1986 VFR750 Carb help

Discussion in '1st & 2nd Generation 1983-1989' started by lvumlow, Sep 26, 2016.

  1. lvumlow

    lvumlow New Member

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    Hello,

    I am new to the forum and just picked up a 1986 VFR750 from someone who let it set for awhile so it fires up but wont idle. I did some basic research and pulled the carb assembly to start the cleanup but I noticed that there is a large vacuum line that was completely disconnected. It appears to go to the four large diaphragm chambers, can someone help direct me as to where this should be connected to? Also, while I was dumping the old stale gas I noticed that the flow would stop as if it the tank was getting vapor-locked and sure enough as soon as I popped open the gas filler lid the gas started flowing well. I cannot see any vapor line at all, there appears to be a large drain line from the filler area but no other lines and I can see a small nipple under to top right of the tank but it has a rubber plug on it which is effectively sealing it. Should that be routed to some type of vapor canister or vent line? I am new to these bikes but common sense tells me that the large vacuum line from the carb should go somewhere and not simply be open like pictured below and since I do see that this bike has a fuel pump so does the tank not have to be vented?

    Any help would be appreciated.

    I was surprised I got the bike to fire at all since it was purchased for $500 as a non-runner that has been sitting for years. After opening up the bottom of the carbs I was surprised that they were relatively clean with only one that looked bad. I pulled the jets and cleaned them and put it back together but still have the same problem whereas the bike runs but will only idle with the choke on so I pulled the carbs and will clean them properly this time.

    Thanks for any help in advance.


    VFR750.JPG

    VFR_Carb.jpg
     
  2. Captain 80s

    Captain 80s Member

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    That is just a vent hose, not vacuum. Sounds like you need to do a more thorough carb clean and make sure all passages are clear. Your idle circuits are still plugged. Do not remove carbs from plenum unless you are experienced and plan on replacing fuel tube o-rings. Consider sending them out to somebody qualified and have them totally gone through.

    Oh, and , very nice bike. That was a score if the right side looks even close to the left (and even if it doesn't!). The shock alone is almost worth the price you paid.
     
  3. lvumlow

    lvumlow New Member

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    Thanks for the tip, I have the whole assembly soaking in gas right now and will try to clean out the journals and jets as one piece. The bike is very clean on both sides and sounds great, cant wait for it to idle so I can take it for a spin. It seems as though some of the circuits are curved (checked for obstructions with a small wire) so are there any tricks to clean those out besides compressed air?

    Any idea about the gas tank venting? Should there be a vent line or will the pump suck the fuel out? I can clearly see the fuel flow slow while dumping the old gas until I opened the cap and do see a vent line on the bottom upper right of the tank but it has a plug on it?
     
  4. Captain 80s

    Captain 80s Member

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    On the 49 state bikes (which is what yours looks like due to the open carb vents), the tank bents through the gas cap. They can get clogged and essentially vapor lock the tank.

    Good carb cleaner then followed by compressed air for the internal circuits should be good for your carbs, sounds like they weren't terrible. Have you removed the fuel screws yet? They have caps installed over them from the factory. In any case, those screws HAVE to come out for a proper cleaning.
     
  5. lvumlow

    lvumlow New Member

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    Again, I really appreciate the advice. I feel much more confident getting in there with some level of guidance. Is there a way to help me identify these "fuel screws" or locations that you speak of? Should I be looking for something that has been covered from the factory (potentially)? I have tried google and cannot find anything conclusive about them using that name. I can provide hi-res pics of the carbs since I have them off the bike. I am going to do my best to clean them out without having to completely disassemble the whole unit and may just get lucky...
     
  6. squirrelman

    squirrelman Member

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    As mentioned, the tube you see is a vent line and must be open to the atmosphere. The filler cap has a built-in vent, black plastic about the size of a dime; blow through it to be sure it flows air.

    Slow jets won't be properly cleaned until you can pass a thin steel wire (.010") clear through, and compressed air isn't in any way sufficient. It's important to find and remove the idle MIXTURE screws so you can blast the passages with carb cleaner and compressed air, then return them to 2 1/2 turns out from fully closed. Do this while the slow jet is removed.

    You also need to check that the throttle plates are synchronized as close as you can get them.

    Study the factory service manual so you have a better understanding of what's what in the carbs.

    The line under the fuel tank is a drain, not a vent.
     
  7. Terry Smith

    Terry Smith Member

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    Take a look at the following link to the ST1100 owners forum, where there is an awesome pictorial of cleaning out a V4 carbs. While I appreciate you have a VFR not an ST, the parts should be very similar.

    http://www.st-riders.net/index.php?topic=10249.0
     
  8. kennybobby

    kennybobby New Member

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  9. lvumlow

    lvumlow New Member

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    Thanks everyone, I think I understand now. The fuel air mixture screws may have Welch plugs in them right and you have to be careful drilling those out. I will check to see if they have already been cleared from previous services and appreciate all the links/info.

    Thanks again! Hope to have it sorted and back in by the weekend. Fingers crossed all goes well.
     
