How do I know my Carburetor needs to be serviced?

Discussion in '3rd & 4th Generation 1990-1997' started by Baystreets510, Jan 21, 2014.

  1. Baystreets510

    Baystreets510 New Member

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    Hey guys sorry I'm still new to the mechanics of my bike, it's a 1991 Honda VFR 750F and I can't find any place that has Manuals for my year... Anyways my symptoms right now is that any sudden acceleration the the throttle my bike dies out.. It idles fine from time to time, and when I was able to accelerate I took out for a test run and it seems fine under 4 RPMs but when it goes above it chokes out and dies. I can be rolling and pop my clutch to start it again but as soon as I hit the throttle I'll choke out. Now I've tries carb cleaner, I haven't checked any tubes and defined not going to rebuild the carb (as I'm still new..) I can slowly accelerate it above 4 RPMs when in idle but not in gear. Before I shell out cash to the mechanics, can anybody help me out with possibilities of what could be wrong? I don't want to be clueless when I bring it to them... Thanks again in advance guys
     
  2. skimad4x4

    skimad4x4 "Official" VFRWorld Greeter

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    Hmm.

    I'm not sure I should attempt to offer any advice on this one as its been ages since I last had a bike with carburettors - but I can see you are not getting much by way of response to your question.

    Please don't see this as me having a go at you, but I rather think you need to offer a lot more information before people on here are able/willing to offer even generic advice.

    When reading through your post, it prompted a whole heap of questions which you probably already know the answer to, but without which, it is nigh on impossible to make sensible suggestions from the other side of the planet.

    You don't say how long you have owned the bike? Whether it has ever been running properly whilst you have owned it? When you last had the bike properly serviced - and when it was done whether it was running fine? Whether the problem arose slowly over many months, or arose suddenly? Whether the bike was running fine pre winter but was unhappy when you tried to restart the bike in Spring. How old is the fuel in the tank?

    You seem to imply its a fuelling problem - and you may be right - but don't say what has led you to this conclusion - certainly problems with spark plugs/leads or electrical issues can also make a bike run roughly?

    As for causes and/or reasons to consider servicing a carburettor it is perhaps self evident when talking about 23 year old components. When the bike was new Honda specified a detailed programme of service intervals designed to ensure things which are likely to deteriorate are addressed before they become problematic, sadly as bikes age people often ignore the servicing and you may be struggling with the inevitable consequences.

    If I was on a tight budget, before calling in a mechanic, I would at least do some basic checks - remove filler cap and with a decent torch inspect the fuel tank for signs of rust - a notorious cause of clogged carburettors. Remove the spark plugs and check for signs of damage - do they look similar? is there any sign of over/under fuelling - there are colour photo charts available online which show what a healthy plug should look like and plugs with defects. If the fuel is many months old, then drain and replace with fresh quality fuel. Check the choke and throttle linkages are not catching.

    Sorry to provide more questions than answers...





    SkiMad
     
  3. GreyVF750F

    GreyVF750F Member

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  4. Guaire

    Guaire New Member

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    Air filter? If an air filter is really clogged, it will bog down the motor under throttle. Open throttle = no air = too rich.
     
  5. Baystreets510

    Baystreets510 New Member

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    Hey guys sorry for the lack of info...

    It's a 1991 Honda VFR750F
    I have owned for 1 year so far and within that year I have only experienced problems a couple of months ago
    I got the bike at 14,000 miles and am now currently at 17,000.
    I have not let the bike sit as I rode all year this year

    I do apologize for the lack of info I will try out the basics that you explained, thanks again I highly appreciate any advice / help
     
  6. squirrelman

    squirrelman Member

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    Drain the float bowls into a paper towel, looking for red rust particles. If the bike won't idle smoothly or needs alot of choke to keep running or won't rev strong from idle up to redline without flatspots or weezing, carbs likely need a cleaning. Could also be related to fuel pump or its relay, dirty filters, etc.
     
