an experiment?  (Do this only if you can reasonably avoid dust storms/clouds.)

What is your sprocket tooth count?  17T Front, 34T rear?  Or, has a 37T been put on the rear?

Stock tire sizes?

Cheers? 

[tensi0n]

sounds good,sparkplugs can tell you alot of things..
also,while your at it,check if your choke works correctly,a incorrect working choke could very well cause your problem,check if they open all the way,by looking into the carb opening on the airfilter side..

Okay I'll make sure the choke is in good order as well.  Thanks for the help!

[martino1972]

Do you only drive uphill? 









Cheers? 

  ....yeah,up hill to his work,and up hill back home...

[tensi0n]

Yes I use gears above third. I don't have a fuel leak. I haven't check the stand pipe yet.  Sounds like its a carb issue.  The air filter isn't clogged. 

If it's a carb issue what do you recommend?  I synced the carbs as good as I could last month and they almost spot on.  What else could it be?

Possible carb issues:
Float height too high
Float valves sticking open
Air screws turned in too far
main emulsion tube cross bleed holes plugged.
Choke butterflies not fully opening.
Slide needles clipped in a too rich position.
Main jet o-ring leaking
Main jet too large.
Main and/or slow jet orifice size altered.

Do your brakes drag?  Wheels spin freely?
Are your tire pressure correct?

Do you only drive uphill? 

Has your air filter gotten wet or oily?

Does your mileage improve if you run a tank without the filter as an experiment?  (Do this only if you can reasonably avoid dust storms/clouds.)

What is your sprocket tooth count?  17T Front, 34T rear?  Or, has a 37T been put on the rear?

Stock tire sizes?

Cheers? 

Well I have to check most of that tomorrow.  The tires are stock sizes.  I'm pretty sure the sprocket is stock, but I'll check...

It's kinda funny that you mention the hill because I do live on one and it's about a 1.5 miles to get from the bottom to the top where I live.  But again...I don't just go up and down till I run out of gas.

[trprbill]

When you fill-up you can lift the nozzle up so the automatic shutoff won't stop you from filling up. Set your trip meter to zero, then when you fill-up next time you can figure out your mileage. My '76 CB750F gets 45-50 mpg so I think your bike will do a bit better.

[rhos1355]

Just thought of something. Might just me p*ssing into the wind. You know if you remove the petcock from the tank there are 2 open ended copper tubes sticking out from the petcock? One is longer than the other. The long one is for erm....normal running so when the fuel level goes below this you have to to switch over to reserve which opens up the shorter tube. Could it be that the PO has put in a longer "long" tube?

just me trying to be overly clever, but think about it if all else fails.