DrMacDaddy's Poor Fuel Mileage Thread

Discussion in '3rd & 4th Generation 1990-1997' started by kennybobby, Nov 2, 2012.

  1. kennybobby

    kennybobby New Member

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    This is a new separate thread to focus on DrMac's 25 mpg fuel mileage issue.

    To summarize:
    He is the 4th owner, 26k miles, stock '93 with no mods, no jet kit.
    It starts on the choke then idles down around 1k when warmed up.
    Uses pure gas, no alkyhol fuel.
    Stock chain and sprockets.
    Throttle butterfly valves appear to snap fully closed.
    Vacuum slides appear to freely slide up and snap closed when released.
    Choke cable seems to open and close normally.

    The old hijacked thread is here:
    http://vfrworld.com/forums/mechanics-garage/18546-poor-fuel-economy-26mpg-help-2.html
     
  2. DrMacDaddy

    DrMacDaddy New Member

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    Thanks for the help with the new thread. I'll be working on the bike now, so I'll have updates later tonight.
     
  3. Scubalong

    Scubalong Official Greeter?

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    Did you check your tires pressure....?
    Air filter is clean...?
    Spark plugs are new?
     
  4. Durk

    Durk New Member

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    Your spark plug is the window to how your engine is running. The condition of them will help you formulate your plan of attack. Or if you post pictures let the vfrw peeps give you suggestions. You could be running overly rich.
     
  5. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    we're doing another thread to say the same stuff all over again?

    How about temperature? The other thread says most of the riding is 2miles to work. Maybe the bike isn't fully warmed up. Possible thermostat stuck open
     
  6. bikeman

    bikeman New Member

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    2 miles! If you want save gas...walk.
     
  7. kennybobby

    kennybobby New Member

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    Stuck Open Thermostat?

    That was the OP from many years ago, not the new fellow.

    Two guys having the same type issue, but several years apart--old thread got hijacked by the new guy and it is too confusing to keep the stories straight...

    But i agree it may be running cold from thermostat stuck open, and since his carbs are sworn to be virgin and seem to be functioning normal, you probably nailed it.

    Hey DrMac, where is your temperature gauge indicating when you are out for a ride?
     
  8. jethro911

    jethro911 Member

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    Being a carbed beast the engine temp won't be as serious as with a fuel injected machine where the computer would keep the mixture rich if it thought the engine was still cold. In this case it could be sticky floats or something sticking in the enrichener circuit. I would tear down the carbs and inspect / clean everything before getting too much further into the other possibilities. The bike is pretty old so this area likely needs some love.
     
  9. Big_Jim59

    Big_Jim59 Member

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    At the risk or repeating myself from the old thread, I had this same issue with an '01 600 Suzuki Bandit. I purchased the bike as a project/wreck. I got is back together but it needed to be sorted out. The bike started, ran fine and idled. Was sluggish in mid range and cleared out on top. Mileage was in the mid 20s for a 600cc motor and that's just wrong. I found the needle jet was missing (just gone). Gas was poring through the engine on #4 but the other cylinders were masking the ill effect. I fixed the jet issue by installing a new one and the mileage jumped to 36 mpg. The plugs should tell the tale. My advice is to go through the carbs. Check all the jet sizes. Check and make sure the choke circuits are closing and above all check for a missing jet. You never know who has been in those carbs before you.
     
  10. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    Fuel does not atomize as well in a cold engine so the user would be force to use a higher throttle position to get the same power output from a cold engine. It doesn't run at the same efficiency and an engine not running a temp would cause poor economy.

    Certainly not the only thing, just one possibility.
     
  11. LostAngel

    LostAngel New Member

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    Same problem as DrMacDaddy

    Hi there all...! New to this forum.

    I bought a 92 750f a couple of days ago, Immaculate "visual" condition black 38000 miles I paid $2000 dollars for it. A great price no doubt.

    Im having exactly the same problem, at (th)irst I thought the fuel gauge was broken.

    But Im averaging 25MPG.

    So yeah Ill check the plugs out first and have a talk with my mechanics up the road.

    This is a huge difference in fuel consumption from the reputed average of 40 MPG.

    After reading all the advice I'm thinking carbs/jets for sure, but hey I'm no expert.

    Thanks for all the feedback, its been helpful.
     