  7. desktopdave

    desktopdave New Member

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    Bet it's the fuel pump or relay. Honda installed one so the bike wouldn't starve at high RPMs.

    I've never owned a '91 VFR but I've had a bunch of other V4 Hondas with that same system. In the older Hondas, testing is easy. Turn on the ignition (don't start the bike) then toggle the kill switch on and off. You should hear the pump audibly 'click' once every time you cycle the kill switch. Any decent bike mechanic will know about these pump setups, they're on a lot of older bikes.

    The fuel pump relay is solid-state, thus makes no sound when switching on and off. It's a little black box, likely mounted on the rear frame under the seat. You can test the relay by process of elimination. Don't recall wire colors, but you short two wires on the relay harness to see if the pump switches on.
     
  8. Baystreets510

    Baystreets510 New Member

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    Update:

    Hey guys so i cleaned my Air Filter (Literally was clumps of dirt on it) and i was able to reach higher speeds, it did not choke out at 4 RPMs but it did sound like it would when i aggressively hit the throttle. (its like it stuttered a bit and jolted me slightly)

    not sure if this helps with any diagnostics lol
     
  9. crustyrider

    crustyrider New Member

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    yeah clean the carbs
     
  10. REEK

    REEK New Member

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    and with clumps of dirt, I'd also get a new air fiter.
     
  11. skimad4x4

    skimad4x4 "Official" VFRWorld Greeter

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    +1 to all the above, especially the relay which I had forgotten about was another pain for some 750's. But in short I guess most of them would be picked up if this had been done..

    "Service Intervals
    Minor service intervals (oil and filter) are every 4000 miles or six months, with a more comprehensive service every 8000 miles and a major seeing-to at 16,000. The latter includes valve clearances and will set you back around £325 including parts, labour and VAT. The VFR750 is getting on a bit, so check for excessive wear in the consumables – chain and sprockets, fork seals, tyres, brake pad and disc wear. Check the bike’s service history to be certain it has been looked after properly."


    Note the "major seeing to at 16K" - It sounds like the poor thing just needs a bit of TLC and you should be good to go.:vtr2:

    Take care and let us know how you get on.




    SkiMad
     
  12. squirrelman

    squirrelman Member

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    Maybe your idle mix adjustment screws are still set lean from the factory. If so, you can experiment by opening each up between 1/2 and 1 turn to look for improvement.
     
  13. kennybobby

    kennybobby New Member

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    Are you using the choke lever to start the bike? What is the idle speed when the engine is warmed up? Does it require the choke lever to be on to keep running up to 4 krpm?

    If you are always using the choke lever to run then the pilot jets are clogged.
    Usually the pilot jets and choke jet clog from debris or old fuel, but it seems that maybe your main jets are clogged or the main needle is varnished and dirty.

    It seems that a carb cleaning is in your fu-chure.
    Everybody can learn to clean carbs if they have motivation and attention to details: http://vfrworld.com/forums/mechanics-garage/22449-cleaning-carbs.html
     
  14. ridervfr

    ridervfr Member

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    You need to remove the carburetors and give them a good cleaning, look at it this way; your gona be so intimate with your viffer, your gona have to buy "her" roses for Valentines day. Seriously though, most of the o-rings and gaskets are gona be perished including your insulator boots along with the clamps. You may have paid very little for her, but its not gona be a free ride! Your bike should start with the inricher and you should be able to :flip: it off after 15 seconds and the bike should take throttle with-oot stalling. You can see this on your tacko, once you have an idle, (after 40 seconds or so, you can snick that sucker into gear and ride with good throttle response.) If your in a cold climate, maybe a minute, or so. You could try removing the fuel mixture screws and juicing them with some carb-cleaner and blowing some air in there, but I garantee that the 0-ring is kaput, thats why your gona need 4 carb rebuild kits for that bike. Nuff Sed Pictures are requied by the way :worthless:
     
