Topic: 1978 CB750 K8 gas mileage seems worse than my 79 CB750K by a long shot

[engraym]

Good evening sohc4 members. 

I finally rode the 78 until it started to sputter due to an empty tank and switched over to the reserve.  It was at 140 miles from a tank filled to the brim...  This comes out to about 35mpg.  my 79 CB would start to sputter around 200 miles.  I had cleaned the carbs, bike has 13000 miles on the clock, timing is dead on but I didn't get a chance to adjust the points within the tolerance of the manual. front brake caliper WAS bound when I bought the bike but I cleaned it up for a temp fix and it was acting as it should. I just put in a stainless piston but have not had a chance to ride since I am waiting for a stock headlight to replace the vetter funjammer on the front end.  140 just seems very low and was hoping some of you guys may have some insight.

Thanks,

Mike

[engraym]

Also some prior history.  the bike was rarely ridden in the past five years.  maybe 500 miles tops.  I put in new plugs, a fuel filter to catch any grime that was clinging to the walls from the old gas in the tank, a new air filter.  Maybe this is just blowing out the cob webs?  tires are in great shape and properly inflated. I did cruise on the highway between 75 and 80mph for a period of time which definitely isn't fuel efficient but 140 miles seems low in comparison to the dohc 750 I had  Jeeze even 34 mpg is bad for my CBR 929RR which has 150hp at the crank haha. 

[cookindaddy]

You will likely get a few suggestions but mine is:

Do your carbs drip any fuel out the overflow tubes when you are stopped? You wouldn't notice that when you are riding of course.

Did you change the tank fuel filter? The one above the petcock right up in the K8 tank? The Honda in-tank fuel filter is *great* and a good one completely stops particles from getting into your carbs which can block the inlet valves open and cause overflows and waste your gas. I took out the in-hose external fuel filter that was on my bike once I replaced the stock fuel filter with a new one from Honda (still available from dealers). No fuel air locks either that way.

My K8 gets 55 MPG the last time I checked earlier this year. My carbs never drip. I am so confident, I never turn off the fuel shutoff when parked. No puddles. No wasted gas.

[Gordon]

Are you going by what the tank is supposed to hold before reserve (4gal), or are you going by how many miles ridden divided by how many gallons it takes to refill the tank?  The latter is the only reliable way to calculate fuel mileage. 

[engraym]

Gordon, i did not refill the tank yet.  this is just a rough estimation of gas mileage on the 4 gallons available before reserve. Even by a rough estimate, 140 seems low to me. 

Cookindaddy,
i will have to view my overflow lines while it is running.  I haven't taken notice to this at all and I always let the bike warm up for a minute or two before I take off. I didn't know about the internal fuel filter and i will be picking one of these up and getting rid of the external filter. 

I still need to pull the plugs to see how they look.  they only have about 150 miles on them so they will be a good indicator on how the machine is running. 

thanks for the help so far guys.  it's appreciated.

Mike

[bollingball]

I agree with Gordon you have to divide by how much fuel it takes to refill to the same level as the first other wise you are just guessing If you are going to guess then just write down any number you want.

[kvallelunga]

Last year I was getting 150 miles to about 3.5 gallons, pretty good I felt. It has fallen to exactly 120 miles per 3.5 gallons. But it does need a tune up, plugs/cam chain tension/ points/condenser/ etc. So your numbers are similar to an out of tune F3.

[cookindaddy]

Hey Mike,
sounds like you are on the right track

[TrueSpin]

For what it's worth, from the sputter to the brim is 3.5 gal in my 750 with a K8 tank. No idea how big the reserve is.

I average about 40mpg in my F with "spirited" riding.

[Gordon]

Gordon, i did not refill the tank yet.  this is just a rough estimation of gas mileage on the 4 gallons available before reserve. Even by a rough estimate, 140 seems low to me. 

You have no idea how much gas you're using before reserve, so searching for a solution to the problem is a little premature at this point.  It's entirely possible there's either an issue with the petcock or somewhere in the fuel lines/routing that's keeping you from being able to use all 4 gallons of gas before needing to switch to reserve. 

If there's some kind of partial blockage anywhere in the fuel delivery system, it could go unnoticed as long as there's enough fuel in the tank pushing down on it, forcing the gas through, but once the level drops low enough it will begin restricting the flow and acting just like you've reached the reserve level. 

How long will it take to go fill the tank up and do the math to find out the actual fuel mileage?  Until you do that we don't even know for sure what problem you're trying to solve.       

[engraym]

I agree with Gordon you have to divide by how much fuel it takes to refill to the same level as the first other wise you are just guessing If you are going to guess then just write down any number you want.

Bollingball,  I know how to calculate gas mileage, i do it every fill up with the S10 and the CBR. As stated before, this was just a ROUGH estimate that seemed very low going by the specifications that Honda gives for the fuel tank capacity. 

The purpose of this thread is to learn some more information about this bike such as the internal fuel filter that I did not know about which if clogged could give a false sense of needing to switch to reserve much earlier than expected.

Gordon, i did not refill the tank yet.  this is just a rough estimation of gas mileage on the 4 gallons available before reserve. Even by a rough estimate, 140 seems low to me. 

How long will it take to go fill the tank up and do the math to find out the actual fuel mileage?  Until you do that we don't even know for sure what problem you're trying to solve.       

Gordon, I will have my next fill up after I receive the headlight assembly for my bike through the mail.  I removed the windjammer that came equipped on the bike and have been scrounging for the correct parts to bring the bike back to stock form. 