  12. DrMacDaddy

    DrMacDaddy New Member

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    I guess I didn't update you all on the condition of the plugs. There was just a very small amount of black soot at the very base of the electrode of the plug, but not very much to speak of. The rest of the electrode had the nice tan color that you should see when pulling used plugs on an engine running well. Perhaps the plugs could have been a little tighter, but other then that they were all in very good condition. Keep in mind that the previous owner had replaced the plugs about 1600 miles ago. The temperature gauge also appeared to operate normally,....and made the full transition from the C mark when the engine was started cold,.....and then slowly moved in the H direction as the engine began to warm up. I ran it until the fan kicked in, and at that point the temp needle was about 2/3rd in the direction of "H".....which seems normal to me. So, I don't think it's a defective or stuck-open thermostat. Also, the previous seller's daily commute was not 2 miles,.....but rather about 35 miles each way. So, his bike was fully warmed up by the time he reached work or got back home at days end. You might have confused that with my statement that he had to fill up the tank every 2 to 3 days,....which simply meant that he had fresh fuel in his tank about every 2 to 3 days. It was only the last 18-24 months that he owned the bike where it sat unused in the garage. He'd go out and start it every 3 weeks or so....and let it run until the fan kicked in. That was about it. So, in summary so far:
    -the plugs are okay
    -the air filter and airbox are both clean of obstructions and working fine
    -the choke lever moves freely in both directions, and fully disengages when no longer needed
    -the tires are always at the exact pressure as indicated on tire wall of the tires and in the owners manual/service manual
    -there is absolutely no front or rear brake drag at all as I freely roll the bike
    -the butterfly valves all close fully, and all appear to open up an equal amount when moving the throttle grip
    -I have used only 91 octane fuel (pure, no ethanol)....and only ride solo (165 lbs total weight)
    -the front and rear sprockets and chain have all been replaced 1600 miles ago with stock components
    -the air filter, front & rear tires, plugs and battery were also all replace 1600 miles ago
    -I use the choke to start the bike when cold, but within 1 min fully disengage the clutch and merely rev mother at 1500 rpm to running smoothly
    -When pulled, the plugs looked just like they should have,...with just a very small amount of soot at base....and rest of electrode tan in color
    -I'm 51 years old, and if anything ride the bike gently rather then aggressively.
    -I consistently use SeaFoam gasoline additive per can directions to help keep the fuel pathway and carbs clean. I also treat with Stabilizer.
    -the bike has plenty of power and off-the-line pep....with steady power throughout the entire powerband.

    All of this,....and yet I am still consistently getting 25 mpg with each tank fill....and have since the day I bought the bike.
     
  13. DrMacDaddy

    DrMacDaddy New Member

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    Man.....LostAngel,....you make me want to break down into tears. I tried and tried to talk my seller down in price, since the bike was a '93 and literally coming up on 20 years of age. He also admitted that he bought the bike with only 5K miles on it,....about 9 years ago,...and only paid $4500 for it. But, I couldn't get him to take a penny less then $3500 cash. So...that's what has had me so uptight and frustrated over the power gas mileage. I do see something quite odd here, though. Out of the three guys here all reported poor gas mileage with their respective VFRs.....we are all getting exactly 25 mpg. What are the odds of that happening? Seems to me that we are all experiencing the same root cause resulting in the same plateau of gas mileage at 25 mpg. Up to this point in time,...I think that Jethro911 is probably throwing out the best advice,....and that is to pull the entire carb assembly and then meticulously tear them apart in an organized fashion.....and then thoroughly clean everything before reassembling. It just wasn't something I thought I was going to have to do after dropping $3500 on a 20 year old bike and retailed new for $6995.
     
  14. kennybobby

    kennybobby New Member

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    Your plugs are not okay--they indicate you are running rich. Since they are all the same it is a common issue, not just related to one carburetor. That was the purpose in looking at the plugs.

    The 3rd gen carbs are unique. They have the largest venturi. They use the round slides like previous gen, but they added a separate decel enrichment circuit (air cut off) with external plumbing/vacuum lines. And they have the most complicated linkages for the choke system.

    If you have a long bladed screwdriver you might try pushing directly on the back of brass tips of the choke pistons and see if you get any movement in toward the carb body. i tried this on an old set of 3rd gen carbs and found that the "closed" position of the main lever did not have enough leverage to fully seat the pistons and i had to manually push on the brass ends to close the circuit. If the pistons are not fully seated, or if the seats are worn, then you will suffer a continuously rich mixture. The fourth gen changed to a more direct action on the choke pistons, so maybe that design change was made for a good reason.