  10. squirrelman

    squirrelman Member

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    No plugs on the idle mixture screws.
     
  11. squirrelman

    squirrelman Member

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    No plugs on the idle mixture screws, so put the drill away ! :sour:
     
  12. lvumlow

    lvumlow New Member

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    Perfect, thanks again...
     
  13. lvumlow

    lvumlow New Member

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    ..just wanted to post up a follow-up

    Soaked the complete carb assembly for a couple of days and cleaned everything up - found two clogged pilot jets. Got everything back together and she runs great. I was impressed how smooth and stable the bike is, going to need some new tires before I hit the canyons but wanted to thank everyone for the input. I have never worked on one of these and it is great to have a resource like this.

    I still have a small pulse at idle but it is barely noticeable so maybe need to chase down a vacuum leak or something but it runs so nice I will be taking it to work tomorrow. Seems like a score for a little work and $500, may just have to keep it...

    Thanks again!
     
  14. ridervfr

    ridervfr Member

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    Nice score on the bike! I wish my 93 had as nice looking bodywork, i paid 500 bucks too. Invest in a carburetor balance tool so you can synchronize them. I have a morgan carb tune and love it. You can make a manometer too, but why bother. When they sold mercury you could buy a nice simple tool that worked well, not any more. Happy riding, love the bike!
     
  15. lvumlow

    lvumlow New Member

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    Rode her in to work today - have to admit is nice to be back on a street bike again (sold my '06 CBR1000 a few years back) but since the VFR has been sitting awhile the tires are pretty hard so I don't have much confidence in the turns. I will be looking for some fresh rubber and could use some suggestions...

    Can anyone suggest a good set of tires for this bike since it is running the 16/18 combo I am not sure what is available?

    What is the largest rear tire I can fit?
     
  16. lvumlow

    lvumlow New Member

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    ...one more question.

    Now that I am getting the bike on my open roads, I am finding the power band to fall off at mid-higher rev's. I feel like my tank is getting vapor locked because I get a large pressure release when I open the gas cap. Also, I have no history on this bike as to where it was jetted for so I noted that I am running 118 main jets with 38 pilots, can anyone confirm if this is within range for sea level riding? The bike feels like it wants more feel when I open the throttle but considering that I just had to clean the carbs, I may have missed something in the main circuits.

    Is there any way to confirm or clean the gas cap vent to ensure it is breathing properly?

    does my jet range sound ok with 118 mains and 38 pilots?

    Again, appreciate any feedback.

    I ordered a new set of Pirelli's today so should have them tomorrow and hopefully and can get this mid-high range flat spot resolved and I will be golden...
     
  17. Captain 80s

    Captain 80s Member

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    Your jetting might be on the lean side considering you have a 4-1 slip on exhaust. And you should not get any pressure release when opening the cap, so you have an issue there.
     
  18. squirrelman

    squirrelman Member

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    Unless someone who knows tuning has dialed out the flat spot, it's fairly common to find one if the stock exhaust has been changed.

    Most likely fix is a properly-selected washer under the slide needle. Read the tuning instructions that K&N/Dynojet furnishes with its kits.
     
  19. lvumlow

    lvumlow New Member

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    Ok, I finally got some fresh rubber on the bike so I could spend a little time in the canyons to get acclimated to the vintage persona and have some information.

    I replaced the fuel filter and no longer feel the flat spot I was referencing to earlier but the bike definitely falls off at/over 7000/7500 rpms. It drives nice and smooth with modest power through the low range (city streets) but if I get to half throttle or more in the higher rpms it definitely falls on its face. I am used to riding a modern fuel injected liter bike so I am trying to be realistic in my expectations but I feel like this bike is too lean on the top end.

    I found a long straight and got the bike in the upper rpms (at a consistent rate) and slowly pulled the choke down a little at a time. What I found is that once the enrichening circuit kicks in the bike comes alive and let me know if my methodology is flawed but this tells me the upper end is too lean.

    I went to the dynojet site get try and get a baseline expectation and my bike has the 4-1 slipon Yoshi exhaust with the airbox mod (looks like holes drilled in the top cover to let more air in) so it would seem that I would be looking at a stage 2 jet kit and they spec for a 124/128 main jet with the slide holes drilled out to a specific size. I realize this may only be an approximation but these specs are a long way from the stock 118 mains that are currently in the carbs.

    Anyone have some suggestions? Should I consider a jet kit that has the needle and other mods or simply go up on my mains until I find a sweet spot?

    Thanks for the advice ...as always, the input has been helpful.
     
  20. Captain 80s

    Captain 80s Member

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    On my 86 700 with a 4 into 1, I went with 125 mains, 42 pilots and the stock needles shimmed up using the same washers as the pilot screws have in between the springs and tiny o-rings. I did not drill my slides. K&N air filter and the air box snorkel is removed. Mid range flat spot is minimal and the bike pulls cleanly and evenly to redline. This is all butt dyno / plug reading, but it runs perfect in my opinion.
     
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