I already know to remove the petcock and inspect the internal filter which maybe be gummed up so this thread although it is premature, it is serving it's purpose of giving me direction to trouble shoot of possible problems.

thanks,

Mike

[engraym]

Got an update for you guys.  Went to my parents today to get the CBR 929RR ready for the new owner who is picking it up later this week. Once i was done with that, I went to work on the 78.

Started off by turning on the gas and as someone mentioned earlier, gas started to come out of one of the overflows.  The carb for cylinder one is where the overflow came from. I started the bike up to see if this would halt the overflow and it did not after 30 seconds.   I then turned the bike off.  While the gas was still flowing,  I open the throttle and closed it a few times and it made the dribble from the overflow fluctuate and then the overflow stopped.  I guess the needle is not seating properly and I will have to pull the bowl off and take a closer look. 

Onto the fuel tank.  I only intended to remove the internal fuel filter I was just made aware of.  With only a gallon of gas in the tank, I removed the petcock and started to drain the tank.  flow was alright until about 1/2 a gallon was left where it was just a trickle.  The filter has to be pretty dirty.

One question I have about the internal fuel filter. On the OEM schematics, it appears this should just come out with the petcock. If this true that it should just come out with the petcock? I had to work at mine for a while to get it to budge and even once free it was still bounded up fairly well  while removing the entire thing. 

Well the filter is out and it is DIRTY.  Had basically a coating of powdered rust on it.  I cleaned it up and the screen looked much better.  While I had this out i figured I would handle minor surface rust issue still in the tank instead of waiting for winter to tackle this.  I went to a Giant and picked up two bottles of Lysol Toilet Bowl Cleaner.  I made sure I picked up the one with Hydrogen Chloride as the main active ingredient and WOW did it do wonders!  Many use nuts a bolts in the tank to knock any rust free but I found it much easier putting 6 foot of chain inside the tank (easier to remove afterwards)

I swish and swoshed it around every 5 minutes for about half an hour and looking inside it looked brilliant.  I then removed the chain while washing the inside with hot soap and water to remove the cleaner.  The cleaner went from a dark "sanitation" blue to a dark "emerald city" green with all the rust it removed.  The chain was oxidized before I put it in and it came out looking like the chain you find on the bulk rolls at any hardware store. 

Once I cleaned the cleaner out I hit it with the air house and once it was mostly dry, dumped some acetone in the tank to remove any water.  once  I drained that out, I sprayed the air hose into the tank while fogging WD40  to cover all the surface. 

Quickly fitted the screen and petcock back in the tank and took the tank to the gas station and filled it to the brim with fuel.  total was 5.210 gallons.   Found 5 bucks laying on the ground as well must be a good day!

Took the tank home and fitted it back on the bike after a good polish and wax and took it for a small test ride. 100 yards into the testride with my half helmet on which has ear flaps, a yellow jacket found its way into the ear flap and stung the "sh(t out of my jaw.  I guess it was just the universe making my day break even after having a spotless gas tank on the inside and finding 5 bucks on the ground. That is a little too much good news for me I guess so the bee made things right.  haha. 

[cookindaddy]

The tank fuel filter when clean and new just slides into place. Getting it out when it is old and crusty sometimes takes a bit of delicate effort. On one of my tanks, a previous owner (PO) had coated the inside of the tank with some kind of sealant without removing the in-tank filter. That one was very hard to get out.

The filter is very fine and does a great job. You can still get these from Honda although they are not cheap, about $35. Given that you cleaned your tank, you might want to install a new filter?

On your original problem, I had suggested checking the in-tank filter to make sure it was there. I was thinking that maybe someone had removed it or it was damaged, letting the crud through to stick your carb inlet valves open, thus causing overflow.

So let us know what your gas mileage is when you can do a proper test.

Too bad about the hornet - ouch.

[dave500]

dohc is more efficient,different tanks,see how yours goes after the clean up,sure your tank hasnt got a giant dent or two filled with bondo?done the float heights or made sure it has stock jetting?

[engraym]

The filter cleaned up extremely well and works very well.  I had drained the gas out of the bottom of the tank into a portable gas container with a shop towel in the neck to catch anything.  it was all clear gas with no particles at all.  After finally removing the screen, i dumped the remaining gas out of the tank into a bucket and it was horrific!  I guess the new gas I put in along with vibrations and just having gas moving around the tank knocked even more lose. 

It's only a few more months before the bike will be going to sleep for the winter.  The filter should be in good shape for the rest of the riding season.  I would rather put a new filter in right before the bike is stored.   

With how slow the fuel was coming out of the tank with at least a quart of gas in it, I wouldn't be surprised if it was still above reserve with the clogged filter only allowing a trickle of gas to pass through the mesh screen. When flipped to reserve gravity takes over again and pushes the gas down. , there could be some remaining crud left in the carbs that is giving me the problem or just a dirty float needle/seal that needs to be cleaned up or replaced. 

The bike was my fiance's father who recently passed away.  It's all original mechanically.  The only thing he changed on it was cosmetic things such as adding a fairing and crash bars/cruiser pegs. The tank is all original paint with no dings or dents and filling it with 5.2 gallons from bone dry confirms the volume is right on.

Still waiting for my headlight as well...  Seller used economy shipping and its been on the road for over a week coming from Indiana and going to Pennsylvania.  Hope it is here before the holiday weekend.  Want to get some riding in on a complete bike!

The bike is definitely rideable and to some people they would be happy with the way it runs.  I would say it is around 90 percent on the testride I took yesterday after everything was put back together. It's the fine tuning that needs to be addressed now to bring that other 10 percent around. 

Very thankful for the help and advice you guys have been giving me.  With the direction I am going with your help and referencing the shop manual, I should be 100 percent soon!