  15. Billy C

    Billy C New Member

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    The most common problem with carbs and prolly 90% of my carb rebuild biz is clogged idle circuits. The new ethanol blended fuels are terrible for clogging these circuits if you don't prep your bike for non riding season (winter for most). The symptom is the bike will not idle well if at all unless choke is applied. Another common symptom is the engine will stumble or die when you try and give it throttle. The slow jets are #40S on these bikes and being bigger than the #38 jets on the earlier V4 models are less prone to clogging but are certainly not immune. Another symptom of carb trouble is loss of top end power. The culprit is a ripped vacuum slide diaphragm. The diap rips and will not lift the vacuum slide out of the bore at higher rpms as it should and this chokes off air/fuel to that cylinder(s). I see this on a fair number of 82 - 88 bikes but have yet to see it on a 3rd Gen Magna or 3rd/4th VFR. Better rubber? Or more likely just not old enough yet to start having this problem. Something I try to spread the word about is NEVER EVER spray chemicals in the intake funnels of your carbs. Folks that don't know any better tend to spray ether (starter fluid) in the intake to try and start a balky engine and it is death to those diaps. Crack the throttle while cranking the engine and it sucks that ether up thru the bottom of the slide and dumps it right on the rubber diap. Another more obvious symptom of carb problems is fuel leaks. We are running 20 years on these bikes and rubber seals were not made to last forever.

    Before blaming your carbs always check for spark at the plugs. Blue spark on all 4 cylinders. Red spark is iffy. It may run but air/fuel under compression requires a very hot spark for good ignition. A red spark may fire it but start the burn late in the cycle resulting in an incomplete burn and loss of power. At high rpms there is less time for the coil to charge and a weak coil just makes this a bigger problem. Also during cranking the voltage from the battery to the coils drops off so a weak coil will not give a hot spark and you will get poor starts.
     
  16. ZEN biker

    ZEN biker New Member

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    Seems air or lack there of is your culprit. Get a new filter. Cleaning works as a hold off but you need a new one if its been clogged with clumps. This lack of air will upset your carb balance. Since this is a fed system, look for the bowl vents and make sure they are clear. They will be one of the ports inside the carb where the throttle plate is. Also, has the bike ever been set up for your elevation? The carbs adjust themselves a bit but if your up 2500ft like where I am then the sea level settings dont work well so you end up with flat spots at best and bogging. Worse is that it out right quits. You will need a few jets to try. #40s is the common so as you go up in elevation you need smaller jets. A good rule of thumb is for every 1000ft up go down 5 jet sizes. So a 40 becomes a 35 at 1000ft up. At 2500ft my cm200t uses #32 jet, it came with a 60 from factory. If your at an odd elevation, either round to the nearest 500ft or figure out the average elevation for around your riding area and then round down if possible. The idea is to have 250ft either side to play with. So you can ride practically anywhere around your place. Dont worry if you exceed the elevation difference as long as you dont do it for days on end. If your going mountain toppi g for a weekend then carry some jets with you so that you always can adjust for best running and riding. Same for going sea side :) jets ussually dont take long to change once you know how and what actually has to come apart.
     
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  17. ridervfr

    ridervfr Member

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    carb diaphrams are incredebly tough, you could almost do that old comedian routine where he put the condom on his head and blew it up with his nose. Have used that gorilla snot to patch up a ripped diaphram and was able to get the bike to run enough to kick it in its arse oot the door. It was a happy ending and as far as I know its still running around. (it was a second gen Magnatoid.) My gen 3 bike got the full oem treatment as far as gasket kits and insulator boots go. You can get K&L too, for less.

    Rubber dont last for ever, insulator boots get hard as rocks and o-rings can turn to dust/mush right in front of you. I go for oem stuff for my own bikes, other peoples, I get creative with.
     
  18. Guaire

    Guaire New Member

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    "Another symptom of carb trouble is loss of top end power. The culprit is a ripped vacuum slide diaphragm. The diap rips and will not lift the vacuum slide out of the bore at higher rpms as it should and this chokes off air/fuel to that cylinder(s)."
    Thanks, Billy C. That might be my problem.
     
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