Carb Re-jetting question

Discussion in '3rd & 4th Generation 1990-1997' started by roger_roger, Mar 15, 2010.

  1. roger_roger

    roger_roger New Member

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    I just picked up a 1997 vfr750 with 16K miles. The bike came with a two brother's pipe on it. I have read that the carbs should be re-jetted to optimize the air flow because of the new pipe. The original owner had died and his sons had no idea if the bike's carbs had been re-jetted. Is there anyway to tell?

    thanks,

    (two days of ownership and 270 miles later, I love this bike...LOVE IT!!!)
     
  2. TOE CUTTER

    TOE CUTTER Mullet Man

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    Remove the air box and lift one of the slides to inspect the needle. If it has a very sharp taper to it in the middle of the needle it has a jet kit. Or remove the carbs and inspect the jets.
     
  3. roger_roger

    roger_roger New Member

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    Toe Cutter: Thanks I will give that a try. I have been wanting to check out the air cleaner anyway and see if it needs replaced. The bike has been sitting for a while, so I wouldn't be surprised if it is the stock air filter anyway. I want to get a K&N or BMC so that I can maximize air in to match the air out the pipe gives me.
     
  4. squirrelman

    squirrelman Member

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    IF it feels good with no flat spots or hesitations anywhere in the rpm range then you needn't worry cuz with or without a jetkit the bike runs fine.

    If someone (RIP) poped for the pipe he probably did the jetting too.
     
  5. roger_roger

    roger_roger New Member

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    The bike does feel pretty good, but compared to my old bike: A single banger bmw 650, this bike has astonishing performance. I probably will have to get a little more comfortable on it before I start noticing any major flat spots. All of this carb stuff came about because it idles slightly rough and it made me wonder if maybe the idle jet was too small for the pipe. The bike, however, has been sitting and had old gas in it. I think I will change fuel filters, air cleaner, do a valve job, and try to sync the carbs before I can really evaluate the bike. ....And I always wonder why I don't have time for anything....
     
  6. squirrelman

    squirrelman Member

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    HOW 'bout a can of SeaFoam or other carb cleaner worked through the system??

    Idle jets are very easily plugged if fuel is allowed to dry up in carbs, and a can of cleaner usually improves running ......if the bike does run.

    You could check valve clearances, but those bikes go for a long time before any clearance adjustments are required.
     
  7. roger_roger

    roger_roger New Member

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    Carb re-jetting issues


    I am going to try the seafoam tomorrow or the next day. I am hoping that this will help smooth her out. The bike does run...oh boy does she run.
     
  8. TOE CUTTER

    TOE CUTTER Mullet Man

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    Rode Doodles bike last weekend, on the back wheel. It does pop a bit but runs well, a slight bump in jetting will cure it , but it does run very well for a stock machine with the same pipe.Would at most run one size larger slow jet.Maybe shim the needles.Just a touch.
     
  9. roger_roger

    roger_roger New Member

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    hmmm, that's seems to make sense. I just poured some seafoam in. I'll give that a tank or two, then dive into some minor carb fixes. I don't mind the popping, but I want to fix this slightly rough idle. Gas mileage seems to be pretty decent so far and performance appears to be top shelf, so maybe just screwing with new plugs, filter, etc will do the trick. Kind of like a 16K service. I am smelling fuel a bit at idle and shut off, too. This leads me to believe that I have some clogging issues in the carbs as well. You guys say seafoam, I'll use seafoam. (plus my Dad uses it in his '62 Power wagon...I don't think they're that much different, right?). Thanks for the help guys...I can't wait to get this bike up to fully maintained and unleash the beast!!
     
  10. epicbeardman

    epicbeardman New Member

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    I can't believe that no one has explained how to check for proper air/fuel mixture. Read the plugs!! They will tell you instantly if the bike is running too rich or too lean.

    I'm willing to bet that your mixture is fine right now, if the bike is running great. If you just go ahead and change the jetting w/o knowing the current air/fuel status, you will almost certainly make it worse. That's what happened to my bike at some point. It has an aftermarket can, just a slip-on, somebody decided that it needed a jet kit. It runs rich. I am currently in the market for a set of stock needles/jets if anyone has an extra set. From what I've gathered in my limited research, these bike do not need to be jetted at all with just a can change. Honda put enough richness in the stock jets for that minor change in exhaust.

    Also, if you want a K&N AF, I've got one to sell. I put a new stock filter in mine, much better filter and no restriction issues. K&N filters are the worst junk out there and really let a lot of dirt into your carbs but if just have to have one, hit me up.
     
  11. TOE CUTTER

    TOE CUTTER Mullet Man

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  12. VT Viffer

    VT Viffer New Member

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    I also have a 4G with a TBR can hanging off the back. Stock jetting, K&N filter. I have the popping on decel (throttle closed to >5% open), and it has a lumpy idle. She's a little cold blooded, but not like a Ducati.

    Just the nature of the beast. Leave the carbs alone!
     