    The air cut off valves cause the pilot mixture to run rich on deceleration and theoretically could cause a rich mixture condition if they failed in the closed postion, but the piston is spring loaded and that doesn't seem as likely as the manual choke pistons being slightly open.

    Is your idle speed at 1500 rpm--the reference in post above is not clear?
     
  15. DrMacDaddy

    DrMacDaddy New Member

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    Once I have the bike warmed up, it always idles smoothly @ 1000-1100 rmp. My reference to 1500-1600 rpm is only when I'm starting the bike when the engine is cold. I pull the choke lever all the way open to start up the bike.....and then back off on it until the engine is running approx 1500 rpm. After about a minute with the choke engage......I then push the choke lever all the way off, and use the throttle to bring the rpms back up to approx 1500 rpm. I manually hold the throttle to keep the engine running in that 1500 rpm range for another minute,....and then let the throttle go back to it's normal resting position. At this point, the bike is running fairly smooth......at around 1000 rpm. From this point, it takes me a few minutes to get my riding gear on and ready to ride. In those few extra minutes.....the bike is just idling in the 1000-1100 range with no choke applied at all. From KB's description of the air cut-off circuitry, I guess I do have an issue. Can I reach those brass pistons without taking any of the carb assembly off the engine....or disassembling any of the carbs? The weather here has now turned very cold (mid 20's to low 30's), so the riding season is basically over for this year. That being the case, I have nothing but time to work on my bike in the evenings and weekends. I do have the 90-93 Honda Service Manual for the bike, so if there's a specific area you want me to read up on, KennyBobby, just let me know. I also have the 90-96 Service Manual in pdf format to browse on my laptop.
     
  16. DrMacDaddy

    DrMacDaddy New Member

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    I had a question about the air cut-off system on the bike, KB. You mentioned that on deceleration, the pilot makes the mixture a little richer. I was curious why they would design it that way. Before this bike, I owned a '95 Kawasaki Vulcan 750cc bike. I loved that bike, except for one issue that drove me crazy. Everytime I began to decelerate, I'd get this howlitzer bang from the exhaust pipe. The first time it happened, I nearly jumped off the bike thinking a piston flew thru the top of the engine. I also had the service manual for that bike, and as I read into the exhaust system section, my attention focused on Kawasaki's approach toward abiding to current EPA regulations. Essentially, they put a reed-valve system on the exhaust stream to ensure that any unburned fuel in the exhaust stream was full combusted before exiting the tail pipe. The huge bang I was hearing was essentially a secondary combustion occurring at the beginning of the exhaust manifold. To test this, I merely removed the reed-valve cover and flipped the reed-valve around. This prevented any air from entering into the exhaust stream. Magically, the howlitzer bangs disappeared. However, it also made the bike impossible to start properly when it was cold. Ultimately, I installed a ball-valve into the hoses leading to the reed-valves on each side of the engine. I opened up the ball-valves in order to start up the bike when cold.....and then closed them when I disengaged the choke lever to begin riding. I'm sure the EPA would not have been happy with my mod, but it certainly took care of the problem. When I logged into a Vulcan forum, I realized that many Vulcan 750 riders suffered from the same problem. I was just curious why the Honda engineers would want to enrich the fuel-air mixture upon deceleration. Any ideas as to what they were trying to accomplish with that strategy?
     
  17. 34468 Randy

    34468 Randy Secret Insider

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    I am certainly not that technically advanced but a couple questions I have for you to consider. If you are using your bike fairly frequently, why the fuel stabilizer? If nothing else, that is an unneccessary expense. Similar thought to the cleaning additive. These, if needed at all, need only be used occassionally. The Stabilizer, only if you are leaving it stored for a longer period of time. If you are using fuel from a busy source, the fuel should be quite clean and fresh. Also, why are you using 91 octain fuel? My manual, albeit for an 06, says to use 86 or higher. If the engine runs well with the lowest octain, then that is probably what you should run. If you start to get pinging, then go up an octain rating until the ping goes away.

    I used to think that the higher the octain rating was, the better the mileage should be. Somewhere on this forum a while back, there was a link to an article about octain ratings for bikes and what was said, made sence to me. That is why I went from mid grade to the lower grade fuel. But seeing as your bike is carburated I may be talking 'meat' and you are describing 'potatoes'. If that's the case, I will go away and drink beer.
     
  18. kennybobby

    kennybobby New Member

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    Enrichment is needed on deceleration to prevent the misfiring and "howitzer" effect of combustion within the exhaust.

    When the throttle is suddenly closed the mixture goes too lean to combust in the engine--so the unburnt mixture gets pushed into the hot exhaust pipes where it explodes and pops loudly.

    So extra fuel is metered in during deceleration such that the mixture will ignite in the combustion chamber and get burned up there instead of in the exhaust pipes.


    You will have to remove the middle side fairings to see the carbs and the brass pins of the choke.

    Undo the 1/4 turn fasteners on the sides plus remove the clips on the center front fairing piece below the radiator. Then you can lift up and swing the bottom edge of the side fairing out. Take care not to break the tabs and slots at the top edge of the side fairing. See chapter 2 for details. Read chapter 6 for fuel system/carbs.


    i read on a forum that Seafoarm is 50% diesel, 35% alcohol, 15% napthalene--not sure any of these will dissolve carb sludge, but it might make a good grease cleaner. Let some parts sit in it to see how well it cleans them. Mixing it with gas and burning it in the engine does what?
     
    Last edited: Nov 12, 2012
  19. DrMacDaddy

    DrMacDaddy New Member

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    Thanks for the input, Randy. Keep in mind that I've only had this bike for about 2 months now, and experienced the really poor fuel mileage from day 1 as I drove away from the seller's location. I filled the bike up to the top as i was leaving his place, and 78 miles later I had already used 2/3rd of a tank of gas. Knowing I wouldn't make it home on that, I filled up again in Emporia, KS, for the remaining 76 miles. When I pulled into my home town, I had again used 2/3rd of a tank of gas. So, I thought I would put in the 91 Octane fuel to just see if my gas mileage improved. It did not. I consistently get 25 mpg on the button. From this point forward, I will definitely be using lower octane fuel since it didn't make any difference.

    As far as the fuel stabilizer is concerned, it was just a recent addition. The temp this morning was a roasting 26 degrees, and the Kansas City area has now shifting into it's cold Winter weather. Not knowing how long I'll be tinkering with this bike before I discover the cause of the poor gas mileage, and the fact that the tank was completely full,....I wanted to make sure I didn't add to my problems if the bike literally sat in the garage for the next 5 months....waiting for warmer riding weather. Who knows...perhaps if the previous owner had used a fuel stabilizer in the gas before he let it sit in his garage for 2 years unused......I probably wouldn't be going thru all of this crap right now. The Stabil product I used was only $2.99,....and I used only 3 oz out of the 12 oz in the bottle. So, the cost was incredibly low for the safety factor it was providing.

    Do I need to limit it to just "1" beer?
     
  20. DrMacDaddy

    DrMacDaddy New Member

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    Thanks KB,....I'll look for those clips in the front fairing that are holding me up from being able to remove the two side mid fairings. Probably the main issue for me is my residual eyesight. Back in January-February of 2003, both of my retinas completely detached from the back of my eyes. It took until November of 2003 before we found a retinal surgeon that felt he could use a laser to weld my retinas back on the rear side of each eye. The right eye was done in November of 2003, and the left eye the following January of 2004. The laser-welding procedure destroys the retinal tissue wherever it touches, and the surgeon had to literally go all around each retina. After that came 7 additional laser treatments to keep them re-attached. Although the reattachment procedure gave me my eyesight back, each laser treatment destroyed more and more of my peripheral vision. So, even though I have enough residual eyesight to ride my bike.....it's incredibly difficult to do anything mechanical that requires strong eyesight into small areas. Like most guys, though, I don't want to throw in the towel. I'll work with what I have left....and hopefully with help from guys like you I'll be able to fix this bad boy. Thanks KB.

    Btw....according to the SeaFoam directions.....it is intended to be used as both a gas and oil additive. It was also created to be used with all internal combustion engines, including motorcycles. Around the top of the metal bottle it has small pics of all the vehicles it can be used with, and one of those pics is a motorcycle. In the past, i always used B12 Chemtool to clean out my bike carbs as an additive,...but have read in several forums that a lot of riders like the SeaFoam product....so that's why I decided to try it this time.
     
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