  13. viffer93

    viffer93 New Member

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    Considering any motor is just a big air pump and we make changes like better breathing air filters and exhaust systems to increase air flow in and out you have to also make adjustments to the carbs. No argument that any motor will run without adjusting the carbs or even syncronizing them but it does not mean you are receiving the benefit of your changes (unless you just did it to say you did it).

    My 93 VFR have a K&N, TBR full exhaust and I installed a Factory Pro jet kit and rode my VFR for a while thinking I dialed it in just fine, until I had the chance to get the bike to someone I trust with the dyno. My settings were off and I made the recomended adjustments (increased main jet, reduced idle jet) and I can now say I am getting the benefit of the air filter and exhaust.

    I believe the following about carbs.
    - If you change the intake and outtake specs of any motor you will affect the settings of the carburation. It will run but not at its optimum.
    - There is always a benefit when any motor is jetted and adjusted properly but it takes time to do and rarely is it done "perfectly" the first time
    - Aftermarket jet kits typically have an adjustable clip on the needle jets. I know the VFR OEM ones do not
    - Take the time to syncronize your Carbs after any ajustments especially valve adjustments.
    - Seafoam works well. I also run a bottle of Techron through my bike in the spring and before I put it away for the winter to keep carbs clean
    - Over time, jets and needles do wear out, get dirty or become enlarged (Fluids are abrasive). We don't notice because it is a change that happens over time but sometimes it is worth replacing the jets and needles to bring things back to OEM spec.
    - I am not good enought to judge carburation by the "seat of my pants" (some people are) take the time to bring it to someone who can show you data from a dyno. Not everyone who runs a dyno knows what they are talking about.
    - When storing the bike take the time to drain the carb bowls and fog. I am sure plenty of people here have not done this and never had a problem. For me it is cheap insurance for the long term.
    - I am not a carb expert and I am not afraid to get help from people who are. There are some people who just love to talk jet kits and carbs who are willing to help. Find them.
     
  14. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    :rolleyes: :popcorn:

    Some things can get a little carried away. If a guy isn't doing track days or drag racing, taking the bike to a dyno and messing trying one jet over another is a costly expense for no noticable gain.

    Reading plugs is over rated. As there are typically 4 systems in a carb (enrichment, idle, slow speed, and main), attempting to read plugs only gives a generalization and tells little about how one "part" of the carb is working over another. Also, some people believe that the removal of lead in fuel has left less of the necessary coloring once used to read the plug not to mention that many states are now mandating up to 15% ethenol in the fuel. Looking at the plugs for consistancy across all four cylinders is more telling than what shade of red a person thinks they see.

    My plugs read me bedtime stories.

    No offense, and welcome new guys - I'm just sayin....
     
  15. viffer93

    viffer93 New Member

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    I agree that one can get obsessed wtih getting it absolutly perfect but the expense is questionable unless you are going to a dealer then it is very expensive.

    Dyno run and analysis $50.00
    New Jets : $3.00 ea from Factory Pro
    Improved throttle response and operation: Priceless

    My opinions are worth what your paying for them
     
  16. epicbeardman

    epicbeardman New Member

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    You can still read plugs for A/F mixture just fine. It's pretty glaringly obvious if the engine is running lean. If you are concerned about false readings from the idle or slow speed jets(?), just run the bike @ 4k rpm in 2nd gear for half a block, hit the kill switch and pull in the clutch while you coast into your driveway, then remove the plugs. Personally, I find that unnecessary.

    There is a lot of dodgey info on there on the interwebs and a lot of $$ to be made off urban myths. (Like K&N filters). What makes people think that the stock air filter is "restrictive"?? Manufacturers are bound by noise and emissions regulations with regards to mufflers/exhaust but there is no govt. regulation or engineering parameter that would cause Honda to put air filters that are too small on their high-performance street bikes. Or do people think that the sales douches @ K&N know better than HRC? Furthermore, if an AF was restrictive, the solution would be a bigger filter size, not bigger holes in the same size filter. Use your heads on this one, guys.

    As you may have guessed, I'm not a big fan of oiled air filters. But don't take my word for it. Spend a minute or two with your mouse clicking and research it. I'm new to this board but not to burning gasoline. Thanks for the welcome...
     
  17. roger_roger

    roger_roger New Member

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    Guys,

    Thanks for the info. I have changed the plugs, air cleaner (went with the stock one), fuel filter, and have ran two tanks with sea-foam additive in them. bike is running better, but still has a lumpy idle. The plugs indicated that bike was running pretty good...maybe a little rich, but not enough for me to worry about it. (read: if I screw with the carbs at this point, then I will make it worse.) I bought the bike last Saturday and you could tell the previous owner had let it sit for a while. The gas had a varnish smell to it and I am willing to bet the carbs were starting to get a little gummy. I have put almost 500 miles on it this last week and it is starting to run better and settle into its own. I think that it will probably get better and better as I ride it.

    Thanks for the info!
     
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